Assessment of small pelagic fish stocks in Ghanaian and adjacent waters

Abstract

a) Five stocks of small pelagic fish important to Ghana are assessed for the period 1990-2017. 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 anchovy (Engraulis encrasicolus), round sardinella (Sardinella aurita), flat sardinella (Sardinella maderensis), bonga shad (Ethmalosa fimbriata) and Cunene horse mackerel (Trachurus trecae). 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 and show some long term decline for four of the five stocks. Catches of horse mackerel have increased. c) Fishing effort data are available for 6 fleets. Effort trends are highly variable but Ghanaian fleets, that account for most of the catch show a marked increase. d) Catch per unit effort data (cpue) suggest that all stocks, except horse mackerel have declined from the 1990 level but may have stabilised or increased slightly in recent years. However, the trends may be over-optimistic as the effort data are not corrected for increases in fishing power. Horse mackerel cpue shows no long term trend. e) A surplus production model is used to assess the stocks using catch and effort data by fleet. The model also accounts for increasing fishing power over time, missing catch data as well as observation errors in the catch and effort data. f) Four of the five stocks show similar downward trends in stock biomass and increasing fishing mortality. They 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. The horse mackerel stock shows no clear trend in biomass and while is appears to be fished above FMSY, the stock status is highly uncertain. g) In all five stocks Ghanaian fleets dominate the fishing mortality with the artisanal fleet having a large impact. Cote d’Ivoire makes a large contribution to fishing mortality on sardinella and horse mackerel. Togo makes a significant contribution to anchovy mortality h) Estimates of the rate of increase in fishing power vary between fleets and species. Most notable is the above average increase in fishing power by the Togo artisanal fleet for anchovy and round sardinella, and the Benin artisanal fleet for most stocks. i) Equilibrium analyses suggest that all five stocks are at high risk of stock collapse at current (2017) rates of fishing, though there is large uncertainty in the case of horse mackerel. 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, fishery model that can be used to investigate management scenarios and economic impacts.