Cohort structure and individual resource specialization in loggerhead turtles, long-lived marine species with ontogenetic migrations

Abstract

Cohort structure and degree of individual resource specialization affect individual- and population-level fitness and ecosystem interactions. These 2 characteristics are difficult to establish for long-lived marine species such as sea turtles. Both characteristics were approached by applying a novel Broken-Stick analysis of Stable isotope Time series (BS-SiTs) framework to longitudinal data series derived from metabolically inert carapace scute tissue of 13 loggerhead turtles Caretta caretta near the coast of Kochi prefecture, Japan. During the subadult stage, δ15N signatures decreased 1.29‰ per 50 µm scute slice away from values associated with neritic habitats and a diet of benthic invertebrates. Following a breakpoint, δ15N signatures stabilized in the range of 10-15‰, corresponding with planktonic feeding adults with an oceanic foraging strategy in the North Pacific. Ontogenetic shifts in δ15N occurred at different chronological points for each individual, implying the presence of a diverse cohort structure within the local coastal population. During the adult stage, within-individual variation in δ15N was <6.1% of the total δ15N variation among sampled individuals. To offer effective protection, management actions should account for this potentially high individual resource specialization at the scale of local coastal populations. Nesting numbers of loggerhead turtles in Japan have been declining since the 1990s. The BS-SiTs framework provides ecological baseline information on ontogenetic shifts in resource use of individuals suitable to support management plans. Specifically, when applied across multiple individuals, the BS-SiTs framework offers a low-cost, non-invasive solution to monitor and compare trends in cohort structure among local coastal populations.