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

## TL;DR

This study uses chemical signatures in green sea turtle bones to track their movement and foraging patterns in Southern California, revealing key life history details.

## Contribution

The novel use of geochemical signatures in bones to determine habitat shifts and foraging patterns in green sea turtles.

## Key findings

- Green sea turtles transition from oceanic to coastal habitats around 6.6 years of age, though some may shift earlier.
- Stable isotope analysis showed that seagrass was not the primary diet of most turtles in coastal habitats.
- Turtles often establish multi-year residency in isotopically distinct coastal areas after recruitment.

## Abstract

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.

We present vital life‐history and demographic data including the oceanic stage duration, timing of ontogenetic habitat shifts, and multi‐year foraging patterns of eastern North Pacific green sea turtles found in Southern California. The findings were obtained by analyzing sea turtle bones (age and growth via skeletochronology; and habitat and diet via stable isotope analysis) in conjunction with data obtained from in‐water monitoring, and provide detailed, multi‐year information on individual green sea turtles.