Reconstructing ontogenetic movements in pelagic sharks coupling ocean models and stable isotope data in incrementally grown tissues

Abstract

Ecological interactions in the marine pelagic environment are difficult to study, mostly because the open-ocean is vast and largely inaccessible. Migration is a common ecological trait in pelagic settings, with large impacts on community structure and dynamics, and ecosystem functioning. However, migratory predators are rapidly declining worldwide, with unclear ecological consequences.Pelagic sharks have declined regionally by > 90% in the past 15 years, largely as a result of overfishing and by-catch. Shark vulnerability to fishery capture depends on individual movements, and on the presence of movement traits across individuals, populations or species, which may implyshared vulnerability. Yet, the movements of pelagic sharks and other migratory oceanic animals are difficult to monitor or reconstruct.Natural-abundance stable isotopes allow retrospective movement reconstruction, by relating the isotopic composition of animal tissues to geographically indexed measurements or predictions of isotopic ratios at the base of the food web (isoscapes). Where incrementally grown, metabolically inert tissues are available, movements can be reconstructed throughout life. However, the application of stable isotopes in bulk tissues to study migration is complicated by mixed baseline and trophic effects and, in pelagic settings, by large uncertainties in the spatio-temporal distributions of isotopic baselines.In this study, I explored how the ontogenetic movements of two pelagic shark model species, the blue (Prionace glauca) and porbeagle (Lamna nasus) sharks, could be reconstructed using modelled global ocean carbon isoscapes, and carbon and nitrogen isotopes in bulk cartilage collagen and single amino acids from vertebrae.To provide a possible solution for poor sampling of isotopic baselines, I developed a process-based, mechanistic carbon isotope model, predicting the likely spatio-temporal distributions of the carbon isotopic composition of phytoplankton across the global ocean.To provide information on pelagic shark life-history traits, I recovered individual-level life history carbon and nitrogen isotope records for bulk cartilage collagen from vertebrae of sharks caught across the North Atlantic. I also recovered comparable carbon isotope records for single amino acids,producing the first compound-specific isotopic dataset of within-individual ontogenetic variance in sharks. Consistent ontogenetic isotope patterns across individuals of each species revealed species level life-history traits. Whilst the interpretation of traits for bulk collagen using modelled isoscapeswas ultimately limited from confounding influences from trophic level change, that of traits for essential amino acids conclusively demonstrated ontogenetic and transgenerational movement traits.During juvenile growth, blue sharks increasingly utilised foraging grounds with more positive carbon isotopic baselines, whereas porbeagles made increasing use of isotopically more negative grounds. Blue shark pupping and maternal foraging occurred in isotopically distinct grounds, with the possibility of natal homing by adult individuals. Pregnant female porbeagles, by contrast,migrated to isotopically distinct foraging grounds prior to giving birth.Isotope-derived information on ontogenetic movements complements tag-derived information over a snap-shot of the entire life of individuals, but explicit isoscape-based geo-location is limited by large uncertainties in isoscape models, and trophic influences on bulk tissue isotopiccompositions.

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