590 



Fishery Bulletin 93(3), 1995 



Id ^^ 



g D oz g Q az g a Q g a Q g a oz g D 

 T4 T5 T6 T7 T8 T9 



Figure 2 



Green turtle, Chelonia mydas, submergence and surface durations 

 by specified time intervals in hours, dawn (0500-0900), day (0900- 

 1700), dusk (1700-2100), and night (2100-0500). 



data were analyzed by dawn, day, dusk, and night 

 (Table 3). The number of surfacings >15 sec was 

 higher at night than at other periods for every turtle 

 tracked at night. The mean surface time was signifi- 

 cantly higher at night than at all other time periods 

 for four of six turtles tracked at night. 



Discussion 



Movements and habitat use 



Habitat use by juvenile green turtles in the Brazos- 

 Santiago Pass was not proportional to available habi- 

 tat. 1,2 The jetty habitat, which contained extensive 

 algal mats, received a disproportionately high 

 amount of use, suggesting that turtles possibly con- 

 gregated there for food. Analysis of stomach contents 



of juvenile green turtles by Landry et al. 3 sup- 

 port the utilization of algal food sources. 

 Turtles in this study were seen feeding on 

 algal growth along the jetties, especially at 

 dusk. The abundance of food may account for 

 the high site fidelity and small core areas. 

 Other studies of similar-size green turtles de- 

 scribed their food source as algae ( Wershoven 

 and Wershoven, 1989; Wershoven and Wer- 

 shoven, 1991), both algae and sea grasses 

 (Burke et al., 1991), and primarily sea 

 grasses (Ogden et al., 1983). Mendonca 

 ( 1983), working with larger turtles (7.8-54.5 

 kg) postulated that green turtles prefer sea 

 grass. Sea grasses were present within 1 km 

 of the jetty habitat of our study area. 4 One 

 turtle from our study entered and remained 

 in a sea grass habitat for over a month; how- 

 ever, no fecal or stomach samples were taken 

 while it was in this habitat. Bjorndal et al. 

 (1991) suggested that green turtles possess 

 gut microflora for digestion of both algae and 

 seagrass and that the relative abundance of 

 microbial species would vary in response to 

 longterm changes in diet. Therefore, turtles 

 could take advantage of a local abundance 

 in either food source. 



All of the turtles in this study had small 

 ranges of movement. A juvenile green turtle 

 tracked at the jetties in 1991 also showed lim- 

 ited movements. 2 The most limited move- 

 ments were at night, suggesting that resting 

 was most common at night. This hypothesis 

 is supported in studies by Bjorndal (1980), 

 Mendonca (1983), and Ogden et al. (1983) 

 who documented resting areas for turtles at 

 night coupled with a shorter range of move- 

 ments compared with activity during the day. 

 Mendonca ( 1983) noted that during summer months 

 the resting sites of green turtles on consecutive nights 

 were within meters of the previous night's rest site. 

 The reason for the westward movements of Tl at 

 night is unknown. The more sheltered jetty habitat 

 adjacent to the cove may have been preferred for rest- 

 ing at night, whereas a broader stretch of the jetty 

 was utilized for daytime foraging activities. Turtles 

 at the jetties apparently have an abundant food 

 source in proximity to resting sites. In contrast, 

 Mendonca (1983) recorded areal ranges of 0.48 to 



4 Quammen, M., and C. Onuf. 1991. Laguna Madre: seagrass 

 changes continue decades after salinity reduction. Rep. 

 to the National Wetlands Center, U.S. Fish Wildl. Serv., Cam- 

 pus Box 39, 6300 Ocean Dr., Corpus Christi, TX 78412, 

 27 p. 



