Long-term trends in the demersal fishery resources of Ghana in response to fishing pressure K.A. Koranteng Marine Fisheries Research Division, Tema, Ghana International Symposium on Marine fisheries, ecosystems and societies in West Africa: half a century of change Dakar, Senegal, June

Introduction The continental shelf of Ghana is narrow, varying in width from 13 to 80 km, and tends to drop sharply beyond the 75 m depth contour; Generally, the area beyond 75 m depth is not trawlable; thus, trawling activities are limited to a small part of the shelf.

Introduction (cont.) Ghana originally enjoyed rich fisheries resources, especially along its western coast due to seasonal upwelling; However, over time, these resources were subjected to increasing exploitation, resulting in a substantial decline in the abundance of targeted species.

Introduction (cont.) : high sea temperatures, low salinity, and a thermocline below its long-term average depth; : global decline in sea temperatures and a rise in salinity; : high temperatures, with salinity low and erratic. Three climatic periods with distinct environmental characteristics have been identified in the Gulf of Guinea for the period 1963 – 1992:

Introduction (cont.) This contribution examines: – Changes in the abundance and distribution of demersal species on Ghana’s continental shelf (assessed through bottom trawl surveys); –Variation over time in species composition of assessed stocks; –The effects of increasing fishing pressure on the resources.

Exploitation The marine fishing industry in Ghana is usually described in terms of three sectors: artisanal, semi- industrial and industrial; Bottom trawling for demersal fishes in Ghanaian waters began about five decades ago; The first industrial fishing trawlers were acquired about four decades ago, principally to fish in distant waters, mainly off Northwest and Southwest Africa.

Exploitation (cont.) The declaration of 200 m EEZs by West African countries in the 1970s forced the industrial trawlers to move ‘back home’ and to fish in Ghanaian waters; This marked the beginning of the apparent over-exploitation of demersal fisheries resources in Ghanaian waters.

Assesments (cont.) Trawling effort in the Ghanaian demersal fishery, 1972 – 1990 (---- semi-industrial; industrial)

Assessments (cont.) The effort increase led at first to a catch increase, then to the predicable decline…

Trawl surveys Between 1956 and 2000, a number of bottom trawl surveys were conducted on the continental shelf and upper slope off Ghana; For this study, the biomass and density of selected groups, and of the total stock were re-calculated for each survey using the NAN-SIS computer programme.

Trawl surveys (cont.) Biomass differences between Guinean Trawling Survey (1963/4; blue) and ‘Guinea 90’ survey (shaded)

Trawl surveys (cont.) The difference in density of all demersal species estimated between the two surveys is statistically significant (P = 0.02), with higher density recorded for the GTS than for the Guinea 90 survey; The reduction in density occurs at all depth ranges except beyond 100 m, where the density estimated from the Guinea 90 survey is higher than that of the GTS; The latter is due mainly to increased biomass of cuttlefish (Sepia officinalis) at these depths.

Trawl surveys (cont. ) Obviously, the changes differed between species groups:

The case of triggerfish From about 1973 and following the decline of the sardinella fishery, there was a dramatic increase in the abundance and landings of triggerfish (Balistes capriscus) in the Ghanaian demersal fishery; From 1979 to 1980, the demersal component of triggerfish, assessed in bottom trawl surveys, was 99,000 t, or 5.6 t per km 2, i.e., about 60% of the demersal fish biomass in Ghanaian coastal waters at that time.

Triggerfish (cont.) Triggerfish declined in abundance from about 1988, when landings were well below estimated ‘potential yield.’ Only a few specimens were caught in surveys conducted in the 1990s; Although monitoring of catches of the species was poor – there were large discards at sea - fishing alone doesn’t seem to explain this decline; Rather, this is attributed to environmental changes somehow related to the ‘climatic’ periodicity described above.

The Ghanaian shrimp fishery (I) The fishery lasted from the late 1960s to the mid 1970s; An average of 720 t of shrimps (mainly Penaeus notialis and Parapeneopsis atlantica) was caught annually; The potential yield of Penaeus notialis alone was estimated to be t per year; The fishery collapsed for various reasons, foremost overexploitation.

The Ghanaian shrimp fishery (II) A new fishery developed, with about 5.4 t taken in 1984 (mainly P. notialis); Landings then increased to a maximum of 317 t in 1995; MSY was estimated at about 350 t per year; The low shrimp-to-fish catch ratio, the high volume of discards (made up mainly of juvenile demersal fish) and the general destruction of the inshore habitat by shrimp trawlers, called for stringent management measures; These measures came too late, and the fishery has (again!) virtually collapsed.

Other groups In three recent surveys (Feb – Sep. 2000), large quantities (up to 624 kg/h of trawling) of previously unreported scallops (Chlamys purpuratus, Pecten jacobeus) were caught, constituting % of the catch of some hauls; Other groups can be expected to appear on the Ghanaian shelf, and in the fisheries, then to wane, as did the triggerfish; Unfortunately, these periods of boom and bust do not offer a substitute to the steady supply previously provided by abundant demersal fishes. Thanks for your attention