A better understanding about the ability of hatchlings to escape nearshore predation and successfully disperse to offshore habitats (e.g., nursery areas at sea) is important for long-term sea turtle population management at both the regional and global scales. Although various monitoring approaches (e.g., counts of individual females, nests, and emerged hatchlings) are utilized to estimate hatchling output from nesting beaches (e.g., Broderick and Godley 1996; Bjorndal et al. 1999; Margaritoulis 2005), little is known about hatchling mortality and dispersal thereafter (Witherington and Salmon 1992; Gyuris 1994, 2000; Pilcher et al. 2000; Stewart and Wyneken 2004; Lorne and Salmon 2007; Whelan and Wyneken 2007; Harewood and Horrocks 2008; Duran and Dunbar 2015; Reising et al. 2015). In addition, rookeries that are fringed by reefs or adjacent to man-made structures (e.g., docks, jetties, piers, wharves) appear to have higher rates of predation (30%–88%, Gyuris 1994, 2000; Pilcher et al. 2000; Reising et al. 2015; Wilson et al. 2018) than other beaches that are not fringed by reefs (0%–7%, Witherington and Salmon 1992; Whelan and Wyneken 2007; Harewood and Horrocks 2008; Duran and Dunbar 2015).

From a population management perspective, information on nearshore hatchling predation rates is important because 1) a more accurate estimate of hatchlings entering into the population can be made and used in population modeling (Crouse et al. 1987; Mazaris et al. 2005), and 2) if predation in nearshore waters is found to be very high, rookery managers might be better off concentrating resources and effort into releasing hatchlings offshore after they have completed a beach crawl, rather than increasing the number of hatchlings emerging from nests onto the beach.

Three methodologies have been used to explore dispersal of sea turtle hatchlings as they move offshore. The first involves following hatchlings at close range by snorkeling (Abe et al. 2000; Hasbún 2002), the second involves attaching line tethers to hatchlings and then following the tether from a watercraft (Witherington and Salmon 1992; Gyuris 1994, 2000; Pilcher et al. 2000; Stewart and Wyneken 2004; Lorne and Salmon 2007; Whelan and Wyneken 2007; Gearheart et al. 2011; Duran and Dunbar 2015; Reising et al. 2015; Hoover et al. 2017), and the third involves attaching miniature ultrasonic “pingers” directly to hatchlings and following their movement though a network of offshore listening stations (Thums et al. 2013, 2016; Hoover et al. 2017; Wilson et al. 2018).

Studies using the snorkeling method are not useful for predation studies because the snorkeler probably deters predators from attacking swimming turtles. The most popular method for studies investigating sea turtle predation immediately after they enter the water is the float-towing method, and it is assumed that towing a float does not affect a hatchling's swimming behavior, but slows its swimming speed (Stewart and Wyneken 2004). It has been reported previously that the slower the swimming speed of a hatchling, the longer it will spend in the predator-dense nearshore water and thus increase its chance of predation (Gyuris 1994). This method is relatively simple, not expensive, and can be used to follow hatchlings far out to sea, and thus can be easily employed in all regions of the world. The miniature tag method, although probably being the least invasive, is relatively expensive and therefore effectively unavailable to be used at many sea turtle rookeries owing to financial constraints.

In this report, we investigate how tethering green (Chelonia mydas) and hawksbill (Eretmochelys imbricata) turtle hatchlings influences their offshore dispersal so that this can be taken into consideration when interpreting data on predation of hatchlings as they swim offshore while towing floats, as hatchlings towing floats may swim slower than float-free hatchlings and thus experience greater rates of predation.

Methods. — This study was conducted at the Chagar Hutang beach, Redang Island, Malaysia (lat 5°48′47″N, long 103°00′30″E) over a 1-wk period during July 2018. All swimming trials were conducted in daylight between 10: 00 AM and 12: 00 noon when the tide was almost full and tidal currents were minimal using sea turtle hatchlings that hatched the previous night and were held in darkened containers for 12–15 hrs before trials commenced. There was no wind and wave height was less than 20 cm during swimming trials.

Hatchlings had a 3-m monofilament nylon line (2-kg breaking strain) tether glued to their carapace with superglue (cyanoacrylate); the other end was attached to a streamlined commercially available LEDfishing float (29 cm in length, 3 g in air; Fig. 1). These floats were chosen because they were easily available, inexpensive to purchase, reliable, and easily seen at night. A tether length of 3 m was chosen to ensure that the float would not attract or deter predators in subsequent trials assessing predation (Gyuris 1994). Once the tether was attached, the hatchling was released on the beach 3 m from the water's edge and it crawled to the water where it began its offshore swim. A snorkeler followed 1–2 m behind the hatchling continuously recording its movement with a video camera (Go Pro Hero 5, Go Pro Inc, San Mateo, CA) held in front by an outstretched hand. Using this technique, it was possible to keep the hatchling continuously in camera view, and because hatchlings almost always swam close to the surface, the surface was in view and the depth the hatchling swam below the surface easily estimated by comparing the hatchling's carapace length to the distance between the swimming hatchling and the water's surface. A kayaker followed 2–3 m behind the snorkeler and the position of the kayak was recorded every 10 sec using a global positioning system (GPS) logger (I-gotU GT-120, Mobileaction, Taiwan). The presence of the snorkeler did not appear to affect green turtle hatchling swimming behavior, but did initially affect hawksbill turtle hatchling swimming behavior (see “Discussion” for details). After 10 min of swimming while towing the float, the hatchling was caught and the tether removed; the hatchling was followed for another 10 min while swimming float-free.

View Figure

• Download as Powerpoint
• Download Figure

Swimming speed was calculated for the last 5 min of towing the float, and the first 5 min of float-free swimming using the GPS time/position data. The accuracy of this method for calculating swim speed was verified by walking a 200 m path and comparing the speed calculated from the GPS data with that from walking the distance being timed with a stopwatch. The 2 methods were within 0.006 m/sec of each other. The video for these swimming periods was replayed at half-speed and the proportion of time spent within 10 cm of the water surface, the proportion of time spent power-stroking (synchronous up and down movement of front flippers), and the power-stroking rate were calculated for green turtle hatchlings.

Only one hawksbill turtle exhibited power-stroking behavior and this was for a very short time. The dominant swimming behavior of hawksbill hatchlings was synchronous backward and forward kicking by the back flippers, with the front flippers tucked on the back of the carapace. Hawksbill hatchlings also swam near the surface, with their carapace skimming the surface. For this reason, hawksbill hatchling swimming behavior was quantified by swimming speed, the proportion of time spent at the water surface, the proportion of time spent back-kicking, and the back-kicking rate.

Swimming behavior attributes while towing a float and float-free were analyzed using paired Student t-tests, using the individual hatchling as the experimental unit. Proportional data were arcsine transformed before analysis. Results are presented as mean ± SE and statistical significance assumed if p < 0.05.

Results. — Trials for 10 green turtle hatchlings from 3 different clutches (mass = 20.8 ± 0.9 g, straight carapace length [SCL] = 46.2 ± 1.1 mm, straight carapace width [SCW] = 35.0 ± 0.9 mm) and 4 hawksbill turtle hatchlings from a single clutch (mass = 12.3 ± 0.4 g, SCL = 33.0 ± 0.6 mm, SCW = 26.6 ± 0.3 mm) were conducted. Towing a float did not affect swimming behavior in green or hawksbill hatchlings (Table 1). In green turtle hatchlings, the proportion of time spent within 10 cm of the surface, proportion of time spent power-stroking, and the power-stroking rate were the same with and without towing a float (Table 1). In hawksbill hatchlings, the proportion of time spent at the surface, proportion of time spent back-kicking, and the back-kicking rate were the same with and without towing a float (Table 1). However, the swim speed of hatchlings was significantly affected by towing a float; green turtle hatchlings towing a float swam at 80% the speed of float-free hatchlings, and hawksbill hatchlings towing a float swam at 50% the speed of float-free hatchlings (Table 1).

Table 1. Swimming parameters of green and hawksbill hatchlings swimming offshore from the beach while towing a float and without towing a float. Power analysis of swimming speed, setting power (1 – β) to 0.90, and type I error (α) to 0.05, and using the means and standard deviations calculated for float-free and float-towing swimming, indicated a sample size of 9 was needed for green turtle hatchlings, and 2 for hawksbill hatchlings to indicate a significant difference.

View Fullscreen

Discussion. — This is the first study to quantify swimming speed and behavior of newly hatched sea turtle hatchlings swimming offshore into the open ocean. A previous study found that small ultrasonic transmitters attached by harness or direct attachment to the carapace via Velcro to 8-wk-old captive green turtle hatchlings swimming in a pool could influence stroke rate and swimming speed but not swimming depth (Hoover et al. 2017). Importantly for studies of offshore swimming and predation of sea turtle hatchlings, in the present study towing a float did not affect swimming behavior as reflected by power-stroke or back-kicking rate or where in the water column hatchlings swam, a finding consistent with previous studies (Witherington and Salmon 1992; Stewart and Wyneken 2004; Hoover et al. 2017). Our green turtle hatchlings spent most of their time swimming within 10 cm of the surface as has been previously reported (Abe et al. 2000), and our hawksbill turtles swam just below the surface as previously reported (Hasbún 2002). However, towing a float did significantly reduce a hatchling's swimming speed, perhaps suggesting that a hatchling towing a float may spend more time in shallower predator-rich waters than unencumbered free-swimming hatchlings, potentially increasing their risk of predation. This needs to be considered when interpreting data on predation rate of hatchlings swimming offshore if the float-towing methodology is used as predation rate data collected from hatchlings towing a float may overestimate the predation rate of free-swimming hatchlings.

The reduction in swimming speed while towing a float was greater in hawksbill turtle hatchlings compared with green turtle hatchlings. This is related to differences in size of the hatchlings of these 2 species. The float being towed was the same in both species so it would have created the same amount of added drag when towed through the water. It is the added drag of the towed float that would cause the reduction in swimming speed. However, our green turtle hatchlings were considerably larger than our hawksbill hatchlings and therefore the ratio of float size to body size was much larger in hawksbill hatchlings compared with green turtle hatchlings. Therefore, towing a float would be more difficult for a hawksbill hatchling than a green turtle hatchling, resulting in the greater reduction in swimming speed of hawksbill hatchlings. Hence when considering future studies using floats to follow sea turtle hatchlings swimming offshore, every effort should be made to keep the float as small as possible to minimize the added drag of towing a float, and this is especially true for species with relatively small hatchlings such as hawksbill turtles. We chose the float we used in the current study because it was easily purchase from the Internet, inexpensive, and had high visibility. There could be a trade-off between float size and visibility; smaller floats might be more difficult to follow. Different types of float may be available in different regions of the world, but minimizing float size while still maintaining visibility should be of primary concern when choosing floats for offshore swimming studies involving sea turtle hatchlings. In future studies using the float-towing technique, the potential effect of float-towing on swimming behavior and swim speed should be evaluated for the type of float being used and species of sea turtle hatchling being studied.

Besides the difference in swimming rate and speed between green and hawksbill hatchlings observed in the current study, there was a difference in their reaction to being handled when the float tether was removed from the carapace. Green turtle hatchlings power-stroked continuously while being handled within the water and took off swimming immediately on being released. In contrast, hawksbill hatchlings stopped all movement while being handled, and remained floating at the surface with their front flippers tucked on top of their carapace and back flippers still at their side for between 30 and 60 sec after being released before they started kicking their back flippers. Similar observations have been observed before in these species and have been attributed to differences in antipredator behavior (Mellgren et al. 2003; Chung et al. 2009a, 2009b).