Abstract
The purpose of this study was to evaluate the effect of the intensive commercial jigging fishery on the western winter-spring cohort of neon flying squid (Ommastrephes bartramii) in the Northwest Pacific Ocean and to estimate the exploitation status of this stock during the period 2005-2015. We applied a Bayesian hierarchical DeLury depletion model to the Chinese jigging fisheries data to estimate the stock abundance and catchability for each year, and sensitivity analysis on daily natural mortality (M) was conducted. The results indicated that M values had great impacts on the overall estimates of stock size. Initial annual population sizes varied from 66 to 662 million individuals with the M value of 0.003-0.01 per day during the study period. O. bartramii suffered from a certain degree of overexploitation in 2008. The proportional escapement values (M=0.003-0.01) were 8.94% to 19.82% in 2008, with an average of 13.74%, which may have led to a low abundance of O. bartramii and annual catch since 2009. As short-lived ecological opportunists, O. bartramii are extremely sensitive to changes in multi-scale environmental conditions, especially when anomalous environmental conditions occur, and significant between-year variations in the initial abundance resulted in O. bartramii suffering from a certain degree of overexploitation in 2010. Although the proportional escapement met the management target of 40% from 2011 to 2015, the stock size and annual catch still fluctuated at relatively low levels. Improved knowledge of the influences of environmental conditions on abundance of the western winter-spring cohort of neon flying squid can contribute to the sustainable management of this stock.
Highlights
The neon flying squid (Ommastrephes bartramii), one of the most abundant and economically important oceanic squid in the family Ommastrephidae, is widely distributed in subtropical and temperate waters of the global ocean (Roper et al 1984, Chen et al 2009, Arkhipkin et al 2015)
The results indicated that M values had great impacts on the overall estimates of stock size
The proportional escapement values (M=0.003-0.01) were 8.94% to 19.82% in 2008, with an average of 13.74%, which may have led to a low abundance of O. bartramii and annual catch since 2009
Summary
The neon flying squid (Ommastrephes bartramii), one of the most abundant and economically important oceanic squid in the family Ommastrephidae, is widely distributed in subtropical and temperate waters of the global ocean (Roper et al 1984, Chen et al 2009, Arkhipkin et al 2015). The North Pacific population is widely distributed between 20°N and 50°N, and comprises two cohorts (the autumn-spawning cohort and the winter-spring cohort) and four stocks: (1) the central stock of the autumn cohort, (2) the eastern stock of the autumn cohort, (3) the western stock of the winter-spring cohort, and (4) the central-eastern stock of the winter-spring cohort (Bower and Ichii 2005). The western winter-spring stock is the main fishing target of the Chinese squid-jigging vessels. This stock annually migrates between spawning grounds (subtropical waters) and feeding grounds (the sub-Arctic domain) (Murata and Nakamura 1998, Ichii et al 2006), and the latter grounds are where the fishery takes place (Fig. 1). Chinese squid-jigging fleets mainly operate in the waters of 150-165°E, where over 80% of the total catch occurs from August to November (Wang and Chen 2005, Chen et al 2008b, Cao et al 2015)
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