The various factors that enter into the catch of fish by a baited longline are combined into equations that permit using the information afforded by the catches to compute linear estimates of abundance, providing that all rates can be estimated reasonably accurately, and assuming they are linear. The equations also serve to show the catches of a baited longline have a maximum in contrast to the asymptote of gill net catches. In practice, application of the suggested methods may not result in precise estimates of abundance, but they will serve to indicate the degree of confidence to be placed on the catch data and what precautions must be observed in interpreting the data, and they may serve to develop knowledge of the behaviour of a species. This knowledge, together with the knowledge of the several rates involved, may serve as a basis for examining fishing tactics.