Jon Atwood, PhD
Department of Environmental Studies
Six of the eight species of sea turtles are currently listed under the U.S. Endangered Species Act as either threatened or endangered. Although many factors, such as egg poaching, development of oceanfront property and ingestion of plastics, are responsible for the decline of sea turtles, drowning in fishing nets is the leading cause of sea turtle mortality. Biologists studying sea turtles commonly mark individuals by attaching plastic or metal tags to their flippers, a practice that has recently been suggested as possibly contributing to the animals' likelihood of entanglement. The effects of flipper tagging on sea turtle net entanglement were studied from June-August 1999 in Bahia de los Angeles, Baja California, Mexico. Twelve recently-captured black sea turtles (Chelonia mydas agassizii) were exposed to a 4-m x 1-m net of 16.5 cm mesh for up to thirty minutes while being temporarily held in tanks at Centro Regional de Investigacion Pesquera (CRIP). Each turtle was tested with 2 metal Monel flipper tags (size 681) on each rear flipper, 2 plastic flipper tags (size mini) on each fore flipper, and no tags during 3 different trials; treatment sequences were randomly assigned to each animal. Of 16 observed instances of entanglement under the experimental conditions 15 (94%) involved tags. There was a significant difference in the frequency with which tagged and untagged turtles were entangled and a highly significant difference in the frequency of entanglement between metal and plastic tags. Plastic tags resulted in entanglement more frequently than expected. No significant difference in frequency of entanglement existed between turtles with metal tags and untagged turtles. Logistic regression found no evidence of a relationship between straight carapace length and likelihood of a tagged turtle becoming entangled. This study suggests that plastic flipper tags increase the likelihood of sea turtle net entanglement, whereas metal Monel tags do not. Further use of plastic tagging as a marking technique should be discontinued.