Understanding the ecology and movements of lamnid sharks through stable isotope analysis and electronic tagging [electronic resource]
Keywords: Biology.

As large, upper trophic level predators, sharks play an important role in structuring and maintaining the function of marine communities in the world's oceans. The life history characteristics of many sharks, and indeed most chondrichthyan fishes (sharks, skates, rays and chimaeras), make them particularly susceptible to anthropogenic stressors such as fishing pressure and habitat disturbance. In addition, the migratory nature of many of these animals often takes them across international boundaries and into areas that lack regulation and/or enforcement, making effective management of these species difficult or impossible. Due to this vulnerability and the important role this group of organisms generally plays in marine ecosystems, sharks need to be managed carefully. The first step to managing a migratory, pelagic species is to understand its role in marine ecosystems. This requires an understanding of the ecology of the species and patterns of distribution throughout its ontogeny and range, as the vulnerability of a species to fishing pressure or habitat disruption may vary based with age and movement patterns. Studying the ecology of large pelagic fish and sharks has historically been very difficult due to their migratory nature and the general inaccessibility of their habitat. However, recent advances in electronic tagging technology and biogeochemical techniques, such as stable isotope analysis (SIA), are providing researchers with an increasingly powerful tool kit with which to study these vagile organisms. I used this combination of electronic tagging and stable isotope techniques to investigate the ecology and trophic ecology of two species of lamnid shark, the salmon shark (Lamna ditropis) and white shark (Carcharodon carcharias). Tagging data indicated that salmon sharks utilized the productive waters of the Gulf of Alaska throughout the year, and that their trophic ecology shifts seasonally as oceanographic conditions and prey availability changes. The distribution of salmon sharks showed a high degree of overlap with commercially important species, indicating the species is at risk of being caught as bycatch in commercial fisheries. Stable isotope analysis of salmon shark vertebrae indicated that they undergo an ontogenetic shift in resource use which corresponded with a shift in distribution. Sharks primarily used oceanic resources during their early life history, and transitioned to using more neritic resources around the age at first maturity. The primary nursery habitat of juvenile salmon sharks was in the North Pacific Transition Zone (NPTZ), with a smaller number using the California Current nursery area during the first year of life, though by year 2 all sharks were using the NPTZ nursery area. As sharks increased their use of neritic habitats, their isotopic niche diminished as they became more specialized on particular habitats or prey. This ontogenetic shift in habitat likely represents a balance between prey availability, vulnerability to predation, and thermal constraints on this endothermic shark. The importance of temperature was apparent in the distribution of juvenile salmon sharks in the California Current as well. The latitudinal distribution of strandings of juvenile sharks on beaches along the west coast indicated that sharks were restricted to warmer waters, and that their range extended north as water temperatures increased at higher latitudes. Strandings increased during periods of peak upwelling as well. SIA indicates that juvenile salmon sharks primarily use the offshore habitats of the California Current, though stranded sharks all moved to productive neritic habitats in the month or two prior to stranding, possibly exposing them to coastal upwelling. SIA results and electronic tag data were used to investigate the migratory behavior of white sharks and found that overall, white sharks appear to forage at a much lower rate when offshore relative to when they are in California, and California appears to be their primary foraging area. I also found evidence of an ontogenetic change in habitat use related to onset of maturity. Low offshore consumption rates and initiation of offshore migrations around the size of first maturity suggests that it is unlikely that foraging is the primary purpose of the offshore migrations. This may indicate that there is an underlying reproductive function to white shark migrations.