This paper develops a Bayesian mark-recapture model for estimating both fishing and natural mortality, and integrates catch-at-age data into the estimation model, to aid in distinguishing between natural and fishing mortality. The model essentially follows tagged cohorts, with the estimated parameters being the fishing and natural mortality vectors for that particular cohort.
The model was tested against simulated data, and performed as expected. Mark-recapture data from toothfish at South Georgia was analysed with the model. The ages covered by the analysed cohorts ranged from 5 to 13. In the model runs, while fishing mortality-at-age was estimated, only one value of the natural mortality was permitted, as allowing both to vary can produce either non-sensical or nonconvergent results. For all the cohorts the model was applied to, a consistent picture emerged. Both natural and fishing mortality were estimated to be lower than those estimated in previous stock assessments using both mark-recapture and catch data. There was additionally some suggestion of agestructure in the natural mortality-at-age.
Abstract:
Trawl surveys are used in CCAMLR waters to estimate abundance of juvenile toothfish (Dissostichus eleginoides) and icefish (Champsocephalus gunnari). Documentation of some past trawl surveys has been insufficient to allow members of WG-FSA to fully interpret the survey results. This document suggests a standard pro-forma for reporting trawl survey results to WG-FSA based on survey reports for bottom-trawl surveys for hoki (Macruronus novaezelandiae) in the New Zealand EEZ.
