Survival of juvenile herring (Clupea harengas membras) after passing through a selection grid in a pontoon trap

Abstract

A common problem in the Baltic Sea and Gulf of Bothnian herring trap fishery is the bycatch of undersized Baltic herring (Clupea harengus membras). By equipping a pontoon trap with a rigid grid with vertical bars, a substantial proportion of the undersized herring can escape from the gear by swimming through the grid. However, it has not been clear whether or not the young fish are injured by their passage through a grid and what their fate is. The purpose of this study was to investigate the short term mortality of herring after passage through a grid. To capture young herring, a selection grid was mounted at the entrance to the fish chamber of a pontoon trap, so that only young herring (approximately 12–18cm in length) could swim in. The trap was then closed and the herring were confined in the fish chamber for seven days before assessing survival rates. The same procedure was performed with control herring which were trapped without passing through any grid. The whole trial was repeated three times. On average about 7% of the young herring selected through a sorting grid died during the experiment. The results demonstrated that passing through a rigid grid did not affect the short term mortality of young herring during the seven days in captivity. The highest mortality (45%) was in fact observed in one of the control groups presumably in connection with extreme temperature fluctuations.