IATTC Fishery Status Report 8 - Comisión Interamericana del Atún ...

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G. SWORDFISH
Swordfish occur throughout the Pacific Ocean between about 50°N and 50°S. They are caught mostly by the longline fisheries of
Far East and Western Hemisphere nations. Lesser amounts are taken by gillnet and harpoon fisheries. They are seldom caught by
recreational fishermen. During the 2005-2008 period the greatest catches in the EPO have been taken by vessels of Spain, Chile,
and Japan, which together harvest about 70% of the total swordfish catch taken in the region. All three have fisheries that target
swordfish, though much of the swordfish taken in the Japanese fishery are incidental catches of a fishery that targets predominantly
bigeye tuna. Other nations with fisheries known to target swordfish are Mexico and the United States.
Swordfish reach maturity at about 5 to 6 years of age, when they are about 150 to 170 cm in length. They probably spawn more
than once per season. Unequal sex ratios occur frequently. For fish greater than 170 cm in length, the proportion of females
increases with increasing length.
Swordfish tend to inhabit waters further below the surface during the day than at night, and they tend to inhabit frontal zones.
Several of these occur in the eastern Pacific Ocean (EPO), including areas off California and Baja California, off Ecuador, Peru, and
Chile, and in the equatorial Pacific. Swordfish tolerate temperatures of about 5° to 27°C, but their optimum range is about 18° to
22°C. Swordfish larvae have been found only at temperatures exceeding 24°C.
The best available scientific information from genetic and fishery data indicate that the swordfish of the northeastern Pacific Ocean
and the southeastern Pacific Ocean (south of 5°S) constitute two distinct stocks. Also, there may be movement of a northwestern
Pacific stock of swordfish into the EPO at various times.
The results of preliminary modeling with MULTIFAN-CL of a North Pacific swordfish stock in the area north of 10°N and west of
140°W indicate that, in recent years, the biomass level has been stable and well above 50% of the unexploited levels of stock
biomass, indicating that these swordfish are not overexploited at current levels of fishing effort. A more recent analysis for the
Pacific Ocean north of the equator, using a sex-specific age-structured assessment method, indicated that, at the current level of
fishing effort, there is negligible risk of the spawning biomass decreasing to less than 40% of its unfished level.
The standardized catches per unit of effort of the longline fisheries in the northern region of the EPO and trends in relative
abundance obtained from them do not indicate declining abundances. Attempts to fit production models to the data failed to produce
estimates of management parameters, such as maximum sustainable yield (MSY), under reasonable assumptions of natural mortality
rates, due to lack of contrast in the trends. This lack of contrast suggests that the fisheries in this region have not been of magnitudes
sufficient to cause significant responses in the populations. Based on these considerations, and the long period of relatively stable
catches in the northern region (Figure G-1), it appears that swordfish are not overfished in the northern region of the EPO.
An assessment of the southern stock of swordfish in the EPO was carried out with Stock Synthesis II (SS2: Ver.1.23b) in 2006 and
produced the following results. The population has undergone considerable changes in biomass and is currently at a moderate level
of depletion. There was strong evidence of one or two large cohorts entering the fishery recently, but their strengths were uncertain.
The trend in spawning biomass ratio (the ratio of the spawning biomass of the current stock to that of the unfished stock; SBR) for
this stock was estimated to have been between about 0.5 and 0.9 during the entire period of monitoring (1945-2003), and to have
decreased to its lowest levels during the mid-1960s and again during the mid-1990s.
The MSY for the southern EPO swordfish stock was estimated to be about 13,000-14,000 t, and the SBR at MSY to be about 0.26.
The spawning biomass was estimated to be well above the biomass corresponding to the MSY.
The average annual catch from the this stock during 1993-2000 was about 7,000 t (range ~ 4,800-8,700 t). It rose to a peak of about
15,000 t in 2002, then declined to about 6,000 t (Figure G-1), which is less than the estimated MSY catch. It is not expected that
catch levels on the order of those observed in the early 2000s would be sustainable.
No attempts have been made to estimate the level of MSY that could be obtained by each fishery operating exclusively. However, it
is likely that the fisheries that capture younger fish (e.g. the longline fisheries of Chile, Japan, and Spain) are less efficient at
maximizing yield.