Diet and foraging niche flexibility in green and hawksbill turtles

We used stable isotopes to investigate isotopic niche size, overlap, and diet composition in green (black and yellow morphotype Chelonia mydas; 50.0 to 95.0 cm curved carapace length, CCL) and hawksbill turtles (Eretmochelys imbricata; 38.5 to 83.0 cm CCL) in a recently described foraging habitat in North Pacific Costa Rica. We measured whole blood stable carbon (delta C-13) and nitrogen (delta N-15) ratios in black (n = 39; mean +/- SD, - 16.54 +/- 0.66 parts per thousand and 14.39 +/- 0.77 parts per thousand), yellow (n = 13; - 15.74 +/- 0.65 parts per thousand and 12.37 +/- 0.55 parts per thousand) and hawksbill turtles (n = 13; - 16.23 +/- 1.34 parts per thousand and 12.63 +/- 0.32 parts per thousand) and skin delta C-13 and delta N-15 values in black (n = 36; - 15.32 +/- 0.79 parts per thousand and 15.16 +/- 0.72 parts per thousand), yellow (n = 12; - 15.38 +/- 0.91 parts per thousand and 13.78 +/- 0.75 parts per thousand) and hawksbill turtles (n = 10; - 14.33 +/- 1.49 parts per thousand and 13.77 +/- 0.29 parts per thousand). Isotopic niche space revealed distinctly higher delta N-15 area in black turtles and significant overlap between yellow and hawksbill turtles, and a recent shift in diet in yellow turtles from omnivory to herbivory. In black turtles, isotopic niche suggests individual specialization during the non-upwelling season and generalization in diet during the upwelling season. Mixing model results suggest that black turtles forage at multiple trophic levels (fish: 34.8 +/- 10.1% of diet and macroalgae: 51.8 +/- 12.8% of diet), while yellow and hawksbill turtles primarily forage on macroalgae (85.0 +/- 6.6% in yellow turtles and 85.1 +/- 5.9% in hawksbill turtles). These results add to a growing understanding that diet in sea turtles is influenced by diet items present in the environment and suggest that black turtles are potential tertiary consumers.

Automated summary

This study used stable isotopes to investigate the isotopic niche of green and hawksbill turtles in a foraging habitat in North Pacific Costa Rica. The results revealed individual specialization and generalization in diet for black turtles, significant diet overlap between yellow and hawksbill turtles, and a recent shift in diet for yellow turtles. Black turtles foraged at multiple trophic levels, while yellow and hawksbill turtles primarily consumed macroalgae.