Size selection during trawl sampling of cod and haddock and its effect on abundance indices at age

Investigations of length selectivity of the standard Norwegian bottom trawl has revealed that small cod and haddock were largely under-represented in the catch. To minimize the effect of length selection on abundance indices at age obtained from surveys, a new standard sampling trawl has been used since January 1989. Based on parallel trawling experiments, conversion factors for comparing catches from the two sampling gears are available from previous work. Using these factors, data from 1983 to 1988 are adjusted to produce indices consistent with those from 1989 and 1990 which were obtained with the new sampling trawl. The new time series appears to be more accurate than the old series, which was biased by large yearly variability in the growth rates of small fish. Cod were more affected than haddock. The validity of combining results from two different sampling trawls into one index time series is discussed.