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Temporal synchrony and variation in abundance of Atlantic salmon (Salmo salar) in two subarctic Barents Sea rivers: influence of oceanic conditions
Abstract
Long-term variation in Atlantic salmon (Salmo salar) stocks was analyzed in two Barents Sea rivers, the Teno and Naatamojoki, that represent the northernmost distribution area of the species. In contrast to most of the North Atlantic area, these rivers are among a group of northern salmon rivers that, despite wide annual variation in catches, demonstrate no consistent trend for declining abundance. Variations in abundance were generally synchronous for the total catch and numbers of 1-sea-winter (1SW) and 2SW salmon during period of 1972-2003. Part of the variation observed in catches could be related to ocean climate conditions as the mean seawater temperature in July during the year of smoltification for the Kola section of the Barents Sea was significantly related to numbers of 1SW, 2SW, and 3SW salmon in the large River Teno. In contrast, NAO (North Atlantic Oscillation) indices were not related to salmon catches. The latest increase (1999-2001) in salmon catches in these rivers reflects both temporarily improved oceanic conditions and past management measures affecting offshore, coastal, and river fisheries.
... numbers by the currents are time-averaged velocities in km/d. a 9-11-year cycle, and Niemelä et al. (2004) suggested an 8-9-year cycle for Finnish, Barents Sea rivers. A cyclic pattern with phases of 8-10 years spanning the period of 1892-1971 was observed in the commercial landings of salmon from the Saint John River, Canada ( Figure 4; Dadswell et al. 1984), and angling catches in Spanish rivers from 1950 to 2010 demonstrated distinct 10-year cycles (Almodóvar et al. 2019). ...
... Birman (1979) demonstrated that the periodicity and abundance of annual adult returns of Atlantic salmon were similar to cyclic annual adult returns of Amur chum salmon, Oncorhychus keta (Walbaum, 1792), and proposed that climatic effects on salmonid populations were common to both the North Atlantic and North Pacific Oceans. Other studies demonstrated that climatic cycling did not seem to cause variations of adult return abundance in certain regions (Niemelä et al. 2004), although recent analysis suggests that annual cycling of returns in this same region was statistically correlated to sea surface temperature (SST) cycles in the Barents Sea (Pasanen et al. 2017). Sea surface temperature variation, rather than NAOI or AMOI cycles, has also been shown to correlate best with rate of marine growth and age at maturity Jonsson et al. 2016) and survival of post-smolts (Olmos et al. 2020). ...
... Wild, smolt to adult return rates in the River Bush, Ireland varied from 3.9 to 12.0% for 1SW and 0.4 to 1.9% for 2SW fish even with commercial exploitation rates in Irish coastal waters of 36.5 to 60.0% (Crozier and Kennedy 1993). Annual adult return abundance in northern stocks, such as at Labrador (DFO 2014), Norway (Niemelä et al. 2004;Otero et al. 2011) and Russia (Zubchenko and Sharov 1993) remained at historic levels. ...
ABSTRACT Adult returns to many Atlantic salmon wild and hatchery stocks of the North Atlantic have declined or collapsed since 1985. Enhancement, commercial fishery closures, and angling restrictions have failed to halt the decline. Human impacts such as dams, pollution or marine overexploitation were responsible for some stock declines in the past, but adult returns to river and hatchery stocks with no obvious local impacts have also declined or collapsed since 1985. Multiple studies have postulated that the recent widespread occurrence of low adult returns may be caused by climate change, salmon farming, food availability at sea, or marine predators but these possibilities are unsupported by stocks that persist near historic levels, loss of stocks remote from farm sites, a diverse marine prey field, and scarcity of large offshore predators. The decline and collapse of stocks has common characteristics: 1) cyclic annual adult returns cease, 2) annual adult returns flatline, 3) adult mean size declines, and 4) stock collapses occurred earliest among watersheds distant from the North Atlantic Sub-polar Gyre (NASpG). Cyclic annual adult returns were common to all stocks in the past that were not impacted by anthropogenic changes to their natal streams. A flatline of adult abundance and reduction in adult mean size are common characteristics of many overexploited fish stocks and suggest illegal, unreported, and unregulated (IUU) fisheries exploitation at sea. Distance from the NASpG causing higher mortality of migrating post-smolts would increase the potential for collapse of these stocks from IUU exploitation. By-catch of post-smolts and adults in paired-trawl fisheries off Europe and intercept adult fisheries off Greenland, in the Gulf of St. Lawrence, and off Europe have been sources of marine mortality but seem unlikely to be the primary cause of the decline. Distribution in time and space of former, legal high-sea fisheries indicated fishers were well acquainted with the ocean migratory pattern of salmon and combined with lack of surveillance since 1985 outside Exclusive Economic Zones or in remote northern regions may mean high at-sea mortality occurs because of IUU fisheries. The problem of IUU ocean fisheries is acute, has collapsed numerous stocks of desired species worldwide, and is probably linked to the decline and impending collapse of the North Atlantic salmon population.
... numbers by the currents are time-averaged velocities in km/d. a 9-11-year cycle, and Niemelä et al. (2004) suggested an 8-9-year cycle for Finnish, Barents Sea rivers. A cyclic pattern with phases of 8-10 years spanning the period of 1892-1971 was observed in the commercial landings of salmon from the Saint John River, Canada ( Figure 4; Dadswell et al. 1984), and angling catches in Spanish rivers from 1950 to 2010 demonstrated distinct 10-year cycles (Almodóvar et al. 2019). ...
... Birman (1979) demonstrated that the periodicity and abundance of annual adult returns of Atlantic salmon were similar to cyclic annual adult returns of Amur chum salmon, Oncorhychus keta (Walbaum, 1792), and proposed that climatic effects on salmonid populations were common to both the North Atlantic and North Pacific Oceans. Other studies demonstrated that climatic cycling did not seem to cause variations of adult return abundance in certain regions (Niemelä et al. 2004), although recent analysis suggests that annual cycling of returns in this same region was statistically correlated to sea surface temperature (SST) cycles in the Barents Sea (Pasanen et al. 2017). Sea surface temperature variation, rather than NAOI or AMOI cycles, has also been shown to correlate best with rate of marine growth and age at maturity Jonsson et al. 2016) and survival of post-smolts (Olmos et al. 2020). ...
... Wild, smolt to adult return rates in the River Bush, Ireland varied from 3.9 to 12.0% for 1SW and 0.4 to 1.9% for 2SW fish even with commercial exploitation rates in Irish coastal waters of 36.5 to 60.0% (Crozier and Kennedy 1993). Annual adult return abundance in northern stocks, such as at Labrador (DFO 2014), Norway (Niemelä et al. 2004;Otero et al. 2011) and Russia (Zubchenko and Sharov 1993) remained at historic levels. ...
... [13,45]). This is true also with salmon populations in northernmost Europe where abundance indices and marine growth rates are correlated with sea surface temperatures in adjacent sea areas of the Barents Sea and Norwegian Sea, but fewer links have been detected with the broad-scale climate indices, like the North Atlantic Oscillation [36]. In addition to survival, temperature may influence the oceanic migration patterns and subsequent timing of maturation and return migration [45]. ...
... All sea age groups of salmon are included in the counts at both Kola and Tuloma. In the River Teno, annual estimated catches of one-sea-winter salmon (in numbers) has been used as a proxy of abundance (see [36]). ...
... Although the oceanic migration of these three salmon populations is not known in detail, young salmon from these rivers spend at least their first months of sea migration in the Barents Sea and thus the temperatures measured at the Kola section are likely reflecting the environmental conditions the salmon are facing at sea [1,36]. Moreover, the temperatures at the Kola section are highly correlated with those measured at other gauging stations along the Norwegian coast of the Barents Sea (Niemelä, pers.comm.; http://www.imr.no/sjomil/index.html). ...
Variation of marine temperature at different time scales is a central environmental factor in the life cycle of marine organisms, and may have particular importance for various life stages of anadromous species, for example, Atlantic salmon. To understand the salient features of temperature variation we employ scale space multiresolution analysis, that uses differences of smooths of a time series to decompose it as a sum of scale-dependent components. The number of resolved components can be determined either automatically or by exploring a map that visualizes the structure of the time series. The statistical credibility of the features of the components is established with Bayesian inference. The method was applied to analyze a marine temperature time series measured from the Barents Sea and its correlation with the abundance of Atlantic salmon in three Barents Sea rivers. Besides the annual seasonal variation and a linear trend, the method revealed mid time-scale (∼10 years) and long time-scale (∼30 years) variation. The 10-year quasi-cyclical component of the temperature time series appears to be connected with a similar feature in Atlantic salmon abundance. These findings can provide information about the environmental factors affecting seasonal and periodic variation in survival and migrations of Atlantic salmon and other migratory fish.
... High rates of exploitation in ocean fisheries have been associated with many stock declines, but the abundance of many stocks nevertheless continues to fall despite great reductions in marine salmon fisheries during the past couple of decades ( Windsor and Hutchinson, 1994;Parrish et al., 1998;Dempson et al., 2001;ICES, 2002). However, some stocks in the Northeast Atlantic, especially in northern Scandinavia (Norway, Finland), fluctuate in a cyclic manner with no declining trend ( Niemelä et al., 2004), and the pre-fishery abundance of the northern stocks has even improved in recent years (ICES, 2003). Management of salmon stocks is often, or should be, dependent on scientific information about the characteristics and status of the stocks, revealed by various research and monitoring programmes. ...
... The large Subarctic River Teno (Tana in Norwegian) in northern Europe (70(N, 28(E) drains a catchment area of 16 386 km 2 into the Barents Sea ( Figure 1), and represents one of the few remaining large salmon rivers with healthy and abundant wild salmon stocks that still supports largescale salmon fisheries in both freshwater and coastal waters ( Niemelä et al., 2004). In addition to the main stem, there are more than 20 spawning tributaries with distinct salmon sub-populations ( Figure 1). ...
... The size and variation of the run size are highly dependent on the preceding smolt output ( Chadwick, 1987;Jonsson et al., 1998), fluctuations in environmental conditions at sea, and marine fishing mortality ( Jonsson and Jonsson, 2004, and references therein). Oscillations in abundance can follow a long period of 20e30 years of high catches ( Bielak and Power, 1986b), or a shorter period of 8 or 9 years, as in the River Teno system ( Niemelä et al., 2004), and can also be common over wide geographical areas ( Dempson et al., 1998). The warmer sea temperatures in the Barents Sea in the late 1990s ( Niemelä et al., 2004) coincided with smolt cohort migrations, which recruited historically high numbers of 1SW, 2SW, and 3SW salmon in the River Teno catches in the years 1999e2002. ...
The abundance of wild Atlantic salmon in the River Teno system has been monitored since the 1970s by estimating salmon catches and juvenile salmon densities at permanent electrofishing sites. Analysis of the time-series has shown significant relationships between juvenile densities (0C and 1C) and subsequent 1SW and 2SW catches. Corresponding significant relationships have been detected between 1SW and 2SW female salmon in the catches and subsequent fry and parr densities. Monitoring juvenile densities allows evaluation of spawning escapement 1 and 2 years earlier, confirming the stock status information provided by catch statistics. These relationships between juvenile abundance and catches suggest that the monitoring programme has included feasible and biologically relevant variables and proper methodologies. Increasing trends were detected in the numbers of 1SW and 2SW salmon in catches between 1977 and 2003. Similarly, fry abundance indicated long-term increasing trends at most sites. Significant relationships were detected between abundances of subsequent sea-age groups in catches (1SW vs. 2SW 1 year later, etc.), indicating that strong smolt year classes influence the abundance of several subsequent sea-age groups, and that such relationships permit forecasting future catches of multi-sea-winter salmon by 1SW salmon catches.
... Evidence exists for dramatic changes in ocean climate conditions in the northwest Atlantic, particularly during the early 1990s (e.g., Colbourne et al. 1997;Colbourne and Anderson 2003), prompting some to suggest that there has been a marine climate regime shift (Drinkwater 2000;Montevecchi et al. 2002). Linkages between ocean climate conditions and variability in abundance or survival have been established in various populations of Atlantic salmon (Friedland 1998;Friedland et al. 2000Friedland et al. , 2003Niemelä et al. 2004). In addition, Chaput et al. (2005) noted strong evidence for a phase shift in Atlantic salmon productivity in the early 1990s that has persisted to date. ...
... For the River Teno, a border river between Finland and Norway that empties into the Barents Sea, environmental variables included in the correlation analysis included mean sea temperature (8C) calculated from average values integrated vertically from 0 to 50 m across all stations of the Kola section in the Barents Sea annually from 1975 to 2002 (Tereshchenko 1996; S. Prusov, Polar Research Institute of Marine Fisheries and Oceanography, unpublished data) and the NAOI. Salmon abundance informa-tion was obtained from published summaries (Niemelä et al. 2004;O'Connell et al. 2005;Chaput et al. 2006;Caron et al. 2007) or archived records maintained by the Gulf Fisheries Centre, Moncton, New Brunswick (Miramichi and Restigouche River data) and refer to numbers of returning individuals. Correlation analyses were performed using abundances of corresponding 1SW (de la Trinité, Teno) or small (Newfoundland, Maritimes) salmon where small salmon are predominately maiden 1SW fish. ...
In many areas of the North Atlantic, populations of salmon (Salmo salar) are now either in a state of decline or extirpated such that concern over the continued survival of the species has been given more attention in recent years despite large reductions in directed ocean fisheries. Previous investigations have established linkages between ocean climate conditions and variability in abundance or survival. However, one avenue not previously explored considers whether changes in marine food webs owing to ever increasing and unsustainable levels of exploitation on many marine species – the so called ‘fishing down marine food webs’ hypothesis – could influence survival and abundance of salmon as a result of shifts in trophic position or changes in energy flows. Since Atlantic salmon are opportunistic feeders during the marine life-history phase, the species lends itself well to studies associated with marine environmental conditions and food web interactions. Here we examine long-term variability in the trophic ecology of Atlantic salmon using analyses of stable isotope signatures of carbon and nitrogen (δ13C and δ15N). Signatures were extracted from the marine growth portion of scales of maiden one-sea-winter fish. Data were obtained from nine Canadian and one north European river (Teno) covering periods extending over three-to-four decades. Significant differences in δ13C and δ15N signatures were found to exist among rivers, as well as among years within rivers. Trends over time in either δ13C or δ15N signatures were evident in only a few situations thus providing little evidence of substantive changes in the trophic ecology of salmon in the North Atlantic. In addition, isotopic signatures were largely invariant in relation to variations in abundance or to various environmental measures characterizing ocean climate conditions in the North Atlantic.
... The spawning age of Atlantic salmon varies from one to five years, calculated as the number of winters spent in the marine feeding areas after their two to five freshwater years in the river [11,12]. Occasionally, salmon are able to recover from their first spawning and return to the sea, and even spawn for a second time [5,13,14], but in the Baltic Sea the proportion of such fish is currently only a few percent [15]. As a nearly semelparous fish, the final body size of the Atlantic salmon depends on its age at maturity. ...
... Factors common to all stocks during the feeding migration include changes in the sea environment, such as the sea surface temperature, conditions in the feeding area, such as the availability of food, and the offshore and coastal fishery. This reasoning has proved correct in several previous studies related to salmon abundance, growth, and migration distance, in which synchronous trends have been related to common environmental factors [6,13,62,63]. The decrease in the proportion of old males has also been observed previous studies , as Järvi [64] reported approximately 11% of the spawners to be 4-6 SW salmon in the 1930s and 1940s, but an analysis sixty years later reported only 0.5-2% of salmon of that age [15]. ...
Origin and age was determined for individual fish caught in offshore catches of Atlantic salmon stocks (Salmo salar L.) in the Baltic Sea over the years 2000–2009. DNA microsatellite loci and smolt age were used to probabilistically assign returning spawners to their stock of origin. Data for this study were based on approximately 2600 catch samples of the five most common wild and four sea-ranched, hatchery-reared stocks. Spawning age, and sex ratio differed both within and between these wild, and sea-ranched groups. The females were mainly (78.7%) two sea-winters old and the males usually (68.7%) only one sea-winter old. In both sexes, the mean age at maturity was lower in the hatchery-reared, sea-ranched stocks than in naturally reproducing stocks. In the 2000s, there was a weak decreasing trend in the male spawning age, but not in that of females. The sex-ratio of the spawners was female dominant in the naturally reproducing stocks, but male dominant in hatchery-reared stocks. Published historical data from two of the same rivers suggest that the majority of males were multi-sea-winter spawners in the 1930s, and variation in the age distribution of the spawners has become narrower and skewed towards a younger age in the present data (2000–2009) compared to the earlier situation.
... While fluctuations in population size can substantially reduce N e , it is especially affected by the cyclic lows (Vucetich et al. 1997;Waples 2002). In the River Teno, numbers of returning salmon are known to oscillate with the ocean climate conditions showing threefold difference between the lowest and highest figures (Niemelä et al. 2004). Given the differences in fecundity, where females can produce 2000– 23 000 eggs depending on size at maturity (Erkinaro et al. 1997), variance in reproductive success among individuals may also have a major impact on N e (e.g.Araki et al. 2007). ...
... The ultimate cause of this temporal instability and small effective population size remained unclear, but two not necessarily mutually exclusive explanations deserve attention: (i) there has been reduction of population size, i.e. bottleneck at or before the time of sampling followed by quick population recovery possibly accompanied with natural supplementation from nearby rivers; (ii) Utsjoki is an incessantly small population subject to strong genetic drift supplemented periodically from nearby rivers, adhering to mainland-island metapopulation dynamics. Our early sampling period (1982–1984) coincides with period of lowest overall catch of salmon in the River Teno since the commencement of monitoring programme (Niemelä et al. 2004). In addition to this, there was exceptionally heavy fishing pressure using nets and weirs along Utsjoki waterway during late 1970's and early 1980's (Niemelä E. personal observations). ...
The evolutionary potential of a population is closely related to two key population genetic parameters, namely the effective population size (N e) and migration rate (m). Furthermore, knowledge of these parameters is required in order to assess potential constraints on local adaptation and for the development of biologically sound management strategies. We addressed these key issues by investigating the temporal and spatial genetic structure of over 2000 adult Atlantic salmon (Salmo salar) collected from 17 sites in the Teno and Näätämö rivers in northernmost Europe with up to five time points spanning temporal intervals up to 24 years (∼4 generations). In all cases except one, local populations were found to be temporally stable within the river system. Estimates of N e were generally a magnitude larger for the mainstem and headwater populations (MS+HW, N e∼340–1200) than for the tributary populations (N e∼35–160), thus explaining the higher genetic diversity and lower divergence of the MS+HW populations compared to tributaries. The overall migration rates to tributaries were low, and in some cases, low enough for local adaptations to potentially evolve, despite their lower N e. Signs of a population bottleneck and natural recruitment from nearby populations were detected in one local population. This highlights a fact which is relevant for the conservation and management of highly substructured population systems in general: that even when the overall census size is large, local populations can be vulnerable to perturbations. To preserve the current and to regain the historical distribution of salmon within the river system, we propose that the status of the total population complex should be evaluated at the local population level rather than from descriptive statistics at the system level.
... The influence of climate at sea on S. salar growth has extensively been studied (e.g. Friedland, 1998;Friedland et al., 2000;Niemelä et al., 2004;Peyronnet et al., 2008). Sea surface temperatures at large scales and global indices such as the North Atlantic Oscillation Index have been observed to be correlated to individual growth. ...
... For example there are almost no individual spending 3 or 4 years in the river or more than two winters at sea such as in Russian populations (e.g. Niemelä et al., 2004). The present temperature-dependent growth function in the river phase used by IBASAM could give rise to older smolt with temperature regimes colder than in the Scorff. ...
... However, other mechanisms may cause synchrony, such as dispersal among populations that reduce the size of relatively large populations and increase that of relatively small ones, or trophic interactions that cause spatial synchrony between the predator population and the prey species (Liebhold et al. 2004). Synchrony in population sizes and recruitment have also been found in many fish species (e.g., Fromentin et al. 2000;Tedesco and Hugueny 2006;Nunn et al. 2007), including salmonids ( Friedland et al. 1998;Niemelä et al. 2004;Pyper et al. 2005). Peterman et al. (1998) examined spatial patterns among survival rate indices and found positive covariation among stocks of sockeye salmon (Oncorhynchus nerka) in the northeast Pacific Ocean at regional scales, but not at a larger, ocean-basin scale. ...
... Similar results are also found for pink salmon (Oncorhynchus gorbuscha) (Pyper et al. 2001) and chum salmon (Oncorhynchus keta) ( Pyper et al. 2002). In the northeast Atlantic Ocean, synchrony in recruitment in Atlantic salmon (Salmo salar) has been reported for two sets of geographically close pairs of populations in Scotland and South Norway (Friedland et al. 1998) and in the Barents Sea ( Niemelä et al. 2004). The synchrony documented in these studies suggests that survival rates are affected by similar environmental processes that operate primarily at confined spatial scales ( Pyper et al. 2005). ...
More synchronous growth was observed between close, than more distantly separated populations of Atlantic salmon (Salmo salar), during both the first and the second year at sea. The marine growth of seven Norwegian populations, located between 60°N and 70°N, were correlated with sea surface temperatures (SSTs) in the Barents Sea, the Norwegian Sea, and the North Sea, and it was found that growth correlated best with the water temperatures in the area located closest to their home river. Growth was also compared with three broad-scale climate indices (North Atlantic Oscillation (NAO), Atlantic Multidecadal Oscillation, and subpolar gyre), with the strongest relationship occurring with the NAO index. However, SSTs explained more of the variability than the climatic indices did. Growth increment for the first year, but not the second year, was higher for southern than northern populations, mainly because of later smolt migration to sea in the north, and hence, a shorter growth season. For multi-sea-winter fish, all populations except one had a negative trend in growth with years for both the first and the second year at sea. For the second year at sea, this was most pronounced after the beginning of the 1980s. This is in accordance with the negative trend in pre-fishery abundance of adult salmon during the same period.
... While fluctuations in population size can substantially reduce N e , it is espe- cially affected by the cyclic lows ( Vucetich et al. 1997;Waples 2002). In the River Teno, numbers of returning salmon are known to oscillate with the ocean climate conditions showing threefold difference between the low- est and highest figures ( Niemelä et al. 2004). Given the differences in fecundity, where females can produce 2000- 23 000 eggs depending on size at maturity ( Erkinaro et al. 1997), variance in reproductive success among individuals may also have a major impact on N e (e.g. ...
... The ultimate cause of this temporal instability and small effective pop- ulation size remained unclear, but two not necessarily mutually exclusive explanations deserve attention: (i) there has been reduction of population size, i.e. bottle- neck at or before the time of sampling followed by quick population recovery possibly accompanied with natural supplementation from nearby rivers; (ii) Utsjoki is an incessantly small population subject to strong genetic drift supplemented periodically from nearby rivers, adhering to mainland-island metapopulation dynamics. Our early sampling period (1982)(1983)(1984) coincides with period of lowest overall catch of salmon in the River Teno since the commencement of monitoring programme ( Niemelä et al. 2004). In addition to this, there was exceptionally heavy fishing pressure using nets and weirs along Utsjoki water- way during late 1970's and early 1980's (Niemelä E. per- sonal observations). ...
... Some of these stressors are natural, while others are caused by constantly expanding anthropogenic activities in rivers and the coastal zone (Forseth et al., 2017;Lennox et al., 2021). With some exceptions in the northern areas (Niemelä et al., 2005), Atlantic salmon (hereon referred to as salmon) populations have declined throughout most of their distribution over the past several decades (Friedland et al., 2009;Jensen et al., 2011;Peyronnet et al., 2007Peyronnet et al., , 2008Todd et al., 2008). Parasites like salmon lice Lepeophtheirus salmonis (Thorstad et al., 2015) and Gyrodactylus salaris (Johnsen & Jensen, 1991), introgression of escaped domesticated salmon (Bolstad et al., 2017;Fleming et al., 2000;Glover et al., 2013Glover et al., , 2019McGinnity et al., 2003;Skaala et al., 2019), river regulations and agriculture practices have all been identified as major threats to the abundance of salmon populations, although their relative importance varies from region to region and over time (Forseth et al., 2017). ...
Wild Atlantic salmon populations have declined in many regions and are affected by diverse natural and anthropogenic factors. To facilitate management guidelines, precise knowledge of mechanisms driving population changes in demographics and life history traits is needed. Our analyses were conducted on (a) age and growth data from scales of salmon caught by angling in the river Etneelva, Norway, covering smolt year classes from 1980 to 2018, (b) extensive sampling of the whole spawning run in the fish trap from 2013 onwards, and (c) time series of sea surface temperature, zooplankton biomass, and salmon lice infestation intensity. Marine growth during the first year at sea displayed a distinct stepwise decline across the four decades. Simultaneously, the population shifted from predominantly 1SW to 2SW salmon, and the proportion of repeat spawners increased from 3 to 7%. The latter observation is most evident in females and likely due to decreased marine exploitation. Female repeat spawners tended to be less catchable than males by anglers. Depending on the time period analyzed, marine growth rate during the first year at sea was both positively and negatively associated with sea surface temperature. Zooplankton biomass was positively associated with growth, while salmon lice infestation intensity was negatively associated with growth. Collectively, these results are likely to be linked with both changes in oceanic conditions and harvest regimes. Our conflicting results regarding the influence of sea surface temperature on marine growth are likely to be caused by long‐term increases in temperature, which may have triggered (or coincided with) ecosystem shifts creating generally poorer growth conditions over time, but within shorter datasets warmer years gave generally higher growth. We encourage management authorities to expand the use of permanently monitored reference rivers with complete trapping facilities, like the river Etneelva, generating valuable long‐term data for future analyses. Using a dataset spanning smolt year classes 1980–2018, we observed a clear temporal decline in growth rate during the first year at sea, with a stepwise reduction over the four decades. In the same time period, we also observed a clear switch from a dominance of 1SW fish to a dominance of 2SW fish, and, more than a doubling in the proportion of repeat spawners in the population. The influence of summer SST on marine growth depended on the length of the time series used, with a negative effect over the longer angling time series, and a positive effect over the shorter fish‐trap time series. Zooplankton positively influenced marine growth while sea lice intensity negatively influenced growth
... Yet, the second peak in 1SW abundance in [2000][2001] resulted in an unpreceded abundance of PS salmon in the Teno system ( Niemelä et al. 2006a), although the coastal catches were generally higher in the latter period ( Niemelä et al. 2012). Since environmental conditions in the Barents Sea are correlated with the variation in salmon catches in the region ( Niemelä et al. 2004;Pasanen et al. 2017), it has been speculated that the increased surface temperature at the Barents Sea be- tween the 1990s and 2000s may have contributed to the better survival of kelts and improved prospects for abundant returns of PS salmon in the early 2000s ( Niemelä et al. 2006a). In addition, a general peak in salmon abundance was evident at the turn of the millennium (Anonymous 2017;ICES 2017). ...
We used >154000 scale samples collected from salmon fisheries in the large River Teno system over a 40-year period to quantify life history diversity and long-term trends. We identified 120 different life history strategies including combinations of smolt (2-8) and sea ages (1-5) and previous spawning events. Most strategies were rare: 60% of individuals matured after one year at sea following 3-5 years in freshwater. Age at maturity has changed with an increase in two-sea-winter (2SW) salmon and previous spawners (PS), and a decline in 3SW fish. Smolt age distribution showed a decreasing proportion of age-3 smolts while that of age-5 increased. Fishing gear and fishing season times selected for fish differing in life-history strategies. Temporal variation in life histories reflects changes both in fisheries and the changing environment. There was an inverse relationship between years spent in freshwater and sea age. Biocomplexity was manifested by the multiple year classes (6-11) present in annual runs that increased with years, reflecting an increase both in PS and sampling effort. The high number of cohorts spawning simultaneously each year indicates strong generation overlap, which has been suggested to maintain genetic diversity and thereby resilience via the portfolio effect.
... spring-summer one year before). Furthermore, this relationship is consistent throughout the latitudinal range examined (a random effect on SST was not supported) and the sign of the effect agrees with previous results across several Norwegian rivers [38,39], and other rivers elsewhere (e.g. Pacific salmon in Alaska, [40]); but differs from what was shown in other areas (e.g. ...
... Based on significant differences in the elemental composition of whole otoliths of smolts, mean classification accuracies were moderately high and extremely high for river management stocks and large river systems, respectively. Previous studies attempting to discriminate among stocks with the aid of natural marks have also achieved high classification rates, but not at the resolution needed to identify oceanic factors influencing differences in recruitment patterns which have been shown to act at regional scales (Friedland 1998;Friedland and Reddin 2000;Niemelä et al. 2004). More recent studies employing chemical marks have been able to discriminate among individual hatchery or stream populations (Flem et al. 2005;Veinott and Porter 2005;Martin et al. 2013), but may not meet the assumptions required to act as natural tags of origin for the purpose of ocean research and management. ...
The utility of otolith elemental fingerprints for discriminating sub-regional stocks of Atlantic salmon was examined. Otoliths were removed from Atlantic salmon smolts collected from three individual river watersheds in the Canadian Maritimes during spring and analyzed for 27 elements using inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma emission spectrometry (ICP-ES). Calcium and minor and trace elements were precisely measured in whole otoliths at concentrations well above detection limits. Six elements (Ba, Pb, Li, Mn, Rb, and Tl) were significantly different among watersheds. Linear discriminate function analysis based on otolith elemental concentrations of Li, Mn, Rb, and Tl correctly classified smolts to their river of origin with an average accuracy of 73%. At a slightly greater spatial scale of large watersheds, correct mean classification rate was 92% based on a fingerprint of four elements (Ba, Li, Mn, and Rb). Results indicate that elemental fingerprints of otoliths can be used to discriminate among river management stocks which may be important in the future since dried or frozen stored otoliths retain their signature indefinitely and otoliths are often available from previous studies. Otolith elemental fingerprints would be effective as a natural tag of a river system or biogeoclimatic zone of origin when applied to the study and management of Atlantic salmon in the North Atlantic Ocean.
... The 200 years (1804-2003) time series of abundance data for Atlantic salmon in the Baltic Sea is based on the all-nation total yield statistics derived from McKinnell and Karlström (1999) and FGFRI (2004). Indirect measures of population abundance, such as harvest data of game (Ranta et al. 1997, Forchammer et al. 1998, Stenseth et al. 1999) and fish (Scarrnecchia 1984, Chadwick 1985, Mills 1989, Gudjonsson et al. 1995, Beaugrand and Reid 2003, Post et al. 2004, Niemelä et al. 2004, Boylan and Adams 2006, Voellestad et al. 2009) are often used as a surrogate of abundance to make inferences on long term population dynamics when direct observational data are not available (Cattadori et al. 2003, Ranta et al. 2008). However, a major concern over the use of such data is that harvesting records tend to underestimate low densities and overestimate high densities as a consequence of variation in harvesting effort. ...
Atmospheric variations on multi-decadal time scales, or climate regimes, can cause significant alterations throughout the physical environment, often with remarkable consequences for animal populations and ecosystems. Here we report that the climate regimes in the Baltic Sea area, which are based on the North Atlantic Oscillation and two more regional oceanographic variables, account for the observed dynamics of Atlantic salmon abundance and size in the sea. The changes in the salmon abundance and size followed changes in the climate regimes in the Baltic Sea area, indicating that high amplitude shifts in the Atlantic salmon performance are closely associated with longer-term patterns in the climate. During maritime, temperate regimes salmon were larger in size but low in abundance, with contrasting characteristics during continental, cold climate regimes. Therefore, Atlantic salmon seem to provide a very sensitive indicator of the biological effects of climateforced changes in the aquatic environment.
... However, marine thermal conditions are already a problem for southern tier salmon stocks in North America and Europe. It stands to reason then that the concomitant increase in northern tier stocks is a result of shifting thermal conditions (Niemela¨et al. 2004;Chaput et al. 2005). The phenological mismatch identified as critical to North American stocks will only be exacerbated in the GOM considering that temperature is expected to increase on the order of 2°C in the GOM over the next century (Fogarty et al. 2008). ...
Abstract Observations relevant to the North American stock complex of Atlantic salmon, Salmo salar L., suggest that marine mortality is influenced by variation in predation pressure affecting post‐smolts during the first months at sea. This hypothesis was tested for Gulf of Maine (GOM) stocks by examining wind pseudostress and the distribution of piscivorous predator fields potentially affecting post‐smolts. Marine survival has declined over recent decades with a change in the direction of spring winds, which is likely extending the migration of post‐smolts by favouring routes using the western GOM. In addition to changes in spring wind patterns, higher spring sea surface temperatures have been associated with shifting distributions of a range of fish species. The abundance of several pelagic piscivores, which based on their feeding habits may predate on salmon post‐smolts, has increased in the areas that serve as migration corridors for post‐smolts. In particular, populations of silver hake, Merluccius bilinearis (Mitchell), red hake, Urophycis chuss (Walbaum), and spiny dogfish, Squalus acanthias L., increased in size in the portion of the GOM used by post‐smolts. Climate variation and shifting predator distributions in the GOM are consistent with the predator hypothesis of recruitment control suggested for the stock complex.
... With only a limited time-series of data available, no Age and fine-scale marine growth of Atlantic salmon post-smolts in the NE Atlantic significant relationship between SST and post-smolt growth was found, and additional information would be required to determine the influence of SST on growth in that area and period. In populations farther north, salmon abundance and marine growth are strongly influenced by sea temperature (Niemelä et al., 2004; Jensen et al., 2011). Although based on only four years of data spread over an 8-year period, the SSB of pelagic fish was significantly and negatively related to post-smolt growth in the Vøring Plateau area. ...
Jensen, A. J., Ó Maoiléidigh, N., Thomas, K., Einarsson, S. M., Haugland, M., Erkinaro, J., Fiske, P., Friedland, K. D., Gudmundsdottir, A. K., Haantie, J., Holm, M., Holst, J. C., Jacobsen, J. A., Jensås, J. G., Kuusela, J., Melle, W., Mork, K. A., Wennevik, V., and Østborg, G. M. 2012. Age and fine-scale marine growth of Atlantic salmon post-smolts in the Northeast Atlantic. – ICES Journal of Marine Science, 69: 1668–1677.
Surface trawls were conducted over a large area of the Northeast Atlantic in 2002, 2003, 2008, and 2009 to collect samples of Atlantic salmon (Salmo salar) post-smolts during their marine feeding migration (n = 2242). The dominant smolt age of wild post-smolts was 2 years, followed by 1- and 3-year-old fish, and a few 4-year-old fish. The average rate of circulus formation in the marine zone of scales was estimated to be 6.3 d circulus⁻¹. Both the age structure and the number of marine circuli in the scales suggest that the majority of the post-smolts originated in rivers in southern Europe. Applying intercirculi distances in scales as a proxy variable of growth rate suggests that putative marine growth rates varied among years, with the fastest growth rates in 2002 and the slowest growth rates in 2008. Further, the first marine intercirculi distances were narrowest in 1-year-old smolts, successively increasing with smolt age, indicating that growth rates during the first period at sea were lowest for salmon of southernmost origin. Growth indices are linked to prevailing environmental and biological conditions.
... The River Teno, with >1200 km of accessible migration routes for Atlantic salmon, forms the border between northern Norway and Finland (70°N). The river system supports one of the largest Atlantic salmon populations in the world (annual salmon catch mostly between 100 and 200 t; Niemelä et al. 2004) and has a drainage area of 16,000 km 2 with an annual mean discharge of approximately 200 m 3 AEs )1 . Underwater videocamera recording was carried out in an area close to the confluence of the River Utsjoki (a tributary to River Teno, 69°54¢N; 27°02¢E), 106 km upstream from the sea. ...
Linnansaari T, Keskinen A, Romakkaniemi A, Erkinaro J, Orell P. Deep habitats are important for juvenile Atlantic salmon Salmo salar L. in large rivers. Ecology of Freshwater Fish 2010: 19: 618–626. © 2010 John Wiley & Sons A/S
Abstract – Juvenile Atlantic salmon were studied by underwater video surveillance and self contained underwater breathing apparatus-diving in deep (i.e. >1.0 m), fast flowing areas of two large river systems (River Teno, River Tornionjoki) in northern Finland. Both video and diving data indicated that young-of-the-year (YOY) salmon (0+) and salmon parr (>0+) readily utilised habitats deeper than 1 m. Young-of-the-year salmon and parr were observed through a range of 0.5–1.9 m and 0.4–2.2 m, respectively. A negative linear relationship between the mean abundance of YOY salmon and mean depth was noted from the diving transects. Salmon parr were similarly abundant throughout the range of depths studied. Video data showed that deep habitats were used throughout the summer (June–August). It was concluded that deep, fast-flowing areas in large rivers may constitute a significant habitat resource for juvenile salmon that has not been traditionally accounted for when estimating salmon production.
... More than 1200 km of river are accessible to migrating adult salmon, including about 25 tributaries. The River Teno is the most productive salmon river in both Norway and Finland with annual river catches between 70 and 250 t (Niemela¨, Erkinaro, Dempson, Julkunen, Zubchenko, Prusov, Svenning, Ingvaldsen, Holm & Hassinen 2004). A more detailed description of the River Teno watercourse is given by Niemela¨, Julkunen & Erkinaro (1999). ...
Abstract Reliability of underwater snorkel counts of adult Atlantic salmon, Salmo salar L., was analysed in the tributaries of the River Teno, close to the spawning period. In small (width 5–20 m) rivers, the replicated total counts of salmon were reasonably precise (CV = 5.4–8.5%), while in the medium-sized river (width 20–40 m) the precision of the counting method was considerably lower (CV = 15.3%). Low precision in a medium sized river was also observed in an experiment using marked live fish where the observation efficiency varied between 36.4% and 70.0%. In a small river, the detection efficiency of artificial fish silhouettes (test salmon) was almost perfect in pools (98%), but decreased in rapids (84%). Separate counts of males, females, grilse and large salmon were usually more variable than total counts, indicating that divers were more capable of locating a fish than properly identifying its sex and sea-age. The behaviour of adult salmon was favourable to conduct snorkel counts, as fish normally stayed still, or after hesitating, moved upstream (>95%of the cases) when encountering a diver. The high observation efficiency (>90%) and precision, favourable behaviour of salmon and congruence between snorkel counts and catch statistics in small rivers suggest that reliable data on Atlantic salmon spawning stock can be collected by snorkeling provided that the environmental conditions are suitable and the divers are experienced.
... The effects of ocean climate on Atlantic salmon growth and survival clearly are both pervasive and complex (Reist et al., 2006). For example, while there has been a general pattern of marked stock declines over the past three decades this does not extend to all northern European rivers (e.g. Niemelä et al., 2004). None the less, our analyses indicate that SST warming of the sub-Arctic North Atlantic Ocean over the past decade has been excessive to the point of constraining, rather than promoting, adult growth condition of multiple stocks within the ICES southern European grouping (Fig. 6a and b). ...
Ocean climate impacts on survivorship and growth of Atlantic salmon are complex, but still poorly understood. Stock abundances have declined over the past three decades and 1992–2006 has seen widespread sea surface temperature (SST) warming of the NE Atlantic, including the foraging areas exploited by salmon of southern European origin. Salmon cease feeding on return migration, and here we express the final growth condition of year-classes of one-sea winter adults at, or just before, freshwater re-entry as the predicted weight at standard length. Two independent 14-year time series for a single river stock and for mixed, multiple stocks revealed almost identical temporal patterns in growth condition variation, and an overall trend decrease of 11–14% over the past decade. Growth condition has fallen as SST anomaly has risen, and for each year-class the midwinter (January) SST anomalies they experienced at sea correlated negatively with their final condition on migratory return during the subsequent summer months. Stored lipids are crucial for survival and for the prespawning provisioning of eggs in freshwater, and we show that under-weight individuals have disproportionately low reserves. The poorest condition fish (∼30% under-weight) returned with lipid stores reduced by ∼80%. This study concurs with previous analyses of other North Atlantic top consumers (e.g. somatic condition of tuna, reproductive failure of seabirds) showing evidence of major, recent climate-driven changes in the eastern North Atlantic pelagic ecosystem, and the likely importance of bottom-up control processes. Because salmon abundances presently remain at historical lows, fecundity of recent year-classes will have been increasingly compromised. Measures of year-class growth condition should therefore be incorporated in the analysis and setting of numerical spawning escapements for threatened stocks, and conservation limits should be revised upwards conservatively during periods of excessive ocean climate warming.
... spring–summer one year before). Furthermore, this relationship is consistent throughout the latitudinal range examined (a random effect on SST was not supported) and the sign of the effect agrees with previous results across several Norwegian rivers[38,39], and other rivers elsewhere (e.g. Pacific salmon in Alaska,[40]); but differs from what was shown in other areas (e.g. ...
Many Atlantic salmon, Salmo salar, populations are decreasing throughout the species' distributional range probably due to several factors acting in concert. A number of studies have documented the influence of freshwater and ocean conditions, climate variability and human impacts resulting from impoundment and aquaculture. However, most previous research has focused on analyzing single or only a few populations, and quantified isolated effects rather than handling multiple factors in conjunction. By using a multi-river mixed-effects model we estimated the effects of oceanic and river conditions, as well as human impacts, on year-to-year and between-river variability across 60 time series of recreational catch of one-sea-winter salmon (grilse) from Norwegian rivers over 29 years (1979-2007). Warm coastal temperatures at the time of smolt entrance into the sea and increased water discharge during upstream migration of mature fish were associated with higher rod catches of grilse. When hydropower stations were present in the course of the river systems the strength of the relationship with runoff was reduced. Catches of grilse in the river increased significantly following the reduction of the harvesting of this life-stage at sea. However, an average decreasing temporal trend was still detected and appeared to be stronger in the presence of salmon farms on the migration route of smolts in coastal/fjord areas. These results suggest that both ocean and freshwater conditions in conjunction with various human impacts contribute to shape interannual fluctuations and between-river variability of wild Atlantic salmon in Norwegian rivers. Current global change altering coastal temperature and water flow patterns might have implications for future grilse catches, moreover, positioning of aquaculture facilities as well as the implementation of hydropower schemes or other encroachments should be made with care when implementing management actions and searching for solutions to conserve this species.
Historical salmon catch records suggest that climatic variability, and more recently human exploitation, control patterns of abundance in Atlantic salmon populations. We present the first long-term (2000-year) reconstruction of Atlantic Salmon population variations based on a Marine Derived Nutrient (MDN) lake sediment record. Our record is constructed from nitrogen isotopes (δ15N) measured from a lake sediment core, which we compare with an escapement index (EI) derived from historic net catch data on major Scottish salmon rivers. We used an isotope mixing model to demonstrate that the N isotope values are likely enriched with MDN and demonstrate that Loch Insh sediments are enriched compared with a control site (Loch Vaa) that has never had exposure to salmon. We demonstrate that current adult spawner returns are around half that of historic values prior to major human exploitation. Before the onset of widespread human exploitation and habitat degradation, large fluctuations in salmon abundance are attributed to variations in North Atlantic sea surface temperature. While our data support published reconstructions of declining Atlantic salmon stocks in Northwest European rivers over the last 1000 years, rather than point to a solely human cause, the human impact appears to be overprinted on larger-scale changes in marine habitat occurring at the transition from the warmer Medieval Climatic Anomaly (MCA) to the cooler Little Ice Age (LIA).
We used over 154 000 scale samples collected from salmon fisheries in the large River Teno system over a 40-year period to quantify life history diversity and long-term trends. We identified 120 different life history strategies, including combinations of smolt (2-8) and sea ages (1-5) and previous spawning events. Most strategies were rare; 60% of individuals matured after 1 year at sea following 3-5 years in fresh water. Age at maturity changed with an increase in two-sea-winter salmon and previous spawners and a decline in three-sea-winter fish. Smolt age distribution showed a decreasing proportion of age-3 smolts, while that of age-5 smolts increased. Fishing gear and fishing season times selected for fish differing in life history strategies. Temporal variation in life histories reflected changes in both fisheries and the changing environment. There was an inverse relationship between years spent in fresh water and sea age. Biocomplexity was manifested by the multiple year classes (6-11) present in annual runs, which increased with years, reflecting an increase both in previous spawners and sampling effort. The high number of cohorts spawning simultaneously each year indicates strong generational overlap, which has been suggested to maintain genetic diversity and thereby resilience via the portfolio effect. Résumé : Nous avons utilisé plus de 154 000 échantillons d'écaille prélevés de pêches au saumon dans le grand réseau du fleuve Teno sur une période de 40 ans pour quantifier la diversité des cycles biologiques et les tendances à long terme. Nous avons décelé 120 stratégies de cycle biologique différentes incluant des combinaisons d'âges de saumoneaux (2-8) et au stade marin (1-5) et d'épisodes de frai antérieurs. La plupart des stratégies étaient rares; 60 % des individus atteignaient la maturité après 1 année en mer suivant 3-5 années en eau douce. L'âge à la maturité a changé avec l'augmentation des saumons ayant passé deux hivers en mer et des frayeurs répétitifs, et une baisse des saumons ayant passé trois hivers en mer. La distribution des âges de saumoneaux est caractérisée par une diminution de la proportion de saumoneaux de 3 ans, alors que celle des saumoneaux de 5 ans a augmenté. Les engins de pêche et le moment des saisons de pêche ont entraîné la sélection de poissons présentant des stratégies de cycle biologique différentes. La variation dans le temps des cycles biologiques reflète des changements tant dans les pêches que du milieu ambiant. Il existe une relation inverse entre les années passées en eau douce et l'âge en mer. Une biocomplexité se manifeste dans les multiples classes d'âge (6-11) présentes dans les migrations annuelles qui augmentent au fil des années, reflétant une augmentation tant du nombre de frayeurs répétitifs que de l'effort d'échantillonnage. Le grand nombre de cohortes frayant simultanément chaque année indique un fort chevauchement des générations, et il a été suggéré que ce phénomène maintiendrait la diversité génétique et, du coup, la résilience par l'entremise de l'effet de portefeuille. [Traduit par la Rédaction]
Atlantic salmon, Salmo salar , are opportunistic feeders, utilising a wide variety of available prey throughout all life stages while feeding at sea. It is during this time that they build up the bulk of their final body size with many fish increasing in weight 1000 - fold or more. Their main prey are different species of fish and fish larvae and planktonic crustaceans, and successful marine feeding is a key factor for enhancing the survival and production of salmon at sea. However, any changes in the marine environment on both spatial and temporal scales may affect the availability of prey and hence impact the subsequent survival and abundance of salmon. In particular, the first few months at sea are often regarded as an important feeding period in order for young salmon to rapidly enhance their growth and reduce their risk of predation. Drawing on past as well as recent studies, we review current knowledge related to similarities and differences associated with spatial and temporal aspects of Atlantic salmon feeding during various stages of their marine migration from nearshore coastal waters and out into the open sea. As salmon are likely affected by ecosystem structure rather than directly infl uence it, we note that future studies should focus on direct linkages between prey abundance and the prey that salmon consume, along with how these associations affect survival and productivity of stocks in the context of a changing climate.
Atlantic salmon spawn in more than 2000 river systems along both sides of the North Atlantic Ocean. A river ' s capacity for producing juvenile salmon is to a large degree dependent on the total amount of suitable freshwater habitat available for rearing, which varies extensively among rivers. This chapter focuses on how landscape and its anthropogenic land use modification contributes to the among - river variation in salmon production. We outline how landscape factors operate at widely varying spatial scales which are hierarchically nested within each other. Due to this hierarchy of river functioning, large - scale factors often override factors controlling salmon production at smaller scales. Human impacts and land use have infl uenced the historical and current status of Atlantic salmon productivity from the global to the catchment and local reach scales. Understanding how factors at different scales infl uence salmon productivity is thus obviously important in salmon habitat conservation and restoration. In light of this we discuss relevant spatial scales for assessment of anthropogenic land use impacts on salmon freshwater habitat and for carrying out habitat restoration strategies.
The 30 year time series analyses revealed large temporal variation in the return rates and a recent increase in abundance of previous spawning Atlantic salmon Salmo salar in the River Teno, northern Scandinavia. The mean proportion of repeat spawners was 7 and 4% in the total Atlantic salmon catch and 9 and 22% in multi‐sea‐winter (MSW) catch component for females and males, respectively. Previous spawners constituted on the average 7% of the catch in mass but up to 20%(31 t) and 30%(19 t) in 2003 and in 2004, respectively. In 1975–2000, the proportion of previous spawners varied between 1 and 6%(3–12% of MSW Atlantic salmon), whereas in 2001–2004, they accounted for 8–21%(16–35% of MSW Atlantic salmon) of the total Atlantic salmon catch. The number of previous spawners in the catch correlated significantly with the preceding numbers of respective 1–3 sea‐winter (SW) maiden Atlantic salmon 2 years earlier. The recent increase in the numbers of 1S1 and 2S1 (1 or 2 years at sea followed by first spawning and 1 year reconditioning period at sea) alternate spawning Atlantic salmon was a consequence of higher numbers of maiden 1SW and 2SW Atlantic salmon in the catches and increased sea temperatures. Similarly, the return rate of 1SW Atlantic salmon to second spawning has improved in recent years. Most previous spawners ascended and were captured early in the fishing season. The smolt and sea‐age combinations of repeat spawners comprised 68 age groups contributing with the annual mean of 15 age groups to the great diversity of the River Teno Atlantic salmon population complex.
The important decline of Atlantic salmon (Salmo salar) across its range during the past three decades, despite numerous management and conservation programmes, is an alarming index of the vulnerability of this species. The following series of papers was produced to summarize current knowledge on specific interactions between biotic and abiotic variables that may contribute to determine the survival of Atlantic salmon. Evaluation of the challenges encountered in spawning grounds (siltation, oxygenation), nursery habitats (substrate, trophic interactions), overwintering habitats (flow conditions, winter feeding opportunities), and coastal and oceanic environments (water temperature, predators, parasites) suggest that all habitats required by Atlantic salmon and all processes that occur in each habitat represent a critical link that allows this species to persist. Management practices employed during artificial fish selection, incubation, and stocking also affect the success of restoration efforts. Because limiting factors may change in time and because our ability to intervene in specific habitats may be minimal, the only strategy within our reach may be to continue gathering information about processes that determine the fragility of Atlantic salmon and, in the light of our findings, to implement scientifically sound actions where and when possible.
Abstract Weekly exploitation rates of Atlantic salmon, Salmo salar L., in the River Utsjoki, Finland, were estimated from catch reports during the 2003, 2004, 2006 and 2007 seasons, and recordings of all Atlantic salmon ascending the river using a submerged video camera array. In all years, mean weekly fishing mortality rates were significantly higher in June than July to August, with a falling trend throughout the fishing season. Owing to overlap in size between one-sea-winter (1SW) and multi-sea-winter (MSW) salmon, the estimated fishing mortality rates were based on all sea-age categories combined. By the second week in June, 18.3–34.7% of large salmon (only MSW) had ascended, compared with 2.2–6.4% of small salmon (1SW and some MSW). Indirectly, the earlier start of ascent of large salmon to River Utsjoki indicated that in-river fishing mortality of MSW salmon is higher than for 1SW salmon. A later opening of the fishing season may be used to reduce the in-river fishing mortality, especially for the MSW component.
The River Alta, northern Norway (70°N), was regulated for hydropower in 1987. Densities of juveniles and catches of adult
Atlantic salmon have been studied since 1980–1981 to examine the effects of regulation. The need to control environmental
variables during electrofishing was emphasized, as flow variables explained up to 42% of the variation in estimated juvenile
densities. The number of spawning redds was counted along the river from 1996 to 2005. The annual number of spawning redds
was correlated with the catch of multi-sea-winter salmon (predominantly females). In the upper 7km section, just downstream
of the power station outlet, juvenile densities were reduced by 80% from pre-regulation levels to minimum levels in 1992–1996.
This was followed by partial recovery during 1997–2005, although not entirely back to pre-regulation levels. In contrast,
the general trend in the middle part of the river was a linear increase in juvenile densities during 1981–2005. Decreased
juvenile densities in the upper section was subsequently followed by reduced catches of adult salmon in this part of the river.
The relative catches of smolt year classes migrating to sea in the upper section was reduced by up to 75% from 1991 onwards.
Spawning and recruitment in the upper section have increased in recent years, probably back to the introduction of catch-and-release
angling and an increase in salmon runs. However, present day smolt production in the upper section is still reduced compared
to the middle part of the river, 18years after regulation. The decreased densities of juvenile salmon in the upper section
were probably caused by several factors, of which stranding mortality due to sudden drops in the water level and increased
winter mortality due to changed environmental conditions, especially reduced ice-cover, may be the most important. In conclusion,
the regulation caused a considerable reduction of the salmon production in the upper 16% of salmon reaches, but did not affect
the salmon population negatively further downstream. This study illustrates that apparently small environmental disturbances
can cause large changes in Atlantic salmon abundance in high latitude populations.
To study smolt behaviour and survival of a northern Atlantic salmon Salmo salar population during river descent, sea entry and fjord migration, 120 wild S. salar were tagged with acoustic tags and registered at four automatic listening station arrays in the mouth of the north Norwegian River Alta and throughout the Alta Fjord. An estimated 75% of the post-smolts survived from the river mouth, through the estuary and the first 17 km of the fjord. Survival rates in the fjord varied with fork length (LF), and ranged from 97.0 to 99.5% km(-1). On average, the post-smolts spent 1.5 days (36 h, range 11-365 h) travelling from the river mouth to the last fjord array, 31 km from the river mouth. The migratory speed was slower (1.8 LF s(-1)) in the first 4 km after sea entry compared with the next 27 km (3.0 LF s(-1)). Post-smolts entered the fjord more often during the high or ebbing tide (70%). There was no clear diurnal migration pattern within the river and fjord, but most of the post-smolts entered the fjord at night (66%, 2000-0800 hours), despite the 24 h daylight at this latitude. The tidal cycle, wind-induced currents and the smolts' own movements seemed to influence migratory speeds and routes in different parts of the fjord. A large variation in migration patterns, both in the river and fjord, might indicate that individuals in stochastic estuarine and marine environments are exposed to highly variable selection regimes, resulting in different responses to environmental factors on both temporal and spatial scales. Post-smolts in the northern Alta Fjord had similar early marine survival rates to those observed previously in southern fjords; however, fjord residency in the north was shorter.
One model for marine migration of Atlantic salmon Salmo salar proposes that North American and southern European stocks (<62 degrees N) move directly to feeding grounds off west Greenland, then overwinter in the Labrador Sea, whereas northern European stocks (>62 degrees N) utilize the Norwegian Sea. An alternate model proposes that both North American and European stocks migrate in the North Atlantic Subpolar Gyre (NASpG) where S. salar enter the NASpG on their respective sides of the Atlantic, and travel counterclockwise within the NASpG until returning to natal rivers. A review of data accumulated during the last 50 years suggests a gyre model is most probable. Freshwater parr metamorphose into smolts which have morphological, physiological and behavioural adaptations of epipelagic, marine fishes. Former high-seas fisheries were seasonally sequential and moved in the direction of NASpG currents, and catches were highest along the main axis of the NASpG. Marking and discrimination studies indicate mixed continental origin feeding aggregations on both sides of the Atlantic. Marked North American smolts were captured off Norway, the Faroe Islands, east and west Greenland, and adults tagged at the Faroes were recovered in Canadian rivers. Marked European smolts were recovered off Newfoundland and Labrador, west and east Greenland, and adults tagged in the Labrador Sea were captured in European rivers. High Caesium-137 ((137)Cs) levels in S. salar returning to a Quebec river suggested 62.3% had fed at or east of Iceland, whereas levels in 1 sea-winter (SW) Atlantic Canada returnees indicated 24.7% had fed east of the Faroes. Lower levels of (137)Cs in returning 1SW Irish fish suggest much of their growth occurred in the western Atlantic. These data suggest marine migration of S. salar follows a gyre model and is similar to other open-ocean migrations of epipelagic fishes.
Over the last decades, the abundances of many Atlantic salmon populations have declined drastically both due to anthropogenic and natural factors. The period when salmon enter the sea for the first time, starting as smolts in the river and subsequently as post-smolts in the fjord, is regarded as one of most critical periods in the salmon life history. In addition, salmon can also suffer from high mortality during their return migration through fjords and estuaries both due to predation and coastal fisheries. Despite an increasing number of recent Atlantic salmon migration studies, few have been conducted in northern areas which host some of the largest and most productive salmon populations in the world.
The main aim of this thesis was therefore by use of electronic acoustic tracking and video observation to study how the environment influence on the behaviour patterns of northern Atlantic salmon during their smolt migration in rivers (Tana and Alta), and as post-smolts and homing salmon during their estuary and fjord migration (Alta). Since northern areas have 24 h daylight during summer, special attention was given to the impact of light intensity on salmon migratory behaviour. This also includes a study of direct impact from light intensities on post-smolt swimming behaviour in a south-west Norwegian fjord (Hardanger Fjord) with distinctive day and night periods. Secondly, the observed behaviour of the northern Atlantic salmon was related to other important environmental factors like temperature, river flow, current, tides and wind. Finally, the survival rate of post-smolt during fjord migration, as well as the fjord residency and migratory speeds of both northern post-smolts and homing salmon, were studied for the first time for a northern salmon population. The results were compared with earlier reported findings from southern populations.
The results showed that the northern smolts, post-smolts and homing salmon migrated during both day and night in both the river and fjord. In contrast, the post-smolts in the south-western Hardanger Fjord showed distinctive changes in day and night behaviour by swimming deeper in the water column during the day than during the night. However, the northern smolt behaviour was affected by river flow and water temperature, and the post-smolts in the estuary and fjord seemed to be affected by the tidal cycle and wind-induced currents. The homing salmon were also periodically affected by wind-induced currents, and similar to the post-smolts, they migrated mainly close to the surface. Further, the homing salmon generally followed the coastline towards the river mouth, and as they approached the estuary, migratory speed was reduced by 75% and the average swimming depth reduced from 2.5 m to 0.5 m. There was no evidence that river entry of these fish was affected by tidal cycles or river flow. The post-smolts used on average only 0.8 days to migrate the first 17 km outward through the fjord (20.5 km day-1). In contrast, the homing adult salmon used 20% more time over the same distance (16.5 km day-1). Finally, the post-smolt survival rate was estimated to be 75% from the estuary and through the first 17 km of the fjord.
The findings of seemingly no difference between day and night migratory behaviour for the northern smolts and post-smolts in the present thesis may be due to the fact that nocturnal migration, as often observed for southern populations, does not provide the northern fish any benefit in regard to sight feeding predators due to the 24 h of daylight. The observed change in day and night swimming depth in the Hardanger Fjord indicated that light intensity may also affect the swimming depth of post-smolts. The relationships between migratory behaviour of northern smolts and post-smolt and water temperature, river flow and tidal cycles found in this thesis, could, as well as the adaptation to the light intensities, be antipredatory strategies.
In total, it seemed like the first-time migrants were more influenced by light, river flow, tidal cycle and fjord currents than the homing salmon. This may be due to their smaller size and higher vulnerability to predation, supported by the fact that 25% of the tagged post-smolts did probably not survive the first 17 km of the fjord migration. The high mortality rate was similar to earlier findings in southern populations during the first few days after sea entry. However, the finding that the homing salmon migrated close to the surface and shoreline, combined with their longer residency in the inner fjord, may greatly have increased their risk of being caught by net fishing targeting salmon along the shoreline in this area.
In conclusion, this thesis indicates that the migration behaviour of northern and southern salmon differ somewhat, and that this is related to local adjustments to the existing abiotic environmental factors typically for the different latitudes, in particular the light regimes. These local adaptations may be due to phenotypic plasticity and/or different genotypes. The study also highlight the post-smolts and homing salmon phases in fjords and estuaries as important bottlenecks of survival in the Atlantic salmon life cycle, and that variation in both natural and anthropogenic factors during these phases may have large impacts on their migration behaviour, performance and subsequently the total return rate of salmon to their home rivers. An evaluation of the impact from subsequent effects of interventions along the coastline in areas with migratory Atlantic salmon is highly recommended in order to avoid any negative effects on the seaward and homing migration. Norwegian Research Council, Norwegian Institute for Nature Research, Directorate for Nature Management, Fishery and Aquaculture Industry Research Fund (Canada), Governor of the County of Finnmark, Memorial Foundation of Kjell Moen, Norwegian College of Fishery Science The papers of the thesis are not available in Munin:
1. Davidsen, J., Svenning, M. A., Orell, P., Yoccoz, N., Dempson, J. B., Niemelä, E., Klemetsen, A., Lamberg, A. & Erkinaro, J.: «Spatial and temporal migration of wild Atlantic salmon smolts determined from a video camera array in the sub-Arctic River Tana», Fisheries Research 74(2005), 210-222 (Elsevier - publisher's restriction). Available at http://dx.doi.org/10.1016/j.fishres.2005.02.005 .
2. Davidsen, J. G., Plantalech Manel-la, N., Økland, F., Diserud, O. H., Thorstad, E. B., Finstad, B., Sivertsgård, R., McKinley, R. S. & Rikardsen, A. H.: «Changes in swimming depths of Atlantic salmon Salmo salar post-smolts relative to light intensity», Journal of Fish Biology 73(2008), 1065-1074 (Wiley - publisher's restriction). Available at http://dx.doi.org/10.1111/j.1095-8649.2008.02004.x . Accepted version of this paper is available in Munin: http://hdl.handle.net/10037/2227
3. Davidsen, J. G., Rikardsen, A. H., Halttunen, E., Thorstad, E. B., Økland, F., Letcher, B. H., Skarðhamar, J. & Næsje, T. F.: «Migratory behaviour and survival rates of wild northern Atlantic salmon (Salmo salar) post-smolts: effects of environmental factors», Journal of Fish Biology 75(2009), 1700-1718 (Wiley - publisher's restriction). Available at http://dx.doi.org/10.1111/j.1095-8649.2009.02423.x . Accepted version of this paper is available in Munin: http://hdl.handle.net/10037/2431 .
4. Davidsen, J. G., Rikardsen, A. H., Halttunen, E., Mitamura, H., Thorstad, E. B., Præbel, K., Skarðhamar, J. & Næsje, T. F.: «Homing behaviour of Atlantic salmon during final marine phase and river entry» (manuscript).
Large fluctuations in the fisheries have been a characteristic feature of great importance in the history of the Norwegian people. Herring periods have been times with large fisheries for herring spawning along the west coast of Norway, alternating with periods when the herring was gone. Herring periods and periods without herring appear to have been of cyclic nature with a periodicity of about a century (Devoid 1963, Dragesund & al. 1980, Skjoldal 1990,øiestad 1990). Towards the end of the last century, a common explanation for such fluctuations was that it reflected variable migration routes and therefore variable availability of perceived stable fish stocks to coastal fisheries. When ICES was established in 1902, a standing committee called the Migration Committee was created to deal with such issues (Sinclair & Solemdal 1988). However, in his classical book, JohanHjort (1914) identified large fluctuations in stock size due to variable recruitment as a major source for fluctuations in the fisheries. Variable recruitment is an inherent property related to the reproductive mode of fishes with high fecundity and which produce a high number of larvae that are part of the planktonic system (Skjoldal & Melle 1989).
Numbers of wild anadromous Atlantic salmon (Salmo salar) have declined demonstrably throughout their native range. The current status of runs on rivers historically supporting salmon indicate widespread declines and extirpations in Europe and North America primarily in southern portions of the range. Many of these declines or extirpations can be attributed to the construction of mainstem dams, pollution (including acid rain), and total dewatering of streams. Purported effects on declines during the 1960s through the 1990s include overfishing, and more recently, changing ocean conditions, and intensive aquaculture. Most factors affecting salmon numbers do not act singly, but rather in concert, which masks the relative contribution of each factor. Salmon researchers and managers should not look for a single culprit in declining numbers of salmon, but rather, seek solutions through rigorous data gathering and testing of multiple effects integrated across space and time.
A total of 174 multi-sea-winter Atlantic salmon (75-115 cm fork length) were radiotagged in the Tanafjord in 1992-1993 and their upstream migration and exploitation in the Tana River (Teno) were studied. Of the tagged fish, 75% and 77% entered the river, and 40% and 69% of them were later recaptured in 1992 and 1993, respectively. The lower 60 km of the river accounted for 36% of the recaptures. Gillnets and weirs took 68% of the fish recaptured in the river in 1992 but only 40% in 1993, the rest being caught by rod and line. Weirs caught more recently entered salmon than gillnets. Rod and line fishery caught smaller fish than gillnets and weirs. There were no differences in the size distributions between the initially tagged salmon, those that entered the river, were recaptured in the river, or the ones survived until spawning. Exploitation rates (nrecaptured fish/nentered the river) were the highest in the upper reaches of the river system.
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We used a multi-stock comparison to identify spatial and temporal characteristics of environmentally driven sources of variability across four decades in the productivity of 29 sockeye salmon (Oncorhynchus nerka) stocks from British Columbia (B.C.) and Alaska. We examined patterns of covariation among indices of survival rate (residuals from the best-fit stock-recruitment curve) and found positive covariation among Fraser River sockeye stocks (southern B.C.) and, to a greater extent, among Bristol Bay stocks (western Alaska) but no evidence of covariation between these two regions or with stocks of other regions in B.C. and Alaska. This indicates that important environmental processes affecting variation in sockeye survival rate from spawners to recruits operate at regional spatial scales, rather than at the larger, ocean-basin scale. The observed covariation in survival rates of Bristol Bay stocks appears to be due to a combination of both freshwater and, to a greater degree, marine processes. Bristol Bay sockeye stocks showed a dramatic and persistent increase in survival rates coinciding with the abrupt changes in the North Pacific environment in the mid-1970s; however, there was little evidence of a similar response for Fraser River stocks.
Autocorrelation in fish recruitment and environmental data can complicate statistical inference in correlation analyses. To address this problem, researchers often either adjust hypothesis testing procedures (e.g., adjust degrees of freedom) to account for autocorrelation or remove the autocorrelation using prewhitening or first-differencing before analysis. However, the effectiveness of methods that adjust hypothesis testing procedures has not yet been fully explored quantitatively. We therefore compared several adjustment methods via Monte Carlo simulation and found that a modified version of these methods kept Type I error rates near . In contrast, methods that remove autocorrelation control Type I error rates well but may in some circumstances increase Type II error rates (probability of failing to detect some environmental effect) and hence reduce statistical power, in comparison with adjusting the test procedure. Specifically, our Monte Carlo simulations show that prewhitening and especially first-differencing decrease power in the common situations where low-frequency (slowly changing) processes are important sources of covariation in fish recruitment or in environmental variables. Conversely, removing autocorrelation can increase power when low-frequency processes account for only some of the covariation. We therefore recommend that researchers carefully consider the importance of different time scales of variability when analyzing autocorrelated data.
Numbers of wild anadromous Atlantic salmon (Salmo salar) have declined demonstrably throughout their native range. The current status of runs on rivers historically supporting salmon indicate widespread declines and extirpations in Europe and North America primarily in southern portions of the range. Many of these declines or extirpations can be attributed to the construction of mainstem dams, pollution (including acid rain), and total dewatering of streams. Purported effects on declines during the 1960s through the 1990s include overfishing, and more recently, changing ocean conditions, and intensive aquaculture. Most factors affecting salmon numbers do not act singly, but rather in concert, which masks the relative contribution of each factor. Salmon researchers and managers should not look for a single culprit in declining numbers of salmon, but rather, seek solutions through rigorous data gathering and testing of multiple effects integrated across space and time.
Summary • Catches of Atlantic salmon Salmo salar L. decreased in the 1980s and 1990s over its entire area in the North Atlantic and smolts were often released for stock enhancement. However, there are questions about their survival and performance relative to fully wild fish. This paper reports on the survival and sea growth of River Imsa salmon released from 1981 to 1999 as 1- and 2-year-old hatchery and wild smolts. • Survival was significantly higher for wild than hatchery fish. Hatchery salmon released as 2-year-old smolts had lower survival, were captured more in coastal than freshwaters, grew more slowly and attained maturity younger than corresponding 1-year-old smolts. • The survival rate of hatchery fish released as 2-year-old smolts, but not 1-year-olds and wild smolts, decreased during the 1980s and 1990s. Growth rates at sea, adult size and the proportion of multi-sea-winter fish of all three groups also decreased over time. • Catches in coastal relative to freshwaters were higher for two- than one-sea-winter fish. Salmon captured in coastal water were greater in length than those captured in rivers. Mean specific growth rate at sea was similar for wild and hatchery salmon released as 1-year-old smolts, and higher than in hatchery fish released as 2-year-olds. • The proportion of two-sea-winter salmon correlated positively with the specific growth rate in the first year at sea. Total capture of wild adult salmon in rivers and Norwegian home waters each year correlated positively with the specific growth rate in the first year at sea. The same correlation held for hatchery fish released as 2- but not 1-year-old smolts. • Synthesis and applications. The coastal fishery was size-selective in reducing the size and age of salmon. Releases of 1-year-old smolts were financially more profitable than those of 2-year-olds. Decreasing production of River Imsa salmon since 1981 was chiefly caused by reduced sea-age at maturity and growth rate at sea of both hatchery and wild fish. A counteracting measure would be to reduce the size selectivity of the salmon fisheries.
Logistic regression is appropriate in cases where the dependent variable is categorical, dichotomous, or polychotomous. It can be used with continuous and/or discrete independent variables. Logistic regression is motivated by the underlying binomial or multinomial distribution of dichotomous and polychotomous dependent variables and transforms the data to explicitly model these distributions. Locally weighted regression scatterplot smoothing or LOWESS regression is used to model the relationship between a dependent variable and independent variable when no single functional form will do. LOWESS regression is motivated by the assumption that neighboring values of the independent variable are the best indicators of the dependent variable in that range of independent values. -from Authors
F. 1999. Cessation of the Norwegian drift net fishery: changes observed in Norwegian and Russian populations of Atlantic salmon. – ICES Journal of Marine Science, 56: 84–95. The aim of this study was to evaluate effects of the ban on the Norwegian coastal drift net fishery in 1989 on the spawning run of Atlantic salmon (Salmo salar) populations in four Norwegian and four Russian rivers. Catches/escapements, size distribution, sea-age proportions, and mean weights of different sea-age groups were collected from official catch statistics and analyses of scale samples. In three of the Norwegian rivers, catches of grilse (1SW) increased significantly after the ban on the drift net fishery. No changes were recorded for multi-sea-winter (MSW) fish. In addition, the proportion of grilse increased in all the Norwegian rivers, and 2SW salmon increased in three of them. The mean weight of grilse increased in all four Norwegian rivers, whereas the mean weight of 2SW fish decreased in the two rivers in southern Norway. The size of 3SW fish did not change. These changes correspond well to actual net selection curves and reported mean weight of drift net catches in different parts of Norway. Trends in the Russian populations were more varying. In the three rivers draining to the Barents Sea, changes were similar to those in the Norwegian rivers. However, these changes were less obvious than those observed in Norwegian populations. The authors conclude that the ban of the drift net fishery significantly affected the structure of the spawning run in Norwegian Atlantic salmon populations. Furthermore, the results indicate that the drift net fishery affected Russian salmon populations in rivers draining to the Barents Sea to a lesser extent than Norwegian salmon, and had no effect in rivers draining to the White Sea.
Sea-age at maturity of Atlantic salmon Salmo salar decreased with increasing values of the seasonal NAOI from February to April. Body mass increment from smolts to adults of one-sea-winter Atlantic salmon increased with increasing NAOI in May at the time when the juveniles moved to sea.
Climate influences a variety of ecological processes. These effects operate through local weather parameters such as temperature, wind, rain, snow, and ocean currents, as well as interactions among these. In the temperate zone, local variations in weather are often coupled over large geographic areas through the transient behavior of atmospheric planetary-scale waves. These variations drive temporally and spatially averaged exchanges of heat, momentum, and water vapor that ultimately determine growth, recruitment, and migration patterns. Recently, there have been several studies of the impact of large-scale climatic forcing on ecological systems. We review how two of the best-known climate phenomena—the North Atlantic Oscillation and the El Niño–Southern Oscillation—affect ecological patterns and processes in both marine and terrestrial systems.
Greenland ice-core data have revealed large decadal climate variations over the North Atlantic that can be related to a major
source of low-frequency variability, the North Atlantic Oscillation. Over the past decade, the Oscillation has remained in
one extreme phase during the winters, contributing significantly to the recent wintertime warmth across Europe and to cold
conditions in the northwest Atlantic. An evaluation of the atmospheric moisture budget reveals coherent large-scale changes
since 1980 that are linked to recent dry conditions over southern Europe and the Mediterranean, whereas northern Europe and
parts of Scandinavia have generally experienced wetter than normal conditions.
Greenland ice-core data have revealed large decadal climate variations over the North Atlantic that can be related to a major source of low-frequency variability, the North Atlantic Oscillation. Over the past decade, the Oscillation has remained in one extreme phase during the winters, contributing significantly to the recent wintertime warmth across Europe and to cold conditions in the northwest Atlantic. An evaluation of the atmospheric moisture budget reveals coherent large-scale changes since 1980 that are linked to recent dry conditions over southern Europe and the Mediterranean, whereas northern Europe and parts of Scandinavia have generally experienced wetter than normal conditions.
In the 1820's, a relatively large harbour seal population may have inhabited the Tana River Estuary. Due to heavy persecution, especially in the 1920's and 1930's, it was reduced dramatically, and today there are probably less than 40 individuals left. There are few interactions between harbour seals and the salmon fishery in the river, most likely because of the low population size. Only 17% of the fishermen questioned, reported harbour seals drowned in fishing nets in the river. The salmon fishermen in Tanafjord reported more interactions, most often with the grey seal, but neither harbour nor harp seal interactions were uncommon. During an experiment using bag nets, 16% of the salmon caught showed scarring from seal attacs.
In Norway, Atlantic salmon, Salmo
salar L., are found in rivers along the coast from the border with the USSR southward to the border with Sweden. In recent years, several salmon populations have been wiped out by pollution, particularly in southern Norway where several important rivers now lack salmon because of the effects of acidification of the water (Jensen and Snekvik, 1972; Leivestad, Hendrey, Muniz and Snekvik, 1976). Another threat to several salmon populations in Norway is the very recent introduction of the fluke Gyrodactylus
salaris to several rivers. This parasite kills salmon parr, and at present the estimated loss of salmon due to G. salaris is between 250 and 350 tonnes of salmon (Johnsen and Jensen, 1985).
Synchronous trends in the abundance of hatchery and wild Baltic salmon populations are examined using correlation analysis.
The river catch of wild adult salmon, the recruitment of age 0+ wild salmon parr, and the recapture rate of tagged hatchery
smolts are positively correlated among rivers in the Baltic Sea. The spatial correlation among populations is greater than
reported for other anadromous salmonids. Baltic salmon populations have undergone low frequency, high amplitude changes in
abundance during the past 200 years. Recent production levels have been supported largely by hatcheries. Efforts to conserve
the remaining wild populations through fisheries regulation appear to be having a positive effect on spawner abundance.
Autocorrelation in fish recruitment and environmental data can complicate statistical inference in correlation analyses. To address this problem, researchers often either adjust hypothesis testing procedures (e.g., adjust degrees of freedom) to account for autocorrelation or remove the autocorrelation using prewhitening or first-differencing before analysis. However, the effectiveness of methods that adjust hypothesis testing procedures has not yet been fully explored quantitatively. We therefore compared several adjustment methods via Monte Carlo simulation and found that a modified version of these methods kept Type I error rates near a. In contrast, methods that remove autocorrelation control Type I error rates well but may in some circumstances increase Type II error rates (probability of failing to detect some environmental effect) and hence reduce statistical power, in comparison with adjusting the test procedure. Specifically our Monte Carlo simulations show that prewhitening and especially first-differencing decrease power in the common situations where low-frequency (slowly changing) processes are important sources of covariation in fish recruitment or in environmental variables. Conversely, removing autocorrelation can increase power when low-frequency processes account for only some of the covariation. We therefore recommend that researchers carefully consider the importance of different time scales of variability when analyzing autocorrelated data.
The aim of this study was to evaluate effects of the ban on the Norwegian coastal drift net fishery in 1989 on the spawning
run of Atlantic salmon (Salmo salar) populations in four Norwegian and four Russian rivers. Catches/escapements, size distribution, sea-age proportions, and
mean weights of different sea-age groups were collected from official catch statistics and analyses of scale samples. In three
of the Norwegian rivers, catches of grilse (1SW) increased significantly after the ban on the drift net fishery. No changes
were recorded for multi-sea-winter (MSW) fish. In addition, the proportion of grilse increased in all the Norwegian rivers,
and 2SW salmon increased in three of them. The mean weight of grilse increased in all four Norwegian rivers, whereas the mean
weight of 2SW fish decreased in the two rivers in southern Norway. The size of 3SW fish did not change. These changes correspond
well to actual net selection curves and reported mean weight of drift net catches in different parts of Norway. Trends in
the Russian populations were more varying. In the three rivers draining to the Barents Sea, changes were similar to those
in the Norwegian rivers. However, these changes were less obvious than those observed in Norwegian populations. The authors
conclude that the ban of the drift net fishery significantly affected the structure of the spawning run in Norwegian Atlantic
salmon populations. Furthermore, the results indicate that the drift net fishery affected Russian salmon populations in rivers
draining to the Barents Sea to a lesser extent than Norwegian salmon, and had no effect in rivers draining to the White Sea.
Marine exploitation rates were estimated for nine Newfoundland Atlantic salmon (Salmo salar L.) populations, separately for small and large salmon size components. Estimates were derived using counts of salmon returning to fish counting facilities rather than from tagging studies and thus adjustments were not required to account for tag loss, handling or tagging mortality, or tag reporting rates. For all stocks combined, the overall marine exploitation rate during the period 1984-1991 averaged 45.3% (29.6-57.1%) on small salmon and 74.2% (57.7-83.7%) on large salmon. These estimates are considered minimum values. Concerns related to declining salmon abundance resulted in the closure of the Newfoundland commercial salmon fishery in 1992. Results are discussed in relation to previous estimates derived from tagging, and highlight the importance of accounting for marine exploitation when examining trends in salmon survival and return data even when commercial fisheries have been closed.
Ocean climate and ocean-linked terrestrial climate affect nearly all phases of Atlantic salmon (Salmo salar) life history. Natural mortality in salmon occurs in two main phases: juvenile stages experience high mortality during freshwater residency and pre-adult salmon experience high mortality in estuarine and ocean environments. Freshwater survivorship is well characterized and tends to be less variable than marine mortality. Sources of marine mortality are poorly known due to a lack of basic knowledge about post-smolt distributions and habits. Coherence patterns among regional and continental stock groups suggest broad scale forcing functions play a more important role in defining recruitment than mortality effects associated with individual rivers. The action of mesoscale regional environment is most prominent during the post-smolt year when survival, maturation, and migration trajectories are being defined. During the early weeks at sea, growth mediated survival defines recruitment patterns. A correlation between sea surface water temperature and survival has been observed for salmon stocks in the northeast Atlantic suggesting temperature either directly affects growth or modifies post-smolt behavior. Age at first maturity is controlled by environmental as well as genetic factors. The abundance of two seawinter spawners in North America is directly scaled to the size of overwintering thermal habitat in the northwest Atlantic, which suggests a link between maturation and environment.
Exploitation rates in different sea fisheries were estimated by tag returns of Atlantic salmon released as hatchery-reared smolts in the River Drammenselv, SE Norway, 1984–1986. The salmon were exploited in distant fisheries at the Faroes and Greenland, in marine fisheries in Norwegian home waters and in the River Drammenselv. The exploitation rate of these fish was low at Greenland, moderate at the Faroes and relatively high in Norwegian home waters. However, the exploitation rate in home waters was lower than for other Norwegian salmon stocks. Exploitation within the River Drammenselv was estimated by combining catch statistics, counts of ascending fish in a salmon ladder and mark-recapture experiments downstream from the ladder. From 1985 to 1988, the rod exploitation rates varied between 0.33 and 0.53.
The effect of climate on the post-smolt survival of North American Atlantic salmon is obscure owing to the difficulty in interpreting the only relationships thus far observed between the abundance of these stocks and climate, which focuses on winter conditions. Placing significance on winter post-smolt survival is contrary to conventional thinking that the spring period is more important, because that is when the post-smolts migrate to sea and transition to ocean life takes place. The pre-fishery abundance for North American stocks was compared to thermal conditions in potential post-smolt nursery areas during the period 1982-1999. Pre-fishery abundance was modeled as a reconstruction of one-sea-winter (1SW) and two-sea-winter (2SW) age salmon populations. Cohort abundance was compared to mean temperature and thermal habitat (sea surface area within a given temperature range) in five index areas. Stock size was negatively correlated with mean sea surface temperature during June. Correlations were comparatively stronger between stock abundance and thermal habitat, further asserting that June conditions - the first month at sea for most stocks in the region - may be pivotal to survival. These correlations suggest that post-smolt survival is negatively affected by the early arrival of warm ocean conditions in the nursery area. Hypotheses related to post-smolt migration, predation, and the availability of suitable prey are discussed. © 2003 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.
Large masses of cold, low-salinity or warm, high-salinity water move with ocean currents in the North Atlantic Ocean, drastically changing the conditions for the biota and affecting the sizes of fish stocks. Oceanic conditions in the Barents Sea seem to be repeated in the Iceland Sea 2 to 3 years later. The sea temperatures in these areas fluctuate similarly with time lags of 2 and 3 years; the correlation coefficients are 0.63 and 0.62 for 2- and 3-year differences, respectively (P < 0.05). The abundance of stocks of Atlantic salmon Salmo salar in rivers in north Iceland show fluctuations similar to those of Atlantic salmon stocks in rivers on the Kola Peninsula in Russia 2 to 3 years earlier. Correlation coefficients of Atlantic salmon stock size in three rivers of the Kola Peninsula and the salmon catch in three rivers in north Iceland were 0.62 to 0.90 (P < 0.01). Similar trends were observed in recruitment and catch of Atlantic cod Gadus morhua and catch of capelin Mallotus villosus in these distant areas. We hypothesize that fish stocks in other areas of the North Atlantic Ocean show similar fluctuations in abundance with time differences based on the rate of movement of the ocean currents.
Landings of North American Atlantic salmon (Salmo salar) over the past century show multidecadal patterns, which most recently characterize unprecedented declines in abundance. Stock size is compared with sea surface temperature (SST) data in the marine nurseries of post-smolt Atlantic salmon. A previously described correlation between stock abundance and winter SST conditions was again documented; however, of more relevance to the survival of salmon post-smolts, a correlation was also observed between abundance and spring SST in the Gulf of St. Lawrence. The relevance of the winter SST correlation was further investigated by considering winter conditions in the freshwater nurseries as a factor causing elevated overwintering mortality of pre-migrant parr. The salmon abundance time series was compared with air temperature and rainfall trends averaged over time and space. Air temperature and rainfall do not appear to be significant environmental variables in shaping salmon recruitment. The timing of smolt runs appears to be out of synchronization with ocean conditions in the post-smolt nursery areas. The relationship between marine and freshwater impacts may change with changing climate conditions. Persistent positive phase forcing in the North Atlantic Oscillation raises the concern that recent declines in Atlantic salmon are, in part, due to global climate change.
A weight-age classification, based on recent catches, was retroactively applied to angled Atlantic salmon (Salmo salar) catches recorded in the log of the Godbout salmon club between 1859 and 1983. Over this period, numbers of salmon caught have fluctuated with a periodicity of 20–30 yr but catches have been maintained. There has been a big increase in fishing effort, and the proportion of two-sea-year to previously spawned fish has increased. The mean weight of these age groups has declined 0.005–0.009 kg∙yr−1. These changes are attributed to the selective effects of commercial fisheries on the stock.
Atlantic salmon (Salmo salar) are distributed over large areas in the north Atlantic Ocean. They usually move very quickly from freshwater to oceanic areas, whereas there is considerable variation among Pacific salmon in early marine movements. In some areas, Atlantic salmon of exploitable size are sufficiently abundant that commercial high seas fisheries have developed. Such areas are off west Greenland, where North American and European fish are harvested, and in the Norwegian Sea, north of the Faroe Islands, where mainly European fish are exploited. Atlantic salmon feed on a wide range of large crustaceans, pelagic fish, and squid in the marine environment, supporting the hypothesis that Atlantic salmon are opportunistic feeders. In the ocean the salmon grow relatively quickly and the sea age when they become sexually mature depends on both genetics and on growing conditions. Natural marine mortality of salmon is highest during the first few months at sea and the major mortality factor is probably predation. However, marine mortality of Atlantic salmon has increased in recent years, apparently correlated with a decline in sea surface temperatures. Similar relationships between environmental conditions and the growth and survival of Pacific salmon have been reported. Atlantic salmon life histories most closely mimic stream-type chinook salmon or steelhead trout among the Pacific species. Finally, Atlantic and Pacific salmon return to their home rivers with high precision and possible mechanisms controlling the oceanic homing migration are presented and discussed.
Closure of the Newfoundland commercial Atlantic salmon, Salmo salar L., fishery in 1992 was the most restrictive measure introduced to help rebuild depressed local stocks of salmon. Here, the effects of the closure are evaluated by analysing trends in abundance since 1984, and estimates of survival in both freshwater and marine environments derived from enumeration of salmon at fish counting facilities. While freshwater production of smolts generally has been maintained, marine survival rates remain low (2–10%), and highly variable. Overall, total stock size differs little from that prior to the closure of the commercial salmon fishery. Spawning escapements have increased by a factor of 2 or 3 in some rivers, but in other areas total returns are lower on average than those prior to the fishery closure. Factors other than exploitation are contributing to lack of stock recovery, resulting in continued conservation concerns.
2003. Origin and migration of wild and escaped farmed Atlantic salmon, Salmo salar L., in oceanic areas north of the Faroe Islands. – ICES Journal of Marine Science, 60: 110–119. We examined the distribution, migration and origin of wild and escaped farmed Atlantic salmon, Salmo salar L., in the northeast Atlantic ocean north of the Faroe Islands based on individual tagging of salmon in this area. Recoveries of wild salmon were reported from homewaters in nine north Atlantic countries, and in a number of different rivers throughout the distribution range of Atlantic salmon. Most tags were recovered in Norway, but relatively large numbers of returns were observed in Scotland and Ireland as well. No fish were recaptured at Faroes. Fish tagged in the autumn tended to return to areas closer to the tagging site than fish tagged in the winter. This strongly suggests that salmon originating from most areas of the distribution range are at some life stage present in this area, but in variable proportions at different times. Most of the salmon returned home to spawn the next autumn, and the fish that stayed for another year originated from northern areas of Europe. All recoveries of farmed salmon were in Norway except one on the west coast of Sweden, suggesting that they could have escaped mainly from Norwegian fish farms. Assessment of the proportion of wild salmon from different countries present north of the Faroe Islands revealed that 40% of the fish were of Norwegian origin, and Scotland and Russia accounted for about 20% each. Four tags of wild fish were reported from Canada, all in the same year they were tagged. This demonstrates that adult Atlantic salmon can cross the north Atlantic ocean in less than 6 months. email: l.p.hansen@nina.no. J. A. Jacobsen: Fisheries Laboratory of the Faroes, PO Box 3051, FO-110 Tórshavn, Faroe Islands.
O } . 1999. Spatial and temporal covariation in the recruitment and abundance of Atlantic salmon populations in the Baltic Sea. – ICES Journal of Marine Science, 56: 433–443. Synchronous trends in the abundance of hatchery and wild Baltic salmon populations are examined using correlation analysis. The river catch of wild adult salmon, the recruitment of age 0+ wild salmon parr, and the recapture rate of tagged hatchery smolts are positively correlated among rivers in the Baltic Sea. The spatial correlation among populations is greater than reported for other anadromous salmonids. Baltic salmon populations have undergone low frequency, high amplitude changes in abundance during the past 200 years. Recent production levels have been supported largely by hatcheries. Efforts to conserve the remaining wild populations through fisheries regulation appear to be having a positive effect on spawner abundance.
Marine exploitation rates were estimated for nine Newfoundland Atlantic salmon (Salmo salar L.) populations, separately for small and large salmon size components. Estimates were derived using counts of salmon returning to fish counting facilities rather than from tagging studies and thus adjustments were not required to account for tag loss, handling or tagging mortality, or tag reporting rates. For all stocks combined, the overall marine exploitation rate during the period on large salmon. These estimates are considered minimum values. Concerns related to declining salmon abundance resulted in the closure of the Newfoundland commercial salmon fishery in 1992. Results are discussed in relation to previous estimates derived from tagging, and highlight the importance of accounting for marine exploitation when examining trends in salmon survival and return data even when commercial fisheries have been closed.
Protein electrophoresis was used to study genetic variation and population structure of anadromous Atlantic salmon in two rivers of northernmost Europe. Eye, liver, and muscle tissues were analyzed from a total of 1540 fish from 23 and 4 locations in Teno and Näätämö Rivers, and 14 proteins encoded by 36 loci were screened. Eight loci were variable, and mAAT-1,2* and PGDH* expressed variation not previously described in Atlantic salmon. All samples from different locations were in Hardy-Weinberg equilibrium at the 5 % significance level. Observed heterozygosities varied from 1.8 % to 4.8 % in different locations, with averages of 3.4 % and 4.3 % for Teno and Näätämö, respectively. Average unbiased genetic distance (NEI 1978) between locations was 0.0023, with a range from zero to 0.013. Hierarchical gene diversity analysis indicated that 0.3 % of the total genetic variation was distributed between the two drainages, 2.6 % among tributary systems within drainages, 3.5 % among sampling locations within tributary systems, and 93.6 % within sampling locations. The results suggest that there is a potential of local adaptations within sampling locations, and that genetic variation within a drainage is not negligible and should be recognized by fishery management practices, but we emphasize that genetic divergence in enzyme loci is only one criterion when defining fishery management units.
The climatic conditions in the Barents Sea are mainly determined by the influx of Atlantic Water. A homogeneous wind-driven numerical current model was used to calculate the fluctuations in the volume flux of Atlantic Water to the Barents Sea which are caused by local wind forcing. The study period is from 1970 to 86. When compared with observed variations in temperature, ice coverage, and air pressure, the results show remarkably good agreement between all three parameters. The climate system of the Barents Sea is discussed with emphasis on the interrelations and feedback mechanisms between air, sea, and ice.
In the arcto-boreal Barents Sea, temperature variability is an important source for the pronounced year-to-year fluctuations in fish recruitment. Sea temperature is closely linked to the volume flux of the relatively warm Atlantic water masses flowing in from southwest, as well as to regional heat exchange with the atmosphere. We examine the relations between Barents Sea temperature, inflow, and North Atlantic scale climate variability. For the last three decades, large-scale climate forcing statistically has accounted for 75% of the variability in the barotrophic inflow, whereas the North Atlantic Oscillation and sea temperature combined statistically explain 55% of the variability in cod recruitment. Our results suggest a chain-of-events relationship between large-scale atmospheric variability, Barents Sea oceanography and the ecology of this highly productive region.
We address the hypothesis that survival of a cohort is directly related to growth rates during the pre-recruit period for
marine fish. This hypothesis is widely accepted, but supporting field evidence has been elusive. Here the connection between
size and year-class strength at the early stages is examined for the commercially important Barents Sea stocks of cod (Gadus morhua), haddock (Melanogrammus aeglefinus), and herring (Clupea harengus). We show that there is a close link between length and year-class strength at the 0-group stage within and among the populations
studied, and that the connection also holds between length at the 0-group stage and strength of recruitment at age 3. Both
length and abundance are closely related to temperature. Interannual variability in the temperature conditions is concluded
to be the underlying cause of the covariability between growth and year-class strength. We hypothesize that for stocks at
the high latitude end of the overall range of the species the environmental signal tends to over-ride density-dependent effects
on growth. High temperature will cause a high production of prey items leading to higher growth rates and higher survival
through the vulnerable larval and juvenile stages. The duration of the high-mortality and vulnerable stages is also decreased
by higher temperature directly increasing the development rate.
Desktop Data Analysis with Systat Upper Saddle River, A study of the climatic sys-tem in the Barents Sea
- L Wilkinson
- G Blank
- C N J Gruber
- B Ådlandsvik
- H Loeng
Wilkinson, L., Blank, G., and Gruber, C. 1986. Desktop Data Analysis with Systat. Prentice Hall, Upper Saddle River, N.J. Ådlandsvik, B., and Loeng, H. 1991. A study of the climatic sys-tem in the Barents Sea. Polar Res. 10: 45–49.
Climatic change in the north-eastern Atlantic and its impact on salmon stocks
- W R Turrell
- R G J Shelton
Turrell, W.R., and Shelton, R.G.J. 1993. Climatic change in the
north-eastern Atlantic and its impact on salmon stocks. In
Salmon in the sea and new enhancement strategies. Edited by D.
Mills. Fishing News Books, Blackwell Scientific Publications
Ltd., Oxford. pp. 40-78.
Seasonal and year-to-year variations of temperature and salinity along the Kola meridian transect
- V V Tereshchenko
Tereshchenko, V.V. 1996. Seasonal and year-to-year variations of
temperature and salinity along the Kola meridian transect. ICES
CM 1996/C:11.
Salmon in the sea and new enhancement strategies. Fishing News Books, Blackwell Scientific Publica-tions Ltd Näätämöjoen lohen poikastiheys ja kasvu. Riista-ja kalatalouden tutkimuslaitos. Finnish Game and Fisheries Re-search Institute. Kalantutkimuksia nro 176
- D Mills
- E Niemelä
- J Erkinaro
- M Kylmäaho
- M Julkunen
- K Moen
Mills, D. (Editor). 1993. Salmon in the sea and new enhancement strategies. Fishing News Books, Blackwell Scientific Publica-tions Ltd., Oxford. Niemelä, E., Erkinaro, J., Kylmäaho, M., Julkunen, M., and Moen, K. 2001. Näätämöjoen lohen poikastiheys ja kasvu. Riista-ja kalatalouden tutkimuslaitos. Finnish Game and Fisheries Re-search Institute. Kalantutkimuksia nro 176. [In Finnish with English abstract.]
Changes in ocean climate and its general effect of fisheries: examples from the north-west Atlantic. In The ocean life of Atlantic salmon: environmental and biological factors influencing survival
- K F Drinkwater
Drinkwater, K.F. 2000. Changes in ocean climate and its general
effect of fisheries: examples from the north-west Atlantic. In
The ocean life of Atlantic salmon: environmental and biological
factors influencing survival. Edited by D. Mills. Fishing News
Books, Blackwell Scientific Publications Ltd., Oxford. pp. 116-136.
Ecological model-ling for fisheries. In Models for multispecies management
- J Giske
- H R Skjoldal
- D Slagstad
Giske, J., Skjoldal, H.R., and Slagstad, D. 1998. Ecological model-ling for fisheries. In Models for multispecies management. Edited by T. Rødseth. Physica-Verlag, Heidelberg. pp. 11–68.
The marine survival and growth of wild and hatchery-reared Atlantic salmon
- N Jonsson
- B Jonsson
- L P Hansen
Jonsson, N., Jonsson, B., and Hansen, L.P. 2003. The marine survival and growth of wild and hatchery-reared Atlantic salmon. J.
Appl. Ecol. 40: 900-911.
Effekter av kraftutbyggingen på laksebestanden i Altaelva: Undersøkelser i perioden
- O Ugedal
- T Forseth
- A J Jensen
- J I Koksvik
- T F Naesje
- H Reinertsen
- L Saksgård
- E B Thorstad
Ugedal, O., Forseth, T., Jensen, A.J., Koksvik, J.I., Naesje, T.F.,
Reinertsen, H., Saksgård, L., and Thorstad E.B. 2002. Effekter
av kraftutbyggingen på laksebestanden i Altaelva: Undersøkelser
i perioden 1981-2001. Altaelva-rapport nr 22.