Ontogenetic niche theory predicts that individuals may undergo one or more changes in habitat or diet throughout their lifetime to maintain optimal growth rates, or to optimize trade-offs between mortality risk and growth. These habitat use decisions often mark transitions between life stages and may lead to the use of potentially suboptimal growth habitats where predation risk is low until critical sizes are reached. Multiple empirical studies have found support for this hypothesis in marine species, but applications to large marine vertebrates are lacking. Here, we combine skeletochronological and stable nitrogen isotope (δ15N) analyses of sea turtle humeri (n = 61) to characterize the growth dynamics of juvenile loggerhead sea turtles (Caretta caretta) during an oceanic-to-neritic ontogenetic habitat shift. The primary objectives of this study were to determine if increased growth is associated with this habitat shift and if growth patterns are maintained in individuals with different patterns of resource use. Humerus bone growth layers were sequentially sampled for stable nitrogen isotope ratios using a micromill. These data were used to characterize individual and intrapopualtion variation in resource use through time.
Based on the pattern of their δ15N data, 23 turtles were classified as discrete shifters (single year transition from oceanic to fully neritic resource use), 14 turtles were classified as facultative shifters (multi-year transition from oceanic to fully neritic resource use), and 24 turtles were classified as non-shifters (no evidence of transition). Mean growth rates peaked at the start of the ontogenetic shift (based on change in δ15N values), but returned to pre-shift levels within 2 years. Turtles generally only experienced 1 year of relatively high growth, but the timing of peak growth relative to the start of an ontogenetic shift varied among individuals (before, n = 14; during, n = 12; after, n = 8). Furthermore, no reduction in growth preceded the transition, as is predicted by ontogenetic niche theory. Annual growth rates were similar between non-transitioning turtles resident in oceanic and neritic habitats and turtles displaying alternative patterns of resource use. These results suggest that factors other than maximization of size-specific growth may more strongly influence the timing of ontogenetic shifts in loggerhead sea turtles, and that alternative patterns of resource use may have limited influence on somatic growth and age at maturation in this species.