Assessment of small pelagic fish stocks in Ghanaian and adjacent waters
“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 Côte
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 trecea).
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. Côte 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….”