Assessment of cod survival in the Irish fly-shoot seine fishery using survivorship pop-up satellite archival tags

Abstract

Fly-shoot seining for demersal species is a commercially important fishery for Irish vessels operating in the Celtic Sea. We conducted a pilot-scale assessment of cod (Gadus morhua) survival in the fly-shoot seine fishery using survivorship pop-up satellite archival tags (sPATs). Supporting information on fish condition including vitality on a modified four-point scale - excellent, good, poor and moribund - was collected for all cod caught during the trial. Ten cod of suitable size for tagging (≥ 55 cm) in excellent and good vitality were tagged and released. Cod in poor and moribund vitality were assumed to have died. A Kaplan-Meier estimator was used to assess cod survival. Of 96 cod caught during the trial, 53 (55%) were ≥ 55 cm with 18 (34%) of these found to be in excellent or good vitality. Tagged fish were representative of the latter component of the catch. All 10 tags reported data with tag deployment period ranging from 2 to 21 days and a mean survival of 10 days. Assuming a 34% survival probability at time zero, survivability gradually decreased to zero at day 20 when all fish were assumed to have died. Barotrauma was likely the main cause of poor survival in this study. Alterations to fishing operations to reduce barotrauma are technically feasible but unlikely to be commercially viable. The sPATs and tagging procedures used in this study generally worked well in estimating cod survival rates, elucidating mortality causes and have potential applications for other species.