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Diel vertical migration (DVM) is a widespread behaviour among many pelagic species, from zooplankton to sharks, and has been widely studied in both marine and freshwater environments. Usually, DVM comprises repeated daily vertical movements through the water column, from shallower at night to deeper during the day. Consequently DVM is perhaps unexpected in benthic predators, nonetheless, DVM has been observed in benthic sharks and freshwater teleosts, where it comprises inshore-offshore migrations over the substrate. However there is no clear evidence of this behaviour in large temperate benthic predators, such as skates. Here we present new observations of DVM in 4 species of skate (Raja brachyura, R. clavata, R. microocellata and R. montagui) that identify it as a general behaviour in this clade. Analysis of 89 depth recording archival tags yielded 674 clear DVM events where skate left daytime deeper waters for shallower night time areas before returning to within 2.5 m of starting depths. Interestingly, these events closely resemble those of central place foragers, where shallow areas are foraging and deeper areas are refuging locations. Behaviour such as this has not been previously recorded in marine benthic predators and the findings suggest DVM might occur in many other benthic species. A broader understanding of DVM in benthic animals will be important in the design of effective boundaries for marine protected areas. These findings also have implications for trophic coupling between deep and shallow benthic zones. Further characteristics of this unexpected behaviour are presented and hypotheses for its occurrence are discussed.
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... The vertical movements of benthic elasmobranchs, such as skate (Rajidae), have received limited attention and generalising patterns from pelagic species is challenging (Humphries et al. 2017;Siskey et al. 2019). Available evidence indicates that in some instances skate undergo DVM in association with diel changes in solar light levels (Peklova et al. 2014;Humphries et al. 2017), perhaps in response to diel rhythms in prey such as Norway lobster (Nephrops norvegicus) and teleosts such as hake (Merluccius merluccius) in the North Atlantic (Aguzzi and Sardà 2008;De Pontual et al. 2012;Brown-Vuillemin et al. 2020). ...
... The vertical movements of benthic elasmobranchs, such as skate (Rajidae), have received limited attention and generalising patterns from pelagic species is challenging (Humphries et al. 2017;Siskey et al. 2019). Available evidence indicates that in some instances skate undergo DVM in association with diel changes in solar light levels (Peklova et al. 2014;Humphries et al. 2017), perhaps in response to diel rhythms in prey such as Norway lobster (Nephrops norvegicus) and teleosts such as hake (Merluccius merluccius) in the North Atlantic (Aguzzi and Sardà 2008;De Pontual et al. 2012;Brown-Vuillemin et al. 2020). In turn, the apparent influence of solar light suggests that lunar phase and photoperiod may be important, as reported for other elasmobranchs (Schlaff et al. 2014) and in marine ecosystems more widely (Migaud et al. 2010;Kronfeld-Schor et al. 2013). ...
... A three-way interaction between sun angle, photoperiod and photoperiod direction was included to examine DVM in relation to solar light levels with changes in day length. Following previous results (Wearmouth and Sims 2009;Peklova et al. 2014;Pinto and Spezia 2016;Humphries et al. 2017), it was hypothesised that skate would occupy deeper depths during the day when solar light levels are higher. Weaker, more concentrated DVM was expected in summer when nights are shorter and brighter. ...
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Trends in depth and vertical activity reflect the behaviour, habitat use and habitat preferences of marine organisms. However, among elasmobranchs, research has focused heavily on pelagic sharks, while the vertical movements of benthic elasmobranchs, such as skate (Rajidae), remain understudied. In this study, the vertical movements of the Critically Endangered flapper skate (Dipturus intermedius) were investigated using archival depth data collected at 2 min intervals from 21 individuals off the west coast of Scotland (56.5°N, −5.5°W) in 2016–17. Depth records comprised nearly four million observations and included eight time series longer than 1 year, forming one of the most comprehensive datasets collected on the movement of any skate to date. Additive modelling and functional data analysis were used to investigate vertical movements in relation to environmental cycles and individual characteristics. Vertical movements were dominated by individual variation but included prolonged periods of limited activity and more extensive movements that were associated with tidal, diel, lunar and seasonal cycles. Diel patterns were strongest, with irregular but frequent movements into shallower water at night, especially in autumn and winter. This research strengthens the evidence for vertical movements in relation to environmental cycles in benthic species and demonstrates a widely applicable flexible regression framework for movement research that recognises the importance of both individual-specific and group-level variation.
... For example, archival tags record depth, temperature and other information at high resolution (Hussey et al., 2015;Siskey, Shipley & Frisk, 2019). These data can be informative about vertical movement patterns (Wearmouth & Sims, 2009;Peklova et al., 2014;Neat et al., 2015;Humphries et al., 2016;Pinto & Spezia, 2016;Humphries, Simpson & Sims, 2017) and movement over relatively large spatial scales (typically in the order of hundreds or thousands of kilometres; Hunter et al., 2005a, Hunter et al., 2005bFarrugia et al., 2016;. However, they are poorly suited to studying movement at fine spatial scales (from metres to tens of kilometres) because of the uncertainty inherent in geolocation algorithms based principally on depth, light levels and/or other oceanographic variables (Hunter et al., 2003;Seitz et al., 2006;Pedersen et al., 2008). ...
... These behaviours are poorly understood in flapper skate, but the use of nursery habitats by immature individuals may contribute towards residency in this group (Kinney & Simpfendorfer, 2009;Speed et al., 2010). Over seasonal and annual timescales, depth time series for three resident immature skate tagged in this study exhibit a pattern of repeated movements around particular depths, which points towards central foraging or refuging behaviour (Humphries, Simpson & Sims, 2017;Papastamatiou et al., 2018). Amongst other elasmobranchs, long-term residency appears to predominate among tropical or subtropical species but has been documented in a few deep-water benthic species in higher latitudes, including the bluntnose sixgill shark (Hexanchus griseus; Andrews et al., 2009), the prickly shark (Echinorhinus cookei;Dawson & Starr, 2009) and white skate (Sousa et al., 2019). ...
Article
• Marine Protected Areas (MPAs) are widely used in marine management, but for mobile species understanding the spatio-temporal scale of management measures that is required to deliver conservation benefits depends on a detailed knowledge of species’ movements that is often lacking. This is especially the case for species of skate (Rajidae) for which relatively few movement studies have been conducted. • In Scotland, the Loch Sunart to the Sound of Jura MPA covering 741 km² has been designated for the conservation of the Critically Endangered flapper skate (Dipturus intermedius), but fine-scale movements within this area remain poorly understood. • A passive acoustic telemetry study which coupled acoustic tagging of 42 individuals and a static array of 58 receivers was conducted from March 2016 to June 2017. Using acoustic detection time series, angler capture–recapture data and depth time series from archival tags, fine-scale movements of individuals were investigated. • Overall, 33 of the 42 tagged individuals were detected. Residency, site fidelity and transiency were documented. Residency around receivers, lasting from 3 to more than 12 months, was documented in 16 acoustically detected individuals (48%) and all life-history categories, but was most noticeable among females. Acoustic detections were associated with depth, salinity and season, but there was no evidence that individuals formed close-knit groups in the areas in which they were detected. • Taken together with historical occurrence records of flapper skate, the prevalence and scale of residency documented here suggest that the MPA is sufficiently large to benefit a notable percentage (38 [24–52]%) of skate found in the study area over monthly and seasonal timescales. This result strengthens the case for the use of MPAs to support the conservation of flapper skate and other skate species that display similar movement patterns in areas of high local abundance.
... For example, mesopelagic fishes, such as most myctophids, gonostomatids, sternoptychids and stomiids, feed at night in the surface layers and migrate at dawn to mesopelagic depths where they digest food, respire and defecate, resulting in the vertical transport of organic matter (Saba et al., 2021;Sutton, 2013). Although less well known, some benthic fishes also perform diel vertical migrations from the seafloor into the water column (Humphries et al., 2017). ...
... Vertically swimming through the water column may also be an effective way to detect the layers with most dense prey, as has been suggested for vertically undulating cod (Bjornsson & Reynisson, 2013 (Poss, 2016). Other benthic fishes that are known to forage in midwater include skate fishes and balistids (triggerfishes) (Escánez & Brito, 2011;González et al., 2014;Humphries et al., 2017). By feeding in midwater and resting and excreting on the seafloor this vertical migration behaviour may result in carbon transport between the pelagic and benthic zones (Sutton, 2013). ...
Article
During pelagic video transects off Santo Antão, Cabo Verde, we encountered the Midwater scorpionfish Ectreposebastes imus in midwater between 300 and 800 m over a bottom depth of about 1000 m. The fish were typically positioned vertically with their heads pointing upwards. These first midwater observations of E. imus suggest migratory (potentially feeding) behaviour into the pelagic realm and hence a possible role of this species in the trophic coupling between the pelagic and benthic habitats in the deep seas of Cabo Verde and elsewhere in its global distribution. This article is protected by copyright. All rights reserved.
... It should be noted that fast movements were likely to be underrepresented in this study owing to the design limitations of the acoustic array. Because of their high energy requirement, fast movements tend to occur at the smallest temporal scale possible for the species (Humphries, Simpson & Sims, 2017), sometimes shorter than the transmission delay of the acoustic transmitter (80-160 s). Moreover, detections of fast movements for an extended period of time were also limited by the size of the array (i.e. S. canicula would exit the array prior to speed estimation). ...
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Shark populations have suffered dramatic declines across the world as a result of overfishing. Marine protected areas (MPAs) can help restore overfished populations; however, their effectiveness largely relies on understanding the ecology of the targeted species. This study investigated the spatial ecology of the intensely harvested but understudied small spotted catshark (Scyliorhinus canicula) through acoustic telemetry in the Cíes Islands, a small MPA in the north-west of the Iberian Peninsula. There were significant effects of diel, seasonal cycles, biotic (sex and total length) and abiotic (bottom sea temperature) variables on the spatial behaviour of S. canicula. The mean residency index was low (0.27) and movement patterns suggested a strong connection with inshore waters. While the probability of presence in the study area was mainly driven by sex (i.e. greater for females), a drastic increase in activity was observed at night (compared with daytime hours). The activity space decreased with larger body sizes. Warmer waters were related to higher activity levels and larger activity spaces. This study provides essential knowledge of the spatial behaviour of S. canicula, with significant implications for the conservation and management of this species. The results indicate that small MPAs may fail to protect the whole range of movements of S. canicula, but suggest a larger protection potential for females. To be effective for S. canicula conservation, MPAs should be appropriately sized and designed for the ranging behaviour of the target species in order to provide total protection. Temporal restrictions on fishing at night mirroring the peak activity pattern of S. canicula could be implemented to limit the probability of its capture.
... Natural diel movement patterns for smooth stingrays are currently unknown, but diel patterns in the use of inshore habitats for other batoid species typically involve use of warmer shallow waters during the night and refuging in cooler, deeper waters while they digest during the day (i.e. diel vertical migration; Wearmouth and Sims 2009;Farrugia et al. 2011;Humphries et al. 2017;DeGroot et al. 2020). Indeed, for several dasyatid ray species, presence in shallow habitats is generally higher at night and nocturnal space use is larger as individuals actively forage for food (Cartamil et al. 2003;Farrugia et al. 2011;Corcoran et al. 2013). ...
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Context It is common for recreational anglers to discard waste produced from filleting catches back into the water, which results in a highly spatio-temporally predictable food subsidy for wildlife to scavenge. However, the behavioural responses of these scavengers has received little attention. Aims We aimed to assess the visitation of a common mesopredatory scavenger in relation to temporal patterns in waste discarding at a boat ramp in south-eastern Australia. Methods Using passive acoustic telemetry, the movements of 13 adult female smooth stingrays (Bathytoshia brevicaudata) were tracked, and patterns in their acoustic detections and duration of time spent in different sections within the study area were compared. Key results Use of the study area was strongly focused around the boat ramp, and peaked during periods of increased provisioning activity (i.e. afternoons and weekends). Environmental variables had limited influence on visitation, suggesting that the use of the area was not likely to be linked to natural behaviours. Conclusions The observed patterns indicated that the movements of smooth stingrays were linked to waste-discard practices by recreational anglers. Implications This study has implications for the management of discard practices for recreational fishing.
... Several of the shark species are pelagic and therefore have the capability to move rapidly between surface and bottom waters and indeed have been tracked doing so in the southwest UK region (e.g., porbeagle; Pade et al., 2009). Moreover, benthic species, including the thornback ray, blonde ray, spotted ray, and small-eyed ray, undertake diel vertical migrations from deep benthic habitat during the day to shallow benthic habitat during the night (Humphries et al., 2017). ...
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Abstract Many sharks, skates, and rays (elasmobranchs) are highly threatened by the activities of commercial fisheries, and a clear understanding of their distributions, diversity, and abundance can guide protective measures. However, surveying and monitoring elasmobranch species can be highly invasive or resource‐intensive, and utilization of non‐invasive environmental DNA‐based methods may overcome these problems. Here, we studied spatial and seasonal variation in the elasmobranch community of the Western English Channel using environmental DNA (eDNA) collected from surface and bottom waters periodically over an annual cycle (2017–2018). In total we recovered 13 elasmobranch species within eDNA samples, and the number of transformed eDNA reads was positively associated with species (hourly) catch data resolved from 105‐year time series trawl data (1914–2018). These results demonstrate the ability of eDNA to detect and semi‐quantitatively reflect the prevalence of historically dominant and rare elasmobranch species in this region. Notably, eDNA recorded a greater number of species per sampling event than a conventional trawl survey in the same area over the same sampling years (2017–2018). Several threatened species were recovered within the eDNA, including undulate ray, porbeagle shark, and thresher shark. Using eDNA, we found differences in elasmobranch communities among sampling stations and between seasons, but not between sampling depths. Collectively, our results suggest that non‐invasive eDNA‐based methods can be used to study the spatial and seasonal changes in the diversity and abundance of whole elasmobranch communities within temperate shelf habitats. Given the threatened status of many elasmobranchs in human‐impacted marine environments, eDNA analysis is poised to provide key information on their diversity and distributions to inform conservation‐focused monitoring and management.
... With the Canary Islands thought to be the southernmost tip of the angelshark range, and thus likely to represent the thermal limit of the species, the availability of deeper, cooler waters surrounding the volcanic archipelago may serve to assist thermoregulation during warmer periods. A number of demersal elasmobranchs have shown such behaviours, moving to deeper waters during the day and only becoming more active in shallow waters during the night(Humphries, Simpson & Sims, 2017;Coffey et al., 2020;DeGroot et al., 2020). ...
Article
• As an increasingly important resource in ecological research, citizen scientists have proven dynamic and cost-effective in the supply of data for use within habitat suitability models. With predictions critical to the provision of effective conservation measures in cryptic marine species, this study delivers baseline ecological data for the Critically Endangered angelshark (Squatina squatina), exploring: (i) seasonal, sex-differentiated distributions; (ii) environmental distribution predictors; and (iii) examining bias-corrected, imperfect citizen science data for use in coastal habitat suitability models with cryptic species. • Citizen science presence data, comprising over 60,000 hours of sampling effort, were used alongside carefully selected open-source predictor variables, with maxent generating seasonal male and female habitat suitability models for angelsharks in the Canary Islands. A biased prior method was used, alongside two model validation measures to ensure reliability. • Citizen science data used within maxent suggest that angelshark habitat suitability is low in coastal areas during warmer months, with fewer occurrences despite a negligible change in sampling effort. The prime importance of bathymetry may indicate the importance of depth for reproductive activity and possible diel vertical migration, whereas aspect may act as a proxy for sheltered habitats away from open ocean. Substrate as a predictor of female habitats in spring and summer could imply that soft sediment is sought for birthing areas, assisting in the identification of areas critical to reproductive activity and thus locations that may benefit from spatial protections. • Model outputs to inform recovery plan development and ecotourism are identified as plausible safeguards of population recovery, whereas the comparison of biased and bias-corrected models highlights some variance between methodologies, with bias-corrected models producing greater areas of habitat suitability. Accordingly, an adaptive framework is provided for the implementation of citizen science data within the modelling of cryptic coastal species distribution.
... Due to limitations in the design of the acoustic array, fast movements were probably poorly detected by the method. For instance, fast movements are highly energy demanding(Humphries, Simpson & Sims, 2017) and might thus occur at a temporal scale shorter than the transmission delay of the acoustic tags (40-160 s) and not be detected by the acoustic receivers. Besides, even if individuals move at high speed for a longer time, the size of the study area ($800 m diameter) might prevent detection of tagged fish (i.e. ...
Article
• Knowledge of the spatial behaviour of aquatic living resources is essential to assess their vulnerability to environmental and anthropogenic stressors and inform efficient management strategies. • Elasmobranchs are particularly vulnerable to exploitation. Within this group of fish, the implementation of species-specific conservation actions has been challenging due to insufficient information on their biology and ecology. • In this study, acoustic telemetry was used to investigate the seasonal variation, diel patterns, and biological and sea temperature effects on the spatial behaviour of the endangered undulate skate, Raja undulata, within a marine protected area in north-west Spain. • Movement and behaviour were mainly driven by diel and seasonal patterns. The presence of the tagged skates in the study area peaked during summer. Space use was highest in late spring and activity was lowest during summer. Skates used more space and were more active during night-time as compared to daytime, when they stayed in sandy bottoms. Sea temperature had a negligible positive effect on activity. • Our work represents an important contribution to the understanding of the ecology of this endangered, yet commercially important skate in Europe, and provides important insights for the implementation of spatial and temporal restrictions aimed at reducing mortality and bycatch of this species.
... While many other species of elasmobranch typically exhibit increased rates of horizontal movement during crepuscular periods (dusk and dawn; Hammerschlag et al., 2017), some species do exhibit increased activity at night. Humphries et al. (2017) found that several species of skates undergo vertical diel migrations as they move from deep off-shore water to shallow inshore water at night and return in the morning. Previous research on a single horn shark H. ...
Article
Examining the movement ecology of mesopredators is fundamental to developing an understanding of their biology, ecology and behaviour, as well as the communities and ecosystems they influence. The limited research on the residency and movements of benthic marine mesopredators has primarily used visual tags, which do not allow for the efficient and accurate monitoring of individual space use. Here, we investigated the residency and movement patterns of Port Jackson sharks Heterodontus portusjacksoni (Meyer 1793) at a breeding aggregation site in Jervis Bay, south-eastern Australia, using passive acoustic telemetry to further our understanding of the movement ecology of these important mesopredators. Between 2012 and 2014, individuals were tagged with acoustic transmitters and their residency and movements within the bay were monitored for up to four years. H. portusjacksoni showed strong preferences for particular reefs within and between breeding seasons. Males had significantly higher residency indices at their favoured sites relative to females, suggesting that males may be engaging in territorial behaviour. Conversely, female H. portusjacksoni exhibited higher roaming indices relative to males indicating that females may move between sites to assess males. Finally, H. portusjacksoni demonstrated temporal variation in movements between reefs with individuals typically visiting more reefs at night relative to the day, dusk and dawn corresponding with their nocturnal habits. This article is protected by copyright. All rights reserved.
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Analysis of spatiotemporal partitioning is pivotal to shed light on interspecific coexistence. Most research efforts have involved large-sized carnivores and their prey species, whereas little attention has been given to ungulate in the predator-free ecosystems. We assessed seasonal activity patterns and spatiotemporal overlap among the Siberian roe deer (Capreolus pygargus tianschanicus) and its sympatric species through camera-trapping from October 2017 and September 2020 in Jeju Island, South Korea. Trap events when compared seasonally, roe deer show higher activity in summer (34.9%), a pronounced low in winter (14.1%), and a moderate in autumn (23.8%) and in spring (26.9%). Roe deer exhibited bimodal activity patterns and had the highest spatiotemporal overlap and composite score with sika deer (Cervus nippon). Our results are among the few available data on the interaction of sympatric species and suggest strong overlapping with sika deer. This study provides important insight into species coexistence in predator-free habitats, which would be important for management initiatives.
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Most batoid fishes have a unique swimming mode in which thrust is generated by either oscillating or undulating expanded pectoral fins that form a disc. Only one previous study of the freshwater stingray has quantified three-dimensional motions of the wing, and no comparable data are available for marine batoid species that may differ considerably in their mode of locomotion. Here we investigate three-dimensional kinematics of the pectoral wing of the little skate, Leucoraja erinacea, swimming steadily at two speeds (1 and 2 body lengths per second, BL×s(-1)). We measured the motion of nine points in three dimensions during wing oscillation and determined that there are significant differences in movement amplitude among wing locations, as well as significant differences as speed increases in body angle, wing beat frequency, and speed of the traveling wave on the wing. In addition, we analyzed differences in wing curvature with swimming speed. At 1 BL×s(-1), the pectoral wing is convex in shape during the downstroke along the medio-lateral fin midline, but at 2 BL×s(-1) the pectoral fin at this location cups into the flow indicating active curvature control and fin stiffening. Wing kinematics of the little skate differed considerably from previous work on the freshwater stingray, which does not show active cupping of the whole fin on the downstroke.
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A sympatric assemblage of morphologically similar predators is expected to exhibit fine-scale habitat segregation, or resource partitioning, to reduce the effects of direct competition. This principle has been well studied for predators in terrestrial ecosystems. In the marine environment, the fine-scale spatial segregation of sympatric species of large predators is poorly understood because detailed movement and behavioural data are often not available across multiple species within the same timeframe. The ways in which co-occurring congeneric predators separate spatially is even less well understood. Medium-sized species of skates (genus Raja) co-occur in temperate habitats of the north-east Atlantic Ocean, share similar morphologies and have distributional ranges that overlap significantly in the western English Channel ecosystem. In the present study, detailed depth time series retrieved from 89 electronic data storage tags attached to 4 species of skate were analysed to determine preferred depth ranges. The 4 species were found to segregate spatially into 2 groups, with one group having a significantly shallower core annual depth range than the other. To our knowledge, fine-scale segregation by depth has not been observed previously. Interestingly, the members of each species group appeared complementary, each group comprising species with different dietary preferences and with a larger and smaller body size. An understanding of how core depth ranges differ and how these species utilise vertical habitat could potentially enable geographic ranges around the coast to be predicted, with important implications for how these species interact with fisheries and Marine Protected Areas.
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8217 days of data downloaded from 47 archival tags, recovered from bigeye tuna (Thunnus obesus) 51-134 cm in length (x-=86.9cm), 0.87-3.44 years of age (x-=1.89years) at liberty from 36 to 851 days (x-=183days) in the equatorial central Pacific Ocean, are evaluated herein. Analyses of depth and temperature records resulted in the classification of three daily behavior types: characteristic, associative (associated with floating objects), and other. For three defined length classes, 54-79.9 cm, 80-99.9 cm, and 100-134 cm, when exhibiting characteristic behavior, the proportions of time and average durations of events were 45.3% (x-=5.1days), 62.6% (x-=8.5days), 79.2% (x-=17.5days), and the average daytime depths and temperatures were 284 m and 12.6 °C, 305 m and 12.7 °C, and 312 m and 12.1 °C, respectively. For the same three length classes, when exhibiting associative behavior, the proportions of time and average durations of events were 9.5% (x-=1.9days), 4.8% (x-=1.9days), and 6.0% (x-=1.8days), and the average daytime depths and temperatures were 101 m and 23.2 °C, 105 m and 23.1 °C, and 74 m and 22.3 °C, respectively. There is a significant positive correlation between the proportion of time fish exhibits characteristic behavior and fish length, and significant negative correlations between the proportion of time bigeye tuna exhibit associative and other behavior with fish length. Behavior and habitat preferences of bigeye tuna should be considered for standardizing catch-per-unit of effort (CPUE) data from both longline and purse-seine fisheries targeting tropical tunas in the Pacific, in an effort to provide more reliable estimates of relative abundance.
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Animal daily routines represent a compromise between maximizing foraging success and optimizing physiological performance, while minimizing the risk of predation. For ectothermic predators, ambient temperature may also influence daily routines through its effects on physiological performance. Temperatures can fluctuate significantly over the diel cycle and ectotherms may synchronize behaviour to match thermal regimes in order to optimize fitness. We used bio-logging to quantify activity and body temperature of blacktip reef sharks (Carcharhinus melanopterus) at a tropical atoll. Behavioural observations were used to concurrently measure bite rates in herbivorous reef fishes, as an index of activity for potential diurnal prey. Sharks showed early evening peaks in activity, particularly during ebbing high tides, while body temperatures peaked several hours prior to the period of maximal activity. Herbivores also displayed peaks in activity several hours earlier than the peaks in shark activity. Sharks appeared to be least active while their body temperatures were highest and most active while temperatures were cooling, although we hypothesize that due to thermal inertia they were still warmer than their smaller prey during this period. Sharks may be most active during early evening periods as they have a sensory advantage under low light conditions and/or a thermal advantage over cooler prey. Sharks swam into shallow water during daytime low tide periods potentially to warm up and increase rates of digestion before the nocturnal activity period, which may be a strategy to maximize ingestion rates. "Hunt warm, rest warmer" may help explain the early evening activity seen in other ectothermic predators.
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Although evolutionary ecologists agree that proximate and ultimate aspects are two sides of one coin, they are seldom interested in studies on physiological and behavioural mechanisms at the base of ecological phenomena. Nevertheless, these mechanisms are objects of selection and evolved to realise adaptive significances. This paper is a plea to bring both fields closer together, and, by means of an example of Diel Vertical Migration of Daphnia, some proximate and ultimate aspects are discussed. It is argued that light changes, not fish kairomone, is the primary cause for an individual to swim downwards at dawn and upwards at dusk. However, what is called a causal factor might differ when ecosystems or individuals are studied. In addition, causality in ecology is not simple, and has the character of a `set of necessary conditions'. To illustrate the importance of proximate analyses in DVM, two basic response mechanisms are discussed: Photobehaviour system 1 and 2. The physiological character of these systems leads to a fixed type of migration or to a phenotypically induced DVM, respectively. The adaptive significance of the first might be a reduction of the hazardous effects of UV radiation and of the second a lowering of mortality due to visually hunting predators.
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Foraging theory predicts that predators adjust their movements according to the spatial distribution of prey. Since prey is often patchily distributed, area-restricted search (ARS) behaviour, characterized by sinuous search paths of predators with increased turning frequency, should be effective in foraging. However, it remains unclear whether ARS behaviour actually enhances foraging success in free-ranging animals, especially in marine animals that forage in a three-dimensional (3D) environment. Here, we reconstructed 3D dive paths of a highly pelagic marine predator, the northern elephant seal (n = 3), with multisensor data loggers that recorded depth, tri-axis acceleration, tri-axis magnetism and swim speed. We identified spatial scales of volume-restricted search (VRS, termed for 3D ARS) behaviour using spherical first-passage time analysis on 3D dive paths, accompanied with quantifying feeding rates in VRS by using mandible accelerometers that recorded feeding events. Seals exhibited VRS behaviour at two spatial scales (radius of spheres): small-VRS (8-10 m) and large-VRS (17-19 m). Most feeding events occurred in VRS zones (78 and 86% for small and large-VRS, respectively), although VRS accounted for a small proportion of bottom phase of dives in distance travelled. This suggests a strong link between VRS behaviour and foraging success. There was a hierarchical structure to the VRS; most small-VRS (95%) were nested within large-VRS (i.e. nested VRS). Importantly, nested VRS had significantly higher feeding rates than non-nested VRS, because nested VRS contained small- and large-VRS with higher and lower feeding rates, respectively. These results suggest that seals forage on mesopelagic prey in a hierarchical patch system where high-density patches at small scales are nested within low-density patches at larger scales. We demonstrated that seals employed scale-dependent, hierarchical 3D movements and that underwater fine-scale sinuous movements (i.e. VRS) were strongly linked to higher foraging success, particularly within nested VRS zones. We suggest that seals enhanced foraging success by employing hierarchical movements that possibly reflect the hierarchical property of prey distribution. Although recent studies advocate that optimal searching behaviour would be scale-independent (e.g. Lévy walk), our study suggests that scale-dependent processes are important components of successful foraging behaviour.
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Rhythmic activity patterns are ubiquitous in animals and in the marine environment a dominant rhythmic activity is the diel vertical migration (DVM) of pelagic organisms, moving or ‘migrating’ from deep waters during the day to shallower waters at night. While this overall pattern of movement is well understood, the cryptic nature of the marine environment has limited the study of fine-scale movements within each phase. Active pelagic predators, such as tuna, perform consistent, predictable large-scale vertical movements; however, the fine-scale movements nested within these larger movements have not previously been investigated in detail. Further, the prey field densities are known to differ significantly between day and night, presenting an opportunity to study differences in foraging patterns between these two phases. Here, using long-term depth time series recorded from 93 bigeye tuna, Thunnus obesus, with electronic tags (18 003 days of data), fine-scale changes in vertical movement patterns between day and night time phases were investigated in the context of the Lévy foraging hypothesis, which predicts a Lévy distribution of move steps during foraging when prey is scarce, but an exponential distribution when prey is abundant and searching is not required. During the day, T. obesus were found to exhibit scale-free movements well fitted by a Lévy distribution indicating optimized searching for sparsely distributed prey. During night-time hours, however, exponentially distributed scale-dependent move step lengths were found to be dominant, supporting a simple, Brownian, movement pattern sufficient where prey is abundant. This study not only confirms the predictions of the Lévy foraging hypothesis but suggests that the identification of Lévy patterns in movement data can be a useful indicator of foraging activity in animals that are difficult to observe directly.
Article
Ninety-six bigeye tuna (88134 cm fork length) were caught and released with implanted archival (electronic data storage) tags near fish-aggregating devices (FADs) in the equatorial eastern Pacific Ocean (EPO) during April 2000. Twenty-nine fish were recaptured, and the data from twenty-seven tags were successfully downloaded and processed. Time at liberty ranged from 8 to 446 days, and data for 23 fish at liberty for 30 days or more are presented. The accuracy in geolocation estimates, derived from the light level data, is about 2 degrees in latitude and 0.5 degrees in longitude in this region. The movement paths derived from the filtered geolocation estimates indicated that none of the fish traveled west of 110°W during the period between release and recapture. The null hypothesis that the movement path is random was rejected in 17 of the 22 statistical tests of the observed movement paths. The estimated mean velocity was 117 km/d. The fish exhibited occasional deep-diving behavior, and some dives exceeded 1000 m where temperatures were less than 3°C. Evaluations of timed depth records, resulted in the discrimination of three distinct behaviors: 54.3% of all days were classified as unassociated (with a floating object) type-1 behavior, 27.7% as unassociated type-2 behavior, and 18.7% as behavior associated with a floating object. The mean residence time at floating objects was 3.1 d. Data sets separated into day and night were used to evaluate diel differences in behavior and habitat selection. When the fish were exhibiting unassociated type-1 behavior (diel vertical migrations), they were mostly at depths of less than 50 m (within the mixed layer) throughout the night, and during the day between 200 and 300 m and 13° and 14°C. They shifted their average depths in conjunction with dawn and dusk events, presumably tracking the deep-scattering layer as a foraging strategy. There were also observed changes in the average nighttime depth distributions of the fish in relation to moon phase.