Offshore Dispersal and Predation of Sea Turtle Hatchlings I: A Study of Hawksbill Turtles at Chagar Hutang Turtle Sanctuary, Malaysia
Sea turtle hatchlings emerge from underground nests at night, rapidly crawling seaward to swim off shore. Once in the water, hatchlings might experience high predation rates while in shallow water before reaching deeper water where encounters with predators, and consequently mortality rates, likely decline. Behavioral studies have described different swimming strategies used by hatchlings to counter nearshore predation. Coastal and oceanographic conditions are also likely to influence dispersal away from near shore to the open ocean. This study assessed predation rates of Hawksbill Turtle (Eretmochelys imbricata) hatchlings as they dispersed from shore at Chagar Hutang Bay, Malaysia and the role surface currents play in the transport of hatchlings off shore in the nearshore environment. An acoustic doppler current profiler was used to measure surface currents, and direct observations of hatchlings swimming off shore were made from a kayak using GPS loggers to track hatchling swimming paths. Six of the 31 hatchlings tracked (19.4%) were predated, most within 50 m of shore, indicating that predators are more abundant in shallower areas of the bay where a coralline-rocky bottom predominates. Survival tended to be greater under dark conditions when moonlight was absent or minimal. We quantified the relative importance of the tidal current in a hatchling's offshore swim, and found that in most cases, tidal surface currents assisted the offshore movement of Hawksbill hatchlings as they dispersed from the beach. These findings provide a better understanding of how sea turtle hatchling dispersal is affected by predation, moonlight, and physical oceanographic conditions at Chagar Hutang Turtle Sanctuary.
Satellite image of Chagar Hutang Bay and its coral cover. Black circled dots on the beach represent the two different points where turtle hatchlings were released (site 1: coralline-rocky site; and site 2: sandy site). Yellow dots indicate the end point location of Hawksbill Turtle hatchlings that survived 30 minutes of swimming without predation, while red dots indicate the end points of hatchlings that were released from site 1 and predated, and the red triangle indicates the end point of the hatchling released from site 2 that was predated. The white marker indicates the location where the ADCP was deployed.
(A) Dispersal lines of surviving (followed for 30 min), and (B) predated Hawksbill Turtle hatchlings followed by kayaking in Chagar Hutang Bay. Chagar Hutang beach and open ocean are indicated with black arrows. Shoreline and cliffs bounding the bay are represented by thick brown lines. The dotted line shows the end of the bay before reaching the open ocean. The dispersal paths of individual hatchlings are illustrated in different colors. Number of tracked hatchlings: surviving (n = 25) and predated (n = 6).
Proportion of surviving and predated hatchlings followed by kayaking and released from different sites along the shore. The number of surviving hatchlings from the total number of hatchlings released per site was 18/19 (94.7%) at the sandy bottom site and 7/12 (58.3%) at the coralline-rocky bottom site. Error bars represent standard errors.
Time series of water level (top), current direction (middle), and current speed at surface (bottom) recorded by the ADCP in Chagar Hutang Bay. Moon phases are represented on top of the figure. The four vertical bars along the figure indicate the times when offshore swimming trials were conducted.
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