The Effect of a Change in Perception of Length Distribution of a Population on Maturity-at-age,

The calculation of proportion mature-at-age and mean weight-at-age from length stratified sampling are dependent on the length distribution of the population. The Dept. of Fisheries and Oceans in St. John's, Newfoundland changed its survey bottom trawl in the autumn of 1995. Comparative fishing experiments between the old and new fishing gears showed that the new gear (Campelen) caught more small fish of most species than the old (Engel) gear. Conversions of the Engel time series to Campelen equivalents results in an increase in the number of small fish in the population. This paper examines the effect of this change on maturity-at-age, mean weight-at-age and spawning stock biomass (SSB) for American plaice (Hippoglossoides platessoides). The shift in the perceived length frequency distribution of the population results in an increase in age at 50 % maturity (the fish appear to be maturing later) and a decrease in mean weight-at-age. Spawning stock biomass calculated from these parameters is higher for the Campelen equivalent data because of the increased abundance at age. Trends in maturity-at-age, mean weight-at-age and SSB over the time period are generally the same for the Engel and Campelen equivalent data. Constructing a time series, which consists of unconverted Engel data followed by Campelen data, can be misleading. The change in estimated maturity and weight-at-age, and SSB has implications for both biological studies and the setting of reference levels