Bias In Estimates Of Growth When Selectivity In Models Includes Effects Of Gear And Availability Of Fish

Stock assessment models use data influenced by distribution patterns that are due to the nonrandom movement of fish, which can create bias in the assessment. For many stocks, length data are used to characterize the age structure of the population, and therefore there is a need for unbiased estimates of growth. Because of the influence of size-selective fishing gear, growth and length-based selectivity are often estimated as part of an assessment model to account for the size selection of gear. However, estimated selectivity can include not only length-based gear selection, but the biological aspects of the spatial availability of the target species. If availability to the fishing gear is a function of age, an approximation of an age-based process as a length-based one can bias growth estimates. The magnitude of the bias would be greater for fish with highly variable growth and for those with strong age-based distribution patterns.