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Article Excerpt
Introduction
Four billfish species (Family Istiophoridae) range the Atlantic Ocean and adjacent waters of the Caribbean Sea and Gulf of Mexico: sailfish, Istiophorus platypterus; blue marlin, Makaira nigricans; white marlin, Tetrapturus albidus; and longbill spearfish, Tetrapturus pfluegeri (Robins and Ray, 1986). In addition to filling the role of apex predators in subtropical and tropical pelagic waters, the Atlantic marlins and sailfish also support recreational fisheries that have been valued in the billions of dollars (IGFA, 1996). Despite their ecological and economic importance, little is known about the basic biology and ecology of these species, especially their growth, reproduction, and movement within the vast, highly dynamic oceanic habitats that sustain them (Holland, 2003).
Most research on Atlantic billfishes has focused on issues of stock structure, population abundance, and fishing mortality as part of stock assessments that, since 1994, have been performed every 2-4 years under the auspices of the International Commission for the Conservation of Atlantic Tunas (ICCAT). Several lines of evidence suggest that Atlantic blue marlin and white marlin belong to single Atlantic-wide stocks (Ortiz et al., 2003; Graves and McDowell, 2003); in contrast, sailfish are managed as western and eastern Atlantic populations. Results of the most recent ICCAT stock assessments (ICCAT, 2002; Restrepo et al., 2003) indicated that: 1) the Atlantic blue marlin population is overfished, with its current biomass about 40% of the size required for maximum sustainable yields (MSY); and 2) the white marlin stock is even more depleted, with its current biomass only 12% of the level to support MSY.
For at least the last two decades, the principal source of mortality on adults of both Atlantic marlin species has been pelagic longline fishing (Uozumi, 2003). This method of fishing deploys a continuous mainline, of up to 60 mi in length, with regularly spaced branch lines which terminate with baited hooks (Bjordal and Lokkeborg, 1996; Sainsbury, 1996). For the most part, billfish are not targeted by longline fisheries; rather, they are caught incidentally as the bycatch of fleets that strive to supply the growing global demand for tunas (Scombridae) and swordfish, Xiphias gladius (Prince and Brown, 1991; Beerkircher et al., 2002). Therefore, understanding bycatch of longline fisheries is an important step in reducing uncertainties in stock assessments and projections, implementing appropriate management measures, and developing new techniques to reduce incidental marlin capture, injury, and mortality.
The main purpose of this paper was to identify and analyze temporal and spatial patterns in Atlantic marlin bycatch as reflected in the Japanese pelagic longlining database, which is the longest-running, most spatially-extensive of its kind (Myers and Worm, 2003). Historical trends in catch-per-unit-effort (CPUE) derived from this data source have been central components of billfish stock assessments, especially for gauging past and present fishing levels and removals relative to MSY (Jones et al., 1998; ICCAT, 2001, 2002; Restrepo et al., 2003; Uozumi, 2003). Our focus here was on variation in marlin bycatch within a 41-yr period (1960-2000) in the subtropical and tropical Atlantic Ocean between lat. 30[degrees]N and 30[degrees]S--where the bulk of Atlantic marlin catches occurs (Uozumi, 2003).
Of particular interest were quantitative changes that were consistent (or not) with the assertions first posed by Uozumi and Nakano (1994) and then repeated in several papers thereafter (e.g. Yokawa and Uozumi, 2001; Uozumi, 2003), that the relatively low Atlantic blue marlin CPUE's obtained by this fishery since the 1980's were not indicative of low population levels, but rather were artifacts of changes in fishing practices.
Specifically, Yokawa and Uozumi (2001) and Uozumi (2003) suggested that the operational switch from targeting mainly surface-associated albacore, Thunnus alalunga, and yellowfin tuna, Thunnus albacares, to targeting mainly the deeper-dwelling bigeye tuna, Thunnus obesus, has meant: 1) the gear only covered the lower limits of the blue marlin's depth distribution, and 2) shifting of fishing effort to focus on eastern Atlantic waters has amounted to movement away from preferred blue marlin habitats. Their implication, therefore, is that a drop in blue marlin catchability was the basis for any observed decline in blue marlin CPUE, and thus it should not be used, without adjustment, as an index of blue marlin abundance. It is unclear why the same argument was not made to explain the much greater declines in white marlin CPUE.
In this paper, we computed annual effort, target species catch, and marlin bycatch levels as well as CPUE and bycatch ratios (i.e. number of marlin caught per 100 target fishes, B:T ratio) for each Atlantic marlin species. More emphasis was placed on patterns of marlin B:T ratios than on CPUE precisely because the equivalency of effort units (Hilbom and Waiters, 1992) over the time series has been questioned. Our objectives were to examine: 1) temporal variation in effort, target catch, and marlin bycatch before, during, and after the fishery's operational switch, 2) spatial changes in fishing effort, marlin CPUE, and marlin B:T ratios, 3) the extent to which marlin B:T ratios have been driven by concurrent changes in target and marlin catches, and 4) the theoretical interrelationships among bycatch and target species catchability and bycatch and target species abundance and how these ultimately affect B:T ratio levels.
Materials and Methods
The data analyzed here were a subset of the pelagic Japanese longline (JLL) data series, provided on request by ICCAT. The JLL data series comprises historical (i.e. from 1956 forward) catch and effort information, aggregated on a monthly basis and at the geographic scale of 5[degrees] latitude by 5[degrees] longitude cell. Effort in the JLL is given as total number of longline hooks deployed and catch as numbers of boated and discarded tunas (several species), swordfish, and istiophorid billfishes. In this investigation, we focused on patterns of blue marlin and white marlin bycatch from 1960 to 2000 and within Atlantic waters between lat. 30[degrees]N and 30[degrees]S. The spatial focus was chosen because Japanese longline bycatch of marlins beyond these latitudes is relatively minor (Uozumi, 2003). Sailfish and longbill spearfish bycatch trends were not considered because, prior to the late 1990's, catches listed as sailfish actually included an unspecified proportion of longbill spearfish (Uozumi, 2003).
Plots were generated to examine annual variation as well as average decadal patterns in: 1) fishing effort (number of longline hooks deployed), 2) target fish catch (i.e....
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