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PLASTIC "ROTOTAGS" MAY BE LINKED TO SEA TURTLE BYCATCH
Throughout the course of our investigations of black [Eastern Pacific Green] sea turtle (Chelonia mydas agassizii) ecology in Bahia de Los Angeles, Baja California, México we have had the opportunity to observe and learn about the interactions between sea turtles and fishing nets and to discuss these observations with local fishermen. Our findings suggest that plastic flipper tags (two-part "rototags" manufactured by Dalton Supplies, U.K.) used by the Gulf of California Black Sea Turtle Project over the past decade may contribute to sea turtle bycatch in a variety of fishing net types.
On two separate occasions in July 1996 we observed black turtles at the surface, entangled in monofilament gill nets of 5 cm mesh size. Both turtles were especially tangled around the dorsal and ventral components of the two-part flipper tags. Both turtles were tagged on both the left and right foreflippers. The turtles were removed from the net, measured, their tags removed, and then released. The fishermen who owned the nets were contacted and interviewed.
Over the next several months, interviews were conducted with local fishermen to learn more about sea turtle bycatch in the area. Through these interviews we learned that several tagged turtles had been accidentally captured in gill nets in the past. All of the fishermen interviewed noted that, in their opinion, the presence of plastic flipper tags increases the chance that a turtle may become entangled in a net, no matter the mesh size. They also noted that sharp carapace abnormalities and epibionts (e.g., Murex sp.) may also contribute to entanglement in nets.
Preliminary observations of both tagged and untagged turtles in captivity indicate that turtles are capable of back-swimming out of nets, provided that their tags do not become hooked in the net. Specifically, we conducted trials in cement holding tanks (8 m diameter) at Campo Archelon, Bahia de Los Angeles, in July 1996. A gill net was stretched across the center of the tank from the floor to above the waters' surface. Of ten turtles tested, only the tagged turtles (n=6) became tangled, while the untagged turtles (n=4) always escaped. Experienced fishermen support these observations by pointing out that untagged turtles literally "bounce off" of gill nets with small mesh sizes.
It is clear that additional research is needed (and we are certainly aware that in many parts of the world, untagged sea turtles are routinely captured in gill nets), but based on our preliminary observations of captive turtles, personal observations, and interviews, it appears that these plastic flipper tags, with their projecting leading edges, may make the turtles more available to entanglement once a net is contacted. As a result, the use of plastic tags by the Gulf of California Black Sea Turtle Project has been discontinued.
Gill nets are abundant in the Gulf of California (e.g., Ruiz-Durá, 1985; authors, pers. observ.) and many of the turtles utilizing Baja California waters are migrating from distant nesting beaches, some as far away as Japan (Bowen et al., 1995; W. J. Nichols, unpubl. data). Unrecorded mortality of tagged turtles in the Gulf will lead to erroneous population assessments and may lead to further declines in this already drastically diminished regional population. We recommend further studies on the effects of tagging on incidental capture of turtles, both on feeding grounds and along nesting beaches.
Ruiz-Durá, M. F. 1985. Recursos pesqueros de las costas de México. Ed. Limusa. México. 131 pp.
Bowen, B. W., F. A. Alberto Abreu Grobois, G. H. Balazs, N. Kamezaki, C. J. Limpus and R. J. Ferl. 1995. Trans-Pacific migrations of the loggerhead sea turtle demonstrated with mitochondrial DNA markers. Proc. Natl. Acad. Sci. USA 92:3731-3734.
WALLACE J. NICHOLS and JEFFREY A. SEMINOFF, Wildlife and Fisheries Sciences, School of Renewable Natural Resources, University of Arizona, Tucson, Arizona 85721 USA and ANTONIO RESENDIZ, Centro Regional de Investigaciones Pesqueras, Ensenada, Baja California, MEXICO.
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