Evaluation of two methods for indexing fish year-class strength: Catch-curve residuals and cohort method

Assessment of fish year-class strength is often a key objective of fish monitoring programs. We used a simulation model to assess the performance of two methods for indexing fish year-class strength from catch-at-age data on adult fish populations: catch-curve residuals and tracking cohorts through time. For this comparison we used three performance metrics: correlation with true recruitment values, ability to identify strong and weak year classes, and prediction of relative year-class strength. Both the catch curve and the cohort methods provided reliable indices of recruitment under some conditions. The catch curve method provided reliable evidence for very strong or weak year classes, whereas the cohort method expectedly was able to reliably index year class strength for even moderate fluctuations in year class strength, and thus would provide managers with more detailed and reliable information about year class strength. However, the cohort method required multiple years of age composition data, whereas the catch curve method requires only one year of data. In addition, performance of the cohort method was reduced when variation in annual sampling catchability was high. Our simulation approach allowed for a detailed understanding of the strengths, weaknesses, and data requirements of both methods. While both methods provide information that is useful for fisheries management, appropriate application of either method for monitoring should depend on management needs, funding availability, and system characteristics.

► Simulation model was used to evaluate methods for indexing year-class strength.
► Cohort method outperformed catch-curve residuals as an index of year-class strength.
► Cohort method required multiple years of age composition data.
► Cohort method provided reliable and accurate measures of year-class strength.