Stock Assessment. Stock assessment of the biomass shows a sharp decline in the early model period to a low level during the 1970s and then subsequently increased to a higher level during the 1980s and 1990s (see figure 15). These changes are an effect of long-term trends in recruitment rather than attributable to the impacts of fishing. Overall, the impact of fishing has reduced the overall WCPO YFT biomass to about 30% of unexploited levels, but this is even higher for Region 4 (see figure 16), where total biomass is believed to have been reduced by over 50% as a result of fishing, particularly purse seining. Year Annual average biomass (‘000 t) Figure 15: YFT Estimate Annual Biomass (‘000 t) for Region 4 of the Western Pacific Ocean Source: Xxxxxxx et al, 2005b Year LL = all longline fisheries; PH/ID = Philippines and Indonesian domestic fisheries; PS assoc = purse seine FAD and log sets; PS unassoc = purse seine school sets Impact (%) Figure 16: Impact of fisheries on YFT Total Biomass in Region 4 of the Western Central Pacific Source: Xxxxxxx et al, 2005b The Philippines/Indonesian fisheries are also considered important, as fish move directly into Region 4 from Region 3 to the west. It is noteworthy that the long line fishery has a relatively insignificant impact, which contrasts with sub-tropical fisheries areas.
Stock Assessment. Since fishing began in the 1950s, bigeye biomass fell to about half its original level by the 1970s and has remained more or less stable since then (figure 17). It is considered that this biomass has only been stabilised by higher than average recruitment and should this fall, then the biomass is likely to suffer another period of decline. Year Annual average biomass (‘000 t) Figure 17: BET Estimate Annual Biomass (‘000 t) for Region 4 of the Western Pacific Ocean Source: Xxxxxxx et al, 2005c 16 Two types of reference points are often now required for fisheries management: the fishing mortality (F) at maximum sustainable yield (FMSY) is used as an indicator of overfishing; and the biomass (B) at MSY (BMSY) is used as an indicator of an overfished state. Fishing mortality is mainly for adults rather than juveniles. Xxxxxxx et al (2005c) determined that there has been strong fishery-related depletion of bigeye in this region of the WCPO of over 60% of the total biomass, mainly due to the longline fishery, although the purse seine fishery has also had a significant impact (figure 18). Year LL = all longline fisheries; PH/ID = Philippines and Indonesian domestic fisheries; PS assoc = purse seine log and FAD sets; PS unassoc = purse seine school sets. Impact (%) Figure 18: Impact of fisheries on BET Total Biomass in Region 4 of the Western Central Pacific Year Source: Xxxxxxx et al, 2005c
Stock Assessment. Skipjack recruitment is highly variable and driven largely by ENSO events. The high proportion of small fish observed from the equatorial purse-seine fisheries during 2004 reflects the currently high levels of recruitment over recent years. Total biomass has increased firstly in the mid-1980s in response to the increase in recruitment and increased again in the late 1990s and has been sustained by strong recruitment throughout 1997−2001 (figure 19). The high current biomass is attributable to the exceptionally high recruitment in 2004. Y e a r Annual average biomass (‘000 t) Figure 19: SKJ Estimate Annual Biomass (‘000 t) for Western Equatorial Region of thx XXX Source: Xxxxxxx et al, 2005a Since the early 1990s, there has also been a general increase in fishing mortality rates in the Western Equatorial Region of the WCPO, although exploitation rates are much lower due to the higher overall level of biomass in this area. This is reflected in the relatively low impact of this region’s fishing effort on the overall WCPO biomass, approaching 10% in recent years (figure 20). Year
Stock Assessment. During the 1980s, two studies assessed Pacific swordfish stocks by applying equilibrium production models to Japanese data. These studies provided similar results by suggesting a MSY for Pacific swordfish to be about 20,000 tones per year (range 18,000-28,000 tonnes). These assessments suggest that the Pacific stock was under exploited in the early 1980s. Now landings of swordfish Pacific-wide are currently closer to MSY but still within safe levels, however both the accuracy of reported landings and the estimates of MSY are very uncertain. Long line fisheries provide the longest available time series of billfish catch. Only recently have billfish been reported on purse seine log sheets; however, indications of catch from this source are likely to be hampered by problems of non-reporting and species mis-xxxxxxfication (Xxxxxx et al., 1996). More recent assessments have also been inconclusive and the condition of the Pacific swordfish stock remains uncertain. Xxxxxx et al (2005) have investigated the possible application of the Multifan-CL framework methodology to stock assessment but as yet do not have any results that might be used to generate management advice.
