Grant: 10-031R
Project Title: Population Structure and Genetic Origin of Hawksbill Turtles in the Key West National Wildlife Refuge (Year 2)
Project Manager: Michael Bresette
Organization: Inwater Research Group (Non-Profit Organization)
Grant Amount: $14,233.00
Completion Date: 2012-05-15

Summary: In the course of ongoing work in the Key West National Wildlife Refuge, Inwater Research Group, Inc. has identified areas within the refuge where hawksbill turtles can be reliably captured and studied. We propose a quantified effort at index locations within the refuge to observe and capture at least 20 hawksbills, collect morphometric data, and sample blood and tissue for sex ratio and genetics. These data will be used in conjunction with an existing long-term data set to test hypotheses on trends in abundance, trends in sex ratios, size-class distribution and natal beach origin. In addition, we will use recapture data for analyses of site fidelity and somatic growth rates. This work is intended to be complementary to work funded by the Sea Turtle License Grant Program on hawksbill turtles found in foraging habitats offshore of Palm Beach County. Flipper and PIT tagging of captured hawksbills may provide information on movements between the refuge and Palm Beach County

Results: This study was designed to be complementary to an ongoing research effort by Larry Wood and colleagues in the offshore waters of Palm Beach County (PBC). That study has provided data on hawksbill sea turtle size classes, growth rates, and genetic origin of an assemblage of hawksbills residing on the reef tract offshore of Palm Beach County and in surrounding waters. A review of the data from that project summarized on the website and in publications resulting from that project reveals both similarities and differences with the KWNWR work.

The PBC work has morphometric data from 138 individuals and shows a size class distribution with a mean of 56.6cm SCL and a peak in the 51-60cm SCL size class. There are no observations of turtles with an SCL of less than 40cm (L.Woods, unpublished data). In contrast, the KWNWR hawksbill assemblage shows a mean SCL of 41.4cm and a peak in the size class distribution in the 40-45cm size class. It is reasonable to hypothesize that many pelagic phase hawksbills from the Caribbean Basin and Central America entrained in the Florida Loop Current first encounter suitable developmental habitat in the shallow reef environments of the Florida Keys. It is possible that individuals from the KWNWR assemblage may migrate to the PBC study area reefs as they attain a larger size, but there have not been any tag returns to date to demonstrate such movements.

One attribute noted in both the PBC and KWNWR studies is a remarkable degree of site fidelity. This trait allows for a greater likelihood of recapturing the same individual, allowing for calculations of somatic growth rates. Growth rate data from the PBC study, based on 29 recaptures, depicted a mean growth rate of 2.6 cm/yr. As is typical, growth rates were higher for smaller size classes, with a growth rate for the 40-45cm SCL size class (the smallest in the PBC study) calculated at 4.2cm/yr (L. Woods, unpublished data). Growth rate data from the KWNWR study is based on 15 individuals. The mean growth rate for the KWNWR hawksbills (which ranged from approximately 29-65 cm SCL) was 7.76 cm/yr. The growth rate for the subadult hawksbills in KWNWR (those individuals with a SSCL over 50cm) was 3.66cm/year. Although that growth rate is based on only 4 individuals, since we encountered relatively few larger hawksbills in KWNWR, the growth rate for that larger subset appears comparable with the PBC hawksbills of similar size.

The PBC study has analyzed 120 samples for genetic origin. Seventeen distinct haplotypes were noted. Results were generally similar to the KWNWR results based on 48 individuals reported here, with natal beaches in both the Caribbean Basin and Central America noted. In contrast to the KWNWR results, there were no contributions from Cuban natal beaches in the PBC data set, and the contribution from Mexican natal beaches is somewhat greater at approximately 80% (L. Wood, unpublished data).

Sex ratio analysis through serum testosterone radioimmunoassay for the PBC hawksbill assemblage showed a sex ratio biased towards females, with a sex ratio of 2.5 females:1 male (Blanvillian 2007), in contrast to the somewhat more strongly female biased sex ratio in KWNWR of 3.3 females: 1 male.

The KWNWR contains a wide variety of habitat types, most of which were apparently utilized by resident hawksbills. While hawksbill turtles are generally thought of as being closely associated with coral reefs and other tropical and subtropical hardbottom habitats, we found a surprising number (47% of observations) to be associated with seagrass dominated habitat types in the KWNWR. Juvenile and subadult hawksbill turtles are highly spongivorous in diet (Leon and Bjorndal 2002, van Dam and Diez, 1997) and we did note that the majority of the seagrass dominated habitats we encountered in the KWNWR had numerous small hardbottom outcrops with associated sponges. We also noticed a tendency of our observations to occur near the boundaries of two habitat types, suggesting that hawksbills in the refuge may use resources from a variety of habitat types, and tend to concentrate at ecotones where those resources may be in close proximity to each other.

In conclusion, it appears that the PBC and KWNWR hawksbill turtle assemblages represent different subsets of the Florida hawksbill turtle population. Additional results from both areas, and possibly other sites as well, will be useful in generating a complete understanding of hawksbill turtles in Florida waters. Of particular interest would be a study of hawksbills inhabiting the deeper offshore reef tract in the Keys.