by P. Showers
The demersal resources constitue not less than 60% of the resources of the shelf and they constitue about 10–15% of the national landings. It is therefore the industrial fleet that exploits the greater part of the demersal fish, operating from about the 10 m isobath to the end of the shelf. The demersal stocks have not been studied in detail as much as the pelagics. The easy accessibility of clupeids and carangids has attracted the attention of many sampling programmes. The demersal nevertheless cover a very wide range of species including the elasmobranchs, teleosts, crustaceans and molluscs, from the near-bottom and controversial bastids to the muddy shrimps.
One major factor affecting the distribution of the stocks and accounting for the fluctuations in biomass estimates is the hydrography of the two main seasons namely the rainy and the dry season.
Balistes capriscus
A decade of 50 ago, the grey trigger fish, Balistes capriscus, was considered the most abundant shelf fish in Sierra Leone. Stromme et al. (1982) stated that the southern boundery of the Guinean stocks of balistes lies off Sherbro Island. Acoustic results of R/V EVRIKA in December 1976 estimated the biomass at 78000 t. The highest estimate of 273300 t was obtained in 1980 and since the 1981 estimate of 100000 t a marked decrease of the species was observed.
The seasonal migrations of balistes over the years show a curious parallel with that of the flying gurnard, Dactylopterus volitans, a species considered to be next in abundance to the trigger fish. Balistes migrates downwards from Guinea in the dry season and do the reserve in the rainy season (nov-may). Accord to Burdia and Kovastsenko (1986), balistes and dactylopterus perform counter migrations in the northern half of Sierra Leone waters.
Sazonov (1987) used the first dorsal spine to age the species and obtained the following: I-16 cm, II-15–24 cm, III-20–26 cm, IV-24–33 cm, V-27–34 cm. The Ford-Walford plot gave growth parameters as follows: L o = 43.59, k= 0.18. Fecundity studies noted that the maturation of oocytes and single spawning occur simultaneously. Absolute fecundity = 181620 ± 85665 relative fecundity = 1198 ± 331 (sample length ranges = 18–22 cm FL). L50 = 13cm.
Balistes is a principal consumer of eggs. Sea urchins and shrimps are the main food items, followed by crabs. Biomass estimates for 1987 and 1990 using “swept area” are 32260 t and 1588 t respectively. This drastic fall in the stock might be attributed to hydrographic changes.
The magnitude of the seasonal stocks (the sea basses) has been considerable through the past decade, their distribution in the northern part has been rather patchy due to the heterogenity of the shelf, but more uniform in the south. Concentrations seem to shift to smaller depths in the rainy season.
Cephalopods and crustaceans
As concerns the cephalopods and shrimps, 29 species at least have been recorded of which four shrimps species show commercial potencials. Shrimp catches have yielded up to 40 kg/hr and this catch rates should be considered moderate: “Hake -4 m” is not a specialised gear for shrimping. Parapenaeus longirostris is the most abundant deep sea shrimp species. It occurs along the shelf between 50–300 m, tout tends to concentrate on silty grounds in the south at depths greater than 200 m.
The length of males increases in depth as follows for females:
72 mm - 50 – 70 m
92 - > 200 m
107 - > 300 m
From the length curves of both sexes it is evident that the growth rates are identical till 57 mm after which there is a divergence and the female superseeds the male. The life span is about 3 years for both sexes. The sex ratio, male/female, varies with depth:
1.1.) from 1 to 50 – 90 m;
1.2.) from 50 – 90 to 200 m;
2.1.) between 200 and 300 m.
Spawning occurs in february and july. Those inhabiting the 50–90 m zone spawn before the ones inhabiting more off-shore waters at 200–300 m.
Parapenaeus longirostris feeds mainly on crustaceans (43%) and polychaets (22%), and to a lesser degree on fish larvae and molluscs. The larvae consists of the fry of Myctophidae, Chlorophthalmidae and Sparidae. In terms of their frequency of occurence the food consists foraminifera (80.6%), shrimps and euphausids (69%), molluscs (66%) and fin fish (59%) (Research Course, Fisheries Department).
The other three commercial shrimp species are: Penaeus notialis, which occurs off the Freetown peninsula, P. kerathurus, occuring primarily north of the Sierra Leone river, and Parapenaeopsis atlantica, occuring mainly in the southern part.
An analytical approach was recently used to assess the MSY for shrimps, based on the Thompson and Bell yield per Recruit model. Data on the life cycle, growth and mortality rates were taken over from Garcia and estimates obtained for senegalese stocks (Garcia et Lhomme, 1977). Total mortality estimates were derived using commercial size categories of local catches: Z = 0.31 (1982), Z = 0.24 (1983). An MSY estimate of 2500 t was obtained and fuether runs of the model using the highest and lowest estimates (F= 0.04 and F= 0.11) gave the corresponding estimates of 4000 t and 2200 t.
A more detailed study of the stocks of cuttlefish, Sepia officinalis was undertaken during the bottom trawling survey in 1987 on board the R/V ATLANTIDA. The high commercial potential of this valuable species has been realised locally and it was suspected that the resources are undergoing very high fishing exploration and thus the need to estimate the magnitude and distribution of this cephalopod on the shelf. Results showed highest catch rates in the 11–20 m zone at night, and in the early hours of the morning between 23-03 hrs; lowest catch rates were in the day around 15 hr. Sex ratio shows a preponderance of males to females in the prospection of 1/3 and length at first maturity was 11.0 cm (male) and 92.5 cm (female).
