An assessment of the west winter-spring cohort of neon flying squid (Ommastrephes bartramii) in the Northwest Pacific Ocean

In the Northwest Pacific, the squid jigging fisheries targeted the west winter-spring cohort of neon flying squid (Ommastrephes bartramii) from August to November. Total annual catch by the Chinese mainland squid jigging fleet during 2000-2005 ranged from 64,100 to 104,200 t. The unique life history of this squid species makes the use of traditional age- or length-structured models difficult in evaluating the effect of intensive commercial jigging on this stock. We fitted a modified depletion model to the Chinese jigging fisheries data to estimate the squid stock abundance during 2000-2005. Monthly biological data were randomly sampled from the five squid jigging vessels during the fishing seasons. Effects of using different natural mortality rates (M) and three different error assumptions were evaluated in fitting the depletion model. Based on sensitivity analyses, the log-normal error model was found to be preferred for the squid assessment. The assessment results indicated that the initial (pre-fishing season) annual population sizes ranged from 199 to 704 million squid with the M value of 0.03-0.10 during 2000-2005. The proportional escapement (M = 0.03-0.10) for different fishing seasons over the time period of 2000-2005 ranged from 15.3% (in 2000) to 69.9% (in 2001), with an average of 37.18%, which was close to the management target of 40%. Thus, the current fishing mortality of the squid jigging fishery was considered to be sustainable. We inferred its annual maximum allowable catch ranging from 80,000 to 100,000 t. This study suggests that the modified depletion model provides an alternative method for assessing short-lived species such as O. bartramii.