Precautionary approach in Baltic herring trawl fishery: the effect of hauling techniques and engine power on unaccounted mortality estimates

摘要

Reduction of fishing mortality of immature fraction offish stock is one of the key principles of the precautionary fishery. In general, the mesh size in cod end is taken as the main parameter allowing regulating the catch size composition and the number of escapees through mesh in trawl fishery. However, in case of clupeids, the vast majority of escapees will die due to injuries after the contact with meshes, thus increasing the unaccounted fishing mortality of younger age groups but almost not affecting the total fishing mortality of fully recruited age groups. In order to find more effective technical measures to regulate the catch composition in Baltic herring trawl fishery the effect of hauling techniques and vessel engine power on the catch composition of herring were explored in the Gulf of Riga in 2002-2004. The method of comparative parallel trawl hauls was used to compare the 300 HP stern trawler (cod end hauling in aft) and 560 HP ramp trawler (cod end hauling in bow). In order to account the possible seasonal effect, investigations were carried out both in autumn and spring. Results indicate that in autumn the estimated unaccounted mortality of young herring was significantly higher for ramp trawler, while in spring the difference between the vessel types was not substantial. It was found, that trawl opening, depending on vessel's engine power, also should be taken into account as additional factor affecting the size composition of herring catch, particularly in autumn. This can be explained, by different vertical distribution pattern of juvenile and matured herring.