Ontogeny of the dispersal migration of green turtle (Chelonia mydas) hatchlings

Journal of Experimental Marine Biology and Ecology 379 (2009) 43–50

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Journal of Experimental Marine Biology and Ecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j e m b e

Ontogeny of the dispersal migration of green turtle (Chelonia mydas) hatchlings Junichi Okuyama a,⁎, Osamu Abe b,1,2, Hideaki Nishizawa a,3, Masato Kobayashi c,4, Kenzo Yoseda b,1, Nobuaki Arai a,3 a

Graduate School of Informatics, Kyoto University, Yoshida Honmachi, Sakyo, Kyoto 606-8501, Japan Ishigaki Tropical Station, Seikai National Fisheries Research Institute, Fisheries Research Agency, Fukaiohta 148-446, Ishigaki, Okinawa 907-0451, Japan Yaeyama Station of the Stock Enhancement Technology Development Center, Seikai National Fisheries Research Institute, Fisheries Research Agency, Fukaiohta 148, Ishigaki, Okinawa 907-0451, Japan

b c

a r t i c l e

i n f o

Article history: Received 18 January 2009 Received in revised form 10 August 2009 Accepted 12 August 2009 Keywords: Current drift Frenzy Release program Sea turtles Wave orientation

a b s t r a c t The purpose of this study was to determine the ontogenic change in offshore dispersal movement of sea turtles from the shore during frenzy to the post-frenzy periods. Migration and wave orientation behaviors that affect the dispersal of green turtles (Chelonia mydas) from the shore were investigated at 1, 7, and 28–56 days of age. The effect of ocean currents on dispersal migration was also measured. Our results indicate that migration velocity decreased as turtles aged. Wave orientation behavior was not significantly different between growth stages, but the directional preferences of individuals toward waves tended to decrease with age, producing winding migration trajectories in the older age groups. Ocean currents affect dispersal migrations of turtles in all age groups, although no significant differences among growth stages were observed. These results indicate a significant decrease in migration distance between turtles in the 1-day and 28–56-day-old age groups; additionally, ocean currents influenced the slower-swimming 28–56-day-old turtles for longer periods of time. These observations suggest that turtles reared for some period of time have a decreased probability of experiencing the same migration route as wild frenzied hatchlings. Thus, sea turtle hatchlings should be released as soon as possible to increase their survival and facilitate natural migration. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Migration is one of the most important characteristics of an organism's behavior. The prevalence of migration in a diversity of organisms demonstrates that it has been repeatedly favored by natural selection (Dingle, 1996). Migration involves specialized behavior that is both qualitatively and quantitatively different from other types of movements, e.g., the open water swimming of salmon smolts prior to their journey to the sea (Dingle, 1996). Among the specialized behaviors that occur during migrations, knowing how migrants change their behaviors and responses to circumstances is essential to better understand the history of life and its evolution.

⁎ Corresponding author. Tel.: +81 75 753 3296; fax: +81 75 753 3133. E-mail addresses: [email protected] (J. Okuyama), [email protected] (O. Abe), [email protected] (H. Nishizawa), [email protected] (M. Kobayashi), [email protected] (K. Yoseda), [email protected] (N. Arai). 1 Tel.: +81 980 88 2571; fax: +81 980 88 2573. 2 Present address: Marine Fishery Resources Development and Management Department, Southeast Asian Fisheries Development Center, Taman Perikanan, Chendering 21080 Kuala Terengganu, Malaysia. Tel.: +60 9 617 5940; fax: +60 9 617 5136. 3 Tel.: +81 75 753 3137; fax: +81 75 753 3133. 4 Tel.: +81 980 88 2136; fax: +81 980 88 2138. 0022-0981/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jembe.2009.08.008

Sea turtles are migratory marine organisms, the juveniles of which undertake passive migrations, drifting pelagically in the oceanic gyre system (Musick and Limpus, 1997). Hatchlings emerge from underground nests, crawl down to the ocean, and swim rapidly away from shore (Lohmann et al., 1997). After emergence, hatchlings undergo a frenzy period that is characterized by nearly continuous swimming for the first 24–36 h. The frenzy period facilitates escape from coastal waters and is a specialized response evolved for the displacement of individuals (Salmon and Wyneken, 1987; Wyneken and Salmon, 1992). During the post-frenzy period, hatchling activity gradually decreases; individuals only swim during the day and are inactive at night (Wyneken and Salmon, 1992). Swimming speeds of green turtle (Chelonia mydas) hatchlings have been reported to decrease within as little as 1 h of retention after emergence (Pilcher and Enderby, 2001). Wyneken (2000) documented that hatchlings released from a hatchery hours or days after emergence may change their swimming rates before they have sufficiently distanced themselves from coastal waters. However, these previous studies were conducted in the laboratory; no field studies have examined how post-frenzy sea turtles disperse and behave when released from the shore. A comparison of dispersal movements between frenzy and post-frenzy sea turtles will elucidate the importance and meaning of the “specialized” frenzy behavior. For offshore dispersal, the following two issues are important. One is the migration velocity, as a rapid migration offshore allows

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hatchlings to avoid predators in the littoral zone. The second is migration efficiency. Even if hatchlings were able to swim quickly, indirect offshore migration would result in individuals remaining within coastal areas for longer periods of time. Almost immediately after entering the ocean, green turtle and loggerhead turtle (Caretta caretta) hatchlings establish a course toward the open sea (Frick, 1976; Salmon and Wyneken, 1987). During this period, hatchlings use refracted waves as orientation cues to guide them toward offshore regions (Salmon and Lohmann, 1989; Lohmann and Lohmann, 1992; Lohmann et al., 1995; Lohmann and Lohmann, 1996). Therefore, the orientation behavior of turtles toward waves is considered important for efficient dispersal. When hatchlings disperse offshore, ocean surface currents have considerable effects on the dispersal process (Frick, 1976; Liew and Chan, 1995; Witherington, 1995). Thus, if the degree of drifting caused by ocean currents changes with turtle growth, turtles reared in captivity for a period of time may be unable to reach nursery areas after passive drifting in the open sea. The objective of this study was to clarify the ontogenic change in dispersal movements between sea turtles released from the shore during the frenzy and post-frenzy periods. We investigated migratory and wave orientation behaviors of turtles and the effects of ocean surface currents on individual dispersal. Three experimental groups of

turtles at three different growth stages were employed: frenzy turtles (immediately after emergence from the nest), turtles retained in captivity for 7 days until they were completely past the frenzy period, and turtles reared in captivity for 28–56 days, which were therefore also past the frenzy period and had larger body sizes compared to hatchlings. 2. Materials and methods 2.1. Experimental animals Green turtle eggs laid on beaches were collected between June and August 2006 at Ibaruma Beach in the northeastern region of Ishigaki Island (Fig. 1a). One hundred and fifty-three eggs from a total of nine nests (Nest A–I, Table 1) were collected and relocated to an artificial beach at Yaeyama Station, Seikai National Fisheries Research Institute, Fisheries Agency. More than 90% of the eggs hatched normally. After emergence, the turtles were divided into three experimental groups and reared for approximately half a day, 7 days, or 28–56 days in 18or 24-L containers. Fewer than five turtles under 28 days of age were housed in a single container and turtles over 28 days old were reared separately. Starting on the third day of rearing, turtles were fed once a day. Diets comprised a mixture of anchovies, mysids, and clams, supplemented with vitamins and calcium. The containers and turtles

Fig. 1. (a) Map of Ishigaki Island, Okinawa, Japan. The framed area shows the experimental region. The white circle and the black asterisk represent the sites at Yaeyama Station, the Seikai National Fisheries Research Institute, the Fisheries Research Agency, and Ibaruma Beach, respectively. Gray lines with numbers and dashed lines indicate water depth contours and the reef edge, respectively. Dispersal patterns of turtles in the (b) 1-day, (c) 7-day, and (d) 28–56-day groups. Solid lines show the migration trajectories of turtles. Each letter represents an individual turtle. For turtle A, the partial dashed line indicates the estimated trajectory of the turtle during the period when it was lost. The dashed arrows show the flow of the ocean surface current monitored by GPS-tracked drifters during the experiment.

J. Okuyama et al. / Journal of Experimental Marine Biology and Ecology 379 (2009) 43–50

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were washed and cleaned almost every night. During rearing, the containers were located in a shaded area. Eighteen healthy-looking turtles (6 turtles in each group) were selected from the reared group and used as experimental individuals. Straight carapace lengths (SCL) and body weights (BW) for the turtles in the three groups are summarized in Table 1. The SCL and BW values for the group reared for 28–56 days were significantly larger compared with the other two groups (Kruskal–Wallis test, H = 12.25, p < 0.01 for SCL, H = 12.59, p < 0.01 for BW; Table 1). After the experiments were performed, all turtles were released at Ibaruma Beach.

nearby boats and swimmers during the frenzy period and do not change course to avoid them (Frick, 1976; Salmon and Wyneken, 1987; Salmon and Lohmann, 1989). The post-frenzy 7-day and 28–56-day-old turtles also did not demonstrate apparent avoidance behaviors. Therefore, we considered the tracking performed in the present study to have had little influence on turtle performance. Tracking was primarily based on visual observations; when visual contact was lost, individuals were tracked by radiotelemetry using radio receivers and a four-element Yagi antenna (FT-290mk-II/AR; Yaesu Musen Co. Ltd., Tokyo, Japan). During the experiment, positional data were obtained for the small boat at 3-min intervals using a GPS receiver (eTrex Legend; Garmin Co. Ltd., Southampton, UK). These positions were assumed to represent those of the observed turtles. Based on these data, we calculated the migration velocity, migration distance, and migration direction for individual turtles. Migration velocity and direction were calculated from differences in turtle locations between sampling intervals. Migration distance was the linear distance from the release point to the position attained at the end of the experiment. In addition, the direction in which a turtle was heading and refracted waves were measured almost simultaneously every 3 min using a hand compass during boat tracking. The heading was defined as the turtle's swimming direction. Visual observations were used to assess the condition and behavioral patterns of turtles. In addition, to determine the effects of ocean surface currents on turtle dispersal, the velocity and direction of the surface current was monitored using a GPS-tracked drifter. The design of the GPS-tracked drifters was modified from the devices used by Ishigami et al. (2006) and Watanabe (2006). The drogue of a drifter was designed to catch the ocean current at a depth of 30 cm, since green turtle hatchlings swim at a depth of approximately 20 cm when dispersing to the open sea (Frick, 1976). In each experiment, four GPS-tracked drifters were employed. Drifters were thrown into the sea at 30-min intervals after initiating the experiment. The GPS-tracked drifters recorded their location every 5 min. The velocity and direction of an ocean surface current was calculated from location differences between the first and second sampling, after each GPS-tracked drifter was released. To clarify differences in surface ocean current characteristics with respect to offshore distance, we represented the ocean current data separately as either coral reef or open water areas.

2.2. Tracking experiment

2.3. Data analysis

Tracking experiments were conducted from October 10 to 18, 2006 (Table 1). To facilitate tracking, experimental turtles towed a fishing bobber (in total: length × diameter = 60 × 12 mm2, 3.56 g in air, −1.36 g in water). The fishing bobber contained a tiny radio transmitter (l×d = 19 × 8.2 mm, 1.0 g in water; MBFT-7M; Lotek Co. Ltd., Newmarket, ON, Canada) with a balance weight. The fishing bobber was tethered with 80 cm of nylon line and attached to the ventral surface of a pygal scute with a fishing hook (modified from Witherington, 1995). Before the experiments, the turtles were placed on a sandy beach 5–8 m away from the water's edge (depending on the tide level) in front of Yaeyama Station (24.272oN, 124.125oE) and allowed to crawl down to the water's edge. We defined the time at which the experimental turtles reached the water's edge as the start of the experiment, although some of the turtles that were reared for 28–56 days were initially inactive and did not immediately crawl down to the sea. All experiments were conducted during the day (0800 to 1700) because nighttime experiments pose a risk for boat stranding on shallow coral reefs. Frick (1976) reported that daytime observations are regarded as a valid record of natural sea turtle hatchling behavior. Therefore, we believe that our daytime experiments are acceptable for elucidating the behaviors of individuals in the three experimental groups. Most of the tracking was conducted in a small boat for 3 h. A snorkel was used when tracking in shallower zones that could not be entered by boat. The observer maintained a distance of approximately 5 m behind the turtle to avoid any influence from tracking. Sea turtle hatchlings reportedly ignore

To determine whether ocean surface currents affected the dispersal movements of turtles, the relationships between turtle heading (forward direction), turtle migration velocity and direction, and ocean surface currents were investigated. The heading direction, migration velocity and direction, and ocean current velocity and direction were extracted four times in each experiment from the data obtained from the subsequent sample, after each GPS-tracked drifter was released. Migration and ocean current velocity and direction were divided into two components, one along the turtle's heading and the other along an orthogonal axis (Fig. 2). The velocity of a turtle's sideways drift caused by the ocean current (Ts), the ocean current causing a turtle to drift sideways (Cs), a turtle's backward drift caused by the ocean current (Tb), and the ocean current causing a turtle to drift backward (Cb) were calculated using the following equations:

Table 1 Summary of experimental conditions and turtles in the three experimental groups. Turtle

Rearing period (day(s))

1-day group A 1 B 1 C 1 D 1 E 1 F 1 Mean±S.D. 7-day group G 7 H 7 I 7 J 7 K 7 L 7 Mean± S.D. 28–56-day group M 28 N 29 O 55 P 30 Q 55 R 56 Mean± S.D.

Nest ID

SCL (cm)

BW (g)

Experiment date

Nest G Nest G Nest H Nest H Nest I Nest I

4.5 4.6 4.5 4.4 4.8 4.8 4.6 ± 0.2a

22 23 23 22 23 23 22.7 ± 0.5a

Oct. Oct. Oct. Oct. Oct. Oct.

10 a.m. 10 p.m. 11 a.m. 11 p.m. 15 a.m. 15 a.m.

Nest F Nest F Nest G Nest G Nest H Nest H

4.8 4.9 4.8 4.9 4.6 4.6 4.7 ± 0.1b

21 23 25 23 24 24 23.3 ± 1.4b

Oct. Oct. Oct. Oct. Oct. Oct.

13 a.m. 13 p.m. 17 a.m. 17 p.m. 18 a.m. 18 p.m.

Nest D Nest E Nest C Nest D Nest A Nest B

5.6 5.1 6.7 5.5 6.2 5.8 5.8 ± 0.5ab

34 33 47 30 43 38 37.5± 6.5ab

Oct. Oct. Oct. Oct. Oct. Oct.

12 a.m. 12 p.m. 14 a.m. 14 p.m. 16 a.m. 16 p.m.

SCL, straight carapace length; BW, body weight. Letters (a, b) indicate significant differences (p < 0.05) between the marked groups by the post hoc Scheffe test.

Ts = b sin½α + ð360−βÞ

ð1Þ

Cs = c sin½γ−ð180 + αÞ

ð2Þ

Tb = a−b ̂ cos½α + ð360−βÞ

ð3Þ

Cb = c cos½γ−ð180 + αÞ

ð4Þ

where â, b, and c represent the estimated forward convection velocity, migration velocity, and current velocity, respectively. α indicates the

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3. Results 3.1. Dispersal patterns and swimming behavior

Fig. 2. Schematic diagram showing the relationships between turtle forward swimming, turtle migration, and the ocean current. White arrows represent the components of the migration along the heading and the orthogonal axis. Gray arrows indicate the components of the ocean current along the heading and the orthogonal axis. “â” Represents the estimated forward convection velocity of the turtle; “b” and “c” represent the velocities of turtle migration and the ocean current, respectively. The estimated forward convection velocity corresponded to the velocity obtained for each turtle when the effect of the surface current was zero, as derived from the regression intercept of the forward velocity in relation to the surface current causing the turtle to drift backward. “α,” “β,” and “γ” Represent the directions of the turtle heading, migration, and the ocean current, respectively. Each number corresponds to the equation number presented in the text.

forward direction of the turtle, and β and γ represent the direction of migration and the direction of the current relative to the forward direction (see Fig. 2 for details). To determine whether migration velocities and distances differed among the three age groups, we used one-way analysis of variance (one-way ANOVA) followed by the Tukey–Kramer HSD post hoc test. Mean angle and r values (directional preference) were calculated for the turtle migration, turtle heading, wave approach, and ocean surface current to determine if differences existed among the three age groups using a standard procedure for circular statistics (Batschelet, 1981). The Rayleigh test (Batschelet, 1981) was used to determine whether turtles in each group were significantly oriented or whether waves and ocean surface currents in each group came from a constant direction. Furthermore, angular differences in these data among the three groups were analyzed by the Watson–Williams test (Zar, 1996). We calculated an ri value (individual directional preference) for each turtle as an indicator of the directional variance of migration and heading within the experiment. The Kruskal–Wallis test followed by Scheffe's test was used to determine whether differences in ri values existed among the three age groups. To investigate the wave orientation behavior of turtles, the wave approach direction at each point was normalized to 0º (Avens and Lohmann, 2003). The angular difference between a turtle's heading and the wave approach was assessed relative to 0º because the direction of wave approach and the headings of individual turtles varied at each sampling location. For each age group, the distribution of these angular differences was analyzed using the V-test (Batschelet, 1981) with an expected direction of 0º (the direction in which the turtles would be expected to orient with regard to the refracted wave). We examined the relationships between Ts and Cs, and between Tb and Cb, in each group using a simple least-squares linear regression model. The significance of the regression model was analyzed by one-way ANOVA. In addition, we evaluated differences in the slopes and intercepts of the regression functions among the three groups using analysis of covariance (ANCOVA) with Cs and Cb as covariates (Zar, 1996). All velocity and distance values are presented as means ± standard deviation and all angle values are presented as means ± 95% confidence interval.

All turtles could be tracked during the experiment, except for turtle A, which was temporarily lost for 35 min. No turtles were attacked by predators. After entering the sea, the 1-day group of turtles moved to the north and then changed direction to the northwest at a point around the reef edge (Fig. 1b). The 7-day group exhibited similar, but slightly wider, pathways compared with the 1-day group (Fig. 1c). In contrast, the movements of the 28–56-day group were not uniform (Fig. 1d). In particular, turtle M temporarily drifted on the ocean surface current toward the beach (Fig. 1d). Upon visual observation, all turtles except turtle M continued to swim using power strokes interrupted by dogpaddling during breathing. Turtle M also swam continuously, but its power strokes were intermittent and its duration of breathing was comparatively longer. The flipper strokes of turtles in the 28–56-day group were obviously slower than those of the other groups. Migration velocities were significantly different among the three age groups (one-way ANOVA, F2,15 = 10.32, p < 0.01). The 7-day and 28–56-day groups demonstrated significantly slower swimming speeds than the 1-day group (Table 2). Migration distances from the beach also differed significantly among the three groups (one-way ANOVA, F2,15 = 7.58, p < 0.01). The 28–56-day group dispersed at significantly shorter distances than the 1-day group during the experiments (Table 2). The migration directions of the three groups are summarized in Table 2. Although the turtles in each group were significantly nonuniform (Rayleigh test, 1-day group: r =0.97, Z= 5.70, p< 0.01; 7-day group: r = 0.98, Z = 5.80, p < 0.01; 28–56-day group: r = 0.93, Z = 5.27, p < 0.01), the migration direction of the 28–56-day group was significantly different from those of the other two groups (Watson– Williams test, between the 1-day and 28–56-day groups: F1,10 = 6.70, p < 0.05; between the 7-day and 28–56-day groups: F1,10 = 5.52, p < 0.05). The ri values tended to decrease as turtles aged (Table 2) and differed significantly between the 1-day and 28–56-day groups (Kruskal–Wallis test, H= 6.89, p <0.05), indicating that the migration direction became variable as turtles aged. Turtles in all age groups maintained a nearly northward heading (Table 2) and all of the groups were significantly nonuniform (Rayleigh test, 1-day group: mean= 347º, r = 0.99, Z = 5.94, p < 0.001; 7-day group: mean= 347º, r = 0.99, Z = 5.93, p < 0.001; 28–56-day group: mean= 4º, r = 0.98, Z = 5.73, p < 0.001). The heading direction of the 28–56-day group was significantly different from that of the other two groups (Watson–Williams test, between the 1-day and 28–56-day groups: F1,10 = 7.99, p < 0.05; between the 7-day and 28–56-day groups: F1,10 = 7.08, p < 0.05). The ri values were not significantly different among the three groups (Kruskal–Wallis test, H = 5.34, p = 0.07), but did exhibit a decreasing trend with increasing turtle age (Table 2). In all 18 experiments, the direction of each migration showed little association with the turtle's heading and tended to be more westerly than the heading direction (Table 2). 3.2. Refracted waves and ocean surface currents The data obtained for refracted waves and ocean surface currents for the three groups are summarized in Table 3. During all days of the experiment, refracted waves arrived from approximately the north. For each group, the direction of wave approach was significantly nonuniform (Table 3). No significant differences in approach angles were observed among the three groups (Watson–Williams test, F2,15 = 1.88, p > 0.05). The surface ocean currents tended to flow southwest in all experiments (Fig. 1b–d). The direction of current flow was significantly nonuniform in the coral reef and open water areas for each group (Table 3). For the 1-day and 7-day groups, no significant

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Table 2 Summary of dispersal performance for turtles in three experimental groups. Turtle

1-day group A B C D E F Mean ± S.D. or C.I. 7-day group G H I J K L Mean ± S.D. or C.I. 28–56-day group M N O P Q R Mean ± S.D. or C.I.

Migration velocity (km/h)

Migration distance (km)

Migration direction

Heading direction

Mean angle (degrees)

ri

Mean angle (degrees)

ri

Mean angle (degrees)

Wave orientation ri

0.85 0.92 1.08 1.11 0.92 0.99 0.98 ± 0.09a

2.00 1.90 2.30 2.50 2.10 2.40 2.20 ± 0.22a

314 335 297 318 315 296 312 ± 27c

0.80 0.77 0.72 0.72 0.72 0.76 0.75 ± 0.04d

346 345 351 354 348 335 347 ± 26c

0.96 0.94 0.95 0.94 0.89 0.86 0.92 ± 0.04

325 309 326 331 345 336 329

0.94 0.90 0.93 0.93 0.94 0.85 0.92 ± 0.03

0.89 0.91 0.86 0.77 0.78 0.78 0.83 ± 0.06b

2.10 2.10 1.90 1.90 2.00 1.70 1.94 ± 0.12

322 310 335 307 319 304 316 ± 27

0.70 0.69 0.66 0.73 0.76 0.68 0.70 ± 0.04

355 339 352 345 353 340 347 ± 26

0.94 0.85 0.83 0.92 0.85 0.87 0.87 ± 0.05

359 338 353 341 333 331 342

0.96 0.81 0.82 0.94 0.88 0.89 0.88 ± 0.06

0.84* 0.69 0.62 0.88 0.65 0.74 0.74 ± 0.10ab

0.80 1.00 1.60 2.20 1.40 1.80 1.47 ± 0.46a

8 312 349 333 1 320 340 ± 30c

0.35 0.54 0.76 0.71 0.68 0.70 0.62 ± 0.15d

19 347 3 355 20 359 4 ± 27c

0.66 0.89 0.93 0.88 0.70 0.83 0.82 ± 0.11

8 338 350 356 6 331 351

0.71 0.89 0.91 0.89 0.71 0.79 0.82 ± 0.09

SD and CI indicate the standard deviation and confidence interval, respectively. ⁎ Data obtained during backward drifting of turtles were omitted from the calculated value. The ri value ranged from 0 to 1.0, with 0 representing complete dispersion of the data and 1.0 indicating all data were concentrated in one direction. The wave orientation angle represents the angular difference between the turtle's heading and the wave approach (see text for details). Letters (a, b, c, d) indicate significant differences (p < 0.05) between the marked groups by the (a, b) post hoc Tukey–Kramer HSD test, (c) Watson–Williams test, and (d) post hoc Scheffe test.

tended to decrease with age, it was not significantly different among the three groups (Kruskal–Wallis test, H = 4.71, p = 0.09; Table 2), indicating that the tendency to orient toward refracted waves was not significantly different among the three growth stages.

differences in current directions were observed between the coral reef and open water areas (Watson–Williams test, 1-day group; F1,10 = 0.65, p > 0.05; 7-day group: F1,10 = 0.56, p > 0.05). In addition, no significant differences in current direction were observed among the three age groups in the coral reef area (Watson–Williams test, F2,15 = 0.99, p > 0.05) or between the 1-day and 7-day groups in the open water area (Watson–Williams test, F1,10 = 0.29, p > 0.05). Current velocity was significantly higher in the open water areas than in the coral reef areas during the experiments for the 1-day and 7-day groups (Wilcoxon test, 1-day group: p< 0.05; 7-day group: p < 0.05). Within the coral reef area, current velocity was not significantly different among the three groups (one-way ANOVA, F2,15 =0.27, p = 0.76); observations of open water area velocities also showed no difference between the 1-day and 7-day groups (t-test, df =10, p = 0.37).

3.4. Relationship between ocean surface currents and turtle dispersal movements According to the relational expressions [Eqs. (1)–(4)] that compare a turtle's heading, migration, and the ocean current (Fig. 2), the velocity at which a turtle drifted sideways (Ts) was correlated significantly with the velocity of the ocean current that was causing the turtle to do so (Cs) in each experimental group (Fig. 4a, c, e). Likewise, the velocity with which a turtle drifted backward (Tb) was significantly correlated with the velocity of the ocean current causing the turtle to do so (Cb) in each group (Fig. 4b, d, f). The regression functions of the velocities of sideways drifting with respect to surface currents (Fig. 4a, c, e) were not significantly different among the three groups (ANCOVA, slope F2,66 = 2.81, p > 0.05; intercept F2,68 = 1.39, p > 0.05). Similarly, no

3.3. Wave orientation Turtles in all three groups were significantly oriented in the direction of the approaching wave (Fig. 3). Although the ri value

Table 3 Summary of the data obtained for refracted waves and ocean surface currents during the experiments. Ocean surface current

Refracted wave

Coral reef area

1 day group (r, Z, p values) 7 days group (r, Z, p values) 28–56 days group (r, Z, p values)

Open water area

N

Velocity (km/h)

Direction (degree)

N

Velocity (km/h)

Direction (degree)

N

Direction (degree)

6

0.29 ± 0.08

6

0.47 ± 0.13

0.26 ± 0.09

6

0.39 ± 0.15

2⁎

0.35⁎

225 ± 29 (0.95, 5.46, < 0.01) 219 ± 28 (0.97, 5.64, < 0.01) 218⁎

6

6

241 ± 46 (0.73, 3.21, < 0.05) 209 ± 33 (0.89, 4.76, < 0.01) 214 ± 45 (0.74, 3.32, < 0.05)

18 ± 27 (0.99, 5.94, < 0.001) 5 ± 27 (0.99, 5.93, < 0.001) 12 ± 27 (0.98, 5.73, < 0.001)

6

0.26 ± 0.06

For directional information, statistical values from Rayleigh tests are presented. ⁎ Only two turtles reached the open water area.

6 6

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Fig. 3. Wave orientation of turtles in the three groups. Each dot within the circular diagram represents the mean value of angular differences (in degrees) between the turtle's heading and the wave approach during the experiment for each turtle. The gray arrow represents the mean angular difference in each group. The dashed line shows the 95% confidence interval. (A) The 1-day group: mean angular difference = − 31º, r = 0.98, p < 0.001, V-test, 95% confidence interval ± 26º; (B) 7-day group; mean angular difference = − 18º, r = 0.98, p < 0.0001, V-test, 95% confidence interval ± 27º; (C) 28–56-day group; mean angular difference = − 9º, r = 0.97, p < 0.0001, V-test, 95% confidence interval ± 27º.

significant differences were detected in the regression functions of the velocities of backward drifting with respect to surface currents (Fig. 4b, d, f) among the three groups (ANCOVA, slope F2,66 = 0.40, p > 0.05; intercept F2,68 = 1.98, p > 0.05). These results indicate no differences in the effects of surface currents among the three groups of turtles.

of age would complete their migratory phase and the necessity for rapid swimming would dissipate. The results of this study indicate that postfrenzy turtles (aged 28–56 days) did not exhibit the vigorous and speedy swimming characteristics of frenzied hatchlings during their dispersal from shore.

4. Discussion

4.2. Wave orientation behavior

4.1. Swimming behavior

Our results demonstrate that turtles in all three groups showed significant orientation toward refracted waves (Fig. 3), which is consistent with experiments performed by Salmon and Lohmann (1989) and Lohmann and Lohmann (1992). However, the 28–56-day group exhibited a significantly different heading direction compared with the other two groups (Table 2). These observations appear paradoxical, since the direction of wave approach was not significantly different among the three groups (Table 3). The heading angle of turtles in the 28–56-day group relative to approaching waves was slightly different from those of the other groups (Table 2, Fig. 3). Additionally, individual directional preference (ri value) tended to decrease with turtle age (Table 2), although it was not significantly different among the groups. These two factors might have caused significant differences in heading direction between the 28–56-day group and the other two age groups. The large variability in heading direction and the reduction in swimming velocity are considered to be the main causes of the winding migrations observed in the 28–56-day group. This altered migratory behavior led to significant differences in migration direction and directional preference between the 1-day and 28–56-day groups. Wild turtles aged 28–56 days are believed to have no need of orienting themselves toward the waves because they may no longer be vigorous locomotors, as described above (Carr, 1986). In addition, Witherington (1995) reported that loggerhead hatchlings swimming offshore do not orient themselves toward wind-generated waves. Nevertheless, note that post-frenzy green turtles (aged 28–56 days) maintained a significant orientation response toward refracted waves when released from shore. This observation implies that turtles aged 28–56 days still possessed the instinct to escape from the littoral area where predators are abundant. However, the results of this study also indicate that older turtles demonstrated reduced motivation to orient toward refracted waves. In the present study, refracted waves came from a nearly constant direction, north or northwest, in all experiments. Previous studies demonstrating wave orientation behaviors of hatchling sea turtles experimentally changed the angle at which waves approached the turtles (Salmon and Lohmann, 1989; Lohmann and Lohmann, 1992). Thus, the significant wave orientation behavior observed in this study was comparatively easy to detect statistically.

During the experiments, all of the 1-day-old turtles demonstrated power stroke swimming, except while breathing. This pattern is similar to the findings of previous experiments that evaluated free-swimming green (Wyneken, 1997) and loggerhead turtles (Witherington, 1995). Although these turtles remained in captivity for half a day after emergence, they showed continuous power stroking, not the erratic movements described by Pilcher and Enderby (2001). The turtles moved at a constant velocity of 0.98 ± 0.09 km/h, which was slower than velocities observed for green turtle hatchlings during the frenzy period in other regions (1.56 km/h; Frick, 1976), and even at other beaches on Ishigaki Island (1.62 km/h; Abe et al., 2000). Although formulating a simple comparison between these results is difficult due to differences in environmental characteristics between the experimental regions, possible causes for slower swimming could include fishing buoys, swimming against a current, and wasting energy (Pilcher and Enderby, 2001). However, these effects are considered to have little impact on the swimming behavior comparisons among these groups. Migration velocities in the 7-day and 28–56-day groups were significantly slower compared with the 1-day group (Table 2). The former groups demonstrated power stroking similar to that observed in the 1-day group, except for turtle M. Meanwhile, flipper stroke frequency in the 28–56-day group was obviously slower. Similar decreases in flipper stroke frequency have been observed between hatchlings and post-24-h hatchlings (Wyneken, 1997), as well as between hatchlings and 3-month-old turtles (Nishizawa et al., submitted). Thus, the decrease in stroke frequency might induce a reduction in swimming velocity. Turtle M temporarily drifted toward the beach (Fig. 1d), a behavior that was notably different from the other individuals in the same age group. Green turtles of 4 and 8 weeks of age have been reported to swim and dive by power stroking (Salmon et al., 2004), indicating that the erratic behavior of turtle M might not be a natural behavioral change associated with the life cycle. Green turtle neonates aged 28–56 days are thought to reside in the vicinity of the Sargassum drift lines and to undergo passive migration (Carr, 1987). Therefore, they would no longer be vigorous locomotors and would not exhibit “specialized behaviors” (Salmon and Wyneken, 1987; Dingle, 1996) like frenzied hatchlings. This is because wild turtles of 28–56 days

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Fig. 4. (Left) Relationship between the velocity of turtle sideways drift and the velocity of the ocean current causing the turtle to do so. (Right) Relationship between the velocity of turtle backward drift and the velocity of the ocean current causing the turtle to do so. In all three groups, the velocity of sideways drifting (y) exhibited a significant correlation with the ocean current causing the turtle to drift sideways (x) (1-day group: n = 24, ŷ = 0.81x + 0.39, R2 = 0.27, one-way ANOVA, p < 0.01; 7-day group: n = 24, ŷ = 1.06x + 0.27, R2 = 0.32, p < 0.01; 28–56-day group: n = 23, ŷ = 1.92x + 0.09, R2 = 0.53, p < 0.01). Furthermore, in all three groups, the velocity of backward drifting (y) displayed a significant correlation with the ocean current causing the turtle to drift backward (x) (1-day group: n = 24, y = 0.97x + 0.03, R2 = 0.59, one-way ANOVA, p < 0.01; 7-day group: n = 24, y = 0.78x − 0.01, R2 = 0.61, p < 0.01; 28–56-day group: n = 23, y = 0.84x + 0.01, R2 = 0.61, p < 0.01). See text for statistical comparisons of the regression functions.

4.3. Effect of ocean surface current on turtle dispersal The lack of an association between the direction of migration and a turtle's heading in all groups demonstrated that ocean surface currents affect migration. Current drift by hatchlings was also reported in several previous studies (Frick, 1976; Salmon and Wyneken, 1987; Liew and Chan, 1995; Abe et al., 2000). Significant linear regressions between the drift velocities of turtles and ocean current velocities demonstrated that turtles drifted more in response to stronger ocean currents (Fig. 4). The results of the comparison of the regression functions indicated that ocean currents affected dispersal migrations similarly in all three age groups. This may create a disadvantage for reared hatchlings when they are released into the sea. Prolonged retention in one place increases the time required for dispersal under the same oceanic

currents because of slower and more erratic swimming. Consequently, the accumulation of ocean current effects would cause turtles to use different migratory routes compared to frenzied hatchlings. 4.4. Ontogenic change in dispersal migration Migration distance is derived from the sum of the migration velocity, heading direction, and ocean current drift. Our results demonstrate that the migration distances of turtles significantly decreased as they grew from hatchlings to the age of 28–56 days. All of the parameters were adversely altered as turtles aged, although some of these changes were not significant (Table 2). Thus, the accumulation of changes leads to a significant decrease in migration distance. The decline in migration distance by post-frenzy turtles, and even the significantly larger body sizes of the 28–56-day-old turtles, indicates the important role of the

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“specialized” frenzy behavior for escaping from the dangerous littoral area, and for overall turtle survival, as specialized behaviors must be favored by natural selection (Dingle, 1996). In the present study, each experiment was conducted for only 3 h during the daytime. Post-frenzy green turtles are known to be inactive at night and the proportion of time spent active for 4–6-day-old postfrenzy turtles during the nighttime declined to about one-quarter of that of frenzied turtles (Wyneken and Salmon, 1992). These facts indicate that the difference in migration distance between frenzied hatchlings and post-frenzy hatchlings reared in captivity for a period of time would increase with time, suggesting that turtles reared for such time periods would have a decreased probability of experiencing the same migration route as wild frenzied hatchlings. 4.5. Management implications for turtle releases For sea turtles, hatcheries are among the most commonly employed management tools throughout the world (Wyneken, 2000). In some cases, hatchlings are maintained in captivity for a period of time before being released into the sea because of human activities such as release programs (e.g., Hewavisenthi, 1993; Pilcher and Enderby, 2001) or the environmental conditions experienced in hatcheries. However, the results of the present study indicate that the dispersal distance from shore by hatchlings decreases with age and the possibility of experiencing the original migration route becomes low when release is delayed. Thus, sea turtle hatchlings should not be held or conserved for even a short period of time. However, in cases in which hatchlings must be held, they should be released as soon as possible to increase their chances of survival and of experiencing a natural migration. Releases should be performed at the approximate location in which the individuals would be found within their original migration route based on their age. Acknowledgements We would like to acknowledge the anonymous reviewers and Dr. T. Yasuda for providing helpful comments that have improved the quality of our manuscript. The members of the Ishigaki Sea Turtle Research Group kindly helped in the collection of experimental samples. We also thank the staffs of the Ishigaki Tropical Station and Yaeyama station, Seikai National Fisheries Research Institute for research assistance and constructive comments. We wish to express appreciation to Prof. Y. Michida at Ocean Research Institute, University of Tokyo for providing the GPS mobile phones and for advice regarding the monitoring of ocean surface currents. Dr. K. Watanabe provided technical advice for the experimental methodology. This study was conducted with the permission of the Okinawa Prefecture (no. 18-45) for collecting eggs and breeding hatchlings. This study was partly supported by the JSPS Research Fellowship for Young Scientists (J.O. grant no. 17 1976) and by the Kyoto University

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