Green Sea Turtle Recruitment in the Eastern North Pacific: Patterns Identified Using Geochemical Signatures in Bones

Within marine systems, nutrient cycling is driven by physical forces that create predictable geochemical gradients. In turn, these gradients are reflected in spatially explicit and chemically distinct foodwebs, creating unique chemical signatures of consumer tissues that are useful for tracking the location and diet of consumers. In the eastern North Pacific, over the past three decades green sea turtles (Chelonia mydas ) have become more commonly observed along the west coast of the United States, particularly along the urban Southern California coast. Understanding the habitat use patterns and basic demographic rates of these turtles is important for resource management. To address these data gaps, we used spatial patterns created by natural geochemical cycling (i.e., marine isoscapes) to inform sea turtle movement and habitat use over time. This was done by analyzing stable isotope values of bone growth layers in turtle humeri and analyzing the values with age and size data obtained through skeletochronology. This approach allowed us to recreate the movements and foraging patterns of green sea turtles in Southern California. We present vital life-history and demographic data, including the oceanic stage duration, timing of ontogenetic habitat shifts, and multi-year foraging patterns. Sea turtles depart the oceanic habitat recruiting to neritic foraging grounds around 6.6 years of age, indicated by nitrogen isotope values (δ¹⁵N), but turtles may do so as early as one year old, or may remain in oceanic zones for much longer. Once settled into isotopically distinct coastal habitats, it was common for turtles to establish multi-year residency, and while many appeared to consume at least some seagrass, stable carbon isotope values (δ¹³C)-a primary indicator of critical habitat-suggested that it was not the primary diet item of most individuals. Collectively, these findings fill information gaps about green turtle life-history, which have immediate application to ongoing regional management efforts.