8.1 Exploitation
8.2 Result of acoustic and trawling surveys
8.3 Causes of the B. capriscus bloom
Since 1970, the occurrence of the trigger fish Balistes capriscus has been reported in various parts of the Gulf of Guinea. It has often been reported by fishermen that whenever B. capriscus occurs, no other fish is found in appreciable quantities.
Before 1969, this species was practically absent from the Gulf of Guinea (ORSTOM, 1976). A sudden increase in the abundance of the trigger fish was observed off Ghana in 1970. The occurrence of this fish was simultaneously reported on the continental shelves of Togo and Ivory Coast. The trigger fish accounted for about 60 percent of the total catch of small trawlers operating in Ghana. It is also reported that between 1972 and 1974, 70 percent of the total catch of trawlers operating from Lomé (Togo) were Balistes capriscus. The occurrence of this species has also been reported off Guinea, Guinea Bissau, Senegal, Nigeria (FIOLENT Survey, 1976)1 and Sierra Leone. It may occur on the shelf of Liberia, but no information is available for that sector.
1 Summary report: FIOLENT 1976 Eastern Central Atlantic Coastal Fishery Resource Survey: Southern sector. Table HD. 2. 10, Page 112. CECAF/TECH.REP/77/2/DAKAR
B. capriscus is exploited only in few countries in the Gulf of Guinea (Ghana and Togo). In other countries, because of its low market value, most of the catches are discarded at sea. It is therefore not easy to determine the total catch of this species.
Acoustic surveys conducted in various parts of the Gulf of Guinea confirm the occurrence of B. capriscus in the area. A high catch rate of 3 081 kg/h was observed between 20 and 50 m depth east of Cape Three Points (Ghana) by the FIOLENT Survey (1976). This survey estimated a trigger fish biomass of 68 300 tons for an area of 1 112 n·mi2 off Ghana. During the survey the catch of B. capriscus in some trawl hauls exceeded 20 tons. Off Guinea Bissau, Stequert (1977) estimates a B. capriscus biomass of 140 000 tons for an area of 1 400 n·mi2, in the sector south of Orongo islands. Results of the 'Ernst Haeckel' Survey in 1976 also confirm a high abundance of B. capriscus off Guinea (Zupanovic and Cissé, Appendix 8; Zupanovic and Cissé, 1977).
To date, the causes for the sudden bloom of this species remain undetermined. It is not known whether the cause is a local phenomenon or a global one. It is also not known whether the species is pelagic or demersal. Some studies on the biology of this species have been initiated in Ghana and Ivory Coast.
