Estimation of the catchability of redfish and blue whiting for survey trawls in the Norwegian Sea

Abstract

Estimates of fish biomass, derived from hydroacoustic registrations are generally dependent on biological information from trawl catches. Catch composition is necessary for accurate quantitative allocation of acoustic energy to the different species present in the water column. For this purpose, the species composition in the trawl catch must be representative of the true species composition in the ocean, which requires that the catchability of the trawl for individual species is known. The catchability is expected to be trawl- and species-specific as a result of variations in fish behaviour in front of the trawl (fright reactions, swimming speeds and other factors) which can affect the chance of fish being caught or escaping. We show that catchability estimates for several species can be simultaneously derived from combined trawl-acoustic samples through a simple and straight- forward optimization method. We apply the method to estimate the catchability of two important pelagic species in the Norwegian Sea, beaked redfish (Sebastes mentella) and blue whiting (Micromesistius poutassou) when fished with a large pelagic trawl, the Gloria 2048.