Trophic Niche Partitioning Of Pelagic Shark Species Based Of Multi-tissue Stable Isotope Ratios

Most pelagic sharks are as top predators affect the abundance and structure of prey populations through trophic cascades and play a crucial ecological role in open-sea ecosystems.However,due to its highly migratory,remote habitat and most of them have been listed as endangered species,the researches of the feeding migratory in their life history were still rare and our knowledge on its diet preference and intraguild trophic interaction remained limited.For marine predators that migrate across large distances,electronic tag tracking approaches are the most widespread and direct methods can provide important spatial information,and are not always appropriate due to expensively cost and have failure rates approaching 50%.Multi-tissue stable isotope analysis is an alternative method that provides feeding migratory information at the different time scales,which according to the stable carbon and nitrogen isotope ratios(δ¹³C andδ¹⁵N,respectively)of multiple tissues with different turnover rates can provide information on the feeding,habitat utilization and trophic niche of consumers during different time periods.In recent years,multi-tissue stable isotope analysis has been frequently reported in the study of trophic niche of biological groups in various sea areas,and is a new direction of traditional stable isotope analysis which has gradually become the powerful methods used for examining the trophic dynamic of organisms during the different time scale.In this study,to explore the feasibility and applicability of multi-tissue stable isotope analysis in the trophic niche research of pelagic sharks,theδ¹³C andδ¹⁵N values of muscle,liver,whole blood,red blood cell（RBC）and plasma of common pelagic shark species from tropical Atlantic and Pacific Ocean were analyzed.Based on the analysis of tissue isotope characteristics and trophic niche partitioning of shark species,multi-tissue stable isotope analysis was verified and improved.Furthermore,it provides a basis for tissue selection,and makes clear whether the selected tissue is representative and can be used as the replacement object of lethal sampling in the work of non-lethal sampling of endangered species.The results are as follows:（1）Theδ¹³C andδ¹⁵N values of muscle,liver and whole blood of blue sharks（Prionace glauca）,longfin mako sharks（Isurus paucus）,crocodile sharks（Pseudocarcharias kamoharai）and shortfin mako sharks（I.oxyrinchus）from tropical Atlantic were analyzed.The results showed similar and high trophic level of I.oxyrinchus,P.kamoharai and P.glauca with the lowest one of I.paucus due to their relatively small body size.Feeding segregation was found between P.glauca and other shark species because of their unique oceanic life history.The largest trophic niche width was observed for I.oxyrinchus,indicating its high prey diversity and ecological plasticity.Furthermore,high trophic niche overlap was found for P.kamoharai and I.oxyrinchus,implying their potential resource competition between the two species.Besides,there was no relationship observed between the differences among tissues in theδ¹³C orδ¹⁵N values and the body size indicated no recent diet shift.Exception was found in I.paucus whose differences in theδ¹³C orδ¹⁵N values between liver and muscle,whole blood and muscle showed significant relationship with the shark fork length,indicating its short-term diet change.Moreover,the similarity ofδ¹³C andδ¹⁵N values and higher metabolic rates of liver and whole blood was found in all 4 shark species studied,implying the potential isotopic proxy of whole blood to the that of the livers in shark non-lethal sampling.（2）Usingδ¹³C andδ¹⁵N values to quantify the niche trajectory and isotopic niche width and overlap from muscle,red blood cell（RBC）,liver and plasma of the silky shark（Carcharhinus falciformis）,blue shark（P.glauca）,smooth hammerhead shark（Sphyrna zygaena）,bigeye thresher shark（Alopias superciliosus）and pelagic thresher shark（A.pelagicus）collected in the eastern tropical Pacific Ocean.Overall,there was no significant linear relationship between isotopic niche trajectory length and body size,indicating the amplitude of dietary variation may not be motivated by the relative body size.These pelagic sharks manifested variable feeding strategies over long time scales,may reflect a response to the changes of climate and prey availability.Trophic dynamics from muscle to plasma（i.e.,long to short time scales）revealed a coastal or oceanic feeding migration strategies among these pelagic sharks.Moreover,the observed differences in the isotopic signatures of these pelagic sharks suggest a significant trophic partitioning.The similar trophic level and inshore feeding behavior were observed in C.falciformis and S.zygaena.P.glauca had the highestδ¹⁵N values which might be related to intake prey with high trophic level and deep foraging,whereas the lowerδ¹³C values of A.superciliosus and A.pelagicus were due to the large proportion of small-medium body size within populations and intake the small size prey.This section further improves the method applicability in trophic dynamics of pelagic sharks by using multi-tissue stable isotope analysis.These results highlight the importance of higher metabolic rate tissues to reveal the trophic dynamic of pelagic sharks and the potential isotopic proxy of plasma to the that of the livers in endangered species non-lethal sampling.Through the above experimental study on the trophic niche of pelagic shark species from the two oceanic areas,not only deepened our understanding in the diet,habitat use and trophic interactions of pelagic shark species,verified the feasibility and applicability of multi-tissue stable isotope technique in examining the trophic dynamic of organisms during the different time scale.The similarity of integrating diet over time-scales between whole blood and liver,and can be considered that using whole blood or plasma instead of liver in the non-lethal sampling of endangered species.These results provided useful reference for further research on feeding ecology of organisms based on multi-tissue stable isotope analysis.