Natural Mortality Estimation Within Statistical Catch-At-Age Models for Gulf of Mexico Red Snapper: A Comparison of Modeling Assumptions

The instantaneous rate of natural mortality is one of the most important population parameter to be estimated for an assessment of a species. Many stock assessments currently use a secondary model to estimate the rate of natural mortality from observable life history characteristics. Statistical catch-at-age models were created using data for red snapper in the Gulf of Mexico. Data available includes 11 catch time series, 11 age frequency time series, 2 effort series and 13 abundance indices.  Models incorporating numerous assumptions about selectivity and natural mortality at age were constructed and parameters estimated. The models were compared and selected using information based criteria. The best-fit model resulted in estimates of natural mortality approaching the lower bounds of the parameter. Models that assumed selectivity parameters for Eastern and Western Gulf of Mexico fisheries were identical fit the data better. Estimated values of natural mortality for age-0 and age-1 red snapper in the Gulf of Mexico using a statistical catch-at-age model are highly dependent upon the modeling assumptions associated with selectivity-at-age for the shrimp bycatch fishery used to fit the age frequency data. Natural mortality estimates for age-2 and older age classes were fairly consistent between models.