Polymodal foraging in adult female loggerheads (Caretta caretta)

To determine whether loggerhead turtles (Caretta caretta) nesting in southeastern USA exhibit polymorphic foraging strategies, we evaluated skin samples for stable isotopes of carbon (dC) and nitrogen (dN) from 310 loggerheads from four locations on the east coast of Florida and epibionts from 48 loggerheads. We found a dichotomy between a depleted dC cluster and an enriched dC cluster. Epibionts from oceanic/pelagic or neritic/benthic habitats were largely consistent with this dichotomy. The bimodal distribution of dC could reflect a bimodal foraging strategy or—because of the potential for confounding among four gradients of dC in marine environments—a polymodal foraging strategy. We integrate our results with results from other stable isotope studies, satellite telemetry, and flipper tags to evaluate potential foraging strategies. Understanding foraging strategies is essential for development of management plans for this endangered species that has suffered a 43% population decline over the last decade. Introduction Loggerhead hatchlings (Caretta caretta) emerge from nests on Atlantic coast beaches of the southeastern USA from Florida to North Carolina and enter the North Atlantic. They swim away from shore, are incorporated into offshore currents, and are carried to oceanic foraging areas (Bolten 2003) where they feed primarily on sea jellies and other invertebrates (Bjorndal 1997; Witherington 2002; Frick et al. 2009). Juvenile loggerheads recruit to neritic foraging areas at sizes between 46 and 64 cm curved carapace length, after about 7–12 years in oceanic habitats (Bjorndal et al. 2000, 2003). In neritic habitats, loggerheads shift to a diet primarily composed of hard-shelled, benthic invertebrates (Bjorndal 1997; Seney and Musick 2007). The original hypothesis was that this shift from the oceanic to the neritic environment is a unidirectional ontogenetic niche shift (Carr 1986). However, anecdotal reports (e.g., Eckert and Martins 1989) began to accumulate indicating that some individuals in neritic habitats may return to oceanic habitats and some may never leave oceanic habitats except to reproduce. Thus, the life history model developed by Bolten (2003) included these possibilities as speculative connections. In a study of large juvenile loggerheads captured in estuaries of North Carolina, USA, satellite telemetry revealed that 10 loggerheads moved off the continental shelf and into oceanic habitats to forage, while 13 remained in neritic habitats (McClellan and Read 2007). Hatase et al. (2002) were the first to confirm that some adult loggerheads use oceanic habitats between nesting seasons. Using stable isotopes (n = 149) and satellite telemetry (n = 5), they discovered nesting loggerheads from two different nesting beaches in Japan had been foraging in either oceanic or neritic waters. Through the use of satellite telemetry, Hawkes et al. (2006) documented Communicated by R. Lewison. K. J. Reich (&) K. A. Bjorndal A. B. Bolten Archie Carr Center for Sea Turtle Research and Department of Zoology, University of Florida, Box 118525, Gainesville, FL 32611, USA e-mail: [email protected]; [email protected] M. G. Frick Caretta Research Project, Box 9841, Savannah, GA 31412, USA B. E. Witherington Florida Fish and Wildlife Conservation Commission, 9700 South A1A, Melbourne Beach, FL 32951, USA C. Johnson Loggerhead Marinelife Center, 14200 US Hwy 1, Juno Beach, Fl 33408, USA 123 Mar Biol (2010) 157:113–121 DOI 10.1007/s00227-009-1300-4