AUTHOR=Peavey Lindsey E. , Popp Brian N. , Pitman Robert L. , Gaines Steven D. , Arthur Karen E. , Kelez Shaleyla , Seminoff Jeffrey A. 

TITLE=Opportunism on the High Seas: Foraging Ecology of Olive Ridley Turtles in the Eastern Pacific Ocean

JOURNAL=Frontiers in Marine Science

VOLUME=4

YEAR=2017

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2017.00348

DOI=10.3389/fmars.2017.00348

ISSN=2296-7745

ABSTRACT=<p>Stable isotopic compositions in animal tissues have been widely used to gain insight into trophic dynamics, especially of mobile aquatic predators whose behavior and dietary preferences are difficult to directly measure. Olive ridley sea turtles (<italic>Lepidochelys olivacea</italic>) range across &gt;3 million km<sup>2</sup> of the tropical and subtropical eastern Pacific Ocean and their trophic ecology in open ocean areas has not yet been adequately described. Individuals feed within biogeographic regions where varying nutrient cycling regimes result in phytoplankton with distinct δ<sup>13</sup>C and δ<sup>15</sup>N values that are assimilated by the turtles. We sampled 346 turtles at-sea between 2003 and 2009 and used bulk tissue (<italic>n</italic> = 346) and amino acid compound specific isotope analysis (AA-CSIA, <italic>n</italic> = 31) to empirically support the conventional understanding that olive ridleys are omnivores. Bulk δ<sup>15</sup>N values did not significantly vary with carapace length, a proxy for age, or with putative sex of adults. We therefore hypothesize that trophic position (TP) does not vary across age or sex. In line with other isotopic studies of this biogeographic scale in the same region, we observed a trend of bulk tissue <sup>15</sup>N enrichment with increasing latitude. Using AA-CSIA to account for δ<sup>15</sup>N baseline shifts among food webs (space), we estimated the TP of adult foragers using two methods. We found that across their eastern Pacific range, olive ridley δ<sup>13</sup>C and δ<sup>15</sup>N niche area varied, but median TP of adults remained constant (~3.1). Using a two-amino acid TP estimation method, we detected a small but notable elevation of TP for olive ridleys on the Costa Rica Dome. This study underscores the value of large-scale in-water olive ridley sea turtle research across oceanic foraging habitats to confirm or challenge anecdotal understanding of trophic roles, susceptibility to environmental change, and critical habitats. Further, it improves our understanding of why this species is now abundant in the eastern Pacific Ocean. A prey generalist with plenty of suitable foraging habitat can recover from the brink of extinction despite the presence of major threats. However, such foraging characteristics may require dynamic open ocean management approaches to meet conservation objectives if threats persist and/or increase.</p>