Acoustic assessment of the biomass of aggregated chokka squid Loligo reynaudii on its inshore spawning grounds on the southeast coast of South Africa in the season closed for fishing

Abstract

Recent attempts to estimate the biomass of chokka squid Loligo reynaudii on the southeast coast of South Africa by acoustic surveys of the inshore jig-fishing area are described. Pilot surveys of aggregations, covering part of the fishing grounds, were conducted in 2019 and 2020, followed by a multiphase survey of the entire grounds within the 50-m depth contour by two vessels in 2021. Biomass estimates were made using previous in situ and ex situ estimates of the target strength of L. reynaudii at 38 kHz, and our knowledge of the physical characteristics of spawning aggregations from acoustic studies over the past two decades. Biomass estimates from the 2021 survey ranged from 343 tonnes (coefficient of variation [CV] 47%) for targets conclusively identified as squid, to 2 365 tonnes (CV 9%) if all targets that could possibly have been squid were included. An estimate of 635 tonnes (CV 20%) is proposed as the most reasonable from the survey, although its worth as an absolute estimate is questionable mainly because of identification and target-strength uncertainties. We conclude that a series of wide-area acoustic estimates could provide an additional index of relative abundance for fitting in the current squid assessment model. This would be useful in managing the jig fishery and for improving our understanding of changes in chokka squid abundance and distribution in response to changes in the environment.