Seminoff, JA, WJ Nichols, and A Resendiz. 1997. Diet compositon of the black sea turtle, Chelonia mydas agassizii, in the central Gulf of California, Mexico. Submitted paper, Proceedings of the 17th Annual Symposium on Sea Turtle Biology and Conservation, Orlando, Florida, March 1997. NOAA Technical Memorandum NMFS-SEFSC-415.
DIET COMPOSITION OF THE BLACK SEA TURTLE, CHELONIA MYDAS AGASSIZII, IN THE CENTRAL GULF OF CALIFORNIA, MEXICO
Jeffrey A. Seminoff1, Wallace J. Nichols1, and Antonio Resendiz2
1Wildlife and Fisheries Science, University of Arizona, Tucson, AZ 85721, U.S.A.
2Centro Regional de Investigacion Pesquera, El Sauzal de Rodriguez, Ensenada, BC Mexico.
The Gulf of California is an important area for development and feeding of black sea turtles originating from nesting beaches of more southern portions of the eastern Pacific Ocean (Cliffton et al., 1982; Alvarado and Figueroa, 1992). As these animals move into the Gulf, they enter an enclosed body of water that is considered a dynamic and productive ecosystem (Brusca, 1980; Pacheco and Zertuche, 1996). Supported by seasonal upwelling of nutrient rich waters, coastal areas of this sea host diverse assemblages of fish (Thomson et al., 1979), invertebrates (Brusca, 1980), marine alga (Norris, 1975), and seagrasses (Felger et al., 1980). Though it is clear that the Gulf of California ecosystem provides a wide variety of potential food resources for black sea turtles, the dietary preferences of this species are poorly understood.
Studies of the closely related green sea turtle (Chelonia mydas) have shown that they are primarily herbivorous, feeding on seagrass and/or marine algae (Hirth, 1971; Bjorndal, 1980). Regarding black sea turtles, Felger and Moser (1985) noted that black turtles captured by the Seri Indians regularly had marine algae and sea grass remnants in and around the mouth region. Furthermore, after dissection of the stomachs of many of these turtles it was noticed that all were filled with sea grass and/or marine algae (Felger, pers.comm.). However, due to the limited distribution of seagrasses in the Gulf of California, it is likely that black sea turtles more commonly utilize non-seagrass food resources. Information presented here was collected during studies in the Bahia de Los Angeles region of the Gulf of California, an area at which seagrasses are absent.
This research required the open-water capture of black sea turtles in the Gulf with the use of entanglement nets. This has been proven a safe and effective method for capturing sea turtles (MendonÃ§a and Ehrhart, 1982). Two entanglement nets (100 m x 8 m) were employed and continually monitored during each netting trial to prevent mortality due to drowning. Upon capture of each turtle physical data was collected and diet analysis was performed. Oral examination was used to recover residual food particles for identification. However, the most effective method was lavage, the esophageal flushing of food components (Forbes and Limpus, 1993). Through gentle injection of clean sea water, recently ingested stomach contents were flushed out. These food particles were then preserved and identified.
A total of 40 turtles were captured from the wild using entanglement nets during June - August, 1996. The average straight carapace length for these turtles was 78.4cm with an average weight of 69.5kg (153 lbs). Lavage samples were recovered from 31 captured turtles. Preliminary analysis suggests that the turtles in Bahia de Los Angeles are primarily herbivorous, with marine algae accounting for 92% of the average volume of lavage samples. Red algae of the genus
Gracillaria was most prevalent (69% average lavage sample volume). The following marine algae species were also recovered: Gigartina sp. (9%), Codium sp. (5%), Sargassum sp (3%), Chaetomorpha sp. (3%), and Ulva sp. (3%). In addition to consuming marine algae, turtles also ingested a variety of marine invertebrate organisms including sponges (4%), squid parts (<1%), tube worms (<1%), hydroids (<1%), and small snails (<1%). Lastly, small amounts of substrate particles (sand, shell fragments, small rocks) were recovered in virtually every sample.
This analysis of diet composition is the first step towards understanding the feeding ecology of the black turtle. Future study will include the analysis of local movement and food availability within individual home ranges. When this information is coupled with diet composition, a more thorough understanding of black sea turtle behavior and feeding ecology will be gained. Furthermore, by learning what resources they are most commonly using and where they are moving on a daily basis, we can begin to make educated management decisions regarding this endangered species.
We wish to thank SEMARNAP (MÃ©xico), Earthwatch, PADI Foundation, Lerner-Gray Foundation, and Wallace Genetic Foundation for their financial and logistical support.
Alvarado, J. and A. Figueroa. 1992. Recapturas post-anidatorias de hembras de tortuga marina negra (Chelonia agassizi) marcadas en Michoacan, MÃ©xico. Biotropica 24:560-566.
Bjorndal, K.A. 1980. Nutrition and grazing behavior of the green turtle, Chelonia mydas. Mar. Biol. 56:147-154. Brusca, R.C. 1980. Common intertidal invertebrates of the Gulf of California. University of Arizona Press, Tucson, AZ. 513pp.
Cliffton, K., D.O. Cornejo and R.S. Felger. 1982. Sea turtles of the Pacific coast of MÃ©xico. Pages 199-209 in K. Bjorndal ed. Biology and conservation of sea turtles. Smithsonian Inst. Press, Wash., D.C.
Felger, R.S., M.B. Moser and E.W. Moser. 1980. Seagrasses in Seri Indian Culture. Pages 260-276 in R.C. Phillips and C.P. McRoy, eds. Handbook of seagrass biology, and ecosystem perspective. Garland STPM Press, New York.
Felger, R.S. and M.B. Moser. 1985. People of the desert and sea: ethnobotany of the Seri Indians. Univ. of Arizona Press, Tucson, Ariz. 438 pp.
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Mendonca, M.T. and L.M. Erhardt. 1982. Activity, population size, and structure of immature Chelonia mydas and Caretta caretta in Mosquito lagoon, Florida. Copeia 1982:161-167.
Norris, J.N. 1975. Marine Algae of the Northern Gulf of California. Ph.D. Dissertation. University of California, Santa Barbara. 575 pp.
Pacheco-Ruiz, I. and J.A. Zertuche-Gonzalez. 1996. The commercially valuable seaweeds of the Gulf of California. Botanica Marina 1:201-206.
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