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Abstract

The vertical behaviour of 44 veteran sea trout Salmo trutta (275-580 mm) in different marine fjord habitats (estuary, pelagic, near shore with and without steep cliffs) was documented during May to February by using acoustic telemetry. The swimming depth of S. trutta was influenced by habitat, time of day (day vs night), season, seawater temperature and the body length at the time of tagging. Mean swimming depth during May – September was 1.7 m (individual means ranged from 0.4-6.4 m). Hence, the sea trout were generally surface oriented, but performed dives down to 24 m. Mean swimming depth in May – September was deeper in the near shore habitats with or without steep cliffs (2.0 m and 2.5 m respectively) than in the pelagic areas (1.2 m). May-September mean swimming depth in all habitats was slightly deeper during day (1.9 m) than at night (1.2 m), confirming that S. trutta conducted small-scale diel vertical movements. During summer, S. trutta residing in near shore habitat progressively moved deeper over the period May (mean 1.1 m) to August (mean 4.0 m), and then reoccupied shallower areas (mean = 2.3 m) during September. In winter (November and February), individuals residing in the innermost part of the fjords were found at similar average depths as they occupied during the summer (mean = 1.3 m). The swimming depths of sea trout coincide with the previously known surface orientation of salmon lice Lepeophtheirus salmonis. Combined with previous studies on horizontal use of sea trout, this study illustrates how sea trout utilize marine water bodies commonly influenced by anthropogenic factors such as aquaculture, harbours and marine constructions, marine renewable energy production or other human activity. This suggests that the marine behaviour of sea trout and its susceptibility to coastal anthropogenic factors should be considered in marine planning processes.

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... The current study was largely limited to the period of the year when the sun is constantly above the horizon at these latitudes (i.e., midnight sun); nonetheless, a diurnal effect on depth use was still evident with fish utilizing slightly deeper depths during periods with higher solar elevation. This coincided with a previous study on veteran sea trout from northern Norway, where a slight difference in depth use between day and night persisted through the year (Eldøy et al., 2017). This may indicate that either sea trout are able to adjust their vertical behaviour to subtle difference in light intensity or diel variation in depth use may represent a more general behavioural pattern rather than an explicit response to daily variation in light (Eldøy et al., 2017). ...
... This coincided with a previous study on veteran sea trout from northern Norway, where a slight difference in depth use between day and night persisted through the year (Eldøy et al., 2017). This may indicate that either sea trout are able to adjust their vertical behaviour to subtle difference in light intensity or diel variation in depth use may represent a more general behavioural pattern rather than an explicit response to daily variation in light (Eldøy et al., 2017). ...
... the fjord system or travel to the open sea was independent of body size (del Villar-Guerra et al., 2014).Future studies on the migratory behaviour of sea trout post-smolts should therefore investigate which factors drive this variability, as this would improve our understanding of the spatial ecology of sea trout, which in turn could enable more efficient management and conservation.While at sea, anadromous salmonids spend most of their time in the upper part of the water column (e.g.,Spares et al., 2012;Strøm et al., 2017), and for sea trout a strong surface orientation is well documented across geographical regions and life stages(Eldøy et al., 2017;Gjelland et al., 2014;Kristensen et al., 2018). In the current study, the sea trout post-smolts spent most of their time in the uppermost 2 m of the water column, with temperature and salinity slightly influencing the depth use of individuals. ...
Article
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Anadromous brown trout (sea trout), Salmo trutta, is currently in decline throughout its range, largely due to anthropogenic stressors in freshwater and marine habitats. Acoustic telmetry was utilized to study the marine migration of sea trout post‐smolts from three populations in a relatively pristine subarctic fjord system. While at sea, the sea trout spent a substantial part of their time close to their natal river, preferred near shore over pelagic habitats and were strongly surface oriented. Despite a fidelity towards local areas, the sea trout utilized various parts of the fjord system, with maximum dispersion >30 km and total migration distance >300 km. Almost half of the sea trout (44%) migrated between river outlets, indicating that a metapopulation approach may be appropriate when managing neighbouring sea trout populations at high latitudes. Furthermore, the different populations displayed different migratory behaviours in terms of distance migrated, dispersion from origin and the likelihood of leaving their home area. This variation in migratory behaviour is likely influenced by spatiotemporal differences in habitat quality between sites, indicating that local habitat variations may promote population‐specific behavioural responses even in relatively confined fjord systems.
... Studies of veteran migrant S. alpinus in a Norwegian fjord have suggested that S. alpinus may use the outer fjord areas more often than S. trutta and that the higher use of the outer fjord could be related to a preference for colder waters (Jensen et al., 2014;Rikardsen et al., 2007a), whereas studies from the Canadian Arctic have demonstrated that even large S. alpinus mainly reside in estuaries Spares et al., 2015). For both species, marine depth use studies have mostly been limited to larger veteran migrants, with results suggesting that both species are surface oriented with occasional deeper dives to depths of more than over 5 m (Eldøy et al., 2017;Kristensen et al., 2018b;Mulder et al., 2020;Rikardsen et al., 2007a). ...
... Most studies of S. trutta and S. alpinus in the marine environment have not focused on the post-smolt life stage, instead focusing on veteran migrants (e.g., Eldøy et al., 2015Eldøy et al., , 2017Harris et al., 2020;Spares et al., 2012). This has resulted in limited specific knowledge of the migratory behaviour of S. trutta and S. alpinus post-smolts. ...
... The dives to deeper depths observed in larger veteran migrant S. trutta and S. alpinus by several studies (Harris et al., 2020;Mulder et al., 2020;Rikardsen et al., 2007a;Spares et al., 2012) were not as prominent a feature of the depth use by the smaller post-smolts in the present study. Also, the mean depths of S. trutta and S. alpinus post-smolts were shallower those than in previous studies of veteran migrants (Eldøy et al., 2017;Kristensen et al., 2018b;Rikardsen et al., 2007a), perhaps as a result of differing diets between life stages (Rikardsen et al., 2007b). ...
Article
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Acoustic telemetry was utilized to track 49 brown trout (Salmo trutta) and 37 Arctic charr (Salvelinus alpinus) first‐time migrants of wild origin [post‐smolts; mean LF (fork length): 169 and 172 mm] in a large fjord in northern Norway. The S. trutta were registered at sea for more than twice the time of the S. alpinus (medians of 54 and 22 days, respectively). Both species were mostly detected near river mouths (>80% of detections) and almost exclusively spent their time (>95%) within the interior 18 km of the fjord. They were surface oriented, with most detections at <1 m depth and S. trutta deeper on average (median mean depths of 0.7 and 0.5 m, respectively). This study concludes that post‐smolts of both species stay closer to the surface and to river mouths than larger veteran migrants. This study emphasizes the importance of river mouths and upper water layers for the survival of both species during their first marine migration.
... The behaviour of the fish within the depth range of the net-pen used in the current study may not be entirely representative of real-life behaviour, where the fish inhabit a larger vertical realm. Nevertheless, our findings are reasonably consistent with those from earlier studies of fish in their natural habitat (Lyse et al. 1998, Rikardsen et al. 2007, Eldøy et al. 2017, Kristensen et al. 2018. ...
... A common solution to this problem is to only occupy the shallow waters with high light intensities when feeding and relocate at greater depths the rest of the time to minimize the risk of being eaten (Magnhagen et al. 2008). This is the driving mechanism behind the behaviour known as diel vertical migration (DVM), which has been observed for sea trout in Rikardsen et al. (2007) and later confirmed by a more recent study by Eldøy et al. (2017). Even though data on light intensity was not collected at our study site, introducing a potential source of error, the DVM was seemingly prominent when looking at the raw data. ...
... Another explanation could be the fact that the depth data was log transformed as well. This was our solution for dealing with the data being aggregated close to the surface and not having negative values, drawing on the methodological response to the same challenge in previous studies like Eldøy et al. (2017). This approach implies less emphasis on the extreme values in the data distribution, the values that seemed to be highly correlated with high surface light intensi- (Table S2b), where β 0 is the intercept,⎯T i EXP is averaged experienced temperature,⎯S t up is averaged surface salinity (0.2 m), and W i is mean mass of start and end sample. ...
Article
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Sea trout are known for seeking out sources of freshwater to rid themselves of salmon lice. However, the effect of natural haloclines in fjords on parasite dynamics is not well understood. We tagged 48 naturally infested wild sea trout with acoustic depth sensors. The fish were kept inside a small net-pen (4 × 4 × 5 m), 12 at a time, in western Norway during 4 separate time periods in spring 2017. The sea trout were relatively highly infested with sea lice (prevalence: 100%, mean ± SD: 68 ± 58 lice fish ⁻¹ ), and a relatively large proportion of the individuals did not survive the trials (25% mortality). The results show that temperature and light were the 2 most important factors explaining the vertical behaviour of the surviving trout. Mobile lice also had a significant effect on depth distribution, where fish with higher abundances of lice were observed at shallower depths. During the 7 d periods in the net-pen, total sea louse abundance decreased from a mean of 68 to 35 fish ⁻¹ . Surface salinity explained this reduction better than the experienced salinity of the individual fish, suggesting that short-time exposure to very low salinities, rather than long-term exposure to moderate salinities, is the driving force behind the effect of haloclines on reduction in sea lice numbers.
... A total of 63.8% of all measurements in the dataset were from depths shallower than 3.0 m (Fig. 2), and this residence at the surface aligns well with other findings of studies of sea trout behaviour in fjords (Rikardsen et al. 2007, Eldøy et al. 2017). This may therefore be a general behavioural characteristic for sea trout in marine environments, as it is for Pacific and Atlantic salmon (Walker et al. 2000, Friedland et al. 2001, Reddin et al. 2004, Tanaka et al. 2005. ...
... The observed behavioural pattern with repeated dives during daytime and resting at the surface during the night (Fig. 3) also has a commonality with findings in DST studies of Atlantic salmon (Reddin et al. 2004(Reddin et al. , 2011 and Pacific salmon (Walker et al. 2000, Friedland et al. 2001, Tanaka et al. 2005, but the behaviour has so far not been documented to a similar detail in sea trout. Eldøy et al. (2017) found a slightly deeper mean depth during day (1.9 m) than during night (1.2 m) in the Snillfjord in central Norway, indicating the presence of some small-scale diel vertical movements there, while Rikardsen et al. (2007) found no difference in depths during daytime and night-time in the Altafjord in northern Norway during a period of continuous light. ...
... The deepest recorded dive of 88 m in the present study is the deepest recorded dive for sea trout reported in the scientific literature; Eldøy et al. (2017) observed dives down to 24 m and Rikardsen et al. (2007) observed dives down to 28 m. All fish in the present study visited depths deeper than 44 m, and generally behaved like the Atlantic salmon studied by Reddin et al. (2011) where the fish visited depths down to 50 m. ...
Article
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We tagged 125 sea trout kelts (length: 460−925 mm) in Danish rivers with positively buoyant, depth- and temperature-sensing data storage tags. Eight tags were recovered from fish that had completed a full marine cycle (exit and return to natal river). Mean duration of the post-estuary marine cycle was 96.1 d (range: 47−142 d). The trout resided at depths of 0−3 m for 63.8% of the time and exhibited a characteristic diurnal behavioural pattern with repetitive dives deeper than 5.0 m during daytime and residency at the surface during night-time. The number of dives increased with day length, but dive duration was unaffected. Mean dive duration increased with water temperatures from 9.79 min at 5−7°C to 79.8 min at 17−19°C, and mean residence depth increased with water temperatures from 1.95 m at 5−7°C to 10.1 m at 17−19°C. The fish showed a marked response to temperatures above 17°C by residing at greater depths and by discontinuing the characteristic dive/surface residency pattern for prolonged periods of time during warm periods. Temperature data indicated that the fish were generally close to land in the beginning of the marine period and had migrated into open sea during summer. Our results suggest that Danish sea trout kelts aim to optimize their growth at sea by exhibiting a characteristic foraging pattern similar to that of Atlantic salmon and by seeking temperatures within the range reported as optimal for growth in the species.
... Several watercourses with partially anadromous populations of brown trout drain into both fjord systems. The Hemnfjord study area is described in detail by Eldøy et al. (2015Eldøy et al. ( , 2017 and Flaten et al. (2016), while the Tosenfjord study area is described by Bordeleau et al. (2018). ...
... The individual migratory behavior of the tagged individuals was relative consistent among years despite some observed yearly variation in sea water temperature and salmon lice prevalence in salmon farms. Previous studies have linked both horizontal and vertical marine responses of anadromous brown trout to variation in seawater temperature (Rikardsen et al., 2007;Jensen et al., 2014;Eldøy et al., 2017;Kristensen et al., 2018). It has been thoroughly documented that open cage salmon farming can lead to the unnaturally high infestation of wild salmonids and alter their marine behavior Finstad et al., 2017). ...
Article
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Despite that the study of individual repeatability is a common topic in behavioral ecology, virtually nothing is known about inter-annual variability in the marine migratory behavior of iteroparous salmonids that can complete multiple feeding migrations in their lifespan. Behavioral data from 38 anadromous brown trout (Salmo trutta), tracked by acoustic telemetry in 2–3 consecutive marine feeding migrations in two Norwegian fjord systems, were analyzed for intra-individual repeatability in key aspects of their marine migration. Individual brown trout displayed significant inter-annual consistency in marine area use and in the timing of marine exit (i.e. when they returned to spawning rivers), but not in the timing of marine entry or the time spent in the marine environment each year. Our study raises new questions about how anadromous brown trout respond to changing conditions and anthropogenic factors in the marine environment. Intra-individual repeatability of brown trout linked to changing environmental conditions should therefore be a focus for future studies.
... In northern Europe, immature trout can also return to freshwater in summer after a short stay at sea. Ionoregulation in sea water at low temperature is arduous, but anadromous trout have been observed at sea during winter and tolerate full salinity seawater at temperatures as low as 1-2°C (Eldøy et al., 2017;Knudsen et al., 2009;Olsen, Knutsen, Simonsen, Jonsson, & Knutsen, 2006). In the brackish Baltic Sea, parr can migrate from freshwater to the Baltic coastal zone without undergoing smolting. ...
... This pattern resembles the strategy followed by some Pacific salmonids, such as pink salmon (Oncorhynchus gorbuscha, Salmonidae) and chum salmon (Oncorhynchus keta, Salmonidae), which both start their seaward migration early in life. Trout from streams with low water level during winter may migrate to a neighbouring watercourse for overwintering (Aldvén & Davidsen, 2017) or stay in marine waters (Eldøy et al., 2017;Olsen et al., 2006). ...
Article
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This paper reviews the life history of brown trout and factors influencing decisions to migrate. Decisions that maximize fitness appear dependent on size at age. In partly anadromous populations, individuals that attain maturity at the parr stage typically become freshwater resident. For individual fish, the life history is not genetically fixed and can be modified by the previous growth history and energetic state in early life. This phenotypic plasticity may be influenced by epigenetic modifications of the ge-nome. Thus, factors influencing survival and growth determine life-history decisions. These are intra-and interspecific competition, feeding and shelter opportunities in freshwater and salt water, temperature in alternative habitats and flow conditions in running water. Male trout exhibit alternative mating strategies and can spawn as a subordinate sneaker or a dominant competitor. Females do not exhibit alternative mating behaviour. The relationship between growth, size and reproductive success differs between sexes in that females exhibit a higher tendency to migrate than males. Southern populations are sensitive to global warming. In addition, fisheries, aquaculture with increased spreading of salmon lice, introduction of new species, weirs and river regulation, poor water quality and coastal developments all threaten trout populations. The paper summarizes life-history data from six populations across Europe and ends by presenting new research questions and directions for future research.
... Although acoustic telemetry is a well-established approach for these types of studies, its application over large spatial areas requires great investments both in economic (e.g., a high number of receivers) and operative terms (e.g., installation and maintenance of the receiver array) [1]. Consequently, most of the telemetry studies carried out so far have focused either on small study areas or on specific portions of large aquatic environments, such as bays or fjords [12][13][14][15]. Only a few studies have investigated fish movement in deep large lakes, due to the complexity and extension of such ecosystems [1,16]. ...
Article
Full-text available
Fish movement into large, deep lakes has been rarely investigated due to the complexity and extent of such ecosystems. Among the different monitoring methods available, acoustic telemetry enables the study of the spatial ecology and behavior of aquatic organisms in lentic environments. In this study, the movement of 69 hatchery-reared adult brown trout (size 43–61 cm) marked with acoustic transmitters was monitored in the large and deep subalpine Lake Lugano (Switzerland and Italy). Trout were tracked for six consecutive months by seven acoustic receivers (March–August 2022), positioned in a non-overlapping array. Trout movement was reconstructed using R packages specific for acoustic telemetry (actel and RSP), which also allowed us to translate tracking information into utilization distribution (UD) areas for each fish. The effects of different environmental variables (rainfall, water discharge of the two main tributaries of Lake Lugano, atmospheric pressure, cloud coverage, and moon phases) on trout movement were tested, but none of these variables seemed to significantly correlate with fish movement. After release, most of the tagged fish exhibited reiterative movements during the initial month, with some maintaining this behavior throughout the entire study period. This spatial behavior can be particularly evident in hatchery-reared fish due to their aggressive and bold attitude. The association of these behavioral traits, shaped by domestication, could expose hatchery-reared fish to high risks and post-release mortality in the wild. Indeed, within a few months after the release, most of the tagged fish were no longer detected by the acoustic receivers. In addition, 26% of the total tagged fish were caught by recreational or professional fishermen.
... Diel movement patterns can often be observed in sea trout behaviour, with varying degrees of strength and consistency, depending on season (Eldøy et al., 2017;Haraldstad et al., 2017). Although insufficient data were available to show any significant conclusions regarding diel movement of sea trout in this study, it was apparent that in some cases, strong diel patterns occurred in the shallow-water areas of the fjord. ...
Article
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As a predatory fish that migrates between freshwater and marine environments, the sea trout (Salmo trutta) is important in linking these systems. This study investigated movement patterns of sea trout in a coastal fjord at the Swedish Skagerrak region from August 2018 to January 2019, using acoustic telemetry, while assessing these against environmental variables across different spatial and temporal scales. Six acoustic receivers were deployed in the fjord and a river, which flows into the upper reaches of the fjord, with the aim of detecting 20 sea trout that had been tagged with acoustic transmitters. Sea surface temperature and winds (east–west) affected movement patterns of the sea trout the most, while changes in atmospheric pressure were also important, but to a lesser extent. Sea surface temperature and atmospheric pressure both had a positive relationship with the number of detections, whereas stronger winds from the east (i.e. from land) resulted in more detections than stronger winds from the west (i.e. from the ocean). In addition, interesting diel (day–night) movement patterns were observed for some fish. A significant positive correlation was also discovered between the weight of the fish and the number of detections. This study offers insight in movements of sea trout that use coastal habitats and how environmental conditions can affect movement patterns in a fjord system. To further our understanding of sea trout movement patterns and connectivity, tracking from river, through fjord, out to sea and at a longer time scale with more variation in fish size would be valuable to understand more about the complex movement dynamics of this important species.
... Despite acoustic telemetry is a well-established approach for these types of studies, its application over large spatial areas requires great investments both in economic and operative terms (50). For this reason, most of the telemetry studies carried out so far have focused either on small study areas or on speci c portions of large aquatic environments, such as bays or fjords (34,35,77,95). A few studies have investigated sh movement in deep large lakes, due to the complexity and extension of such ecosystems (2, 50). ...
Preprint
Full-text available
Fish movement into large, deep lakes has been rarely investigated due to the complexity and extent of such ecosystems. Among the different monitoring methods available, acoustic telemetry enables to study spatial ecology of aquatic organisms and to investigate their behavior in lentic environments. In this study, movement of 69 hatchery-reared adult brown trout (size 43–61 cm) marked with acoustic transmitters was monitored in the large and deep subalpine Lake Lugano (Switzerland and Italy). Trout were tracked for six consecutive months by 7 hydrophones (April-August 2022), positioned in a non-overlapping array. Trout movement was reconstructed using R packages specific for acoustic telemetry (Actel and RSP), which also allowed to translate tracking information into utilization distribution (UD) areas for each fish. The effects of different environmental variables (rainfall, water discharge of the two main tributaries of Lake Lugano, atmospheric pressure, cloud coverage and moon phases) on trout movement was tested, but none of these variables seemed to correlate with fish movement. Although trout marked with acoustic transmitters were hatchery-reared fish, the extent of their total movement throughout the study period was unexpected, with many trout moving for more than 100 km in few months. The extent of such movement patterns is potentially related to an explorative behavior, which is more pronounced in hatchery-reared fish due to their more aggressive and bolder attitude. The association of these behavioral traits, shaped by domestication, could expose hatchery-reared fish to higher risks and post-release mortality in the wild.
... Puget Sound bull trout, Salvelinus confluentus, migrate downstream at about the same time of year as cutthroat trout (Quinn and Losee 2022) and are similarly shoreline-oriented (Hayes et al. 2011) but stay in salt water only a few months before migrating back upriver (Goetz et al. 2021). In the Atlantic Ocean, anadromous brown trout and Arctic char were close to the surface, with > 50% of char detections between 0 and 1 m, and > 50% of sea trout detections between 1 and 2 m (Rikardsen et al. 2007), consistent with other evidence that sea trout are near the surface (Eldøy et al. 2017). In contrast, post-smolt Atlantic salmon made frequent deep dives, often > 100 m (Einarsson et al. 2018). ...
Article
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Anadromous salmonid species vary in their use of open ocean, coastal, and inland marine waters. To better understand this diversity in behavior and habitat use, 120 coastal cutthroat trout, Oncorhynchus clarkii clarkii, were caught in southern Puget Sound, Washington, USA, tagged with acoustic transmitters, and 95 were detected by a network of receiver stations. Despite sufficient time to reach other parts of the Salish Sea where many receivers operated, none was detected beyond southern Puget Sound, indicating localized movements. Within southern Puget Sound, fish were detected at 34 of 127 receivers in marine sites throughout the year but especially in spring and fall. Most detection events (between first and last detections at a given receiver) were brief (60.5% were ≤ 2 h and 76.8% ≤ 6 h), indicating movement along the shoreline. However, 823 events (12.8%) exceeded 12 h and 222 (3.4%) exceeded 24 h at a receiver, indicating longer occupancy at certain sites and by certain individuals. The detections also indicated that cutthroat trout were active throughout the 24-h period, but they moved more often at night and less often in other periods than would occur by chance, and they moved more often on ebbing and flooding tides and less often at slack periods. Fish with pressure-sensitive transmitters were almost always (97.3% of records) within 2.5 m of the surface and 76.8% between 1 – 2 m, despite deeper and shallower waters nearby where they could have been detected. The data provide new insights into the behavior of this species, whose marine ecology has not been extensively studied, and differs markedly from the region’s other native salmonids.
... Amongst anadromous individuals the distance and duration of the marine migration varies extensively, both among and within populations. Typically, brown trout at sea remain close to the surface and reside in near-shore coastal and estuarine waters for just a few weeks over the summer months [23][24][25], and amongst those that undertake a short sea-sojourn, some may return as immature juveniles in order to overwinter in freshwater, without returning to spawn [15,17]. In contrast however, examples of brown trout migrating extensive distances from their natal rivers have also been documented [26,27], as well as year-round sea migrations being commonly observed [28][29][30]. ...
Article
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Background Anadromy comprises a successful life-cycle adaptation for salmonids, with marine migration providing improved feeding opportunities and thus improved growth. These rewards are balanced against costs from increased energy expenditure and mortality risk. Anthropogenic-induced environmental changes that reduce benefits and/or increase costs of migration e.g., aquaculture and hydropower, may therefore result in adaptations disfavouring anadromy. We tagged brown trout (Salmo trutta) smolts (N = 175) and veteran migrants (N = 342), from five adjacent riverine populations located in Sognefjorden, the longest Norwegian fjord-system supporting anadromous brown trout populations (209 km). Over four years, 138 acoustic telemetry receivers were deployed to track migrations of tagged individuals from freshwater and throughout Sognefjorden. Detected movements were used to fit migration models and multi-state mark-recapture models of survival and movement for each life-stage. Seaward migration distance was modelled to examine the fitness consequences from alternate migration strategies, with these models used to simulate the extent of fjord-use by individuals and accompanying growth, fecundity and survival consequences. We compared these findings with mark-recapture data collected prior to aquaculture and hydropower development. Results The telemetry data revealed that the outermost-fjord region was utilised by all populations albeit by few individuals. However, historical recaptures were located at a greater distance from the river mouth (87.7 ± 70.3 km), when compared to maximum migration distances of present-day counterparts (58.6 ± 54.9 km). River of origin influenced observed migratory behaviour and differential survival was estimated for each population and life-stage. The simulations based on telemetry-data models revealed a 30% and 23% difference in survival among populations for smolts and veteran migrants, respectively. At the individual-level, a long-distance migration strategy was rewarded with enhanced fecundity. However, the main contribution to population-level fecundity was overwhelmingly derived from middle-distance migrants, due to higher mortality rates and limited numbers of long-distant migrants. Conclusions We conclude that present-day anadromy is precarious, but potential risk varies considerably between life-stages and populations, even within a single fjord system. Our findings suggest that selection for extended migration is under pressure, we therefore stress the importance of monitoring and management actions to secure genetic variation pertinent to preserve fitness gains of anadromy. Supplementary Information The online version contains supplementary material available at 10.1186/s12862-023-02179-x.
... Wild Atlantic salmon have been observed swimming at mean depths of 0.5−2.5 m with occasional dives to 30 m (Davidsen et al. 2013). Sea trout have mean swimming depths between 2.0 and 2.5 m nearshore with dives down to 24 m (Eldøy et al. 2017). Farmed salmon spend the majority of their time between depths of 2 and 4 m, with those having a higher parasite load exhibiting a preference for deeper waters (Bui et al. 2016). ...
Article
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Planktonic salmon louse Lepeophtheirus salmonis salmonis larvae produced at salmon farms spread to and infect both wild and farmed salmonids. Understanding and forecasting the production and distribution of these larval stages from farms is critical to aquaculture management. However, model forecasts are based on available data and therefore include parameters with limited empirical support. This investigation examined salmon louse fecundity with a focus on batch egg clutch size by collecting lice from farmed Atlantic salmon Salmo salar at multiple farms and from wild Atlantic salmon and sea trout S. trutta captured at field sites throughout Norway. The data were analyzed with mixed effects models and total length of female lice was identified as the primary determinant of clutch size. Further analysis revealed that female louse total length is partially explained by temperature at sampling. However, if the temperature at sampling is spatially or temporally disconnected from rearing temperature, it may not be possible to predict the total length of a louse using temperature. The fecundity investigation further found that 66% of female lice on farmed salmon were sexually mature, and 10% of these were not egg-bearing. In comparison, 73% of adult female lice on sea trout were sexually mature, and 40% of these were not egg-bearing. Our results indicate that salmon louse production forecasts would be improved by incorporating female louse sexual maturity and a clutch size parameter that is related to total length of female lice.
... Trout in western and eastern Norway have been shown to occupy depths down to 50 m, but most fish are near the surface (Jonsson 1989). Another study on the depth preferences of brown trout in Norwegian fjords found their depth presence to be between 1 and 3 m below the surface (Eldøy et al. 2017), which more closely reflects these results. Hanssen et al. (2022) showed that smolts in Lake Evanger have a predominantly nocturnal migration; observations in this study that they moved deeper during the day suggests that they are refuging rather than actively migrating. ...
Article
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To understand the predator–prey interactions during this transitional migration, we tracked 22 Atlantic salmon (Salmo salar) smolts and their most prevalent predator, brown trout (Salmo trutta) (N = 21), and recorded their depth use in a basin of Lake Evanger, Norway, with acoustic telemetry during May 2020. Both salmon smolts (mean ± SD: 3.8 ± 3.3 m) and trout (2.9 ± 1.7 m) were distributed relatively shallow in the lake despite depths in the area largely exceeding 30 m. Both species were deeper at midday and smolts tended to be deeper in the water earlier in the migration, overlapping less with trout early in May, but as daily daylight increased and water temperature warmed vertical distribution of smolts and trout increasingly overlapped. Based on depth traces from the tags, only seven were detected at the end of the lake and confirmed to make it through. Despite the behaviour of the salmon smolts mostly matching with predictions of the risk allocation hypothesis, it seems a large share of the tagged smolts succumbed to predation.
... It may smoltify at a size of 12-25 cm (about 10-200 g weight) and make marine foraging migrations during late spring and summer to enhance the growth and reproductive potential (Elliott, 1994), that is at the time when both temperature and salmon lice production increases (Serra-Llinares et al., 2016;. In the marine environment, sea trout typically move actively around in the fjord system or in near-coastal areas, mostly remaining within 10-20 km from the river outlet (Flaten et al., 2016;Gjelland et al., 2014), where they feed in the upper water column, and therefore potentially overlap with the highest salmon lice concentrations (Eldøy et al., 2017;Johnsen et al., 2014). Moreover, sea trout are exposed to lice infestations over a much longer period compared to the Atlantic salmon (Bjørn et al., 2007;Thorstad et al., 2016). ...
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The abundance of the parasitic salmon louse has increased with the growth in aquaculture of salmonids in open net pens. This represents a threat to wild salmonid populations as well as a key limiting factor for salmon farming. The Norwegian ‘traffic light’ management system for salmon farming aims to increase aquaculture production while securing sustainable wild salmonid populations. However, this system is at present solely focusing on mortality in wild Atlantic salmon, while the responses of sea trout with different ecological characteristics are not included. We analyse lice counts on sea trout from surveillance data and use Bayesian statistical models to relate observed lice infestations to the environmental lice infestation pressure, salinity and current speed. These models can be used in risk assessment to predict when and where lice numbers surpass threshold levels for expected serious health effects in wild sea trout. We find that in production areas with the highest density of salmon farms (West coast), more than 50% of the sea trout experienced lice infestations above the levels of expected serious health effects. We also observed high lice infestations on sea trout in areas with salinities below louse tolerance levels, indicating that fish had been infested elsewhere but were returning to low‐saline waters to delouse. This behavioural response may over time disrupt anadromy in sea trout. The observed infestations on sea trout can be explained by the hydrodynamic lice dispersal model, which provides continuous estimates of lice exposure along the whole Norwegian coast. These estimates, which are used in Atlantic salmon research and management, can also be used for sea trout. Synthesis and applications. Wild sea trout, spending its entire feeding migration in fjords and coastal areas, is at higher risk than wild Atlantic salmon to lice infestations from industrial salmon farming. The high levels of lice infestation we observed on sea trout question the environmental sustainability of the current aquaculture industry in areas with intensive farming. We discuss the complex responses of sea trout to salmon lice and how the Norwegian ‘traffic light’ management system may include data on sea trout.
... However, even during high peaks, trout were present in the upstream reaches of the river where depths < 2 m would not have provided suitable refuge to supersaturation > 130% at the surface based on 9.7% compensation per metre depth (Pleizier, Nelson, et al., 2020). Solar azimuth had a profound influence on depth use by the trout, suggesting some diel variation in individual patterns of habitat selection consistent with what has been observed for anadromous trout (Eldøy et al., 2017;Kristensen et al., 2018). The bam model suggested that trout were deeper when the sun was highest by about 50 cm, in the middle of the day, and shallower during night-time by about 35 cm, relative to the intercept. ...
Article
Total dissolved gas supersaturation from dams and power stations is a chronic freshwater pollutant that is toxic to animals with aquatic respiration. Laboratory ecotoxicology experiments have revealed some capacity for captive fishes to saturoregulate by moving deeper, but field ecotoxicology research is largely lacking. We instrumented 94 brown trout in the Rysstad basin of the Otra River, Norway with depth sensor acoustic transmitters and monitored their movements for 10 months. We found that the depths used by the trout largely protected them from the effects of total dissolved gas supersaturation, which ranged from 96‐133% total gas pressure during the study. The depth use of fish was affected by sun position, lunar phase, and spatial position in the river (i.e. available depth), and there was a weak effect of total dissolved gas supersaturation, which was counterintuitive (i.e. positive slope). Depth traces of the fish revealed that nine fish died during the study, mostly coinciding with the first wave of supersaturation, consistent with observations of untagged dead fish with signs of gas bubble trauma found on the river bottom during this period. Overall, tagged trout exposure to total dissolved gas supersaturation depended on their use of depth but responses to waves of extreme supersaturation at supraphysiological levels were weak and biologically insignificant, with individual variation and spatial position in the river most important in the model. Exposure to total dissolved gas supersaturation was mediated by individual differences in habitat use, which may be linked to activity and other traits that determine overall vulnerability to exposure to total dissolved gas supersaturation.
... Prior to intensive aquaculture, the main hosts of salmon lice were wild Atlantic salmon and brown sea trout Salmo trutta, which predominantly swim in surface waters (Eldøy et al. 2017, Hedger et al. 2017. Heuch (1995) suggested diel migration was a mechanism best suited to maximise encounter rates with these largely solitary wild hosts that could be located at various depths in the upper 10 m. ...
... This is in accordance with sea trout intensifying their food search as temperatures increase during spring and summer (Klemetsen et al., 2003). Fish also swam deeper during spring and summer, which can be associated both with different habitat use and that the trout seek out colder water temperatures optimal for growth when surface temperatures rise (Eldøy et al., 2017;Kristensen et al., 2018). Fish tagged in the sea had larger home ranges, utilized a larger range of depths and had higher activity than fish tagged in the river. ...
Article
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Marine reserves can protect fish populations by increasing abundance and body size, but less is known about the effect of protection on fish behaviour. We looked for individual consistency in movement behaviours of sea trout in the marine habitat using acoustic telemetry to investigate whether they represent personality traits and if so, do they affect survival in relation to protection offered by a marine reserve. Home range size had a repeatability of 0.21, suggesting that it represents a personality trait, while mean swimming depth, activity and diurnal vertical migration were not repeatable movement behaviours. The effect of home range size on survival differed depending on the proportion of time fish spent in the reserve, where individuals spending more time in the reserve experienced a decrease in survival with larger home ranges while individuals spending little time in the reserve experienced an increase in survival with larger home ranges. We suggest that the diversity of fish home range sizes could be preserved by establishing networks of marine reserves encompassing different habitat types, ensuring both a heterogeneity in environmental conditions and fishing pressure.
... Prior to intensive aquaculture, the main hosts of salmon lice were wild Atlantic salmon and brown sea trout Salmo trutta, which predominantly swim in surface waters (Eldøy et al. 2017, Hedger et al. 2017. Heuch (1995) suggested diel migration was a mechanism best suited to maximise encounter rates with these largely solitary wild hosts that could be located at various depths in the upper 10 m. ...
Article
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Light is a fundamental environmental cue which influences the migration of many marine organisms. For the salmon louse Lepeophtheirus salmonis , light is believed to drive the diel vertical migration behaviour of their planktonic larvae. Salmon lice are of critical importance to the salmonid industry due to the damage they cause to wild and farmed hosts. Salmon lice larvae have an eyespot and are positively phototactic, yet how light intensity alters their vertical distribution remains unclear. Here, we tested how light intensity (0, 0.5, 10 and 80 µmol m ⁻² s ⁻¹ ), dispersal duration (1, 4 and 12 h) and release point (surface or bottom) influenced the vertical migration of salmon lice nauplii and copepodids under controlled conditions in experimental columns. Overall, higher light intensity increased the proportion of nauplii that aggregated at the surface. Copepodid behaviour differed from that of nauplii, as they swam upwards in both light and fully dark conditions, and surface aggregations increased with dispersal duration. Results from the experiments did not support the existing view that light strongly influences the vertical position of copepodids in the water column. Combined with previous work, our results reveal that salmon lice larval stages display different vertical responses to light, temperature and salinity, which may be explained by the different strategies of nauplii (maximise survival and dispersal) and copepodids (maximise host-finding success). Our results have implications for salmon lice dispersal models, where responses of copepodids and nauplii to light are currently parametrised by the same equations. Implementing stage-specific behaviours towards light may improve the outputs of dispersal models.
... Being a mainly epipelagic fish, salmon spend majority of their time in the surface waters of the sea. Wild out-migrating smolts mainly occupy the top 1-3 m of the water column (Davidsen et al., 2009;Thorstad et al., 2012;Eldøy et al., 2017). Another salmonid, the seatrout (Salmo trutta) occupies the upper 3 m of water 90% of their life (Klemetsen et al., 2003;Rikardsen et al., 2007). ...
Article
Buoyancy regulation is a fundamental process by which pelagic fish maintain their position in the water column in an energy efficient way. Buoyancy has been largely overlooked as an important factor in Atlantic salmon production as salmon have a physostomous swim bladder that they fill by gulping air from the surface. The speed and ease at which the swim bladder can be filled and emptied by salmon has made understanding how much it contributes to buoyancy difficult to calculate. Here, we used an “increased excess mass test” to investigate the maximum neutral buoyancy depth in seawater of farmed post-smolt Atlantic salmon across a range of sizes. The test involved adding small weights to salmon over time to make them heavier. The salmon compensate for the extra weight via gulping air at the water's surface to fill their swim bladders and maintain neutral buoyancy. We monitored the swimming behaviour of salmon to determine the weight at which they could not maintain neutral buoyancy and used this to calculate the maximum neutral buoyancy depth (MNBD). For postsmolt salmon of 175–2400 g, the MNBD in seawater ranged from 21 to 24 m, with some variation with fish size: the average MNBD of 175 g salmon was 3.2 m shallower than 2400 g salmon. Fish body density also influenced MNBD. The individual with the lowest recorded body density (1.043 g/cm3) could achieve neutral buoyancy 11.2 m deeper than the individual with highest recorded body density (1.065 g/cm3). We found that salmon weigh approximately 2.65% of their weight in air in seawater. Depth-modified cages are becoming increasingly popular in Atlantic salmon aquaculture; farming fish deeper requires knowledge of the basic limits of salmon buoyancy and our results will inform guidelines that ensure optimal welfare in these new cage types.
... This increase in depth resulted in relatively stable temperature use, which suggests that fish were regulating their temperatures, perhaps avoiding cold temperatures which may limit metabolism, activity or growth. Similar behavioral thermoregulation has been inferred in sea trout Salmo trutta (Rikardsen et al. 2007, Jensen et al. 2014, which also progressively reside in deeper waters as SST increases throughout the marine feeding season (Eldøy et al. 2017). Other anadromous species such as Chinook salmon Oncorhynchus tshawytscha ex hibit patterns of vertical movement as SSTs warm, which allow them to maintain a relatively consistent temperature (Hinke et al. 2005). ...
... Average swimming depths were slightly shallower at night than in the daytime, indicating smallscale diel vertical movements. These results agree with other studies investigating the behaviour of wild trout in the marine environment (Lyse et al. 1998, Eldøy et al. 2017, Kristensen et al. 2018. However, most of these studies have focused on veteran migrants, mainly due to technological constraints such as tag size for telemetry studies. ...
Article
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We investigated the effect of salmon lice Lepeophtheirus salmonis(Krøyer 1837) infes-tation on the survival and behaviour of wild trout post-smolts (average fork length = 180 mm) during their marine migration. Comparisons of the marine migratory behaviour were made betweenan artificially infested group (n = 74) and a control group (n = 71) in an area with low natural liceinfestation pressure. Artificial infestation was estimated to cause 100% prevalence and a meanintensity of 65 lice fish−1(mean relative intensity of 2.4 lice g−1fish). Survival analysis showed limited statistical power but revealed lice-induced mortality, with an estimated hazard ratio of 2.73(95% CI = 1.04−7.13) compared to the control group, when data from a previous pilot study wereincluded. Surviving individuals in the infested group additionally responded by residing closer tofresh water while at sea, and by prematurely returning to fresh water. On average, infested fishreturned to fresh water after only 18 d at sea, while control fish spent on average 100 d at sea. Theresidency in the inner part of the fjord and the premature return to fresh water represent an adap-tive behavioural response to survive the infestation, at the probable cost of reduced growth oppor-tunities and compromised future fitness.
... The tagged sea trout were comparable in size (54-91 cm) with kelts from other Danish studies (30-87 cm in [17] and 54-79 cm in [18] but larger than sea trout kelts studied elsewhere (e.g. 29-59 cm in [19] and 28-58 cm in [39]. Seven of the tagged kelts were, however, comparable in size (54-60 cm) with the large individuals included in studies of sea trout elsewhere while Fish 1 was substantially larger (91 cm). ...
Article
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Background: Understanding fish movements and migrations are paramount for management and conservation efforts. By applying Hidden Markov Models (HMMs) on records from electronic tags, migration routes of tagged fish can be reconstructed and new insights to the movement ecology of a species can be gained. We demonstrate the usability of HMMs on a widespread, iteroparous salmonid (sea trout, Salmo trutta) in a complex marine area with highly variable temperatures and salinities within small geographic distances. Understanding how the highly adaptable sea trout cope with these complex conditions could shed new light on factors driving the movement ecology of salmonids. Migration tracks of fish migrating at sea are reconstructed by applying an HMM on temperature and depth records from eight wild post-spawned sea trout from four Danish rivers. Results: The fish migrated at sea for 47–142 days. Estimated positions of all fish were close to the coast (<100 km) throughout the marine period, but migrations along coastlines up to 580 km away from the natal river occurred. Seven of eight individuals resided in or actively migrated into stratifed or shallow marine areas that heat up fast during spring, while all eight individuals resided in deeper and more heterogeneous areas that heat up slow during summer. All fish entered the Skagerrak (located between Denmark and Norway) at some stage during summer. Migrations were directed into less saline areas during the first 15 days at sea for all individuals. Mean linear progression of the fish was 16 km day−1 (range 0–58 km day−1 ). Conclusions: The results corroborate the expectation that sea trout are more coastally orientated than other salmonids, but also suggest that longer migrations occur in the seas surrounding Denmark compared to elsewhere. This could be a consequence of the fish seeking out habitats with optimal conditions (e.g. salinity, temperature, predation and foraging options) for growth in different parts of the year. The coinciding movement from shallow or stratifed marine areas that heat up fast during spring to deeper, more well-mixed areas that heat up slow during summer suggested that some habitat selection had occurred. These results shed new light on factors infuencing marine migrations in salmonids and demonstrate how HMMs can expand our knowledge on behaviour and movement ecology of marine fishes
... 27.5-58.0 cm in Eldøy et al. (2017) and 37.0-51.2 cm in Rikardsen et al. (2007). ...
... There were significantly more excursions from the reserve during day than night, implying greater horizontal movement during day. Salmonids have shown great differences in movement rates contrasting day and night (Alanärä, Burns, & Metcalfe, 2001;Candy & Quinn, 1999;Eldøy et al., 2017;Goetz, Baker, Buehrens, & Quinn, 2013), and it has been shown for steelhead trout (Oncorhynchus mykiss) that horizontal movement rates increase twofold during daylight compared to night in the marine habitat (Ruggerone, Quinn, Mcgregor, & Wilkinson, 1990). This may lead to a higher exposure to fishing during the day. ...
Article
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• The extent to which no‐take marine reserves can benefit anadromous species requires examination. • Here, we used acoustic telemetry to investigate the spatial behavior of anadromous brown trout (sea trout, Salmo trutta) in relation to a small marine reserve (~1.5 km²) located inside a fjord on the Norwegian Skagerrak coast. • On average, sea trout spent 42.3 % (±5.0% SE) of their time in the fjord within the reserve, a proportion similar to the area of the reserve relative to that of the fjord. • On average, sea trout tagged inside the reserve received the most protection, although the level of protection decreased marginally with increasing home range size. Furthermore, individuals tagged outside the reserve received more protection with increasing home range size, potentially opposing selection toward smaller home range sizes inflicted on fish residing within reserves, or through selective fishing methods like angling. • Monthly sea trout home ranges in the marine environment were on average smaller than the reserve, with a mean of 0.430 (±0.0265 SE) km². Hence, the reserve is large enough to protect the full home range of some individuals residing in the reserve. • Synthesis and applications: In general, the reserve protects sea trout to a varying degree depending on their individual behavior. These findings highlight evolutionary implications of spatial protection and can guide managers in the design of marine reserves and networks that preserve variation in target species' home range size and movement behavior.
Article
Brown trout is a partially migratory salmonid that makes use of diverse habitats to maximise growth and fitness. One of the most substantial threats to brown trout is infection with pathogens from open net‐pen fish farming, which creates hotspots for pathogen reproduction and transmission. Western Norway is a global hotspot for both fish farming and wild salmonids, which generates conflicts due to the impacts of the farms on the behaviour, survival, and fitness of salmonids that overlap with farming activities. In this study, we tagged adult brown trout (>35 cm) at two spatiotemporal intervals that corresponded to two different life history stages: springtime in the river when trout were completing overwintering and summer in the fjord when trout were in their marine feeding phase. The tagging revealed three different behaviours, fish that remained in freshwater, fish that migrated between freshwater and the fjord, and fish that remained in the estuary. Although some trout moved >100 km to the outer fjord areas, most trout remained relatively close to the river. Depth sensor transmitters in a subset of trout also revealed that the trout remained in the upper water column. Most of the horizontal and vertical movements therefore resulted in spatial overlap with fish farming for the migratory trout, but not for resident trout that remained in the estuary or in freshwater. Findings reveal the challenges of managing a fish with such behavioural plasticity but the urgency of recognising how important inner fjord habitats are for migratory brown trout.
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Anadromous brown trout (Salmo trutta) in the northeastern Atlantic Ocean, and coastal cutthroat trout (Oncorhynchus clarkii clarkii) in the northeastern Pacific Ocean, are analogs in many ways. Both species display similar patterns of complex life histories and behavioral flexibility, are iteroparous and facultatively anadromous and occupy nearshore coastal marine habitats where numerous populations often mix. These characteristics create specific challenges for management and conservation that have been complicated by inadequate scientific attention in some areas. Both species are declining across their native range, and their ecology make them particularly vulnerable to habitat destruction, fishing, and climate change. Here, we review the available literature to compare the biology, ocean ecology, and management practices of these two species. We highlight ecological similarities and differences between the species and identify current knowledge gaps suggesting future research needs and management actions. Using a comparative approach, the review aims to promote and facilitate knowledge exchange between anadromous trout research communities from opposite sides of the globe to improve management and conservation of these species and stimulate the production of management plans specific to anadromous trout.
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Anadromous Arctic charr Salvelinus alpinus is a cold-adapted salmonid that is vulnerable to climate warming and anthropogenic activities including salmon farming, hydropower regulation, and pollution, which poses a multiple-stressor scenario that influences or threatens populations. We studied the horizontal and vertical behaviour of Arctic charr tagged with acoustic transmitters (n = 45, mean fish length: 22 cm) in a pristine, subarctic marine area to provide insights into the behaviour of first-time migrants. Tagged fish spent up to 78 d at sea, with high marine survival (82% returned to their native watercourse). While at sea, they utilized mostly near-shore areas, up to 45 km away from their native river. Arctic charr showed large variation in migration distance (mean ± SD: 222 ± 174 km), and the migration distance increased with body size. Although the fish displayed a strong fidelity to surface waters (0-3 m), spatiotemporal variation in depth use was evident, with fish utilizing deeper depths during the day and in late July. These results represent baseline data on Arctic charr’s marine behaviour in a pristine fjord system and highlight the importance of near-shore surface water as feeding areas for first-time migrants. Furthermore, the observed dependency on coastal areas implies a vulnerability to increasing human-induced perturbations, on top of impacts by large-scale climate change in marine and freshwater habitats.
Technical Report
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As part of building of a new highway (E6) between Trondheim and Værnes in central Norway, plans are in place to fill part of Hellstranda beach, a part of the river Stjørdalselva estuary, with boulders. An extension of the road intersection Værneskrysset is also included in the plans for the new highway, which may ultimately have an impact on the old riverbed east of Langøra sør. The purpose of our study was to use electronic tagging of individual fish with acoustic transmitters (acoustic telemetry) during August 2020 to September 2021 to better understand the behavior and space use of individual anadromous brown trout (sea trout, Salmo trutta) and Atlantic salmon (Salmo salar) at Hellstranda beach and in the old riverbed at Langøra sør. Information from this study was used to evaluate possible negative impacts of the road building on these fish species and develop and evaluate mitigation measures that may reduce these negative impacts. The results showed that the planned intervention area at Hellstranda is an important feeding area for sea trout. Sea trout were observed in this area year-round, but with spring and summer as the most important periods. Most sea trout veterans (sea trout that have been at sea for at least one season before being tagged) stayed in the area from late April to late September, while most trout smolts (first-time migrants) were registered in May. The old riverbed east of Langøra sør is also an important feeding area for sea trout, and in this area, they were also commonly detected during the winter. Monitoring of sea trout smolt with a fyke net in river Gråelva, a tributary to river Stjørdalselva, showed that smolts from this river migrated downstream during the whole period when the trap was in operation (15 April – 28 May). In the rotary screw trap at Sona bridge in the main river (upstream of river Gråelva), the first trout smolts were caught in mid-April, but most smolts were caught in the last half of May. Most sea trout smolts arrived in the estuary during the night (77% of tagged smolts). During the tracking period, the sea trout at Hellstranda and in the old riverbed at Langøra sør remained in the brackish water in the upper water layers of the water column (mean swimming depth 1.5-2.0 m) and not in the more saline water layer closest to the bottom. The Atlantic salmon spent only a short time at Hellstranda and in the old riverbed at Langøra sør when they migrated from the river to their feeding grounds at sea. In general, both salmon kelts and smolts stayed less than a day in the river estuary. Although the residence time in the river mouth for salmon was short, the transition phase can still represent an important part of the seaward migration, particularly for salmon smolts. In comparison to adult stages of the salmon’s life cycle, it is more demanding for smolts to regulate salt balance when move from freshwater to sea water. This makes them more vulnerable also to other negative impacts, and this is a critical phase of their life cycle. The monitoring of the salmon smolt migration in Stjørdalselva watercourse showed that salmon smolts from the tributary Gråelva migrated out during the first half of May, while salmon smolts from the main river and tributaries upstream of river Gråelva mainly migrated out during the last half of May. The seaward migration most likely continued into June, but the monitoring was terminated on 28 May due to the start of the angling season in the river. Most salmon smolts (69%) arrived the estuary at night. Three salmon were tagged in the river mouth in August 2020 when they returned from the marine feeding migration. These fish used the entire survey area until October, after which they migrated upstream in the river - probably to spawn. Excluding one salmon tagged in December, the remaining salmon were all tagged in river Stjørdalselva prior to the seaward migration in May 2021. Consequently, there are only few data available on spatio-temporal habitat use of returning salmon kelts at Hellstranda and in the old riverbed at Langøra sør. The significant increase in sea trout activity levels in May to July at both Hellstranda, the old riverbed at Langøra sør and in the main river channel running through the estuary showed that all three areas are important feeding areas for sea trout in the summer. Loss of feeding areas in the estuary, due to the planned development of new E6, will likely increase competition for food in the remaining areas around Hellstranda and in the old riverbed. As favorable estuarine feeding areas in river Stjørdalselva already have been heavily modified, mainly due to the airport development in the 1950s, further interventions will worsen the situation for sea trout. Furthermore, both the current and a previous study of area use for the sea trout at Langøra sør, indicated that the habitat in the old riverbed is especially important for the youngest sea trout veterans at low water temperatures in winter and early spring. It is therefore strongly recommended to reduce the degradation of the estuary to a minimum and then, as far as possible, to compensate for degraded areas corresponding to what is lost. For Atlantic salmon, the habitat alterations in parts of the estuary will have less consequences than for sea trout since the salmon spent a significantly shorter time in this area than the sea trout. Nevertheless, the transition zone with brackish water is important, especially for salmon smolts, and mitigating and compen-satory measures for the benefit of sea trout will thus also be favorable for salmon. As the mapping of the area use for sea trout and salmon has shown that sea trout in particular use the area actively, the following mitigating measures are recommended during the construction phase: 1) Monitor, on a day-to-day basis, behavior and stress level of adult sea trout in the river estuary to be able to adjust specific activities related to the road construction since these may have an extra-large impact on sea trout in the area. 2) Avoid blasting and backfilling in or nearby the estuary between 21:00 - 06:00 during the main migration period (15 April - 1 June) for brown trout and Atlantic salmon smolts. Three different measures are recommended to mitigate the negative effects of the completed development: 1) Reduce the degradation of the estuary to a minimum strictly necessary for the construction of a new E6. 2) Adapt the new beach zone along Hellstranda so it consists of shallow sand and silt areas that are exposed at low tide towards. 3) Increase the opening between the old riverbed at Langøra sør and the main river so as to ensure that fresh water from the river and marine water from the fjord can still flow in and out. Further, the current survey supports previously proposed compensation measures. As the survey showed that the shallow sand and silt areas, which are exposed at low tide, are important feeding areas for sea trout in particular, compensatory measures must correspond to such a habitat. Two suggestions include construct-ion of new tidal flats influenced by brackish water and do hence satisfy this requirement. 1) Construction of a new shallow sea area with tidal flats between Sjetéen and the airport runway. 2) Restructuring of the river outlet by constructing a new pier from Billedholmen.
Technical Report
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Most river mouths from larger watercourses are today canalized or otherwise exposed to human intervention and there is a continuous pressure for coastal development in the remaining areas. Estuaries, defined as the transition zone between freshwater and seawater, are important habitats for anadromous brown trout (Salmo trutta), often termed sea trout. We have limited knowledge about sea trout migrations between freshwater and the sea, and especially about its use of estuaries. The aim of this report is therefore to document the sea trout's migrations between river, estuary and fjord in the heavily modified Nidelva, the use of the estuary in the almost pristine Gaulosen and migrations between freshwater and the marine fjord in the creek Klefstadbekken on Byneset. All three watercourses drain into Trondheimsfjorden. Migration and area use for sea trout tagged in Gaulosen, the river Vigda (near Gaulosen) and Nidelva were mapped using electronic tags that emit an acoustic signal (69 kHz) which is detected by a network of automatic listening stations (acoustic telemetry). The range of the transmitters is typically 200-400 meters, enabling mapping over larger areas. Sea trout leaving and returning to the creek Klefstadbekken, between Nidelva and Gaulosen, were recorded using Passive integrated transponder (PIT) tags. These are passive fish tags that have a short range (<1 m) and are only registered when the fish passes PIT antennas mounted in the river. In general, sea trout tagged in Nidelva stayed in the estuary for longer periods (weeks) only during March and April, prior to migration to the fjord. There were few fish with occasional visits to the estuary during winter. Sea trout in Gaulosen used the estuary actively throughout the year, but with the highest numbers of individuals present during winter, spring and autumn and fewer during summer. Gaulosen stands out from the Nidelva by having large areas of tidal-affected soft bottom flats, which are known to offer good feeding opportunities for sea trout (e.g. polychaeta and smaller fish). The sea trout in the Gaulosen estuary mainly stayed close to the surface with a mean swimming depth of 2.4 m (range = 0.1 - 25.6 m). Swimming depths were affected by the season of the year and fish body size. During December to June (which was the period with comparable data), swimming depths was shallower in the estuary (mean = 2.9 m, range = 0.4 - 25.6 m) than in the fjord (mean = 5.7 m, range 0.5 - 14.0 m). Six of the 33 sea trouts (18%) that were tagged in Gaulosen in the spring, migrated during the summer of the same year to the rivers Nidelva (n = 4) or Orkla (n = 2). Furthermore, one fish tagged in Vigda during autumn 2018 was registered in Nidelva during summer 2019. The summer migrations to Nidelva and Orkla of fish tagged in Gaulosen during the spring, indicate that Gaulosen is an important habitat for sea trout during winter and spring. This applies not only to individuals from Gaula, but also to sea trout from other nearby watercourses. The results emphasize the importance of taking care of existing shallow brackish water habitats with soft bottom beds. Brown trout were observed migrating from Klefstadbekken to the marine fjord during all months except March and April when the PIT antenna was not operational due to ice. The brown trout were recorded migrating back to the creek during all months except January, March and April. Most brown trout were remaining in the fjord for one to two days before returning to the creek. From 29 sea trout re-released into Nidelva after being used as brood stock for production of fry at the hatchery at Lundamo, ten individuals (29%) disappeared from the river at the same site as they were released, most likely due to predation. Only one fish (3%) performed a successful feeding migration to the marine fjord and back.
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The in-land and nearshore fish farming is facing capacity limitation and onshore push-out regulations. Huge technological innovations are rapidly evolving toward developing competitive Offshore fish farming. These technological innovations are mainly targeting to innovate new farming concepts that dynamically stable, reliable and compatible with offshore environmental loads and conditions. The dynamic operational behaviour of each farming concept is quite complicated. It is a combination of reinforcing behaviours (Loads, cage deformations, welfare issues, e.g. escaping, stress-related disease) and leveraging behaviours (Biofouling-cleaning, Deterioration-maintenance) and all influenced by fluctuating and harsh environmental loads and conditions. Therefore, the purpose of this paper is to analyse the context of offshore fish farming and explore quantitative descriptions of its reinforcement and leveraging behaviours. The context analysis is a well-known method within systems engineering methodology to illustrate and extract critical interfaces. The context analysis is considered as the first step in building simulation model to quantify the impact of systems interfaces on the entire farming economics, i.e. income and cost.
Technical Report
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The marine migration and habitat use of 269 anadromous brown trout Salmo trutta (often termed sea trout, total length 169-890 mm) and 55 Arctic Charr Salvelinus alpinus (270-480 mm) from five watercourses draining to the marine fjords Saltdalsfjorden and Skjerstadfjorden were studied from 2016 – 2018. The fish were tagged with acoustic transmitters and their movements and marine area use recorded by automatic listening stations. Additionally, feeding ecology was compared between anadromous Arctic charr (n=26) and brown trout (n=128) from one of the five watercourses, Lake Botnvatnet. Mean sea trout smolt age varied from 3.2-3.9 years, with fish from the River Lakselva and Lake Kosmovatnet at Valnesfjord and River Lakselva at Misvær exhibiting younger mean ages at smoltification. Individuals from River Saltdalselva and Lake Botnvatnet had the highest smolt ages. Mean sea trout smolt total length varied from 14.9 – 18.2 cm, with the largest individuals captured in River Laksåga at Sulitjelma. The western part of the marine Skjerstadfjord contained the key habitat areas occupied by the sea trout veterans during their summer feeding migrations, while sea trout postsmolts and Arctic charr veterans mainly fed in the Saltdalsfjord. However, both species and life stages utilized the whole fjord complex. Significant variation occurred in the spatial distribution and time spent in the marine environment among the tagged individuals, among groups of fish from the different watercourses and among species. The main period for the fjord migration of sea trout was May-August, while Arctic charr were found in the marine environment during June and July. Sea trout veterans from River Saltdalselva stayed an average of 66 days at sea in 2016 and 74 days in 2018. Sea trout veterans (58 days, 2016; 63 days, 2017) and postsmolt (55 days, 2017) from Lake Botnvassdraget spent longer at sea than the Arctic charr (36 days, 2017) from the same watercourse. Sea trout from River Laksåga (Sulitjelma) stayed an average of 49 days at sea (2017). Marine residence time for the sea trout from River Lakselva (Misvær) and River Lakselva and Lake Kosmovatnet (Valnesfjord) were not estimated since these fish repeatedly migrated back and forth between the freshwater habitat and the marine estuary throughout the year. Tagged fish of both species were observed in the vicinity of five Atlantic salmon sea cage farm sites, however, these stays were in general brief. Return rates for both sea trout and charr to the river where they were originally tagged varied (23-75%) among locations and years. Fish that were not registered back in the watercourse were either known or presumed to have been recaptured and killed, had migrated to other watercourses, had lost the acoustic mark/had a tag failure or had died during the seaward migration. In the period April 2016 - August 2019, 10 % of the tagged sea trout and 7 % of the tagged Arctic charr were reported recaptured, with 89 % of the sea trout and 100 % of the Arctic charr recaptured by rod and reel. Sea trout from Lake Botnvatnet mainly fed on marine fish and shrimps, while the Arctic charr had a broader diet, with a frequent occurrence of prey of freshwater origin in their stomachs. These were most likely fed upon in estuaries. The amount of fish in the diet increased with the sea trout’s body length. In 2017, a fish trap was installed in Lake Botnvatnet. In total, 1476 sea trout and 104 anadromous Arctic charr were captured migrating up to the watercourse. Within the sea trout, 75% had wounds from the salmon lice (Lepeophtheirus salmonis), 44% were infested with 1-10 adult lice and 9% had more than 10 lice. For Arctic charr, the numbers were 5%, 11% and 0%, respectively. An analysis of the genetics of the sea trout showed that sea trout from the five different watercourses were all genetically differentiated. Within River Saltdalselva, the largest of the five watercourses, individuals from three distinct areas were compared and they all originated from the same genetic group. Genetic analyses of sea trout scales sampled by recreational fishermen within the fjord system reviled that 45% of the captured fish originated from River Saltdalselva, 8% from Lake Botnvatnet, 2% from River Lakselva (Misvær) and 1% from River Laksåga (Sulitjelma). The remaining fish originated from unknown watercourses. The effective population size of the sea trout from Lake Botnvatnet (N0=48,7), was the only one which was lower than the recommended minimum level of N0=50 for conservation purposes. A level of N0=50 or greater is needed in order to have a genetic healthy population for the short term (5-10 generations). Possible explanations for the low number can be historic intensive gillnet fishing at the site during the period 1900 ~ 1960, and the fishing pressure in the fjord system in recent years.
Thesis
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The marine life of sea-run brown trout (Salmo trutta - hereafter termed sea trout) was investigated with a combination of acoustic telemetry, positively buoyant DSTs and reconstructed migration tracks. The tagged trout migrated longer distances with higher progression rates than seen for the species in marine areas elsewhere. Evidence of low fjord-survival and specific ranges of temperatures and salinities used by the fish, suggested that the observed behaviour could enable the fish to utilize areas with more favorable predator/prey abundances and distributions of metabolic influencers such as temperatures and salinities. The lowest in-fjord survival recorded for sea trout so far was seen in the Limfjord system – a shallow fjord system with a western exit 110 km from the river mouths of the tagged fish and an eastern exit 120 km from the river mouths. The fish entered the fjord gradually, but 98 % of surviving individuals left it within a short window of time along the same route with the highest progression rates observed for the species so far. This behaviour could reflect important local adaptations to the dangerous system. A characteristic diel diving pattern was observed for Danish sea trout kelts while at sea. The fish performed repetitive dives down to a maximum recorded depth of 88 m during daytime and exhibited residency close to the surface during nighttime. Temperatures within the range reported as optimal for growth (12 – 17 degrees C) were widely used by the fish while at sea. The fish resided almost exclusively in areas with waters warmer than the mean for the region when mean temperatures were below 11 degrees C and almost exclusively in temperatures colder than the mean when mean temperatures surpassed 15 degrees C. Temperatures above 17 degrees C were avoided almost entirely in spite of being predominant at surface levels in the region during July and August. Some fish switched behaviour from the otherwise stable diel dive/surface residency behaviour and avoided the surface almost entirely when surface temperatures surpassed 17 degrees C while others had migrated into areas less exposed to summer heating. Similar behaviour has not been observed on sea trout or other salmonids so far, calling for further studies in light of climate change. Migration tracks of the fish were reconstructed with a Hidden Markov Model. The tracks revealed kelt migrations from shallow or stratified marine areas during spring into deeper and more well-mixed areas during summer. Long migrations up to 580 km from natal river mouths occurred and residency at a minimum of 130 km away from the natal river was seen at some point for all individuals. Migrations were confined to shelf seas, and no fish surpassed a distance of 100 km from the nearest shoreline at any time, although migrations along the southern Norwegian coast, close to the boundary between the North Sea and Atlantic Ocean occurred. Migrations were directed into less saline areas during the first 15 days at sea for all individuals, but no clear salinity pattern was observed after this period. The floating ability of the DSTs enabled recovery of 42 % of all DSTs of which 25 tags were from kelts that had made it to sea and died there. Comparison of surviving and non-surviving individuals revealed that individuals that ended up surviving the entire marine period had higher weight/length ratios when tagged and established the characteristic diel diving behaviour faster while at sea than the fish that died at sea. Non-surviving fish had a mean survival time of 14.3 days at sea, and the first three weeks at sea were critical for kelts. The findings presented in the thesis suggest that the high degree of adaptability and behavioural variability exhibited by brown trout in freshwater extends into the marine environments. The longer and faster migrations recorded in Danish sea trout could reflect adaptations to utilize the most optimal balance between predation, prey availability, temperatures and salinities in the complex seas surrounding Denmark. The results, thus, improve our understanding of sea trout and salmonid behaviour in marine environments, and point toward new areas for future research to delve into.
Article
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In trying to deal with the problematic salmon louse Lepeophtheirus salmonis in salmon aquaculture, strategies to better prevent infestations are gaining traction. Successful prevention requires an accurate understanding of the environmental influences that alter the distribution of the planktonic stages of lice in the water column in space and time. Here, we tested the salinity preferences of nauplii and copepodid larval stages using step salinity column experiments. Under consistent temperature and lighting conditions, we created step gradients using a bottom layer of full salinity (34.7 ppt), with an upper layer of equal or lower salinity (~34.7 to 16 ppt). Lice entered the column in the lower layer and dispersed for 1 h before their position was recorded. Both nauplii and copepodids increasingly avoided the overlying layers as they became more brackish. However, the strength of avoidance differed between nauplii and copepodids. Nauplii almost completely avoided salinities below 30 ppt. For copepodids, there was a more gradual decline in the proportion preferring the less saline overlying layer, and the presence of some individuals occurred even at 16 to 20 ppt. Both stages aggregated at or just below the halocline, with no aggregation evident in isohaline columns at the same depth. For nauplii, clustering within the halocline was particularly strong. When integrated into a sea lice dispersal model, the new salinity preferences we determined markedly altered dispersal patterns in scenarios when salinity gradients were present. Our results have implications for the mapping of salmon lice larval behaviour and dispersal, with benefits for aquaculture planning and management.
Technical Report
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The marine migration and habitat use of 340 anadromous brown trout Salmo trutta (often termed sea trout, total length 135-730 mm) and 14 Arctic Charr Salvelinus alpinus (350-480 mm) from four watercourses drain-ing to the marine fjords Tosenfjorden and Bindalsfjorden were studied from 2015 – 2017. The fish were tagged with acoustic transmitters and their movements and marine area use recorded by automatic listening stations. Mean sea trout smolt age varied among the watercourses from 3.1-3.8 years, with fish from the Flostrømmen and Urvold estuaries exhibiting younger mean age at smoltification. Individuals from River Storelva had the highest smolt age. Mean sea trout smolt total length varied from 15.3 – 17.9 cm, with the largest individuals captured in the Urvold watercourse. The inner part of the marine Bindalsfjord and the outer part of the marine Tosenfjord were the key habitat areas occupied by the sea trout and Arctic charr during their summer feeding migrations. However, both species utilized the whole fjord complex. Significant variation occurred in the spatial distribution and time spent in the marine environment among the tagged individuals, among groups of fish from the different water-courses and among species. The main period for the fjord migration of sea trout from Urvold and Åelva watercourses was Mai-July, however some seatrout from Åelva utilized Flostrømmen estuary and Osan dur-ing the whole winter. Except during the spawning period in October, sea trout veteran migrants (fish that had spent at least one season at sea before tagging) from River Storelva were found in the marine environment at all times of year. Sea trout veterans from Lake Urvold and the Flostrømmen estuary stayed an average of 52 days at sea, while the young sea trout (postsmolt and second time migrants) from River Leirelva spent a mean of 73 days at sea. The Arctic char spent on average 34 days at sea with their seaward migration occurring during the last half of May and their return to fresh water around mid-June. Tagged fish of both species were observed around three Atlantic salmon sea cage farm sites, however, these stays were in general brief. The sea trout that stayed in the Flostrømmen estuary during the whole summer season encountered water masses with changing salinities ranging from brackish (0.5-29 ‰) to marine (30-35 ‰). During April and May, estuarine salinities were in the high range and similar to those in the marine fjord. Consequently, there was no difference in the experienced levels of salinity in the spring between the estuarine and marine feeding sea trout. From the Urvold watercourse, 79% of the Arctic charr and 78% of the sea trout veterans returned to the freshwater habitat after the marine migration, while 69% returned to River Åelva. From the group of young sea trout (postsmolt and second time migrants) from River Leirelva, 35% returned back to the River. Fish that were not registered back in the watercourse were either recaptured and killed, had migrated to other water-courses, lost the acoustic mark/had a tag failure or died during the seaward migration. In the period April 2015 - December 2018, 8.2 % of the tagged sea trout and 29 % of the tagged Arctic charr were recaptured with 95 % of the sea trout and 100 % of the Arctic charr being taken by rod and reel.
Article
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A total of 230 anadromous Salmo trutta (brown trout) were sampled in five sheltered coastal fjords (or sea lochs) on the Isle of Skye, Scotland, U.K., in 2016 at varying distances from active Atlantic salmon Salmo salar farms. Statistical models were developed to investigate potential correlations between salmon lice Lepeophtheirus salmonis burdens on S. trutta hosts and their proximity to S. salar farm cages. Significant correlations were found between lice burdens and fish fork length and proximity to the nearest S. salar farm. The probability of the presence of L. salmonis on fish hosts increased with fish host size and with distance from the nearest S. salar farm, but total lice burdens were highest in fish sampled near S. salar farms and decreased with distance. The proportion of different life‐cycle stages of L. salmonis were also dependent on S. salar farm proximity, with higher juvenile lice numbers recorded at sites near S. salar farm cages. These results highlight the complexity of the relationship between S. trutta and L. salmonis infections on wild fish and emphasise the requirement of further research to quantify these effects to better inform conservation and management strategies, particularly in areas of active S. salar farm facilities.
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Host−parasite systems are often characterised by a co-evolutionary arms race, with avoidance behaviour being the first line of defence for hosts. In aquatic ecosystems, the rapid rise of fish farming has elevated host abundance, altering the context of host−parasite interactions. Behavioural defences in host fish may adapt to combat infection pressure in the captive environment. We tested whether farmed Atlantic salmon Salmo salar altered their swimming depth in response to the ectoparasitic sea louse Lepeophtheirus salmonis. Parasite loads were manipulated on individual fish, which were implanted with internal tags that recorded swimming depth. During daylight hours, salmon exhibited identical swimming depths irrespective of parasite load. However, fish with higher parasite loads (12−18 lice fish−1) swam deeper at nighttime, compared to fish with no or moderate parasite loads (0−6 lice fish−1). As infective sea louse copepodids are concentrated near the surface, our results suggest that the preference for deeper water in fish with higher parasite loads is an avoidance mechanism to prevent further infestation. Host behavioural responses to parasites are predicted to shift with changes to the system and artificial selection of host phenotypes; our study provides the first evidence of a parasite avoidance response in salmon held in sea-cages.
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Abundance and depth distribution of planktonic sea lice were investigated in relation to hydrodynamics and diurnal solar insolation at a salmon farm in Sundalagið, Faroe Islands. Plankton surveys were conducted by surface tows with a plankton net around the farm and by using a plankton pump at 1, 4 and 6 m depth in a fish cage. The entire sample content was investigated under a stereomicroscope and sea lice were identified. Sea lice of the species Lepeophtheirus salmonis and Caligus elongatus were present at the farm. Nauplii dominated the planktonic stages (>95%) while copepodids were absent from most samples. The highest observed copepodid density was 0.3 ind. m−3, which is within the range found in open water. No diurnal vertical distribution pattern was observed for salmon lice nauplii in the net cages, which were most abundant in the top meter of the water column, and density decreased with depth. At 1 m depth, nauplii density was inversely proportional to the current speed at the same depth. From this relation, and the abundance of adult female sea lice on the farmed fish, the in situ nauplii production was calculated to be within 26−68 nauplii female−1 d−1. The lower end of this range is similar to production rates suggested by laboratory studies at similar temperatures (7.8°C).
Article
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An understanding of when and where sea trout Salmo trutta L. are located at sea is essential to the effective management of local populations and in evaluating their vulnerability to salmon lice and other anthropogenic threats. Here we review the available literature on sea trout life-history strategies, behaviour and habitat use in the marine environment, including feeding, growth, survival and homing. There is considerable variation in life-history strategies among individuals and populations and in the timing and duration of marine migration(s). Females tend to adopt the anadromous strategy more than do males. Smolts typically leave rivers in spring (March–June in European rivers), but also at other times of the year. Post-smolts may remain at sea during the summer and return to freshwater to over-winter; adults thereafter spend summers at sea and winters in freshwater, or they can remain at sea until they later return to freshwater for spawning. Sea trout frequently are recorded at sea during winter and can tolerate full-salinity sea water at water temperatures as low as 1–2 °C. Sea trout often remain within 80 km of their river of origin, but also may undertake longer-distance marine migrations (>500 km). The duration and timing of marine migration both are likely governed by trade-offs between mortality risk and growth potential in different habitats, and the most beneficial strategy may vary among individuals and populations. Reduced marine growth and increased marine mortality will reduce the benefit of marine migrations and may result in selection against anadromy.
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There is increasing interest in documenting and explaining the existence of marked intraspecific variation in metabolic rate in animals, with fishes providing some of the best-studied examples. After accounting for variation due to other factors, there can typically be a two to three-fold variation among individual fishes for both standard and maximum metabolic rate (SMR and MMR). This variation is reasonably consistent over time (provided that conditions remain stable), and its underlying causes may be influenced by both genes and developmental conditions. In this paper, current knowledge of the extent and causes of individual variation in SMR, MMR and aerobic scope (AS), collectively its metabolic phenotype, is reviewed and potential links among metabolism, behaviour and performance are described. Intraspecific variation in metabolism has been found to be related to other traits: fishes with a relatively high SMR tend to be more dominant and grow faster in high food environments, but may lose their advantage and are more prone to risk-taking when conditions deteriorate. In contrast to the wide body of research examining links between SMR and behavioural traits, very little work has been directed towards understanding the ecological consequences of individual variation in MMR and AS. Although AS can differ among populations of the same species in response to performance demands, virtually nothing is known about the effects of AS on individual behaviours such as those associated with foraging or predator avoidance. Further, while factors such as food availability, temperature, hypoxia and the fish's social environment are known to alter resting and MMRs in fishes, there is a paucity of studies examining how these effects vary among individuals, and how this variation relates to behaviour. Given the observed links between metabolism and measures of performance, understanding the metabolic responses of individuals to changing environments will be a key area for future research because the environment will have a strong influence on which animals survive predation, become dominant and ultimately have the highest reproductive success. Although current evidence suggests that variation in SMR may be maintained within populations via context-dependent fitness benefits, it is suggested that a more integrative approach is now required to fully understand how the environment can modulate individual performance via effects on metabolic phenotypes encompassing SMR, MMR and AS.
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The use of internal telemetry has greatly advanced fisheries research in the past two decades, permitting researchers to observe movements and distributions of organisms in their natural environment. For many years, the prevailing opinion has been that internal tags should not weigh more than 2% of the dry body weight of a fish. Some studies indicate that tags weighing up to 12% dry body weight do not have a negative effect on swimming performance, but few authors have examined impacts to fish physiology and health. This study investigated how tags greater than 2% body mass affected the mortality, tag retention, swimming performance, and physiological indicators of stress in brook trout (Salvelinus fontinalis Mitchill). No mortality was observed between treatment groups, but tag retention was lowest in the heavy tag treatment group in which 80% (12 fish) lost tags. Swimming performance and physiological indices of stress were not significantly impacted by tag mass. Fish tagged with heavy tags showed slower growth in the 3 weeks after tagging, but growth rates appeared to recover by the end of the study, although this result may be confounded by tag loss. The results of this study suggest that for brook trout, the 2% rule is a highly conservative guideline that can be substantially extended but should not exceed 7% body weight due to concerns about tag retention.
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Salmon farming increases the abundance of salmon lice, which are ectoparasites of salmonids in the sea. Here we review the current knowledge on the effects of salmon lice on wild sea trout. Salmon lice feed on host mucus, skin and muscle, and infestation may induce osmoregulatory dysfunction, physiological stress, anaemia, reduced feeding and growth, increased susceptibility to secondary infections, reduced disease resistance and ultimately mortality of individual sea trout. Wild sea trout in farm-free areas generally show low lice levels. In farm-intensive areas, lice levels on wild sea trout are typically higher, and more variable than in farm-free areas. Lice on wild sea trout are found at elevated levels particularly within 30 km of the nearest farms but can also extend to further ranges. Salmon lice in intensively farmed areas have negatively impacted wild sea trout populations by reducing growth and increasing marine mortality. Quantification of these impacts remains a challenge, although population-level effects have been quantified in Atlantic salmon by comparing the survival of chemically protected fish with control groups, which are relevant also for sea trout. Mortality attributable to salmon lice can lead to an average of 12-29% fewer salmon spawners. Reduced growth and increased mortality will reduce the benefits of marine migration for sea trout, and may also result in selection against anadromy in areas with high lice levels. Salmon lice-induced effects on sea trout populations may also extend to altered genetic composition and reduced diversity, and possibly to the local loss of sea trout, and establishment of exclusively freshwater resident populations.
Article
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The marine migration of post-spawning anadromous fish remains poorly understood. The present study examined survival and progression rates of anadromous brown trout Salmo trutta L. after spawning (kelts) during downriver, fjord, and sea migration. Kelts (n = 49) were captured in the Danish River Gudenaa, tagged with acoustic transmitters and subsequently recorded by automatic receivers. Kelts spent on average 25 d moving down the 45 km river and through the brackish fjord. The fish entered the Kattegat Sea between 14 April and 30 May. Eighteen of the 49 kelts disappeared in the river and fjord during outward migration, likely due to mortality. Survival was not significantly related to gill Na+/K+-ATPase activity, suggesting that physiological adaptation to saltwater may be less critical for adults compared to juveniles (smolts). Of the 31 fish that entered the Kattegat Sea, 45% survived and returned to the fjord. The duration of the entire marine migration, from leaving to entering the river, was on average 163 d. The fish returned from the Kattegat Sea to the fjord between 22 July and 21 October. Upon return, the fish spent 1−90 d passing through Randers Fjord, with most individuals completing the reach within 4 d, suggesting that the kelts spent limited time foraging after returning to the fjord. The total survival during the entire marine migration, including the fjord, was a minimum of 29%. Our study provides data that are important for management of anadromous brown trout, and the high survival highlights that kelts may represent a valuable resource for both population reproduction and recreational fisheries.
Article
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The biology and ecology of anadromous brown trout (Salmo trutta) at sea is poorly understood. This study provided information on spatial and temporal distribution of sea trout in the ocean. The behaviour of 115 individuals (veteran migrants, 270–700 mm) was tracked by using acoustic telemetry in a fjord system during April–September in 2012–2013. Overall, fish spent 68% of their marine residence time close to river mouths (<4 km). Most fish registrations (75%) were in nearshore habitats, but pelagic areas were also used. The maximum migration distance of tagged fish was categorized as short (<4 km from river mouth, 40% of fish), medium (4 – ∼13 km, 18% of fish), or long (>∼13 km, 42% of fish). Long-distance migrants had poorer body condition in spring prior to migration, used pelagic areas more often, and returned earlier to fresh water than short- and medium-distance migrants. Marine residence time was 7–183 days and was positively correlated to body length and smolt age, but negatively correlated to the date of sea entry.
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ABSTRACT: Salmon lice Lepeophtheirus salmonis Krøyer may affect survival and growth of anadromous salmonids through physiological stress and/or behavioural changes. Using acoustic telemetry tracking, we investigated the behaviour of 30 infected sea trout Salmo trutta throughout the summer in a fjord with very high salmon lice infection pressure. Most of the tracked sea trout adopted a movement pattern expected to suppress salmon lice infestation, as they showed a strong preference for fresh or brackish water, spending most of the time close to a river outlet or even migrating into the river. Highly infested sea trout preferred shallower depths, associated with lower salinity. The fish lost to predation stayed further away from the river outlet than non-predated fish, and were likely subjected to a stronger infection pressure. Half of the tracked group were treated with a salmon lice prophylaxis, emamectin benzoate. The effect of treatment on infestation was monitored in a separate group held in a sea cage and found to be moderate; the mortality in this group was associated with infestation by motile lice stages. In contrast, treatment was not found to have an effect on tracked fish behaviour. It is likely that some physiological and behavioural responses to high salmon lice infection pressure may be present even after a prophylaxis treatment, in particular when the treatment is given after exposure to salmon lice infection. We conclude that increased salmon lice infection pressure associated with altered salmon farming practice may have the potential to influence the marine behaviour and growth of sea trout.
Article
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An overview of electronic tags that can be used in fish research is given, including radio and acoustic transmitters, data storage tags (DST, also termed archival tags), pop-up satellite archival tags (PSAT) and passive integrated transponder tags (PIT-tag). Fish telemetry is a term used to describe the application of these methods. Typically, an electronic tag is attached to a free-swimming fish, and information on position, movements and/or measurements of environmental and physiological parameters can be recorded wirelessly by use of a mobile receiver or stationary loggers. For most methods, the fish need not to be recaptured to achieve data. However, DSTs record and store information on environmental and/or physiological parameters in the tag, and therefore need to be retrieved for downloading data. In the case of PSATs, stored data is transferred to satellites when the tag loosens from the fish and pops up to the surface, and in addition, the pop up position is recorded. The developments of telemetry methods have provided opportunities to reveal previously unknown information on fish behavior, habitat use and migrations in fresh water, estuaries, near-coastal areas and oceans, especially since extensive long-term data can be collected repeatedly from individual fish. Detailed information on fish behaviour and migrations is needed to better understand, protect and manage fishes in freshwater and marine systems. The development of successful management measures depends on knowledge of where fish reside and migrate during the day, season and year. There has been a tremendous increase in the use of electronic tagging methods, especially during the last 10-20 years. In addition to descriptive and ecological studies, the methods have been used to assess effects of for instance hydropower production, other river regulations, migration barriers, protected areas, fishing regulations, catch-and-release angling, hatchery-rearing, fish aggregating devices (FADs), water pollution and aquaculture. The main methods for attaching electronic tags to fish are 1) surgical implantation in the body cavity, 2) external attachment, and 3) gastric insertion via the mouth. Potential negative handling effects are inflammations, infections, tag expulsion, altered behavior, decreased swimming performance, reduced feeding, reduced growth and increased mortality. The catch, handling and tagging procedures should have minimal effects on the fish. If not, an anomalous behaviour caused by the tagging may be recorded instead of the natural behaviour, and the study is a failure from a scientific point of view. Furthermore, optimal anaesthetic and tagging methods are required to meet the ethical standards for use of experimental animals, and to ensure fish survival and welfare.
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In this study, we validated a scale-reading method estimating age and growth in brown trout Salmo trutta in wild, landlocked, stream-dwelling populations from mountain headwaters in the Elbe catchment area of the Czech Republic. The values estimated from scale reading were compared with measured values, collected using a mark-recapture program over eight consecutive years. The age-corrected absolute percentage error was 10.71%, primarily because the ages of the oldest individuals according to scale reading were underestimated, and the ages of juvenile individuals were slightly overestimated. The back-calculated length was slightly underestimated (the mean error was −4.60 mm), but it was not significantly different from the real measured length. This study suggests that in cold mountain headwaters, scale reading is a sufficiently accurate method for age and growth estimation in juvenile brown trout; however, the results for adult individuals must be taken with caution.
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Physiological criteria for identifying fully functional smolts are presented and environmental factors are identified which influence these. Methods are described for optimizing time, age and size at release, and data are presented on methods of environmental manipulation to alter smoltification to fit specific resource management needs. Guidelines are presented for management strategies to improve survival of Pacific Oncorhynchus sp., and Atlantic salmon Salmo salar, and anadromous (steelhead) rainbow trout S. gairdneri.-from Sport Fishery Abstracts
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Acoustic telemetry was used to monitor the early marine migration of Atlantic salmon Salmo salar from two rivers of Passamaquoddy Bay, a coastal system with numerous Atlantic salmon farms and weirs for Atlantic herring Clupea harengus on the border of Canada and the United States. Monitoring at fixed sites, active tracking, and systematic searches for tagged fish were combined. The migration success of hatchery-reared (N = 96) and wild (N = 38) smolts out of the estuaries was high (range, 90–97%), and the overall success of postsmolts moving out of Passamaquoddy Bay and into the several passages leading to the Bay of Fundy was reasonable (range, 71–88%). Estuary transit times were usually rapid (
Article
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The seasonal marine feeding pattern of sea trout was studied from March to December 2001 in two large fjords situated at latitudes 69°N and 66°N in the northern part of Norway. Despite low sea temperatures and high salinity, sea running trout were caught during all sampling occasions in both fjords. The trout had fed extensively on marine crustaceans (shrimps, amphipods, and krill) and polychaetes during early and late winter, and had a stable or increasing condition factor during this period. In summer and autumn, the trout fed predominantly on fish, mainly juvenile herring. Food consumption rates were lowest during late autumn and early winter (October–December) in both fjords, while trout in the southernmost fjord had the highest consumption rates from April to August and trout in the northernmost fjord from May to September. These patterns in both fjords matched the seasonal variations in condition factor and relative lipid content of the fish during the same periods. The marine winter migration of these northern sea trout populations appears to be a feeding migration in which the fish maintain or increase their body condition, representing a previously undocumented alternative to the more common life history strategy of over-wintering and starvation in freshwater at the northern distribution of this species.
Article
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Young from the diadromous trout population in Vangsvatnet lake, Norway, showed a significantly higher rate of downstream migration than that of the landlocked trout from Lønavatnet lake, Voss river system. The differences seemed not to be due to environmental factors, but to differences in genetics. The reason may be the strong selection pressure for maintenance of position in rivers above waterfalls. /// У молди диадромной популяции форели в озере Вангсватнет (Норвегия) обнаружены значительно более высокие скорости миграции вниз по течению, чем скорость миграции пресноводной форели из озера Лёнаватнет в бассейне р. фосс. Различия по-видимому оперелятся не экологическими факторами, а генетическими. Причиной может быть сильнй пресс селекюии, направленный на закрепление позиций в реке вьше водопадов.
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The food of migratory Salmo trutta in coastal marine waters along the Norwegian Skagerrak coast varied significantly with age, season and habitat of the fish. The main prey categories in terms of frequency of occurrence were fishes followed by crustaceans, surface insects and polychaetes. Seasonal variation in diet and within habitats was found, supporting the view that brown trout is an opportunistic feeder. An ontogenetic niche shift was observed with post-smolts feeding on inshore and shallow water prey communities, while larger brown trout fed mainly on pelagic fishes.
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Abstract – Avian predation on emigrating wild and domesticated sea trout smolts was investigated in a fjord in the western Baltic Sea. In April 1997, 50 domesticated and 50 wild smolts were intraperitoneally tagged with radio-transmitters and released in a small coastal stream. Predation was recorded by signal interception in an estuarine breeding colony of cormorants and herons near the outlet of the stream. Of the 78 emigrating smolts, 51 (65%) were recorded as eaten. Predation rates were significantly higher among small than large smolts and significantly higher among domesticated smolts. The first 2 days after entering the sea, both wild and domesticated smolts suffered a severe daily predation rate (range 20–34%). The results support the hypothesis of a transient period immediately after exposure to full-strength sea water, where smolts experience an elevated risk of predation. A transient increase in postsmolt mortality may be found also in moderately saline environments (20–23 ppt).†
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In a fjord in northern Norway, both immature and maturing individuals (170-500mm fork length) of anadromous Arctic charr Salvelinus alpinus and anadromous brown trout (sea trout) Salmo trutta were captured up to 5000m offshore. Both species had a high feeding intensity and an almost exclusive piscivorous diet, with herring Clupea harengus as the dominant prey, an apparently opportunistic response to a high density of small-sized herring in the pelagic zone. (c) 2005 The Fisheries Society of the British Isles.
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Hatchery-reared Atlantic salmon Salmo salar (n = 25) and wild anadromous brown trout (sea trout) Salmo trutta (n = 15) smolts were tagged with coded acoustic transmitters and released at the mouth of the River Eira on the west coast of Norway. Data logging receivers recorded the fish during their outward migration at 9, 32, 48 and 77 km from the release site. Seventeen Atlantic salmon (68%) and eight sea trout (53%) were recorded after release. Mean migratory speeds between different receiver sites ranged from 0.49 to 1.82 body lengths (total length) per second (bl s-1) for Atlantic salmon and 0.11-2.60 bl s-1 for sea trout. Atlantic salmon were recorded 9, 48 and 77 km from the river mouth on average 28, 65 and 83 h after release, respectively. Sea trout were recorded 9 km from the release site 438 h after release. Only four (23%) sea trout were detected in the outer part of the fjord system, while the rest of the fish seemed to stay in the inner fjord system. The Atlantic salmon stayed for a longer time in the inner part than in the outer parts of the fjord system, but distinct from sea trout, migrated through the whole fjord system into the ocean.
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Lepeophtheirus salmonis is a common ectoparasite of wild and sea-farmed salmonids. Mean development time of eggs was 419.1 hours (17.5 days) at 5°C, 207.1 hr (8.6 days) at 10°C, and 130.8 hr (5.5 days) at 15°C. Development from the first nauplius to the infectious copepodid stage took 222.3 hr (9.3 days) at 5°C, 87.4 hr (3.6 days) at 10°, and 44.8 (1.9 days) hr at 15°C. Development from the egg to the adult male took 40 days, and from the egg to the adult female 52 days at 10°C. No egg development occurred at 10‰ salinity. At 15‰ eggs developed but failed to produce active nauplii. At 20-30‰ active nauplii were produced, but copepodids were only obtained at 30‰. Copepodids survived for <1 day in waters with a salinity of 10‰ or less. At 15-30‰ and temperatures of 5, 10, and 15°C average survival times ranged between 2-8 days. -from Authors
Chapter
This chapter describes the different aspects of the physiology of smolting salmonids. Many salmonids—the fish of the genera, Oncorhynchus, Salmo, and Salvelinus—are anadromous and undergo a distinct transformation prior to seaward migration. Several morphological and physiological changes occur during smolting. Silvering is because of the synthesis of two purines–guanine and hypoxanthine. The needle-like crystals of these substances are deposited in two distinct skin layers: one directly beneath the scales and the other deep in the dermis adjacent to the muscles. The salmonids, as many other temperate-zone animals, have a seasonally changing physiology that is manifested in the cycles of growth, precisely timed migrations, and the seasons of reproduction geared to the most favorable seasons for the birth and development of the young. There is a circannual rhythm of physiological changes associated with the parr–smolt transformation. After juvenile salmon reach a certain critical size, they become smolts with a capacity to hypoosmoregulate and grow in seawater and behavioral changes result in a downstream migration.
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We give suggestions for the presentation of research results from frequentist, information-theoretic, and Bayesian analysis paradigms, followed by several general suggestions. The information-theoretic and Bayesian methods offer alternative approaches to data analysis and inference compared to traditionally used methods. Guidance is lacking on the presentation of results under these alternative procedures and on nontesting aspects of classical frequentist methods of statistical analysis. Null hypothesis testing has come under intense criticism. We recommend less reporting of the results of statistical tests of null hypotheses in cases where the null is surely false anyway, or where the null hypothesis is of little interest to science or management.
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The aim of this study was to test the hypothesis that hatchery brown trout Salmo trutta smolts, with 50% reduced or no feeding over the last 5 months before release, were more likely to migrate to the sea than individuals with standard feeding ratios. The juvenile fish were divided into three groups 176 days before release: (A) with no feeding, (B) with 50% and (C) with 100% feeding. To study their seaward migration, 40 fish from each feeding group were tagged with acoustic transmitters and tracked by automatic listening stations in the River Nidelva, Trondheim, Norway, its estuary and in the nearest marine environment. At the time of release, mean condition factor was significantly lower in group A and the fish from groups A and B had higher levels of Na+, K+-ATPase. Significantly more fish from group A migrated to the sea, but the rate of downstream progression from release to the estuary did not differ between the three groups. In conclusion, the S. trutta smolts with no access to food in the last 176 day before release were more likely to migrate to the sea. Fish from all three feeding groups, however, appeared to smoltify and had the same rate of downstream progression to the estuary. This indicates that differences in migratory behaviour between individuals from the three feeding groups begin from the time when the fish reach saline waters. It is suggested that feeding in hatcheries has to be greatly reduced (by 50% or more) over several months to have a pronounced effect on the migratory behaviour in S. trutta.
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To study migration performance and return rates of hatchery brown trout Salmo trutta smolts the first 5 months after release, 50 fish in each year (fork length, LF , 158-288 mm) were in two subsequent years tagged with acoustic transmitters and recorded by automatic listening stations in the River Nidelva (central Norway), its estuary and in the marine environment. More than half of the smolts became anadromous migrants (52% in 2011 and 70% in 2012). The fish spent longer time in the estuary than in the marine environment and the results suggest that migratory behaviour of S. trutta smolts is not only restricted to be resident or anadrome-lacustrine, but that there is also an intermediary strategy of estuarine feeding. There were no differences in LF or mass between groups of smolts with different migration patterns. Return rates from the sea within the first 5 months after release were in both years 16%. Median progression rate in the river was 0·090 LF s(-1) but decreased significantly as the smolts entered the estuary (0·015 LF s(-1) ). The long residential time in the estuary may increase the risk of negative effects of anthropogenic activities in estuaries, such as harbours and industrial development, and special attention should be given to evaluate effects of such activities.
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The migratory behaviour and spatial area use of sympatric Arctic charr Salvelinus alpinus and brown trout Salmo trutta were investigated during their marine feeding migration. The likelihood of finding individuals of both species in the inner or outer fjord areas was dependent on water temperature in the inner area (especially for S. alpinus), the temperature difference between the inner and outer areas (especially for S. trutta) and fish fork length (both species). The strongest predictor was the water temperature in the inner area, and particularly S. alpinus left this area and moved to the outer areas with increasing temperatures in the inner area. At 8° C in the inner area, the likelihood of finding S. alpinus in the outer areas was >50%. This predictor had a smaller effect on S. trutta, and the likelihood of finding S. trutta in the outer areas only started to increase at around 14° C. The relationships between temperature and area use did not correspond to the species' optimal growth temperatures, but to their previously documented temperature preferences. Individuals of both species used mainly the littoral fjord areas, and to a lesser extent the pelagic areas. In conclusion, temperature differences between the inner and outer marine areas probably resulted in the segregated area use between the species, because water temperatures or factors influenced by temperature affected their migratory behaviour and habitat use differently. The results indicate that increased marine temperatures with global warming may lead to increased spatial overlap between S. trutta and S. alpinus, which again may lead to increased interspecific competition during their marine phase, and with S. alpinus probably being the more negatively affected.
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Proposed lists of definitions and terms designating the different life stages of the Atlantic salmon (Salmo salar, L.) and the migratory form of the trout (Salmo trutta, L.) are presented in English, with equivalents for the basic terms in the languages of the Atlantic salmon and migratory troutproducing countries in Europe and North America. The basic terminology lists include definitions for alevin, fry, parr, smolt, post-smolt, adult fish, previous spawner and kelt. for both species.
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Diel migrations between habitats containing different levels of food abundance is a common phenomenon among marine organisms, both vertebrate and invertebrate. We hypothesize that in many cases this behavior constitutes a response to diel changes in the relationship between potential feeding rates and predation risks in the different habitats. For planktivores that locate their prey by sight (such as juvenile sockeye salmon) and that in turn are subject to predators that use sight to locate them, the diel time profiles of potential feeding rate and predation risk in near-surface waters may be determined largely by the relative densities of prey at the two trophic levels. A simple model of aquatic predation leads us to hypothesize the existence of brief "antipredation windows" for feeding at dawn and dusk. If this hypothesis is valid, then the optimal behavior for pelagic planktivores is to migrate into surface waters to feed during these two daily windows and to migrate to deeper, less illuminated waters ...
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After hatching in the littoral zone, bluegill sunfish Lepomis macrochirus fry migrate to the pelagic zone to feed on zooplankton. Fry returned to the littoral zone in 4 different Michigan lakes at a relatively constant size of c12.5 mm standard length. Several years are spent feeding in the littoral zone vegetation before the bluegill again shifts to feeding on zooplankton, first in the water column above littoral vegetation and subsequently in the true pelagic zone. This shift to feeding on zooplankton occurred at a specific body size within a lake, but the size ranged from 50-83 mm among 5 different lakes. Size at which the shift occurred was directly correlated with the density of the major predator of the bluegill in these lakes, largemouth bass Micropterus salmoides. The bluegill is faced with a growth (or feeding) rate-predation risk trade-off in making these habitat shifts. Analyses of stomach contents, growth of small fish caged in the pelagic zone, and predictions of foraging rates in both habitats all indicate that the pelagic zone is the more profitable for all size classes of bluegill. Risk of predation by largemouth bass was 40-80 times as great for small bluegills in open water as for those in vegetation. Bluegill can facultatively respond to changes in feeding rates and predation risk. -from Authors
Article
Premature return of sea trout (Salmo trutta) was studied by collecting ascending trout in a fish trap in the mouth of the Lonningdalselven River, western Norway. Postsmolts ascended the river in June and older migrants in July. Ascending fish were heavily infested with salmon lice (Lepeophtheirus salmonis) and many were in poor physical condition. Louse infestations differed between the two groups of anadromous trout. Postsmolts had a median infestation intensity of 206 lice, mainly juveniles. Older migrants carried a median intensity of 43.5 lice, with a high proportion of preadult and adult lice. Forty-one percent of the postsmolts that ascended the river migrated back to the sea, after a median river stay of 37.5 days. These fish had recovered from the severe louse attack but experienced a median decrease of 23.5% in body mass and no increase in length. All older migrants stayed in the river for the rest of the season. Low mortality (3.6%) was observed in prematurely ascending postsmolts but 19.7% of the older migrants died within the ist week of their return.
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The vertical distribution of pelagic nauplii and copepodids of the salmon louse Lepeophtheirus salmonis Krøyer was studied in large enclosures in the sea. Copepodids, which infect salmonid hosts, displayed a distinct diel vertical migration pattern. They gathered near the surface during the day, and spread out into deeper layers at night. Nauplii showed only small differences in depth between night and day. Copepodid distribution seems to be controlled by light intensity; no effect of either salinity or temperature was found. This migration pattern, which is the reverse of that of wild salmonids, may increase the number of parasite–host encounters as hosts will swim through populations of sinking (nighttime) and rising (dawn) parasites every 24 h. Because caged salmon feed at the surface during the day, they are likely to be more exposed to infective copepodids than wild fish.
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We used hydroacoustics to examine diel changes in the vertical distributions of rainbow smelt, Osmerus mordax, in Lake Memphremagog, Quebec/Vermont. Our objective was to evaluate hypotheses linking diel vertical movements of fish with light levels. Smelt distributions were also monitored from June through October (1988 and 1990) to evaluate seasonal changes in their behavior. A strong relationship (r2 = 0.83) between ambient light intensities and the upper fish layer in the water column was observed. Fish depth was also related to the depth of the thermocline during the night and when surface water temperatures were > 18 °C. The most characteristic feature was the strong avoidance of light levels > 0.1 μW/cm2. However, we found considerable variation in lower light levels experienced by the whole fish population. The results suggest that existing models of anti-predation behavior relating light and fish depth are consistent, with some limitations, with patterns of diel vertical migration in rainbow smelt.
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High levels of hybridization between Atlantic salmon Salmo salar and brown trout Salmo trutta have been reported in the River Driva. This study presents the underlying mechanisms of development of seawater (SW) tolerance and marine migration pattern for S. salar×S. trutta hybrids. Migrating S. salar×S. trutta hybrid smolts caught in the River Driva, Norway (a river containing Gyrodactylus salaris), displayed freshwater (FW) gill Na(+) , K(+) -ATPase (NKA) activity levels of 11·8 µmol ADP mg protein h(-1) , which were equal to or higher than activity levels observed in S. salar and S. trutta smolts. Following 4 days of SW exposure (salinity 32·3), enzyme activity remained high and plasma ion levels were maintained within the normal physiological range observed in S. salar smolts, indicating no signs of ion perturbations in S. salar×S. trutta hybrids. SW exposure induced an increase in NKA α1b-subunit mRNA levels with a concurrent decrease in α1a levels. Salmo salar×S. trutta post-smolts migrated rapidly through the fjord system, with increasing speed with distance from the river, as is often seen in S. salar smolts. The present findings suggest that S. salar×S. trutta smolts, as judged by the activity and transcription of the NKA system, regulation of plasma ion levels and migration speed more closely resemble S. salar than S. trutta.
Article
Development, growth, and survival data derived from laboratory experiments are provided for Lepeophtheirus salmonis , a common ectoparasite of wild and sea-farmed salmonids. The mean development time of eggs was 419·1 hours (17·5 days) at 5°C, 207·1 hours (8·6 days) at 10°C, and 130·8 hours (5·5 days) at 15°C. Development from the first nauplius to the infectious copepodid stage took 222·3 hours (9·3 days) at 5°C, 87·4 hours (3·6 days) at 10°C, and 44·8 (1·9 days) hours at 15°C. Development from the egg to the adult male took 40 days, and from the egg to the adult female 52 days at 10°C. No egg development occurred at 10‰ salinity. At 15‰ eggs developed but failed to produce active nauplii. At higher salinities (20–3‰) active nauplii were produced, but copepodids were only obtained at 30‰. Copepodids survived for less than 1 day in waters with a salinity of 10‰ or less. At higher salinities (15–30‰) and temperatures of 5,10, and 15°C average survival times ranged between 2 and 8 days.
Article
Synopsis The general biology and pathology of Lepeophtheirus salmonis and Caligus elongatus and the prevention and treatment of such ‘sea-lice’ infestations on farmed salmonids are described from the literature and original observations. The life-cycle of L. salmonis and probably also that of C. elongatus comprises the egg and 10 stages separated by moults, namely, two nauplius, one infective copepodite, four chalimus, two pre-adult and the adult (male and female) stages. Water temperature greatly affects the rate of development, especially for early larval stages. Heavy infestations of wild fish seem rare, and lice are lost fairly rapidly in freshwater. In Scotland at least L. salmonis shows a succession of generations on farmed salmonids; generation time is about six weeks at 9–12 C. Post-chalimus stages of C. elongatus may exchange between farmed salmonids and wild fish (especially gadoids). Epizootics (particularly with L. salmonis) cause great damage to salmonids in Norwegian and Scottish farms largely through feeding on host skin. The dermis is oedematous and haemorrhaged where lice feed, and blood seeps between scales; deaths probably result from osmoregulatory failure. Whilst prevention of infestation is difficult, a bath treatment for 1 h with 1 ppm of the organophosphorus compound Dichlorvos is effective against post-chalimus stages of L. salmonis on caged salmonids. Side effects are minimal and clearance rates from fish tissues satisfactory, but treatment may be required every 3–4 weeks.
Book
Destruction of habitat is the major cause for loss of biodiversity including variation in life history and habitat ecology. Each species and population adapts to its environment, adaptations visible in morphology, ecology, behaviour, physiology and genetics. Here, the authors present the population ecology of Atlantic salmon and brown trout and how it is influenced by the environment in terms of growth, migration, spawning and recruitment. Salmonids appeared as fresh water fish some 50 million years ago. Atlantic salmon and brown trout evolved in the Atlantic basin, Atlantic salmon in North America and Europe, brown trout in Europe, Northern Africa and Western Asia. The species live in small streams as well as large rivers, lakes, estuaries, coastal seas and oceans, with brown trout better adapted to small streams and less well adapted to feeding in the ocean than Atlantic salmon. Smolt and adult sizes and longevity are constrained by habitat conditions of populations spawning in small streams. Feeding, wintering and spawning opportunities influence migratory versus resident lifestyles, while the growth rate influences egg size and number, age at maturity, reproductive success and longevity. Further, early experiences influence later performance. For instance, juvenile behaviour influences adult homing, competition for spawning habitat, partner finding and predator avoidance. The abundance of wild Atlantic salmon populations has declined in recent years; climate change and escaped farmed salmon are major threats. The climate influences through changes in temperature and flow, while escaped farmed salmon do so through ecological competition, interbreeding and the spreading of contagious diseases. The authors pinpoint essentialproblems and offer suggestions as to how they can be reduced. In this context, population enhancement, habitat restoration and management are also discussed. The text closes with a presentation of what the authors view as major scientific challenges in ecological research on these species
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Through visual observations and stomach analyses, the predation from Saithe (Pollachius virens L.) and cod (Gadus morhua L.) on seaward migrating salmon- (Salmo salar L.) and trout (Salmo trutta L.) smolts was studied. The results indicate that some saithe specialise in preying on smolts and that wild and hatchery smolts were eaten at the same rate.
Article
During the periods 1956–1963 and 1967–1970 traps were operated to catch upstream- and downstream-migrating sea trout, Salmo trutta L. A total of 15 788 sea trout were tagged, using Carlin tags. The number of recaptures made in the traps was 4481, of which 1796 were recaptured more than once. The distribution of the 2122 recaptures in the sea provides a picture of the sea-migration pattern. Of the sea recaptures, 52.8% were reported as within a distance of 3 km from the river mouth, compared to 0.7% more than 80 km away. All the different size-groups of sea trout were represented among both the long-distance and the short-distance migrants. The results of this study of sea trout migrations are discussed in relation to the published results for Atlantic salmon, Salmo salar L., and sea charr, Salvelinus alpinus (L.), from the same river. The four highest values recorded for mean distance of daily travel away from the river were 20, 8, 8 and 6km day−1 by smolts and 6, 6, 5 and 5km day−1 by larger-sized sea trout. Recaptures of tagged sea trout in rivers other than the Vardnes totalled 506, of which 306 had been tagged as smolts. The calculated minimum percentage of stray is 15.5%. The proportion of sea trout from the Vardnes river that actually spawn in other rivers is not known. No significant difference in length distribution was found between the sea trout caught in the Vardnes river and those caught in other rivers. An hypothesis concerning the selective advantages of straying by anadromous salmonids living in small rivers is discussed.
Article
By use of acoustic telemetry, the present study showed that both riverine anadromous brown trout (sea trout) Salmo trutta and Arctic charr Salvelinus alpinus in a north Norwegian river descended the river within the first 4 months after spawning in late September and spent long parts of the remaining winter period in the estuary and also possibly partly in salt water. This contradicts the general assumption, based on studies of lake-dwelling populations, that both species, and in particular S. alpinus, overwinter and spend 9–11 months in fresh water at northern latitudes and the rest of the year in salt water.
Article
The diet of sea trout in some of the sea lochs of the west coast of Scotland was investigated. The contents of 986 sea trout stomachs from the Loch Etive area (1970–1973), and 291 stomachs from the Loch Eil area (1964–1973), were examined and the composition of the diet, seasonal changes and the effect of trout size were analysed using frequency of occurrence, dry weight and number of organisms methods. Benthic feeding (crustacea and annelids) was more important in winter while midwater and surface organisms (young fish and insects) were preferred in summer. Young fish (mainly clupeids and sand eels) featured more in the diet of larger trout (≤21 cm) than in the smaller size range (≥21 cm). Surveys of 24-h were completed in June and September, 1972, and indicated that availability of food was the main factor influencing the presence or absence of trout. Bottom feeding was greatest during the day while the amount of midwater and surface feeding tended to increase between sunset and sunrise.