Sca mounts and tuna in the Tropical Eastern Atlantic.

It is well known by fishermen that large biomasses of tuna are sometimes concentrated on sea mounts. Many sea mounts arc known in the eastern tropical Atlantic. This phenomenon has surprisingly never been studied by scientists. The present paper makes a review of tuna catches by the international tuna fleets during recent years (1980 to 1987) in the immediate vicinity of each of the sea mounts known in the area. Several thousand tons of tuna can be taken yearly on some sea mounts. Other sea mounts are apparently always poor in tuna, even when they arc located in regular fishing areas. The major biological characteristics of the association between sea mount and tuna are analyzed: three species - yellowfin (Thunnus albacares), skipjack (Katsuwonus pelamis) and bigeye tuna (Parathunnus obesus) - are usually taken, in similar proportions on the sea mounts. Those tuna are smaller on the sea mount compared to adjacent areas. Some hypotheses are developed to explain the causes of this concentration phenomenon and the dynamics of this aggregation, using the daily catch and effort statistics of fisheries, and the results of regular and sonic tagging. The prospects for future research on this topic of tuna aggregation on sea mounts, and the rational exploitation of those structures are discussed. As the exact locations and depth of the sea mounts arc presently easily known by satellite imagery, it should be possible, in the immediate future, to know the exact positions of all the sea mounts in the area, which could help the fishermen to fish more efficiently.