Estimation of mortality rates for Gulf Menhaden

The Gulf Menhaden ( Brevoortia patronus ) stock assessment uses an age-structured statistical catch-at-age model that includes natural mortality rates estimated from a tagging study. These estimates are based on work by Ahrenholz et al. in the 1970s. In this work, we develop contemporary analytical methods to analyze data from a 19-year tagging study conducted from 1970 to 1988 in which adult and juvenile Gulf Menhaden were tagged, released, and recovered in fish processing plants. We constructed a Bayesian mark-recovery model using the negative binomial distribution to estimate natural mortality, catchability, and an overdispersion factor. We then explored sensitivity analyses to evaluate the precision of parameter estimates. The base model estimated a constant instantaneous natural mortality for Gulf Menhaden of 1.082 y ˗1 (95% CI: 1.03 y ˗1 to 1.13 y ˗1 ), which is comparable to the estimate by Ahrenholz (1981) of 1.0935 y ˗1 . The sensitivity analyses indicated that the parameter estimates were robust to assumptions. The inclusion of updated natural mortality rates based on a longer time series and newer tag-recoveries models will directly improve the assessment model for the Gulf Menhaden stock and enhance management reference points and advice.