Balistes capriscus is a member of the coastal strip sparid community (Longhurst, 1969), During the Gulf, of Guinea Trawler Survey (GTS, 1963-64) the species was encountered regularly but in small numbers in three sectors, namely over hard bottoms located respectively off Guinea, Ghana and Gabon. As of 1970 the Ghanaian triggerfish population had expanded to become the dominant benthic fauna whereas in 1969 and 1970 triggerfish made up only three percent of the biomass along a transect visited monthly at right angles to Tema. The species expanded simultaneously westward, where it affects two-thirds of the Ivorien continental shelf, and eastward where triggerfish have been reported in Togo, Benin and Nigeria as far as the mouth of the Dodo River (5°40'E - 4°50'N). A similar phenomenon occurred off Guinea, but nothing of the sort has been reported off Gabon. The acoustically estimated biomass over the Ghanaian continental shelf in 1976 was 68 300 t (Fiolent survey). A trawler survey in February 1978 (Chalci I) led to an estimate of over 8 000 t of Balistes or more than one-quarter of the total benthic biomass, over the entire Ivorien continental shelf.
Studies of Balistes biology were begun in Ghana in 1976 where the species is marketed (Ansa-Emmim, Appendix 3), and in Ivory Coast where it is discarded. The Ghanaian Balistes catch, mostly from 8-12 m trawlers, rose from 3 031 t in 1972 to 8 826 t in 1976. Balistes accounted for 80-90 percent of the catch of this class of vessel (Appendix 3).
Considerable modifications in the distribution of benthic biomasses were observed to coincide with the appearance of Balistes. Figure 3 shows the mean annual distribution of the biomass calculated from systematic surveys along the Bassam transect (Ivory Coast) in 1966-67 (Troadec et al., 1969). The biomass involves an early maximum at 35 m, a minimum between 40 and 60 m, and a second maximum at 80 to 100 m. This biomass distribution corresponds rather well to the differential distribution of croaker and bream communities according to depth (Figure 4).
Figure 5 shows the biomass distribution along this same transect, during the Bassam 19 survey (February 1967) and the Chalci I survey in February 1978. Whereas the biomass distribution in 1967 is typically bimodal and there are no Balistes, by 1978 there is only one maximum from 40-60 m consisting essentially of Balistes and which appears to coincide with the ecological niche occupied by semipelagic eurybathic species such as Brachydeuterus auritus.
Little is as yet known about the biology of Balistes capriscus. The first observations were carried out in 1972 and 1973 in Togo by Beck (1974); the juveniles (8-12 cm) are pelagic. Specimens from 17-20 cm are plankton-eaters and those of larger size have a much more varied diet. Beck also noted the presence of breeding Balistes in November 1973 and the total absence of reproduction in September, October and January. Ansa-Emmim (Appendix 3) points out that the most advanced stages of maturity are observed at the close of the year. He indicates sharp seasonal fluctuations in availability and points out that trawler yields drop sharply during the cool season. This was also noted during the August 1978 Chalci I survey in Ivory Coast.
Given the lack of adequate data on the distribution and biology of the species, the Working Party recommends:
- that the biomasses be monitored as much as possible, either by means of commercial statistics where these represent abundance, or by special surveys (trawler, acoustic),- that the geographical and spatial extent of the phenomenon be pinpointed, and
- that biological studies of this species be intensified, particularly studies of stomach content for all size ranges and for the various, seasons, so as to distinguish the trophic links with other species.
