Catchability estimates and their application to the red grouper (Epinephelus morio) fishery of the Campeche Bank, Mexico

We applied a length-based additive catchability model that accounts for sev eral sources of variation, namely time (i.e. years, months), density-depend ence effects, and different fishing fleets. The model is based on 1) a tran sition matrix and 2) population length-structured data expressed as catch p er unit of effort. Other sources of variation were estimated as anomalies f rom the average pattern and incorporated as additions to the slope of the c atchability-at-length equation. The catchability model was applied to the r ed grouper (Epinephelus morio) fishery of the north continental shelf of Yu catan, a demersal fish resource exploited by three different fleets. A sigm oidal shape catchability-at-length function was fitted on the basis of grou per population biology and behavior, particularly reproductive aggregation. Catchability of immature fish was constant but increased with size for adu lt fish, especially during the reproductive season. Time- and density-depen dent catchability responded to reproductive behavior and the allocation of fishing effort. When differences between fleets were incorporated, catchabi lity differences emerged. The catchability model has the ability to identif y the main properties of the fish resource and fishery that affect the rela tion between fishing effort and population abundance; it may therefore be h elpful as an alternative stock assessment tool.