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Report2015

Överlevnad hos återutsatt röding vid trollingfiske

Norrgård, Johnny; Sandström, Alfred; Alenius, Beatrice

Abstract

Recreational fisheries are becoming increasingly important in Europe, particularly in freshwater systems. A significant proportion of the catch in this fishery is often released back into the water. The main reasons for returning fish is that its size deviates from the legal size window, that the daily catch quota already is full or that the fish is released due to ethical concerns. In this study we investigated the release mortality of Arctic char (Salvelinus salvelinus) in Swedish Lake Vättern. The Arctic char population is currently recovering from several years of decline, at least partly due to overexploitation in the fishery. The fisheries on Arctic char and many other fish species (trout, salmon etc.) in Lake Vättern used to be dominated by the commercial sector, but during the last 10-15 years the majority of the catch instead is landed by the recreational fishermen, particularly in trolling. A significant proportion of the catch in this fishery is released (68 %). We studied the potential release mortality in the trolling fishery in Lake Vättern in the late parts of the summer of 2012. We did visual observations of released Arctic char caught in trolling fishery, and marked a subsample of the released individuals with acoustic transmitters in order to follow their fate over a longer time period after their release. Our results show that roughly one third of the released fish died. The most critical period was the first hour after release. Ten percentage of the released fish died within a couple of minutes after release due to injuries related to the fishing method, 19 % was exposed to predation from birds while staying close to the surface and 2 % died after diving down from the surface to deeper waters. Our study was conducted in late summer when fishing intensity is the highest. During this time of the year water surface temperature is the highest, and Arctic char are normally caught at 20-40 meters water depth below the metalimnion. We also conducted a simplified stock analysis to assess the influence of release mortality on the population of Arctic char. We compared two scenarios: a release mortality of 30 % and 0 %. We assumed that fishing effort, size selectivity and catch was on the same level as in the year 2010 and used data from a fish monitoring program that same year for tuning. Our analysis show that without the mortality associated to release of fish in the recreational fisheries the fishing mortality would decrease with 25 %, the catches in the fishery would increase with 13 % and the biomass of Arctic char would increase with 9 %. We conclude that release mortality has the potential to dampen the ongoing recovery rate of the Arctic char stock and that this phenomenon deserves higher attention among fishermen as well as managers. Further attention should be allocated to developing measures to mitigate this problem and to develop guidelines for handling of fishes that are released, preferably in collaboration with local fishermen.

Published in

Rapport från Vätternvårdsförbundet
2015, number: 118Publisher: Vätternvårdsförbundet