Proceedings of the 17th Annual ISTS: turtle mortality

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.