Stock assessment of the autumn cohort of neon flying squid ( Ommastrephes bartramii) in the North Pacific based on past large-scale high seas driftnet fishery data

In the high seas of the North Pacific, large-scale squid driftnet fisheries targeted the autumn cohort of neon flying squid ( Ommastrephes bartramii) from May to mid-September. Total annual catches during 1982–1992 ranged from 102,000 to 209,000 t (149,000 t on average). To assess the effect of intensive commercial driftnet fishing on this cohort, we assessed the stock based on past driftnet fishing data. We used three different methods to estimate stock size during the high fishing season (July) for the driftnet fishing ground between 170°E and 145°W during 1982–1992. We found remarkable agreement in average stock estimates based on the three methods, with estimates of 380,000 t by the swept area method, 367,000 t by the DeLury method, and 333,000 t by the non-equilibrium surplus-production method. Of the three estimates, the swept area estimate was likely to be the most reliable because of the more realistic assumptions and reduced uncertainty of parameters in this method. The relative fishing mortality F/ F MSY derived from the production model was 0.8–1.2, which was around the appropriate (maximum sustainable yield) fishing level (1.0), even during 1987–1990 when catches were large. The proportional escapement (number of squid alive at the end of the fishing season as a proportion of those that would have been alive had there been no fishing) was 18–64%, with an average of 42%, which was around the management target of 40%, even during this period. Thus, the large-scale driftnet fishery is considered to have been sustainable. Possible adverse effects of the driftnet fishery on food webs in the North Pacific are discussed.