This paper develops an overview of the problems in estimating tuna abundance using purse seine fishery catch and effort data. This study is done on the purse seine fishery operating in the eastern inter-tropical Atlantic. Various potential biases in the CPUE biomass relationship are identified and discussed. Changes in the size of the fishing zone are first examined. A major problem in interpreting the CPUE is due to the large increases in fishing power observed for most purse seiners during recent years. While this increase in fishing power is due to various factors, technical factors among others, it is difficult to evaluate properly. The nature of the CPUE biomass relationship as a function of the level of effort exerted is discussed. As this relationship is not linear, it may introduce serious bias in the CPUE as a function of the effort which is exerted. Furthermore, the massive introduction of artificial floating logs, often maintained by auxiliary vessels, is discussed. Potential methods to estimate abundance using catch and effort data are discussed. The use of auxiliary variables in the abundance index is also discussed.
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