Population parameters, biannual cohorts, and assessment in the Pagellus bellotti (Sparidae) fishery off Ghana

Dissection of length-frequency data from research cruises using mixture analysis shows that two cohorts of Pagellus bellottii (Sparidae) enter the fishery off Ghana each year. The separate cohorts may be followed in time until their size distributions merge at about 3 years of age. Beyond this age, mixture analysis suggests that the fishery includes a further 3 age groups. The cohort spawned before the major coastal upwelling in June has faster growth and higher natural mortality than the cohort spawned in the minor January upwelling. Possible reasons for these differences are discussed. This analysis provides values for growth and mortality which can be used in a preliminary long-term yield assessment. Current bottom trawling on the Ghanaian shelf uses 60-mm or smaller mesh sizes, which catch fish below the age of maturity: our analysis suggests that although the fishing mortality rate could be increased, the risks associated with larger meshes would be lower. (DBO)