Research Document 1997/140
The effects of sampling time on age-composition test fishing samples
By R. Tanasichuk
Test fishing data collected since 1972 from all major stock assessment regions was used to determine if age compositions varied over time during the test fishing programs. This analysis was conducted to improve the performance of the age-structured model, particularly in the Prince Rupert District. Analysis consisted of fitting logistic regressions of the proportion of age 2+ herring (the usually dominant age-class) on sampling date. The proportion of age 2+ fish changed over time by at least 20% in 8% of the fishing season/assessment region groups. It is difficult to interpret these results. For the three major herring stocks above Vancouver Island, test fishing ends substantially before spawning ends so spawners are likely not sampled completely. Detailed analysis of the age composition data for Port Simpson/Big Bay and Kitkatla indicated that the age compositions, based on test fishing samples, are consistent between areas for only 3 of 18 years in this assessment region.
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