Release mortality of wild Atlantic salmon in coastal pontoon-trap fishery in the northern Baltic Sea

Abstract

The survival and injuries of wild Atlantic salmon released by coastal fishers from pontoon traps was examined in the early summer of 2020 in the Gulf of Bothnia in the northern Baltic Sea. A total of 491 salmon caught with pontoon traps was tagged with dart tags. By the end of December 2020, a total of 62 tag recoveries (13%) were received. Sixty-two per cent of the tag recoveries were obtained from the sea area, mainly from the commercial fishers in the Bay of Bothnia near northern river mouths. The remaining tags were received from river fishers. It took an average of 22 days from tagging to the capture (range 1–91 days), and the longest distance a tagged salmon travelled was more than 700 km. The mortality of salmon caused by capture and release was estimated to be a maximum of 25–31% for multi-sea-winter (MSW) salmon and a maximum of 24% for one-sea-winter (1SW) salmon. Most of the tagged fish were visually in good condition, although some injuries were observed. When two trap emptying techniques were compared, salmon caught in the Bothnian Sea with an emptying chute showed more injuries than salmon caught in the Bay of Bothnia with a lifting bag. Our results suggest relatively low release mortality for Baltic salmon. Furthermore, we show that disentangling release mortality from combined tagging and handling mortality is crucial in tagging studies.