Scale damage and survival of cod and haddock escaping from a demersal trawl

Abstract

Cod (Gadus morhua L.) and haddock (Melanogrammus aeglefinus L.) escaping through the meshes of a cod-end (135 mm diamond meshes) or through a metal grid sorting device mounted in the foremost part of the cod-end, were withheld in cages (2x2x5m) covering the cod-end during trawling. The scale loss of the escaped fish was examined and compared to that of cod and haddock from a control group. On average, less than 1% of the total body surface of cod was injured, while haddock, particularly those smaller than 40 cm, showed substantial scale loss. Cages containing escaped fish were released from the trawl by means of an acoustic releaser and kept for observation (UTV) at sea bottom for 12 to 16 days. No mortality was found for cod, and the mortality of mesh and grid selected haddock was less than 10%. Methodological problems that may have lead to errors in the mortality estimates are discussed.