Niche overlap and trophic position estimates of multiple sea turtle species cold-stunned in the northwest Atlantic

Juvenile Kemp’s ridley ( Lepidochelys kempii ), loggerhead ( Caretta caretta ) and green ( Chelonia mydas ) sea turtles regularly strand following seasonal cold-stunning events in southern New England, but much of their pre-stranding ecology is poorly understood. To determine general trends in foraging preferences, habitat usage, and trophic position we used a combination of bulk (BSIA) and compound-specific (CSIA) stable isotope analyses. Carbon (δ ¹³ C) and nitrogen (δ ¹⁵ N) BSIA was performed on whole blood (n = 240) and muscle (n = 340) samples collected from stranded individuals of all three species. Sulfur (δ ³⁴ S) BSIA (whole blood, n = 24; muscle n = 32) and δ ¹⁵ N CSIA (whole blood, n = 7; muscle n = 25) were applied to a subset of samples to further infer habitat use and trophic information. Stable isotopic niche overlap and muscle isotope-based trophic position (TP) estimates were similar for the three species, with green (TP _BSIA =2.6, TP _CSIA =2.9) and loggerheads (TP _BSIA =3.1, TP _CSIA =3.3) having the lowest overlap and greatest TP separation. Kemp’s ridley had intermediate estimated TP (TP _BSIA 2.8, TP _CSIA 3.1) and a high degree of overlap with the two other species. Turtle straight carapace length (SCL) was the main predictor of intraspecific stable isotope variation, and δ ¹⁵ N increased with SCL for all species and both tissue types. These patterns are consistent with ontogenetic TP increases as well as transition from oceanic to nearshore isoscapes. Higher intraspecific variation in BSIA-based TP estimates relative to CSIA-based estimates as well as variable inter-tissue stable isotope offsets provide evidence of individual variability in residency in nearshore waters prior to stranding.