Reduction of Bluefin Tuna and Undersize....
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GRANT NUMBER:  NA57FD0031           NMFS NUMBER:  

REPORT TITLE:  Reduction of Bluefin Tuna and Undersize Swordfish Bycatch in Atlantic Longline Fisheries

AUTHOR: Steven A. Berkeley and Randy E. Edwards


AVAILABLE FROM:  National Marine Fisheries Service, Southeast Region, 9721 Executive Center Drive, North Koger Building, St. Petersburg, FL 33702.  PHONE: (813) 570-5324


There were two primary objectives of this research: (1) to determine times, depths, areas, temperatures, and water temperature profiles of the water mass associated with longline catches of yellowfin tuna, bluefin tuna, undersize swordfish, and other bycatch species; and (2) to determine the feasibility of using lighter test leaders to allow giant bluefin tuna to break free while still retaining target species. 

When this proposal was submitted, the southern incidental bluefin tuna quota was being taken annually, usually within 3 or 4 months, after which bluefin could no longer be legally landed. This meant that bluefin caught after the closure were being discarded, frequently dead.  The depressed condition of the bluefin stock meant that these discards represented a significant loss to the spawning stock and potential for recovery as well as a substantial economic loss to the fishery. However, in recent years, either due to changes in regulations or because of reduced abundance, the quotas have not been taken, and discards have been minimal. During the course of our MARFIN project we encountered only a single bluefin tuna in 61 sets of longline gear. Because of the rarity of bluefin encounters, and thus the near elimination of the discard problem, it was impractical for us to pursue objective 2 of the original proposal. Although we designed and tested a "weak link" system that would have allowed us to test various strength leader material without losing any fish as a result, we never conducted these experiments.

The investigators attached "hook timers" to gangions and time--depth recorders to the mainline of longlines deployed by cooperating yellowfin tuna fishermen operating in the Gulf of Mexico.  In addition, expendable bathythermographs were deployed at the start and end of each set and haulback to characterize the thermal structure of the water column.  These instruments allowed the investigators to identify the time, depth, and water temperature in which hookups occurred.  Up to 43 nautical miles of mainline were deployed, with soak times of over 24 hours.  By understanding the fishing operation, the investigators were able to develop gear and attachment techniques that did not interfere with or slow down the commercial fishing operation.

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