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Understanding the mechanisms linking oceanographic processes and marine vertebrate habitat use is critical to effective management of populations of conservation concern. The basking shark Cetorhinus maximus has been shown to associate with oceanographic fronts – physical interfaces at the transitions between water masses – to exploit foraging opportunities resulting from aggregation of zooplankton. However, the scale, significance and variability of these observed associations have not yet been established. Here, we quantify the influence of mesoscale (10s – 100s km) frontal activity on habitat use over timescales of weeks to months. We use animal-mounted archival tracking with composite front mapping via Earth Observation (EO) remote sensing to provide an oceanographic context to individual shark movements. We investigate levels of association with fronts occurring over two spatio-temporal scales, (i) broad-scale seasonally persistent frontal zones and (ii) contemporaneous mesoscale thermal and chl-a fronts. Using random walk simulations and logistic regression within an iterative generalised linear mixed modelling (GLMM) framework, we find that seasonal front frequency is a significant predictor of shark presence. Temporally-matched oceanographic metrics also indicate that sharks demonstrate a preference for productive regions, and associate with contemporaneous thermal and chl-a fronts more frequently than could be expected at random. Moreover, we highlight the importance of cross-frontal temperature change and persistence, which appear to interact to affect the degree of prey aggregation along thermal fronts. These insights have clear implications for understanding the preferred habitats of basking sharks in the context of anthropogenic threat management and marine spatial planning in the northeast Atlantic.

The use of archival depth telemetry as a means of remotely assessing the reproductive rates of free-ranging fishes is explored. This is achieved by electronically tracking the vertical movements of individual female small spotted catsharks Scyliorhinus canicula in the natural environment, whilst simultaneously evaluating the temporal and vertical distributions of egg-laying in this species. Distinctive patterns of short-term (0·3-3·7 h), shallow-water activity are documented in the time-depth profiles of female S. canicula that occur at an appropriate depth (1·0-2·3 m) and periodicity (every 10-12 days) to represent egg-laying behaviour. Putative egg-laying behaviour was exhibited simultaneously by two individually tracked female S. canicula during late-spring and early-summer. The results highlight that, provided species behaviour is suitable and complementary methods such as previous data, laboratory experiments and field surveys can be used to validate the patterns observed, archival depth telemetry offers an unobtrusive means by which egg production and egg-laying behaviour of free-living fishes can be estimated. As precise information regarding life-history parameters is difficult to obtain for free-ranging fish species, this technique could be used to improve the parameterization of species demographic models that are relevant to the management of wild fish populations.

Within-species sexual segregation is a widespread phenomenon among vertebrates, but its causes remain a topic of much debate. Female avoidance of male coercive mating attempts has the potential to influence the social structure of animal populations, yet it has been largely overlooked as a driver of sexual separation. Indeed, its potential role in long-term structuring of natural populations has not been studied. Here we use a comparative approach to examine the suitability of multiple hypotheses forwarded to account for sexual segregation (i.e., activity budget, predation risk, thermal niche–fecundity, and social factors) as drivers underlying sex-specific habitat use in a monomorphic model vertebrate, the small-spotted catshark, Scyliorhinus canicula. Using this hypothesis-driven approach, we show that year-round sexual habitat segregation in S. canicula can be accounted for directly by female avoidance of male sexual harassment. Long-term electronic tracking reveals that sperm-storing female catsharks ...

Commercial fishing and climate change have influenced the composition of marine fish assemblages worldwide, but we require a better understanding of their relative influence on long-term changes in species abundance and body-size distributions. In this study, we investigated long-term (1911‐2007) variability within a demersal fish assemblage in the western English Channel. The region has been subject to commercial fisheries throughout most of the past century, and has undergone interannual changes in sea temperature of over 2.01C. We focussed on a core 30 species that comprised 99% of total individuals sampled in the assemblage. Analyses showed that temporal trends in the abundance of smaller multispecies size classes followed thermal regime changes, but that there were persistent declines in abundance of larger size classes. Consistent with these results, larger-growing individual species had the greatest declines in body size, and the most constant declines in abundance, while abundance changes of smaller-growing species were more closely linked to preceding sea temperatures. Together these analyses are suggestive of dichotomous size-dependent responses of species to long-term climate change and commercial fishing over a century scale. Small species had rapid responses to the prevailing thermal environment, suggesting their life history traits predisposed populations to respond quickly to changing climates. Larger species declined in abundance and size, reflecting expectations from sustained size-selective overharvesting. These results demonstrate the importance of considering species traits when developing indicators of human and climatic impacts on marine fauna.

Summary 1. Megaplanktivores such as filter-feeding sharks and baleen whales are at the apex of a short food chain (phytoplankton‐zooplankton‐vertebrate) and are sensitive indicators of sea-surface plankton availability. Even though they spend the majority of their time below the surface it is still not known how most of these species utilize vertical habitat and adapt to short-term changes in food availability. 2. A key factor likely to control vertical habitat selection by planktivorous sharks is the diel vertical migration (DVM) of zooplankton; however, no study has determined whether specific ocean-habitat type influences their behavioural strategy. Based on the first high-resolution dive data collected for a plankton-feeding fish species we show that DVM patterns of the basking shark Cetorhinus maximus reflect habitat type and zooplankton behaviour. 3. In deep, well-stratified waters sharks exhibited normal DVM (dusk ascent‐dawn descent) by tracking migrating sound-scattering layers characterized by Calanus and euphausiids. Sharks occupying shallow, inner-shelf areas near thermal fronts conducted reverse DVM (dusk descent‐dawn ascent) possibly due to zooplankton predator‐prey interactions that resulted in reverse DVM of Calanus . 4. These opposite DVM patterns resulted in the probability of daytime-surface sighting differing between these habitats by as much as two orders of magnitude. Ship-borne surveys undertaken at the same time as trackings reflected these behavioural differences. 5. The tendency of basking sharks to feed or rest for long periods at the surface has made them vulnerable to harpoon fisheries. Ship-borne and aerial surveys also use surface occurrence to assess distribution and abundance for conservation purposes. Our study indicates that without bias reduction for habitat-specific DVM patterns, current surveys could under- or overestimate shark abundance by at least 10-fold.