Patricia Zárate
Understanding Population Connectivity of the Green Turtle (Chelonia mydas) in the Galapagos Islands Through Stable Isotopes and Genetics
SUMMARY
The Galápagos Islands, located 1000 km from mainland Ecuador, hosts one of the most important rookeries for green turtles in the East Pacific Ocean (Green 1994, Zárate & Dutton 2002, Seminoff 2007). It is one of the few places in the world where a sea turtle nesting aggregation comprises both resident and migrant turtles (Green 1984b, 2003, Zárate 2007, Seminoff et al. 2008). Galápagos nesting aggregations include individuals from multiple foraging grounds, often separated by hundreds or thousands of kilometers, similar to other green turtle populations (Green 1984b, Harrison & Bjorndal 2006, Seminoff et al. 2008).
The use of stable isotopes, a non-invasive technique, provides insights for determining trophic level, identifying food sources, and assessing migratory behavior of species with wide geographical ranges (Peterson and Fry 1987, Hobson and Clark 1992, Rubenstein and Hobson 2004). Stable isotopes have successfully been used to assess trophic status and foraging ecology of sea turtles species (Jones and Seminoff 2013).
I used stable isotope analysis to evaluate foraging ecology of green turtles in the Galapagos Islands. I collected skin samples from green turtles at four nesting beaches and three foraging grounds. I compared d13C and d15N values of green turtles from foraging grounds to potential prey items to establish dietary composition and trophic level of turtles at foraging and nesting grounds in the Galápagos. I also compared my results of stable isotopes from local foraging grounds and nesting beaches with stable isotope values available in the literature for green turtles in other eastern Pacific foraging areas to determine whether I could identify individual green turtles nesting in the Galápagos as either residents or migrants.
Based on my results, I concluded that green turtles in Galápagos are not exclusively herbivorous, and this result was supported by earlier studies on esophageal contents. Green turtles are known to consume animal matter in other areas, particularly in the eastern Pacific (Jones & Seminoff 2013). I found that the isotopic composition of green turtles overlapped considerably between Galápagos nesting beaches and foraging grounds, as expected. The use of Galápagos foraging grounds by Galápagos nesting females (residents) has been recorded in previous studies (Green 2003, Zárate 2007, Seminoff et al. 2008, Zárate unpubl. data).
I expected that because females nesting in Galápagos use foraging areas within and outside the Galápagos, the isotopic values would reflect the location of the foraging area. However, I found no distinctive groups or clusters within the nesting aggregation that could be interpreted as different foraging strategies. In addition, I compared d13C and d15N values at different locations throughout the eastern Pacific, which revealed extensive overlap of values among regions. Mean d13C and d15N values from Costa Rica (Golfo Dulce and Cocos Is.), Colombia (Gorgona Is.), and Perú (Paracas and oceanic waters) fall within the range of values recorded for Galápagos foraging grounds. Therefore I could not use stable isotope values to distinguish between nesting females using local (residents) and those using foreign foraging grounds (migrants).
This is the first study to report stable isotopes values of d13C and d15N of green turtles at nesting and foraging grounds and their potential prey items in the Galápagos. Little information exists to date regarding the marine isoscape pattern for the East Pacific region. Baseline studies in the region and elsewhere are needed for a more accurate interpretation of stable isotopes in the study of sea turtle ecology.
Although this study has provided valuable knowledge about green turtle isotopic composition in foraging and nesting grounds of the Galápagos Islands, additional studies integrating stable isotope analysis with long term foraging ecology and movement data (satellite telemetry or flipper tagging) are recommended to increase our understanding of turtle foraging ecology, population dynamics, and functional roles.
