The influence of gear selectivity and spawning behavior on a data-poor assessment of a spawning aggregation fishery

Abstract

We applied several data-poor techniques to perform an assessment of the Gulf corvina (Cynoscion othonopterus) fishery in the Gulf of California from 1997 to 2012 and to investigate the effects of gear selectivity and age-dependent variation in spawning frequency on estimates of sustainability in spawning aggregation fisheries. The length composition of the catch varied significantly among years but showed no clear directional pattern. However, the average length was above the long term average after the implementation of a regulation that standardized mesh size of nets, and the average length of fish captured after the implementation was significantly higher than during previous periods. Results using three simple metrics based on catch length compositions indicated that fishing activities were sustainable due to the exclusion of juveniles from the fishery and the targeted harvest of adults at the optimal length. However, the low proportion of older, fecund fish in the fishery is a serious cause for concern. Modeled estimates of spawning potential ratios (SPR) were consistently higher when spawning frequency was assumed to be age invariant and were significantly higher after the implementation of gear regulations. However, SPR values only reached levels above 35%, a common reference point for sciaenid fishes, during the current fishing period (2010–2012) under conditions of age invariant spawning frequency. Results of this study support previous claims that suggest estimates of reproductive potential are highly sensitive to age-dependent variation in spawning frequency and imply that such details related to spawning behavior require more attention, particularly for fisheries that target spawning aggregations. Our results also suggest that spawning aggregations can be harvested sustainably through conventional regulations if juveniles are excluded, fish are harvested at optimal length, and older, fecund individuals are protected from harvest. Given the uncertainty of the status of the corvina fishery based on the discordant results of this study and the inherent vulnerability of the species to overfishing, We recommend the precautionary approach be applied to management decisions until more robust information is acquired on stock abundance, the relationship between spawning frequency and age or length, and reproductive output.