In cooperation with the U.S. Fish and Wildlife Service
Occurrence of the Diamondback Terrapin (Malaclemys terrapin littoralis) at South Deer Island in Galveston Bay, Texas, April 2001–May 2002
By Jennifer L. Hogan
U.S. Geological Survey
Open-File Report 03–022
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Contents
Abstract
Introduction
Purpose and Scope
Description of Study Area
Previous Studies
Acknowledgments
Field Surveys and Data Collected
Occurrence Surveys
Nesting and Basking Surveys
Biometric Data
Habitat Data
Occurrence of Terrapins
Terrapins Captured
Terrapin Nesting
Terrapin Characteristics
Terrapin Habitat
Suggestions for Further Study
Summary
References Cited
Figures
| 1. | Map showing location of South Deer Island in Galveston Bay, Texas | |
| 2. | Digital orthophoto showing South Deer Island and locations of field surveys, April 2001–May 2002 | |
| 3. | Photograph showing modified crab trap with chimney | |
| 4–5. | Graphs showing: | |
4. |
Catch per unit effort (CPUE) of Diamondback terrapins and by-catch at South Deer Island, Texas, April 2001–May 2002 | |
5. |
Correlation of monthly catch per unit effort of (a) terrapins and crabs and (b) terrapins and fish at South Deer Island, Texas, April 2001–May 2002 | |
| 6–7. | Photographs showing: | |
| 6. | Female Diamondback terrapin covering her nest site on South Deer Island, Texas | |
| 7. | Diamondback terrapin egg from a nest on South Deer Island, Texas | |
| 8. | Graph showing female and male straight-line carapace lengths of Diamondback terrapins captured at South Deer Island, Texas, April 2001–May 2002 | |
| 9. | Boxplots showing size data from Diamondback terrapins captured at South Deer Island, Texas, April 2001–May 2002 | |
| 10. | Graph showing correlation of straight-line carapace length with mass of Diamondback terrapins captured at South Deer Island, Texas, April 2001–May 2002 | |
| 11. | Boxplots showing condition factor of female and male Diamondback terrapins captured at South Deer Island, Texas, April 2001–May 2002 | |
| 12–14. | Photographs showing variation in: | |
| 12. | Color and markings among Diamondback terrapins captured at South Deer Island, Texas, April 2001–May 2002 | |
| 13. | Carapace color among Diamondback terrapins captured at South Deer Island, Texas, April 2001–May 2002 | |
| 14. | Plastron color among Diamondback terrapins captured at South Deer Island, Texas, April 2001–May 2002 | |
| 15–17. | Graphs showing: | |
| 15. | Frequency of occurrence of female and male Diamondback terrapins with lesions captured at South Deer Island, Texas, April 2001–May 2002 | |
| 16. | Frequency of occurrence of female and male Diamondback terrapins with barnacles and algae captured at South Deer Island, Texas, April 2001–May 2002 | |
| 17. | Maximum water temperature recorded manually at South Deer Island and recorded by National Ocean Service data logger in Galveston Channel during terrapin collection periods, July 2001–May 2002 | |
|
18. |
Historical photographs (a, b) and digital orthophoto (c) of South Deer Island, Texas, from 1930, 1958, and 1995 | |
Tables
| 1. | Summary of terrapin field surveys, South Deer Island, Texas, April 2001–May 2002 |
| 2. | Summary of terrapin capture and biometric data, South Deer Island, Texas, April 2001–May 2002 |
| 3. | Maximums of field parameters recorded during periods of terrapin collection, South Deer Island, Texas, April 2001–May 2002 |
| 4. | Vegetation species or substrate with at least 20 percent relative frequency along six transects, South Deer Island, Texas, April 2001–May 2002 |
Abstract
One-hundred thirty-five terrapins were captured (116 individuals, 19 recaptures) in occurrence and nesting and basking surveys at South Deer Island in Galveston Bay, Texas, during April 2001–May 2002. Along the southern shore of the island, terrapins were captured in crab traps placed in shallow water offshore; in the lagoon on the island, terrapins were captured by hand while walking along the shore. In the nesting and basking surveys, areas of the island with continuous patches of exposed shell typically were checked twice a day for nesting and basking terrapins, except during the last 2 months of the study, when more intensive monitoring was conducted. The occurrence surveys resulted in 119 terrapins captured in crab traps and nine captured by hand around the lagoon. The nesting and basking surveys yielded seven terrapins. Only one was nesting, although the nest is believed to be the first documented terrapin nest found in Texas. Three terrapins were captured swimming near shell beaches, and three more were captured basking on shell beaches. The most terrapins were captured during April–May 2001 and April–May 2002. Biometric data collected on terrapins captured showed size dimorphism on the basis of sex. The median female straight-line carapace length was 17.8 centimeters, 1.4 times larger than the median male carapace length, 13.1 centimeters. The median female mass was 1,021 grams, 2.9 times larger than the median male mass, 354 grams. Intra-specific variation occurred in the colorations and markings of terrapins captured. Anomalies—lesions, deformities, barnacles, and algae—were common among terrapins captured. Both lesions and deformities were more common among males than females. Dominant vegetation or substrate associated with terrapin habitat included saltwort, slender seapurslane, seabeach orache, shell, and woody debris.
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