Comparison of Statistical Catch-At-Age and Brownie Model Stock Assessment Methods for Saginaw Bay Walleyes

Estimates of mortality rates and stock size are critical for managing fish stocks. For walleyes in Saginaw Bay, Lake Huron, application of a Brownie model to tagging data has been the principle means for assessing dynamics of the population. Recently, a Statistical-catch-at-age (SCAA) model was developed. Both models offer many of the same metrics, but are based on different approaches and data. Having both allows us to compare and contrast the methods and evaluate the use of tag-derived metrics as auxiliary information in SCAA. Analysis indicates considerable differences in the two models for exploitation rates, forcing us to reconsider exactly what is represented by tagging. Model comparison demonstrates that the SCAA method provides opportunities to include additional fisheries and extraction forms that are not encompassed by the tagging and the recreational fishery alone. There are differences in stock size estimates and uncertainty in those estimates as well. The SCAA is highly sensitive to the choice of natural mortality supplied and using those from the Brownie model  results in more year specific values of total mortality. In general, it appears that SCAA methods are more robust for this particular stock but the integrated version may be superior to either model alone.