Proceedings of the 17th Annual ISTS: turtle mortality
- wallacejnichols
- March 4, 1997
Nichols, WJ, JA Seminoff, A Resendiz, and A Galvan. 1997. Apparent sea turtle mortality due to flipper tags. Submitted paper, Proceedings of the 17th Annual Symposium on Sea Turtle Biology and Conservation, Orlando, Florida, March 1997. NOAA Technical Memorandum NMFS-SEFSC-415.
APPARENT SEA TURTLE MORTALITY DUE TO FLIPPER TAGS
Wallace J. Nichols1, Jeffrey A. Seminoff1, Antonio Resendiz2, and Anthony Galvan1
1Wildlife&FisheriesScience,SchoolofRenewableNaturalResources,UniversityofArizona,Tucson,AZ 85721, U.S.A.
2Centro Regional de Investigaciones Pesqueras, Ensenada, Baja California, México
ABSTRACT
Plastic flipper tags used to identify black turtles in the central Gulf of California apparently contribute to incidental capture of the tagged turtles in gill nets. This assertion is based on field observations, interviews with local fishermen, and a study on captive tagged and untagged turtles. We recommend discontinuing the use of plastic tags on sea turtles in favor of the closed-loop metal alloy tags or other non-invasive identification techniques.
INTRODUCTION
A variety of methods have been used to mark and/or tag turtles in order to learn about their movement, nesting behavior, population size, growth rates, and age-specific mortality. The most common method of sea turtle identification is flipper tagging. The preferred tags are usually made of metal alloy, however several projects have chosen to use plastic roto-tags due to their superior retention. Many authors have discussed tag retention as a criterion for tag choice (Balazs, 1982; Henwood, 1986; Limpus, 1992; Alvarado et al., 1993), however few studies have considered the effect of tagging on incidental capture in marine fisheries.
OBSERVATIONS
In July of 1996, we observed two tagged black turtles at the surface, entangled in monofilament gill nets of 5 cm mesh size. Both of the turtles were especially tangled around the dorsal and ventral portions of the two-part plastic tags. Both of the turtles were tagged on their left and right foreflippers. The turtles were removed from the net, measured and released. The fisherman who owned the nets was contacted and interviewed. The direct observation of these turtles tangled by their tags and the discussion with the fisherman led us to investigate the possibility of tag mediated bycatch of sea turtles in the area.
METHODS
To determine the likelihood that the plastic tags facilitated capture in gill net, we conducted preliminary trials in the sea turtle holding tanks at the Sea Turtle Research Station in Bahia de Los Angeles, Baja California. A four meter segment of gill net was suspended across the middle of the tank. The net used was of the same mesh size (6 inch stretched) and material (monofilament) as those used by fishermen in the bay. The net was equipped with proper float lines (top) and lead lines (bottom) to ensure that it hung in the water in a manner similar to the larger nets used by local fishermen. In a series of 30 minute trials, six tagged and five untagged turtles were introduced to the tank individually. Interactions with the net were recorded and note was taken of number and location of tags on each turtle.
RESULTS
Throughout the trials, captive turtles were extremely adept at swimming backwards in order to remove themselves from nets. Usually, this reverse motion is performed in combination with extension of their foreflippers anteriorly as they reverse. Most entanglements occurred during this movement. It is likely that in the wild a similar reverse motion is performed. It appeared that once the net was wrapped once around the disk or message portion of the tag, the turtle would either have to pull the tag out, drown, or be captured by a fisherman. Of the six tagged turtles, three became permanently tangled in the net. In each case this was due to the presence of the flipper tag. Of the five untagged turtles, none became permanently tangled in the net. Further investigations are being conducted using inconel tags.
DISCUSSION
Based on the results of our brief study, personal observations, and interviews it seems that the two part plastic flipper tag, with its projecting leading edge (top half and bottom half), may make turtles more available to entanglement in a net if they should come into contact with one. Fishermen interview consider a turtle with a plastic flipper tag vulnerable to any net, even those with small mesh sizes. The central Gulf of California is an area where gill nets are particularly abundant. We recommend that further studies be pursued to investigate the effect of tagging on sea turtle bycatch. We also recommend that an alternative to plastic roto-tags be considered by those currently using this style of flipper tag. Standard inconel, titanium, or monel tags all result in a closed, oval loop after application. If the tang (point) of the tag is sealed (bent over) fully as it should be, there is nothing to the tag that can materially contribute to entanglement in a net. While tagging studies can provide invaluable information about a sea turtle population, it is understandable that tagging sea turtles may continue simply as a matter of historical inertia. It is critically important that widespread tagging efforts do not contribute to turtle mortality. It is additionally important to know how tagging methods may have affected the behavior and survivorship of individual animals in studies that model population dynamics and estimate abundance. Tag-less studies utilizing photo identification (McDonald et al., 1996) as well as the mapping of permanent, natural abnormalities such as scars and carapace notches combined with PIT tagging (McDonald and Dutton, 1996) should be considered as viable alternatives.
ACKNOWLEDGMENTS
We would like to thank Bety Resendiz, SEMARNAP, INE, Earthwatch, and all of the fishermen of Bahia de Los Angeles for their patient support of our research and conservation efforts.
LITERATURE CITED
Alvarado, J., A. Figueroa, C. Delgado, M.T. Sanchez, and E. Lopez. 1993. Differential retention of metal and plastic tags on the black sea turtle (Chelonia agassizii). Heretolp. Rev. 24:23-24.
Balazs, G.H. 1982. Factors affecting the retention of metal tags on sea turtles. Marine Turtle Newsletter 20:11-14.
Henwood, T.A. 1986. Losses of monel flipper tags from loggerhead sea turtles, Caretta caretta. J. of Herpetol. 20:276-279.
Limpus, C.J. 1992. Estimation of tag loss in marine turtle research. Wildl. Res. 19:497.
McDonald, D. And P.H. Dutton. 1996. Use of PIT tags and photoidentification to revise remigration estimates of leatherback turtles (Dermochelys coriacea) nesting in St. Croix, U.S. Virgin Islands, 1979-1995. Chelonian Conserv. Biol. 2:148-152.
McDonald, D.L., P.H. Dutton, R. Brandner, and S. Basford. 1996. Use of pineal spot ("pink spot") photographs to identify leatherback turtles (Dermochelys coriacea). Herpetol. Rev. 27:11.
