A robust method for compiling trawl survey data used in the assessment of central Baltic cod ( Gadus morhua L.)

Annual stratified bottom trawl surveys have been used since 1982 to estimate cod abundance in the central Baltic Sea. Catch at age from national research vessels is included in a common database, but data compilation is hampered by large differences in fishing power between vessels, mainly due to differences in gear size and design. In this study, indices of cod abundance and biomass are developed from arithmetic mean catch rates per depth stratum scaled by stratum areas, but with national trawl catches corrected for gear specific fishing power. The correction factors were estimated from catch rates of adult cod of all ages aggregated to minimise the effects of age determination discrepancies. The established index of spawning stock biomass did not correlate significantly better with VPA estimates than simpler indices ignoring depth stratification and fishing power. However, the method is preferable because it considers factors known to influence survey trawl catches.