A fleet based surplus production model that accounts for increases in fishing power with application to two West African pelagic stocks

Assessments of many West African fish stocks rely on fishery dependent catch and effort data. Typically, these treat the catch data as error free and some assume that fishing power does not change over time. To address these issues we develop a fleet based surplus production model that accounts for increases in fishing power. It allows errors both in effort and catch data so avoiding the assumption that catch data are exact. Mean annual fleet fishing power increase can be estimated when data from multiple fleets are available provided it can be specified for at least one fleet. The model is tested using simulated data and then applied to western stocks of anchovy (Engraulis encrasicolus) and bonga shad (Ethmalosa fimbriata) in the Fishery Committee for the Eastern Central Atlantic (CECAF) area. Both stocks appear to be over-exploited and near to collapse. Corrections for fishing power are important in the anchovy assessment and help to explain conflicting trends in the data. Uncertainty in the assessments is explored with a range of sensitivity tests.