In the Brazilian skipjack tuna (Katswonus pelamis) pole and line fishery, large seasonal oscillations of catch per unit of effort (CPUE) has been observed. As an index, CPUE should reflect variation in the coefficient of catchability (q) as well as in the abundance of the exploited populations. In opposition to the interannual variations, seasonal variations in the population abundance are unimportant in the conceptual framework of most fishery assessment models. Therefore, a plausible simplification would be assume the analysis of factors affecting seasonal oscillations of CPUE as equivalent to the analysis of the seasonal oscillations of the coefficient of catchability. In the Brazilian skipjack tuna fishery, recruitment, migration, oceanographic circulation, sighting of schools, variation in the food availability and variations in the number and dimension of the schools have been identified as the main factors affecting seasonal oscillations in q. The study of these oscillations is important in order to identify and understand also the interannual oscillations in q which may prevent the use of CPUE data in the fishery assessment. Problems of interannual oscillations in q are generally related to the choice of the unit of effort. In the case of the skipjack tuna fishery, the estimation of an index reflecting the search effort (e. g. swept area), seems to be essential for the establishment of a more suitable unit of effort to be used in its stock assessment.