Stronger average decline in late-autumn positive-catch intensity during prolonged catch decline in the Korean common squid jig fishery

We examined whether the long-term decline in the Korean offshore jig fishery for common squid Todarodes pacificus was uniform across the fishing season. Daily vessel reports to the National Federation of Fisheries Cooperatives for 2000–2023 were decomposed into the conditional proportion of squid-positive records and positive-catch intensity per reported vessel-day. On fixed month-by-seablock (0.5° statistical block) support, recorded performance (squid catch per reported vessel-day, including days on which only other species were caught) declined by 2.69% yr - ¹ in summer (June–August) and 5.64% yr - ¹ in autumn (September–December). In the separate vessel-, seablock-, and month-fixed-effect model of positive catches, intensity declined by 4.01–4.96% yr - ¹ in June–August and 8.47–9.38% yr - ¹ in late autumn (October–December). These are average trends, with recent annual rebounds and sharp late-autumn decreases in the squid-positive proportion. The intensity gradient persisted under alternative event definitions, stronger fixed effects, period cutoffs, and targeting restrictions, but its magnitude decreased by 34% when catches below 200 kg per daily equivalent were excluded. Seasonal declines also persisted within fixed fishing grounds and matched vessels. Catch timing advanced relative to reported-activity timing under every season window examined, although its absolute trend depended on the window. Because reported vessel-days do not measure total fishing effort and zero-catch trips were not consistently observed, these results describe a fishery-dependent observation process, not an abundance index or biological encounter probability. The late-autumn contrast remains sensitive to reporting selection.