Movements of Kemp's Ridley (Lepidochelys Kempii) and Green (Chelonia Mydas)
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Movements of Kemp's Ridley (Lepidochelys kempii) and Green (Chelonia mydas) Sea Turtles Using Lavaca Bay and Matagorda Bay 1996 - 1997 By Maurice L. Renaud and Jo A. Williams FINAL REPORT Submitted to the Environmental Protection Agency Office of Planning and Coordination Region 6 Dallas, Texas TABLE OF CONTENTS List of Tables ii List of Figures iii Executive summary v Introduction 1 Methods 2 Study Area 2 Capture and Holding of Sea Turtle 3 Radio Sonic Telemetry 3 Satellite Telemetry 4 Study Period Ranges 5 Submergence and Surface Durations 6 Environmental Data 6 Results 6 Capture and Tagging of Sea Turtles 6 Kemp's Ridley Sea Turtles 7 Short Range Movements 7 Long Range Movements 8 Study Period Ranges 9 Submergence Behavior 10 Surface Behavior 10 Green Sea Turtles 11 Short Range Movements 11 Long Term Movements 11 Study Period Ranges 13 Submergence Behavior 13 Surface Behavior 14 Environmental Data 14 Discussion 15 Sea Turtle Movements 17 Seasonal Occurrence 17 Habitat Preference 19 Abundance 20 Satellite Data 21 Submergence and Surface Durations 22 Conclusion 24 Literature Cited 26 i List of Tables Table 1. Measurements and tracking information for seven Kemp's ridley and four green sea turtles tracked in Matagorda Bay, TX.................................. 31 Table 2. Home range (95% utilization distribution) and core area (percent utilization determined by Home Range Program) to the nearest square kilometer, for of Kemp's ridley and green sea turtles in Lavaca and Matagorda Bays, 28 May through 5 November 1996.................................... 32 Table 3. Submergence and surface durations (in seconds) for all turtles combined, for each species, and for individual sea turtles radio-tracked in 1996........ 33 Table 4. Frequency of submergence durations, by time category, for all turtles combined, for each species and for individual turtles radio-tracked in 1996...................................................................................................................... 34 Table 5. Time spent submerged and at the surface for all turtles combined, for each species, and for individual turtles radio-tracked in 1996...…………….......................... 36 Table 6. Frequency of surface durations by second categories for all turtles combined, for each species and for individual turtles radio-tracked in 1996...................................................................................................................... 37 Table 7. Measurements of air and water temperature, surface water salinity, cloud cover, sea state, wind speed and direction in Matagorda and Lavaca Bays from May 28 to November 5, 1996........................................... 39 ii List of Figures Figure 1. Study area encompassing Lavaca Bay and West Matagorda Bay................ 40 Figure 2. Movements of radio-tracked subadult male Kemp's ridley sea turtle 4191 (33.5 cm SCL) from 28 May - 27 June 1996......................................... 41 Figure 3. Movements of radio-tracked subadult male Kemp's ridley sea turtle 5601 (32.8 cm SCL) from 28 May - 1 August 1996. Home range (95% utilization distribution) is outlined with a dotted line and core area (58% utilization distribution) is outlined with a solid line........................... 42 Figure 4. Movements of radio-tracked subadult female Kemp's ridley sea turtle 5001 (34.3 cm SCL) from 22-28 June 1996.................................................... 43 Figure 5. Movements of radio-tracked subadult male Kemp's ridley sea turtle 4811 (31.5 cm SCL) from 24 July - 2 August 1996....................................... 44 Figure 6. Movements of radio-tracked subadult female Kemp's ridley sea turtle 5801 (43.9 cm SCL) from 30 July - 30 August............................................... 45 Figure 7. Movements of radio-tracked subadult female Kemp's ridley sea turtle 4391 (40.4 cm SCL) from 28 May - 29 September 1996. Home range (95% utilization distribution) is outlined with a dotted line and core area (54% utilization distribution) is outlined with a solid line.................. 46 Figure 8. Home range (95% utilization area) and core area (64% utilization distribution) for all Kemp's ridley sea turtles combined; 28 May to 29 September 1997............................................................................................. 47 Figure 9. Movements of radio-tracked subadult male Green sea turtle 5201 (37.5 cm SCL) from 20 June - 01 August 1996.............................................. 48 Figure 10. Movements of radio-tracked subadult female Green sea turtle 4602 (29.9 cm SCL) from 20 June - 05 November 1996. Home range (95% utilization distribution) is outlined with a dotted line and core area (64% utilization distribution) is outlined with a solid line.................. 49 Figure 11. Movements of satellite-tracked subadult Green sea turtle 7299 (36.3 cm SCL - sex unknown) from 1 October 1996 - 11 June 1997. Southerly movement is shown on map A and northerly movement is depicted on map B............................................................................................ 50 iii Figure 12. Movements of satellite-tracked subadult Green sea turtle 8009 (35.0 cm SCL - sex unknown) from 2 October 1996 - 28 January 1997.................... 51 Figure 13. Home range (95% utilization distribution) and core area (53% utilization distribution) for all radio-tracked green sea turtles combined; 20 June to 2 October 1997................................................................................. 52 Figure 14. Monthly mean values for air and water temperature (Celsius), salinity (parts per thousand), and wave height (ft), in Matagorda and Lavaca Bays, from 28 May through 5 November 1996............................................... 53 Figure 15. Wind direction in Matagorda and Lavaca Bays from 28 May through 5 November 1996, by month. The percent of time the wind was blowing in a given direction for each month is summed by wind direction............................................................................................................... 54 iv EXECUTIVE SUMMARY Understanding the habitat needs of sea turtles is recognized as an essential element in successful recovery of their stocks in the Gulf of Mexico (Thompson et al., 1990). Until recently, little research had been conducted on sea turtle populations in Texas, even though Texas inshore waters provide important habitat for Kemp's ridley, Lepidochelys kempii, green, Chelonia mydas, and loggerhead, Caretta caretta, sea turtles. A preliminary evaluation of the temporal and spatial distribution of Kemp's ridley, green, and loggerhead sea turtles in Lavaca and Matagorda Bays was made 1) to determine if turtles are locally restricted or part of a more extensive group of turtles, 2) to describe their association with specific habitats, (i.e., channels, oyster reefs, seagrass beds, and other areas in the bay) and with geographic areas of the bay, and 3) to address the potential risk of exposure posed to turtles by discharge water from the Formosa Plastics Plant containing potentially toxic substances. Eight sea turtles (6 Kemp's ridleys and 2 greens) equipped with radio and sonic transmitters were released in Matagorda Bay, Texas, and tracked for 3-18 days a month, from May through November 1996. Three additional turtles (1 Kemp's ridley and 2 greens) were fitted with satellite transmitters and released near Indian Point, in northwest Matagorda Bay, Texas. As of 14 May 1997, one green turtle was still being tracked by satellite telemetry. Sizes and weights of turtles ranged from 31.5-58.4 cm SCL and 4.0-21.7 kg for Kemp's ridleys and from 29.9-37.5 cm SCL and 3.3-7.6 kg for greens. Typically, radio-tracked sea turtles remained within 3 km of a 20 km stretch of coast along the west side of Matagorda Bay, from Indian Point in the north toward Port O'Connor in the south. However, one Kemp's ridley did cross Matagorda Bay and enter Carancahua Bay and Tres Palacios Bay. Two green turtles, monitored with satellite telemetry beginning in October 1996, increased the v distance of their daily their movements during November and December, after the passage of cold fronts and subsequent cooling of bay waters. Bay water temperatures decreased from monthly summer means near 30o C to less than 17o C in November and December. The turtles apparently sought warmer water when they moved into the Gulf of Mexico on 18 December 1996 and 1 January 1997, and relocated offshore where temperatures were 13-23o C. Both turtles moved to the south. One was in water depths between 10 and 20 fathoms just south of Corpus Christi Bay by 10 January 1997. On 17 January it was an additional 200 km to the south, in the same depth zone, just below the Texas- Mexico border. Its movement was monitored southward for another 70 km when before signals from its transmitter ceased on 30 January 1997. The second green turtle moved approximately 260 km by 2 February 1997; nearly 40 km south of Port Mansfield in water depths of less than 10 fm. It returned to Matagorda Bay, on 2 April 1997. Since its return to inshore waters in March 1997, it has remained in inshore waters and moved through Espiritu Santo Bay toward Corpus Christi, TX. The last recorded position of this turtle was in Aransas Bay on 11 June 1997. It is still being tracked. We disproved the hypothesis that turtles would use all parts of the bay equally. Data collected during 1996 demonstrated that Kemp's ridleys mostly utilize the shallow areas along the western perimeter of Matagorda and Lavaca Bays. Central