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Article Excerpt
ABSTRACT--Billfish movements relative to the International Commission for the Conservation of Atlantic Tunas management areas, as well as U.S. domestic data collection areas within the western North Atlantic basin, were investigated with mark-recapture data from 769 blue marlin, Makaira nigricans, 961 white marlin, Tetrapturus albidus, and 1,801 sailfish, Istiophorus platypterus. Linear displacement between release and recapture locations ranged from zero (all species) to 15,744 km (mean 575, median 119, SE 44) for blue marlin, 6,523 km (mean 719, median 216, SE 33) for white marlin, and 3,845 km (mean 294, median 98, SE 13) for sailfish. In total, 2,824 (80.0%) Billfish were recaptured in the same management area of release. Days at liberty ranged from zero (all species) to 4,591 (mean 619, median 409, SE 24) for blue marlin, 5,488 (mean 692, median 448, SE 22) for white marlin, and 6,568 (mean 404, median 320, SE 11) for sailfish. The proportions (per species) of visits were highest in the Caribbean area for blue marlin and white marlin, and the Florida East Coast area for sailfish. Blue marlin and sailfish were nearly identical when comparing the percent of individuals vs. the number of areas visited Overall, white marlin visited more areas than either blue marlin or sailfish. Seasonality was evident for all species, with overall results generally reflecting the efforts of the catch and release recreational fishing sector, particularly in the western North Atlantic. This information may be practical in reducing the uncertainties in Billfish stock assessments and may offer valuable insight into management consideration of time-area closure regulations to reduce bycatch mortality of Atlantic billfishes.
Introduction
The istiophorid billfishes are large, highly migratory predators, whose range and movements encompass vast geographical areas. For instance, blue marlin, Makaira nigricans, and sailfish, Istiophorus platypterus, extend to tropical and subtropical waters worldwide, while white marlin, Tetrapturus albidus, range throughout the Atlantic Ocean.
Owing to their highly migratory behavior, billfish stocks are often shared among many countries and, therefore, require regional and international management cooperation (Prince and Brown, 1991). In 1966, the International Commission for the Conservation of Atlantic Tunas (ICCAT) was established with a mandate to facilitate cooperative research, data collection, and management of tunas and tuna-like species (e.g. billfishes) within the Atlantic Ocean.
Over recent decades the landings of some Atlantic billfish species have decreased in size and number, a development largely attributed to increased commercial and recreational fishing activities (Restrepo et al., 2003). The most recent ICCAT stock assessments concluded that Atlantic blue marlin biomass is about 40% of the level required to produce maximum sustainable yield, while the situation for white marlin is more dire at about 20% (ICCAT, 2006). Sailfish numbers in the western North Atlantic are presently stable, but those in the eastern Atlantic appear to be decreasing (ICCAT, 2006). In the U.S. Atlantic management area, commercial harvesting of istiophorid billfish species has been prohibited since 1988. However, mortality resulting from incidental bycatch continues as multinational longline fisheries target tunas (Scombridae) and swordfish, Xiphias gladius, throughout the Atlantic (ICCAT, 2001; 2004).
Concerns for overexploitation and effective management of these highly migratory predators warrant a thorough understanding of their biology, ecology, and associated movement patterns. Mark-recapture studies using conventional streamer tags have assisted in defining the spatial and temporal characteristics for movement and migration of istiophorid billfish populations worldwide (Squire, 1972; Squire and Nielsen, 1983; Miyake, 1990; Pepperell, 1990a; 1990b; Scott et al., 1990; Van Der Elst, 1990; Prince et al., 2002; Hoolihan, 2003). Mather (1963), Bayliff
and Holland (1986), McFarlane et al. (1990), and Prince et al. (2002) provide reviews and tagging technique information.
Conventional tagging of billfish in the United States was initiated in 1954, following development of in-water tagging techniques for large highly migratory species (Mather, 1963; Scott et al., 1990). At present, there are two major constituent-based tagging programs largely responsible for billfish mark-recapture studies in the Atlantic-the Cooperative Tagging Center (CTC), based at NOAA's National Marine Fisheries Service (NMFS), Southeast Fisheries Science Center (Miami, Fla.), and The Billfish Foundation (TBF; Ft. Lauderdale, Fla.). Each program relies on volunteer tagging and recapture reporting by recreational and commercial fishermen.
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Worldwide, from 1954 through 2005, these programs and their progenitors have documented the tagging of 266,448 istiophorid billfish. Recapture encounters have revealed Atlantic blue marlin completing transatlantic, transequatorial, and interoceanic movements, while white marlin have demonstrated transatlantic crossings (Ortiz et al., 2003). Sailfish generally remain closely associated with coastal zones having shallow continental shelves, although longline catch records do indicate a nominal presence in some mid-Atlantic offshore areas (Uozumi, 1996; Kim et al., 1998).
Currently, ICCAT recognizes single Atlantic-wide stocks for blue and white marlin based on mark-recapture studies and genetic evidence suggesting population mixing (ICCAT, 2006). For sailfish, separate eastern and western stocks are recognized and defined by a boundary loosely associated with the mid Atlantic Ridge (long. 40[degrees]W, North Atlantic; long. 20[degrees]W South Atlantic); a decision based on distribution, morphology, genetic analyses, and mark-recapture data (ICCAT, 2006).
ICCAT has managed all Atlantic billfishes by separating presumed stocks along somewhat arbitrary boundaries falling within seven management areas used for statistical data collection purposes (Table 1, Fig. 1). Within some western ICCAT management areas, NMFS has further defined ten "U.S. domestic" subareas, which are targeted by U.S. recreational and commercial fleets and have a finer spatial resolution to suit NMFS requirements for data collection (Table 1, Fig. 1). Istiophorid billfish catches are relatively "rare" events, when compared to large-sized schooling scombrids (Prince and Brown, 1991). Therefore, partitioning data sampling areas into finer-scaled subunits should enhance our comprehension of the spatial distribution and movement patterns of Atlantic billfishes.
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Our study was conducted to analyze...
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