Trophic ecology of green sea turtles in a highly urbanized bay: Insights from stable isotopes and mixing models

Abstract

The green turtle, Chelonia mydas, is a circumglobal species that is susceptible to overexploitation as a food resource, incidental mortality in fisheries, and coastal foraging habitat degradation, all of which have contributed to its listing as Endangered on the IUCN Red List. Efforts to recover regional green turtle populations have been hampered by a lack of information on their biology. In particular, temporal patterns of diet intake and habitat use in neritic foraging areas are not well understood. Historical paradigms suggest that adult green turtles are obligate herbivores with diets consisting of seagrasses and/or marine algae. However, these insights are largely based on conventional diet analysis techniques that only yield snapshots of recently consumed foods. Stable isotope analysis has been used to determine contributions of various potential food resources to a consumer's diet, and this approach is commonly applied to identify diet composition and long-term trophic relationships of a species. In this study, we measured the stable carbon (δ 13C) and nitrogen (δ 15N) isotope values of 86 green turtles and seven putative prey species (e.g., algae, seagrass, invertebrates) collected from 2003 to 2008 in San Diego Bay, California, USA, an urbanized coastal bay in the temperate eastern Pacific Ocean. The δ 13C and δ 15N values in skin of green turtles in this study ranged from − 18.9‰ to − 13.7‰ and 11.0‰ to 19.3‰, respectively, whereas the values for potential foods ranged from − 22.6‰ to − 11.5‰ for δ 13C and 10.4‰ to 15.9‰ for δ 15N. We applied a leading multisource stable isotope mixing model (Stable Isotope Analysis in R), to determine the main contributors to, and annual variation in, green turtle diet based on comparisons of isotope values of turtles and putative prey species. Mixing model outputs indicated that green turtles consumed an omnivorous diet during all years of this study. Mobile invertebrates had the greatest median dietary distribution (38%), whereas seagrasses (26%) and sessile invertebrates (12%) were also found to be major dietary contributors. Red algae and green algae were also identified as feasible prey species, although at reduced levels. When coupled with information on prey species' spatial distributions, these data also provide insights about the types of habitats visited by foraging green turtles. Local seagrass pastures appear to be of high value, serving as a major food resource and providing habitat for other green turtle prey. Protection of the remaining seagrass habitat in and around San Diego Bay is thus considered essential for local green turtle management, and restoration of degraded seagrass habitats in this highly urbanized bay should be considered a top conservation priority.