Mortality and injuries of haddock, cod and saithe escaping throught trawl meshes and sorting grids

Abstract

Mortalities and injuries of the gadoids haddock (Melanogrammus aeglefinus L.), cod (Gadus morhua L.) and saithe (Pollachius virens L.) were studied after escapes from a bottom trawl through 135 mm cod-end meshes and a Sort-X sorting grid with 55 mm bar space. Two full-scale trials in the Barents Sea were carried out in 2000 and 2001, using commercial trawlers and bottom trawls. The fish that escaped were sampled using small-meshed cages attached to a cod-end cover and a cover enclosing the sorting grid. Trawl-caught controls were sampled by removing the cod end and attaching the cage directly to the cod-end extension. In the 2001 trial, control fish were also sampled in fish traps. The timing of sampling during trawling was determined by acoustic closing and releasing devices. Survival rates of cod and saithe that escaped through the cod end and sorting grid were 100%. Mortality rates of haddock were 26.2 to 50.4% (cod-end escapees), 1.6 to 20% (grid escapees), 4.1 to 26.5% (trawl-caught controls) and 0% (trap-caught controls). The haddock mortality decreased with increasing fish length in all groups, with a peak in the mortality k of the mesh escapees whose girth was approximately the mesh size circumference. Cod and saithe had significantly fewer skin and fin injuries than haddock, and the frequency of skin injuries increased generally towards the tail. Scale loss was also related to size, decreasing with increasing fish length in all groups. Fish that escaped from grids had fewer skin and fin injuries than the mesh and control groups. Causes of mortality and injuries are discussed and potential improvements in the experimental procedures are suggested.