Origin of Loggerhead Turtles Stranded in the Northeastern United States as Determined by Mitochondrial DNA Analysis

Abstract

-Loggerhead turtles (Caretta caretta) inhabit coastal waters of the Atlantic between Virginia and Massachusetts on a seasonal basis, but the nesting populations that rely upon this foraging habitat have not been determined. In this study, we examined mitochondrial DNA control region sequences to determine the origin of 82 stranded loggerheads, comparing their haplotypes to those observed among major nesting colonies of the Atlantic and Mediterranean. Our results indicated that the stranded animals originated from three demographically independent nesting areas: northeast Florida/North Carolina (25%), southern Florida (59%), an Quintana Roo, Mexico (16%). The data provide a demographic link between nesting populations and the Atlantic coast feeding ground aggregate and indicate that strandings in the northeastern United States remove individuals from these three nesting colonies, two of which show evidence of serious decline. al of Herpetology, Vol. 35, No. 4, pp. 63846, 2001 i t 2001 Society for the Study of Amphibians and Reptiles i of Loggerhead Turtles Stranded in the Northeastern United The loggerhead turtle (Caretta caretta) is distributed circumglobally in temperate and subtropical waters. Like most marine turtles, loggerheads undertake a series of migrations and use a variety of habitats at different stages of 2 Corresponding Author. Present address: Department of Biological Sciences, University of Delaware, Newark, Delaware 19716, USA; E-mail: krb@udel.edu their life cycle (Musick and Limpus, 1997). Because there are many difficulties involved in tracking turtles at sea, several aspects of loggerhead life history are unclear. In particular, information on migratory habits and distribution of juvenile and subadult turtles is limited, and our knowledge of movements of adults between n sting and feeding grounds is incomplete. These missing components of life history make life cycle (Musick and Limpus, 1997). Bet ere are many dif iculties involved in i g turtles at sea, several aspects of loggerlife history are unclear. In particular, ination on migratory habits and istribution j e ile and subadult turtles is limited, and o ledge of movements of adults between 638 This content downloaded from 207.46.13.146 on Tue, 23 Aug 2016 05:38:56 UTC All use subject to http://about.jstor.org/terms LOGGERHEAD TURTLE ORIGINS it difficult to define management problems and implement recovery plans in habitats outside of the loggerhead nesting range. One such habitat includes coastal waters of the northeastern United States (a juvenile/subadult feeding ground), where turtles are threatened by incidental capture in fisheries, marine debris, recreational activities, and dredging of harbors or bays (National Research Council, 1990). In 1995 alone, loggerheads accounted for 55% of the marine turtle strandings (N = 664) between Virginia and Massachusetts (W. Teas, NMFS, pers. comm.). It is not clear whether these strandings are removing individuals from the larger nesting colonies in southern Florida or, alternatively, are impacting smaller, endangered nesting colonies such as those found between northeast Florida and North Carolina. Furthermore, it is uncertain whether mortality in northeast U.S. waters impacts nesting populations beyond U.S. borders. To determine the natal origin of foraging populations, marine turtle researchers have used molecular markers to define rookery population structure (Broderick et al., 1994; Bass et al., 1998). The focus of these studies has been mitochondrial DNA (mtDNA), a nonrecombining extranuclear genome inherited maternally. In effect, mtDNA markers observed in nesting populations are employed as tags to identify the origin of turtles found in distant foraging habitats. The use of genetic markers to identify the origin of migratory animals was developed by fishery biologists to assess the stock composition of anadromous fish caught in oceanic fisheries. Pioneering studies examined shifts in genotypic frequencies between riverine spawning populations (as revealed with protein electrophoresis) to document the origins of salmon impacted by a coastal fishery (Grant et al., 1980; Pella and Milner, 1987). These mixed stock analyses were based on a maximum likelihood (ML) algorithm. These same algorithms are applicable to mtDNA data (Xu et al., 1994). Significant haplotype frequency shifts between marine turtle rookeries allow an estimate of relative contributions from source populations via ML statistical analysis (Norman et al., 1994; Bowen, 1995). Encalada et al. (1998) analyzed mtDNA control region sequences to assess the population structure of loggerhead colonies nesting on Atlantic Ocean and Mediterranean Sea beaches. This rookery analysis included 249 samples from turtles nesting on beaches in Georgia, North and South Carolina, three regions of Florida, Mexico (Quintana Roo), Brazil (Bahia), and Greece (Kiparissia Bay). These beaches represent the major nesting sites of loggerheads in FIG. 1. Six demographically independent nesting populations (Caretta caretta) as defined by Encalada et al. (1998). These nesting populations were considered as possible sources of loggerheads in northeastern U.S. waters (NE US). Study site locations in NE US are listed in Table 1. the Atlantic Ocean and Mediterranean Sea. Based on a 391 base pair (bp) portion of the control region, 10 different haplotypes (designated by letter codes A-J) were identified among these nesting aggregations. Comparing haplotype frequencies between nesting aggregates, Encalada et al. (1998) distinguished six demographically independent nesting units: (1) northeast Florida to North Carolina; (2) south Florida; (3) northwest Florida; (4) Quintana Roo, Mexico; (5) Bahia, Brazil; (6) Kiparissia Bay, Greece (Fig. 1). The survey by Encalada et al. (1998) provides an appropriate dataset to estimate the origin of loggerheads that inhabit feeding grounds in northeastern U.S. waters. Management of sea turtle foraging habitats has been hampered by limited information on marine turtle natural history. In this study, we estimated the origins of loggerhead sea turtles in a foraging habitat by analyzing mtDNA haplotypes of 82 animals stranded between Virginia and Massachusetts. Determining the sources of turtles in northeastern U.S. waters is an important conservation priority given increased human incursions in the region (National Research Council, 1990; National Marine Fisheries Service and U.S. Fish and Wildlife Service, 1991). Wildlife managers are aware of separate nesting and foraging locations (which may be hundreds or thousands of kilometers apart), but specific migratory links between these critical habitats remain unclear. It is not known whether specific nesting turtles display fidelity to particular foraging habitats or whether feeding populations are composed of migrants from nearby nesting colonies. In part because of limited information on turtles in foraging habitats, conservation efforts have focused on protecting adult females and hatchlings at nesting sites. Although this is 639 This content downloaded from 207.46.13.146 on Tue, 23 Aug 2016 05:38:56 UTC All use subject to http://about.jstor.org/terms K. RANKIN-BARANSKY ET AL. TABLE 1. Locations of stranded loggerhead turtles (Caretta caretta) sampled in this study (N = 82). All samples were collected by Sea Turtle Stranding and Salvage Network members and forwarded to our lab. Sample location Number of samples Virginia Beach, VA 1 Eastville, VA 1 Anne Arundel County, MD 3 Calvert County, MD 1 Assateague Island, MD 1 Sussex County, DE 1 Atlantic County, NJ 1 Cape May County, NJ 5 Monmouth County, NJ 1 Ocean County, NJ 1 Nassau County, NY 1 Suffolk County, NY 14 Barnstable County, MA 51