Population ecology data for marine turtles are limited as we know little about their basic life histories such as juvenile stage duration, age-at-maturation, and longevity. During their long, complex life cycle, turtles migrate between and occupy widely disparate habitats, each with spatially explicit threats and survival rates. Understanding the demographics and basic ecology of animals occupying potentially detrimental habitats is important for conservation and population recovery, especially for endangered species. Biogeochemical analysis of tissues sampled opportunistically from dead animals allows for reconstruction of their habitat use, foraging ecology, and demographics, thereby generating more reliable population abundance and trend assessments. To investigate age-at-maturation and duration and survivorship of endangered North Pacific loggerhead turtles (Caretta caretta) in a region of the east Pacific known for high-turtle bycatch and categorized as a sink habitat, we applied skeletochronology and stable isotope analysis (SIA) on humerus bones from dead-stranded turtles we collected from Playa San Lázaro, Baja Sur California (BSC), Mexico. We estimated the age of ~150 turtles and their residency duration in this region, and used SIA of ~280 annual bone growth layers from 45 of these turtles to recreate multi-year habitat use patterns and assess timing of ontogenetic shifts between distinct foraging grounds.
We found that reproductively valuable juveniles range in ages from 3 to 24 years, and therefore may spend > 20 years in this high-bycatch, sink habitat, and could result in low (~10%) stage survivorship given the previously estimated annual mortality rate (~11%) for loggerheads in this region (Seminoff et al. 2014). We also provide the first empirical evidence for age-at-maturation (~25 years) for loggerheads foraging in the eastern Pacific. Focusing on stable nitrogen isotope (δ15N) values from known-locations samples in two foraging areas, the oceanic central North Pacific (CNP) and the neritic east Pacific region near BSC, we found δ15N values differed significantly (p<0.0001) and by 5‰ (CNP 10.7±1.9‰ vs. BSC 15.7±1.1‰). We then observed two patterns of change in the regionally distinct isotope values within the bone growth layers, indicating a bimodal size/age distribution in the timing juveniles underwent an ontogenetic habitat shift from the CNP to the BSC region (42.7±7.2 cm and 7.5±2.7 yrs vs. 68.3±3.4 cm and 15.6±1.7 yrs). This has implications for the stage survivorships for this segment of the population. Later settlement by some turtles to this sink habitat results in shorter residency durations and increased stage survivorship, a hopeful indicator for North Pacific loggerheads.