Direct Tests of the Efficacy of Bait and Gear Modifications for Reducing Interactions of Sea Turtles with Longline Fishing Gear in Costa Rica

Progress Reports (PDF): FY 2004, FY 2003

Project Overview
The objective of the proposed research is to determine the efficacy of a bait or gear modification that could significantly reduce the incidental capture of marine turtles in longline fishing gear. In addition, with the use of pop-up satellite archival tags, it is anticipated that this research will help to refine estimates of sea turtle survivorship post-capture and release from longline fishing gear.

Bait and gear modification experiments will be conducted on commercial Costa Rican longline fishing boats operating out of several Pacific coast ports of Costa Rica. Within the Costa Rican Exclusive Economic Zone there is an extensive (>400 boats) artisinal longline fleet targeting mahimahi and tunas. Vessels set about 500-800 hooks per day, with a shallow mainline set (generally 5 meters) and length of the gangeons usually around 6 meters. The catch rate of both sea turtles (predominantly olive ridleys) and the target fish species is exceptionally high. An average of approximately three turtles are captured per set, or about one turtle per 180 hooks and catch rates of 15-30 turtles per set do occasionally occur. Because of the shallowness of the mainline, hooked turtles can reach the surface to breath and sea turtle mortality rates at haul back have been zero (based on limited data), even though about 90% of the turtles are hooked in the mouth.

Given the high hooking rates of both the target species and sea turtles, this fishery appears to be an ideal arena in which to test the efficacy of bait modifications that may reduce turtle interactions, but that is not likely to effect catch rates of target fish species.

Proposed Activities
Tests of bait modifications - Behavioral data from research conducted with captive sea turtles at two National Marine Fisheries Services facilities suggest that soaking baits in blue dye may be a simple modification that can be used to reduce or eliminate sea turtle interactions with longline fishing gear. Initial experiments revealed that both green and loggerhead turtles initially avoided eating food items dyed blue. To test the efficacy of dyed bait in actual field trials researchers will place observers on at least four longline vessels for the entire trips per each treatment (e.g., blue-dyed bait). This would result in approximately 40 observed sets and should result in approximately 120 turtle interactions. Sets will be randomly alternated between those using blue-dyed bait and those using untreated bait. Preliminary modeling shows that this number of longline sets will allow a 40% reduction of turtle interactions due to individual bait treatments to be discerned to the 0.05% level of statistical significance. Researchers also anticipate a 40% reduction in sea turtle interactions with longline fishing gear during these trials.

Tests of gear modifications - Researchers plan to test modifications to fishing gear that are designed to be less detectable by sea turtles and thereby reduce or eliminate their interaction with fishing gear. Specifically, use of transparent or counter-shaded floats, use of dulled hardware, and use of a hook shield that may prevent turtles from successfully biting hooks. The gear modification chosen for these trials will depend upon the most recent information available on the most likely method of turtle deterrent. By placing observers on at least four longline vessels for the entire trips per gear treatment, researchers can then compare catch rates of sea turtles caught in either modified or unmodified fishing gear to determine efficacy of gear modifications in reducing turtle interactions.

Survivorship and migration of post-release turtles - Through the use of pop-up satellite archival transmitters (PSATs) researchers hope to better estimate survivorship of turtle post-released from longline fishing gear. PSATs are currently in use for this same purpose under a separate but related PFRP-funded project headed by Dr. Richard Brill and colleagues. With the addition of electronic tagging of turtles (n=10) caught in longline gear and released, we will be able to provide much-needed valuable information on post-release mortality.

Funding for this 1-year project to be awarded in early 2003.

Principal Investigator:

Dr. Yonat Swimmer
National Marine Fisheries Service
Honolulu Laboratory
Kewalo Research Facility
1125B Ala Moana Blvd.
Honolulu, Hawaii 96814
USA
Phone (808) 592-2813
FAX (808) 592-8300
email: Yonat.Swimmer@noaa.gov

This page updated August 22, 2006