Size-related mortality of herring (Clupea harengusL.) escaping through a rigid sorting grid and trawl codend meshes

Abstract

Studies on herring, Clupea harengus (L.), were conducted in the northern Baltic to assess whether the mortality of herring escaping through a rigid sorting grid (12 mm bar spacing) placed in front of the codend was diVerent from that of herring escaping from a 36 mm diamond mesh codend. Escapees were collected into netting cages, and subsequently transferred into large holding cages (85 m 3 ) where they were held for up to two weeks to assess mortality. 76‐100% of small (<12 cm) and 44‐83% of large (12‐17 cm) escapees were dead after 7 days. The 14-day mortalities were 96‐100% and 77‐100% for small and large escapees, respectively. In the spring experiments, survival of large herring that escaped through the grid was 7‐18% higher than that of codend escapees and, in the autumn experiments, was 2‐7% higher. Survival of small escapees was not improved by the use of a sorting grid. Observations on escapees suggested that they suVered considerable loss of scales during capture, and that injured skin areas were often seriously infected within a few days of escape. Herring caught by seine and handline, used as controls, were generally in good condition, but they also experienced mortality. After two weeks their cumulative mortality reached 9% in spring and about 55% in the autumn. On the basis of our results, we argue that a considerable part of the mortality observed in herring escapees may be attributed to mechanical contacts with the trawl netting and exhaustion during the capture process, and that damage can occur in the net long before fish reach the codend. ? 1996 International Council for the Exploration of the Sea