Size-class partitioning and herding in a foraging group of green turtles Chelonia mydas

Abstract

To collect data on green turtles Chelonia mydas near the Marquesas Keys, Florida, USA, we conducted haphazard, unmarked, nonlinear transect (HUNT) surveys from a moving vessel. During HUNTs, we recorded green turtle locations and made opportunistic captures. We found a unique foraging assemblage of subadult and adult green turtles in open-water seagrass habitat (3 to 5 m deep) at the eastern Quicksands, west of the Marquesas Keys. At an adjacent area in the Marquesas Keys (Mooney Harbor), we observed juvenile green turtles foraging in shallow seagrass habitat (< 2 m). During 267 km of HUNTs, 370 green turtles (153 adults, 216 subadults, 1 juvenile) were recorded from the eastern Quicksands. At the Mooney Harbor site, 190 juvenile green turtles were sighted during 309 km of transects. Green turtles captured at the eastern Quicksands were adult and subadult animals that ranged from 69.3 to 108.5 cm straight carapace length (SCL; mean ± SD = 88.4 ± 10.6 cm, n = 31). Green turtles captured in Mooney Harbor were juveniles ranging from 27.0 to 59.3 cm SCL (mean = 44.0 ± 7.8, n = 41). Six repeatable, linear transects were surveyed during 3 sampling events at the eastern Quicksands. During these transects, 238 green turtles were observed. These spatial data were used in a nearest-neighbor analysis, which indicated that the distribution of green turtles at the eastern Quicksands was non-random and clumped. We hypothesize that adult and large subadult green turtles use deeper water habitats than juveniles, and this size-class partitioning may be due to differing habitat requirements and predation risk. Our analyses indicate that green turtles found at the eastern Quicksands form foraging herds.

Ontogenetic habitat shifts of green turtles (Chelonia mydas) suggested by the size modality in foraging aggregations along the coasts of the western Japanese main islands

“…The size distributions of the three FG, however, clearly demonstrated a characteristic bimodality with a low frequency of turtles of 50-70 cm SCL, similar to reports from FG in Shoalwater Bay in eastern Australia (Limpus et al, 2005), and in eastern Taiwan (Cheng and Chen, 1997). Bresette et al (2010) also reported a bimodal size distribution of pooled green turtles from Mooney Harbor and the eastern Quicksands, west of Key West, Florida, USA, although the bimodality was not noted in their study. Interestingly, the peak sizes of the present size distributions were consistent among all three FG.…”

“…In several FG, green turtles were known to demonstrate size-partitioning of habitats as a result juveniles inhabiting shallower waters, and larger turtles inhabiting deeper water (Balazs, 1980;Bresette et al, 2010;Koch et al, 2007;Limpus et al, 2005;López-Mendilaharsu et al, 2005). Such size-partitioning of habitats has been explained by the minimization of predation risk.…”

The Ecology and Migrations of Sea Turtles 8. Tests of the Developmental Habitat Hypothesis

“…One Bermuda green turtle was recaptured in St. Lucie County, Florida, fitted with a satellite transmitter and subsequently tracked to foraging grounds west of the Marquesas Islands (50 km W of Key West) where it remained for at least eight months (D. Bagley, personal commun.). This area west of the Marquesas has recently been shown to serve as adult foraging habitat for C. mydas (Bresette et al, 2010).…”

Habitat selection by green turtles in a spatially heterogeneous benthic landscape in Dry Tortugas National Park, Florida

“…Once we brought a turtle aboard the boat, we took standard measurements, including curved (CCL) and straight (SCL) carapace lengths from the anterior notch to the last posterior marginal scute, and took photographs to document carapace and skin ano malies. Based on external assessment of animal size and tail size, we categorized them into subadult-sized (65−90 cm SCL; Bresette et al 2010) and adult-sized (> 90 cm SCL) females or males; male turtles of which we took tail measurements (n = 5) had cloaca-tip lengths ≥ 5.5 cm. We tagged each animal with a passive integrated transponder (PIT) in the right shoulder and affixed an individually numbered flipper tag to each front flipper.…”