Assessment of demersal fish stocks in Ghanaian and adjacent waters

Abstract

a) Five stocks of demersal fish important to Ghana are assessed for the period 1990-2016. These are based on the stock area as used by FAO that also includes catches from Cote d’Ivoire, Togo and Benin. The stocks assessed include bigeye grunt (Brachydeuterus auritus), sea breams (Dentex spp), threadfin (Galeoides decadactylus), red Pandora (Pagellus bellottii) and croakers (Pseudolithus spp). b) Ghana accounts for the largest proportion of the total catches for all stocks from the four countries in the assessment unit. Total catches are variable but do not show a long term trend for three of the five stocks. There is evidence of a recent decline in catches of sea breams and an increase for threadfin. c) Fishing effort data are available for 17 fleets. Effort trends are highly variable but Ghana industrial trawl shows a marked increase which is also reflected in high catches of some species for this fleet. d) Catch per unit effort data (cpue) suggest that all stocks have declined from the 1990 level but may have stabilised or increased in recent years. However, the trends may be over-optimistic as the effort data are not corrected for increases in fishing power. e) A surplus production model is developed to assess the stocks using catch and effort data by fleet. The model also accounts for increasing fishing power over time and missing catch data. Simulation testing of the model shows that it is able to recover the correct parameter values. f) All five stocks show similar downward trends in stock biomass and increasing fishing mortality. They all appear to be fished above FMSY and the current biomass is below BMSY. The estimates of FMSY are uncertain but the estimated stock status relative to MSY is robust to different modelling assumptions. g) In four of the five stocks Ghanaian fleets dominate the fishing mortality with the industrial trawl having a large impact on sea breams and red Pandora. Cote d’Ivoire and Benin make a large contribution to fishing mortality on croakers but otherwise only comprise a small portion of the mortality on other stocks. h) Estimates of the rate of increase in fishing power vary considerably between fleets and species. Most notable is the above average increase in fishing power by Ghanaian and Benin industrial trawl fleet fishing for seabreams and red Pandora. A smaller increase but similar pattern is also evident in the Ghanaian artisanal hook and line fleet. i) Equilibrium analyses suggest that all five stocks are at high risk of stock collapse at current (2016) rates of fishing. Much of the catch appears to be dependent on in year production (recruitment and growth). j) Despite uncertainty in the estimates of BMSY and FMSY, the model described in the analysis provides the basis for a multi-fleet, mixed fishery model that can be used to investigate management scenarios and economic impacts.