Feeding Ecology Of Jumbo Flying Squid(Dosidicus Gigas) In The Eastern Pacific Ocean Inferred By Biochemical Tracers

The jumbo flying squid(Dosidicus gigas)is a pelagic cephalopod,highly abundant,and endemic to the eastern Pacific Ocean.It is one of the main target species of cephalopod fishery of Chinese distant water fisheries.D.gigas plays a vital role in the marine ecosystems of the eastern Pacific Ocean,not only as a voracious predator but also as a valuable source of prey items of large marine fishes and mammals.The biomass of D.gigas is significantly influenced by climate and environmental changes,although an increasing tendency of the yield was observed.Previous studies have been carried out to investigate the population age and genetic structure,evolutionary history,reproduction and development,and other aspects of basic biology of this squid species.However,the feeding ecology of D.gigas is mainly dependent on stomach content analysis,and the information of the trophic and migration patterns among squid from different geographic origins and response and behaviors to climate change is still limited.The overarching goal of this dissertation is to investigate the population structure,the intraand inter-subpopulation trophic niche partitioning and the mechanism of response of D.gigas to climate change.The novel information may have important implications for understanding the trophic positions and feeding strategies of D.gigas in the marine ecosystems of the eastern Pacific Ocean and provide a beneficial reference for sustainable development and management of this resource.In this dissertation,D.gigas samples were collected during various commercial jigging vessels operating in 2009,2013,2014 and 2015 in the different waters of the eastern Pacific Ocean.Based on characteristics of different soft and hard tissues(muscle,statolith and gladius),we used biochemical tracers,age determination,and morphometric analysis to investigate the feeding ecology of D.gigas.Carbon and nitrogen isotope ratios(?13C and ?15N)and fatty acid profiles were identified in squid from the waters of equatorial central eastern Pacific and off the exclusive economic zone of Peru and Chile.Comparative analysis was used to evaluate the geographic variations in tracers and to identify the suitable discriminatory variables among origins.We analyzed gladii stable isotope ratios,amino acid ?15N values and morphometric characteristics of feeding structures to evaluate food and habitat partitioning between females and males during ontogenesis.The morphometric characteristics of the gladius were analyzed,allowing for a comparison between the morphometric variables and environmental conditions.The gladii of squid from three harvest locations were cut successionally by the different growth curves of gladius proostracum.Comparative analysis was used to evaluate the differences in trophic positions and foraging strategies.The ?13C and ?15N values along the proostracum were used to reconstruct the feeding variations of D.gigas and to evaluate the potential effects of 2009-10 El Ni?o event on the variability of their trophic patterns.The results can be summarized as follows:(1)Significant spatial variations were found in isotopic values and fatty acid profiles in squid muscle tissues,possibly because of different food availability and/or oceanographic conditions(e.g.temperature and primary productivity)that each group experiences at a given location.The stepwise discriminant analysis indicated that ?15N,C16:1n7,C17:1n7,C18:2n6,C20:1 and C20:4n6 were effective variables at differentiating origin.The combined use of stable isotope and fatty acid analyses could trace geographic origins of D.gigas.This study indicated the spatial heterogeneity in food selection of D.gigas.(2)Females and males exhibited similar gladius bulk ?13C values,indicating that both sexes shared the same overall habitat and perhaps similar movement patterns.In contrast,females had higher gladius bulk ?15N values and larger feeding structures than males,suggesting greater foraging abilities in females to catch prey of higher trophic level to maximize energy intake.The results were in consistent with the observed higher relative trophic position indicators in females,which were predicted from amino acid ?15N values in gladius.These mechanisms likely reduce intraspecific competition and promote ontogeny of sexual niche partitioning.Our study reveals some underlying mechanisms of sexual segregation in D.gigas during ontogenesis and identifies how females and males coexist during most of their life cycle.(3)Significant differences were found between geographic subpopulations.Water of a high temperature and sufficient food availability was more appropriate to the longitudinal growth of the proostracum.A longer conus could provide sufficient reinforcement for fin locomotion to accommodate the higher speed current field.Based on these results,the growth curves of gladius proostracum for each subpopulation were established.(4)The gladii of squid from three harvest locations were cut successionally by the different growth curves of gladius proostracum.The gladius bulk ?13C and ?15N values showed inter-individual variations in trophic and movement patterns in all geographic subpopulations.At subpopulation level,the muscle amino acid ?15N values showed different trophic positions in squid among origins.The squid collected off Chile has relative higher trophic positions with trophic niche overlap with squid from the waters off Peru,while the squid from the equatorial central eastern Pacific had a specific trophic niche.(5)Strong variations in ?13C and ?15N values along the proostracum were observed for all squid,changes that could be determined by temporal variation of foraging as well as migration among regions with distinct baselines.An El Ni?o event could decrease the variation of trophic patterns of squid through compressing the space and time for foraging and migration.These results support our general hypothesis that a time-based consecutive sampling of the gladius can back-calculate an ontogenetic shift and the possible migration patterns of squids more precisely and comprehensively and suggest that the isotopic values of D.gigas might be influenced much more by their migration behavior.This study reveals the different food sources,ontogenetic foraging strategy,and trophic relationships among geographic subpopulations.We also showed the different trophic positions,foraging strategy and the response of D.gigas to climate change at individual and subpopulation levels.The novel information may have important implications for selecting the appropriate management approach,enhancing the control ability of the resources and improving the decision-making power of China in the international regional fishery resources management organization.Meanwhile,this study established the scientific research methods for feeding ecology studies of oceanic squid species based on the soft and hard tissues,provided a beneficial reference for feeding ecology studies of other short life species and ecosystem-based management.