Use of Pop-up Satellite Archival Tags to Demonstrate Survival of Blue Marlin (Makaira nigricans) Released from Pelagic Longline Gear

Abstract

Blue marlin (Makaira nigricans) sup(MSY) and that current harvests are port commercial and recreational more than twice the replacement yield, fisheries throughout the tropical and further contributing to the decline of subtropical waters of the Atlantic the stock (ICCAT, 2001). A reduction in Ocean. The species is taken in directed fishing mortality of approximately 60% recreational and artisanal fisheries is needed simply to halt the decline in in several areas and constitutes an stock abundance (Goodyear, 2000). incidental catch of the widespread One means of reducing fishing mor­ commercial pelagic longline fishery. tality on blue marlin, without severely Although blue marlin comprise only a impacting catches of target species of small fraction of the catch of the pelathe pelagic longline fishery, is to release gic longline fishery that targets tunas those blue marlin that are alive at the and swordfish, this fishery accounts time longline gear is retrieved (hauled for the majority of fishing mortality back). Jackson and Farber (1998) reon Atlantic blue marlin (ICCAT, 1997; ported that 48% of blue marlin caught 2001). in the Venezuelan longline fishery are Atlantic blue marlin were last asalive at the time of haulback. Data from sessed in 2000 by the Standing Committhe U.S. observer program between tee for Research and Statistics (SCRS) 1992 and 1996 indicate that 66% of of the International Commission for the blue marlin were released alive from Conservation of Atlantic Tunas (ICthe domestic longline fishery (Lee and CAT). The assessment indicated that Brown, 1998) and U.S. National Marine the total biomass of Atlantic blue marFisheries Service (NMFS) data and lin is only about 40% of that necessary mandatory pelagic longline logbook to produce maximum sustainable yield submissions between 1987 and 1991 indicate that 59.8% of blue marlin are released alive from commercial pelagic longline gear (Cramer, 1998). ICCAT has been encouraging the release of live blue marlin for several years, and in 2000 the Commission mandated that all live blue marlin and white marlin be released from com­ mercial longline and purse seine ves­ sels. However, for such a management measure to significantly reduce fishing mortality, released animals must have a reasonably high postrelease survival rate. Little information exists about postrelease survival of blue marlin, espe­ cially of animals taken on pelagic longline gear. In general, recovery rates of billfish tagged with conventional (streamer) tags have been quite low (<2%; Jones and Prince, 1998; Ortiz et al., 1998). This observation is consis­ tent with high postrelease mortality, although low recovery rates could also result from tag shedding and a lack of reporting recovered tags (Bayley and Prince, 1994; Jones and Prince, 1998). Results of acoustic tracking studies of blue marlin captured on recreational gear suggest that postrelease survival over periods of a few hours to a few days is relatively high, although mortalities have been noted (reviewed in Pepperell and Davis, 1999). More recently, popup satellite archival tags (PSATs) have been used to study postrelease survival of blue marlin taken in a recreational fishery. Graves et al. (2002) attached nine PSATs to blue marlin caught on recreational gear off Bermuda. Eight of the tags detached from the animals and reported as expected after five days; net displacement and direction, tag incli­ nation, and temperature data for all eight individuals were consistent with postrelease survival for five days. In this note, we present the results of a study evaluating the postrelease survival of blue marlin from pelagic longline fishing gear in the western North Atlantic. We include analyses of the movement and behavior of these animals for tagging periods of both five days and thirty days.