Relationship between catch, effort, CPUE and local abundance for non-target species, such as billfishes, caught by Indian Ocean longline fisheries

This paper analyses the local relationship between effort, catches, catch per unit effort (CPUE) and abundance of target species (such as tunas) and of non-target species (such as billfishes). The Indian Ocean longline fisheries are taken as an example. This paper evaluates the potential bias in the relationship between local CPUE and abundance when fisheries are increasing their fishing effort. A presentation of the Indian Ocean longline fisheries is carried out. A statistical analysis of CPUE is conducted using a generalized additive model which tends to indicate that the local effort is an important component in the statistical behaviour of the local CPUE. A migratory model in which both resources and fisheries are mobile was built. This model simulates the combined exploitation of two species, a target and a bycatch species, both fished at increasing intensity. This model confirms the potential bias as a result of the concentration of fishing effort in areas of high density of the target species. It also suggests that the CPUE of bycatch species may be more heavily biased because of their status. It is recommended that local fishing efforts should preferably be taken into account in order to calculate the CPUE of both target and non-target species.