5. POTENTIAL RESOURCES AND PRESENT STATE OF EXPLOITED STOCKS

5.1 Nominal Catch Data

In spite of extensive statistical activities of the Statistical Unit of the Fisheries Division since its establishment in 1978, long-time series of detailed catch statistics are not readily available in Sierra Leone.

The first Frame Survey was conducted during 1972 and 1973, and the second was conducted in May 1982. The Frame Survey provides the most basic statistics of the artisanal fisheries, that is, the number of fishing boats, and such statistics are used in the Catch Assessment Survey for estimating the total catch in each district.

The reported nominal catch data for the combined artisanal and industrial fisheries in the marine sector are not fairly well allocated by commercially important exploited species. The total marine catches are said to have steadily increased from about 29 600 t in 1971 to about 118 960 t in 1982 (see Table 8). Soviet fishing fleet accounted for more than 60% of the catch of foreign vessels. It is shown in Table 8 that foreign fleet caught about 106 200 t in 1980, 106 200 tin 1981 and 82 400 t in 1983. Soviet frozen fish production for the period 1980–83 and the 12% share for Sierra Fishing Company are shown in Tables 9–12. It is shown that the Soviet catch increased very rapidly between 1975 and 1978, as indicated in Table 13. The pelagic species (bonga, Sardinella and Ilisha africana) contribute more than 68% of the artisanal catch. The catch trends of pelagic fish species for the artisanal fishery are shown in Table 14 and the relative contribution of various species to artisanal landings is given in Table 15.

The collection and compilation of nominal catch data for the artisanal fishery is made more difficult by use of local fish names. In several instances a local name refers to two or more species (see Appendix 4). In these circumstances, estimates of catch magnitudes of various species are biased.

The trawl fishery has the following demersal target species: croakers (Pseudotolithus), soles (Cynoglossus), Pomadasys jubelini, various sparids and threadfin (Polynemidae). Besides, the trawl catch includes a significant proportion of the family Carangidae.

The importance of various demersal fish species in the trawl fishery during the late 1970s, before trawling was intensified, is illustrated in Table 15 (Italian inshore trawlers in 1976) and Table 16 (Ghanaian trawl fleet in 1978). It is evident that the croakers (Pseudotolithus), threadfins (Polynemidae), grunters (Pomadasyidae), seabreams (Sparidae), tonguefishes (Cynoglossidae), contributed significantly to the trawl landings.

The annual total shrimp catch from the industrial vessels is now put at about 1 800 t (head-on weight). It is noted that in 1982 and 1983, Sierra Fishing Company (SFC) landed 960 t and about 1 100 t, respectively. Marine Development Company (MDC) landed about 780 t and 640 t for 1982 and 1983, respectively. The total annual shrimp catches for Marine Development Company for the period 1980–82 are given in Tables 16–18. It can be shown from the same Tables that MDC obtained more than US$ 1 million from its shrimp landings.

5.2 Catch Per Unit Effort (cpue) Data

Information on fishing effort expended to obtain a certain quantity of catch is scanty. In most instances only the weight of the catch and the value of fish are recorded. Information on number of gillnets, castnets, hooks, beach seines, etc., used is still inadequate. Additionally, there are no data on catch rates and discards from shrimp trawlers or other major gears operated by the industrial fleets, but the discards are likely to be considerable. Reliable data on separate catch and effort magnitudes in the creeks, estuaries, coastal lagoons and inshore waters are still lacking. However, general trends in fishing effort exerted by the artisanal fishery are given in Tables 3, 4, and 5. Additionally, fishing trends in the bonga fishery are given in Tables 19 and 20.

a The 1980 total shrimp production (heads-on weight) byMarine Development Company alone is 1 143.2 t

Information on fishing effort of the industrial fleet could be derived from the list of trawlers and shrimpers licensed to fish, but there are no reliable historical data on the actual number of vessels which have fished in a particular period; neither is there reliable information on the duration of fishing. Inference on catch rates can be made on some Soviet vessels which fished in Sierra Leone waters in 1979 (see Table 21). Information on catch rates of shrimp by Sierra Fishing Company vessels in 1982 is also given in Table 22. Additionally, catch and effort data for Marine Development Company in 1983 are given in Table 23. The average yearly shrimp catch per vessel was about 64 t heads-on for Marine Development Company compared to 72 t (heads-on) for SFC in 1983.

5.3 Population Structure and Parameters

Recent data on population structure (i.e., length/age frequencies, sex ratio, mean size, etc.) of individual species in the catch made by the artisanal inshore fishermen is not available. Information on selectivity of mesh sizes (range 50.8–76.2 mm) used in the shallow inshore waters by the canoe fishermen is missing. Since the creeks and estuaries are nursery grounds for some species, it is difficult to assert that the small mesh-sized gillnets and castnets used by the artisanal and coastal inshore fishermen harvest mostly mature individuals.

a The 1981 total shrimp production (heads-on weight) byMarine Development Company alone is 803.3 t

There is already valuable information on some aspects of the biology of a number of commercially important fish species, but the main limitation in the use of this information in management models is that some of the research studies have been of rather limited duration and geographical scope. In many instances the examination of fish catch for length frequencies, age, sexual maturity and fecundity has been focused on catch of industrial fleets. Therefore, the management picture is made more difficult by the fact that the artisanal coastal inshore fishermen and the industrial fleets fish the same stocks, but most probably taking different age and size groups. Nevertheless, management of coastal fisheries may be still possible if changes are monitored in population parameters which are sensitive to increasing fishing intensity.

Fortunately, there are some data based on studies made in the late 1950s when the recent modifications in population structure of fish species had not yet occurred. In the late 1950s, trawling was just being introduced and the exploitable fish stocks had a good proportion of large-sized old fish.

Longhurst (1983) gives production estimates for plankton, benthos and various fish groups (demersal and small pelagics) on the continental shelf of Sierra Leone. Allen (1971) assumed exponential mortality and estimated the ratio P/B (total production (P) mean biomass (B)). Léêque, Durand and Ecoutin (1977) considered the relation between total production, mean biomass and total mortality (Z). The relationship P/B = Z is valid for many fish species with life spans less than five years.

a The 1982 total shrimp production (heads-on weight in tons)by Marine Development Company alone was 780.7 t

If dB/dt = P - ZB, it can be shown that under steady state conditions P - ZB = 0. Hence P/B = Z and for a virgin stock P/B = M (M is natural mortality).

Pauly (1980) discusses the relationship between von Bertalanffy function (VGBF) and natural mortality (M). Longhurst (1983) used the formulation given by Pauly (1980) to estimate M from maximum length (L_oo), the growth rate (K) and the mean environmental temperature (T). The vital population parameters estimated by Longhurst (1983) are given in Table 24.

5.4 Biomass and Potential Estimates of Finfish Species

The estimates presented are based on the results of several acoustic and exploratory fishing surveys which have been conducted in Sierra Leone waters. Acoustic surveys have enabled the study of bottom types, distibution of species by depth and determination of biomass by species groups. Exploratory fishing has facilitated determination of species composition, total catch magnitudes, vital biological factors of available species, rates and catchability. Finally, the exploratory surveys have provided information observations on temperature, salinity, dissolved oxygen levels and turbidity, all of which affect the distribution of fish. A summary of information concerning various surveys conducted in Sierra Leone since 1979 is given by van der Knaap (1985) and shown in Appendix 5. Additionally, references to all scientific expeditions listed in Appendix 5 are also given in the bibliography of this report.

Guinea Trawling Surveys (GTS) were conducted in Sierra Leone waters between August 1963 and June 1964 (Williams, 1968). There have been several joint USSR/Sierra Leone fishery investigations (Ivanov and Overko, 1983). The joint USSR/Sierra Leone fishing surveys, undertaken between 1976 and 1982, were aimed at obtaining the data base to enable an evaluation of the State fishery resources on the continental shelf of Sierra Leone. Results of the joint USSR/Sierra Leone fishery investigations are given by ATLANTNIRO (1983). Fish resource surveys have also been made with R/V DR FRIDTJOF NANSEN between May 1981 and March 1982. Biomass estimate of fish stocks in Sierra Leone waters, based on the results of R/V DR FRIDTJOF NANSEN are given by Stromme (1983) and CECAF (1984). Additionally, Longhurst (1983) gives mean biomass estimates of sciaenid community, estuarine sciaenid sub-community and sparid community.

On the basis of the results of the GTS, the relative abundance of total fish on the continental shelf of Sierra Leone is 500–1 000 kg/ha. The “observed density” based on fish catch for the sector comprised by southern Guinea and northern Sierra Leone for the depth zones 15–50 m and 50–200 m is about 52 kg/ha and 40 kg/ha, respectively (Williams, 1968). The “observed density” for the sector (southern Sierra Leone/northern Liberia) for depth zones 15–50 m and 50–200 m is about 38 kg/ha and 33 kg/ha, respectively (Williams, 1968).

The estimated standing crop (total fish) given by Williams (1968) is as follows:

ATLANTNIRO (1983) gives biomass estimates based on the following “echo-trawling” surveys of abundance: (a) June 1977 - SRTM ‘Prognoz’; (b) February-March 1978 - SRTM ‘Vykhama’; June-July 1979 - SRTM ‘Langust’; February 1980 - SRTM ‘Prognoz’; June 1980 - SRTM ‘Vykhama’; February, July-August 1981 and February 1982 - SRTM ‘Belogorsk’.

The triggerfish (Balistes) appears to have maximum biomass during the rainy season (May-October). Its estimated biomass during the rainy season of 1979, 1980 and 1981 was 245 600 t, 273 000 t and 83 500–126 500 t, respectively (ATLANTNIRO, 1983). During the dry season of 1979, 1980 and 1981, the estimated biomass was 20 000 t, 12 400 t and 3 000 t, respectively. If one also considers the results of the surveys conducted by the R/V MONOCRYSTALL and R/V DR FRIDTJOF NANSEN, the biomass of Balistes can be put at 80 000-100 000 t (ATLANTNIRO, 1983; CECAF, 1984).

The Carangidae and clupeoids have significant biomasses on the continental shelf of Sierra Leone. According to the acoustic survey conducted by ATLANTNIRO using R/V MONOCRYSTALL in February/March 1981, false scad (Decapterus rhonchus) accounts for 40% of the coastal small pelagics. The clupeoid species form 25% and the other Carangidae constitute 35% of the total coastal pelagics.

^a The catch rates are based on daily catches of eight USSR trawlers of 80 hp, covering 42% of totalannual trawl catch of 11 000 t. For good fishing performance the average catch is 6.2 t per fishingday.

The acoustic survey by R/V MONOCRYSTALL covered an area of about 11 992 km² and for this area the total fish biomass was put at 478 700 t (ATLANTNIRO, 1983). The estimated biomasses by the major species groups were:

The biomass estimates, based on the survey of R/V MONOCRYSTALL, can be compared with estimates made with the results of R/V FRIDTJOF NANSEN (May 1981/March 1982). The fish resource survey of R/V DR FRIDTJOF NANSEN covered an area of about 14 942 km² (Strømme, Saetersdal and Gjøsaeter, 1982). It is necessary to raise the estimates of the R/V MONOCRYSTALL by a factor of 1.2 and compare them with biomass given by Strømme (1983). The comparative biomasses are given in Table 25. Results of the acoustic survey by R/V MONOCRYSTALL, giving density distribution of fish, are shown in Figure 4.

It should be noted that the biomass estimates shown in Table 25 do not include fish in waters of less than a depth of 20 m. According to preliminary calculations, the total biomass of commercial fish species in the shallow waters (less than 20 m) is 24 600 t (ATLANTNIRO, 1983).

The bulk of the fish resources on the shelf of Sierra Leone consists of pelagic fish species (Sardinella, Decapterus, Trachurus and Balistes.

In June 1984, a combined survey using the R/V MONOCRYSTALL and R/V EVRIKA, was carried out in Sierra Leone waters. Since details on this surevey are not available, the results of the survey are not considered here; but CECAF (1985) gives some estimates based on this survey.

5.4.1 Potential of shrimp resources

The shrimp stocks of the Sherbro Division are mainly exploited by Sierra Leone and Liberia. There are also shrimpers of other countries which are licensed to fish in the EEZ of Sierra Leone. There are three shrimp species: (i) the Guinea shrimp (Parapenaeopsis atlantica), occurring mainly in the southern sector of Sierra Leone; (ii) the southern pink shrimp (Penaeus notialis), which is the most abundant species and occurs off the Freetown Peninsula, especially around Banana Island; and (iii) the Caramote prawn or tiger shrimp (Penaeus kerathurus) occuring primarily north of Sierra Leone River. The main fishing grounds for shrimps are:

• the coastal sector between Sherbro Islands and Cape Mount with an area of about 518 km² (200 mi²) (Garcia and Lhomme, 1980); and

• the coastal sector between Cape Mount and Cape Monrovia with an area of about 389 km² or 150 mi² (Garcia and Lhomme, 1980).

Garcia and Lhomme (1980) estimate the mean optimum yield of the Sierra Leone-Liberia shrimp resources to be 1 800 t. Optimum yield can be defined as the point at which marginal yeild equals 10% of the cpue recorded at the time the fishery was launched (Troadec and Garcia, 1981). An estimated optimum yield of 1 800 t was based on catch and effort statistics of Senegalese vessels fishing in the Sierra Leonean/Liberian sector between 1969 and 1976. The catch and effort trends are shown in Table 26 and Figure 5. The reported shrimp catch by Sierra Leone in 1983 was about 1 800 t. Shotton (1983) estimated the Liberian shrimp catch to be about 470 t. Thus, the total reported shrimp catch in 1983 amounted to about 2 300 t.

The industrial shrimp fishery of Sierra Leone is dominated by two fishing companies: (i) Sierra Fishing Company (SFC) which operated 14 shrimp trawlers in 1983; and (ii) Marine Development Company (MDC) which operated 10 shrimpers in 1983.

The trends in penaeid shrimp catch and finfish by-catches for Sierra Fishing Company are given in Table 27.

All the shrimp trawlers are of the USA-Gulf of Mexico double-rigged type. These shrimpers have the following vessel characteristics: 23–26 m long; 80–130 GRT; and 300–400 hp.

Detailed information on the economic performance of the industrial shrimp fishery is given by Willmann and Frielink (1985). The wholesale value of the shrimp exports of the two companies was estimated on the basis of New York Green Sheet prices by commercial size categories quoted by Shotton (1983) in a study on the Liberia shrimp fishery. Data on the size composition of the 1983 catch by commercial categories were provided by Sierra Fishing Company. It was assumed that the landings of Marine Development Company showed the same size composition as those of Sierra Fishing Company. On the basis of these prices and size composition a wholesale price of US$ 10.9 million was estimated for the Sierra Leone shrimp exports in 1983.

Figure 4 Demsity distribution of fish on the Sierra Leone continental shelf based on ATLANTNIRO (1983). The acoustic survey results of the R/V MONOCRYSTALL

Figure 5 Simulation results of the yield per recruit model of the Sierra Leone shrimp fishery - based on Willmann and Frielink (1985)

Apart from the shrimp catch, the 24 trawlers of the two companies landed also about 1 870 t of fish by-catch of commercial quality, a part of which was sold domestically. The value of the finfish landings has been estimated at US$ 930 000.

At present, shrimping is still a good business in Sierra Leone because of high average catch rates by the trawler fleet and rather low labour costs. The total annual profit of the industry is in the order of US$ 2.8 million (including revenues of about US$ 0.93 million from finfish by-catch), and the return on investment is estimated at 23% (Willmann and Frielink, 1985).

The Sierra Leone economy does not appear to benefit much from the shrimp industry. The shrimp industry generates some positive employment and income effects through the demand for locally produced supplies, the execution of domestic repair and maintenance works. Gross income created in this way may amount to an additional US$ 1.2 million.

The major part of the expenditures in local currency can be met by the sales proceeds of the finfish by-catch on the domestic market. The companies might, therefore, not transfer hard currency earnings realized in the shrimp export markets. Indeed the foreign exchange entering the country from the export of small shrimp and finfish by-catch amounted to US$ 0.6 million in 1983 (according to the Bank of Sierra Leone). In the same year, the FOB value of shrimp exports declared by the two companies amounted to US$ 3.3 million, the difference being retained by the companies for servicing their debts on imported capital goods such as vessels and plant machinery (Willmann and Frielink, 1985).

Source: Garcia and Lhomme (1980)

Willmann and Frielink (1985) used a bio-economic simulation model to assess the optimum shrimp trawler fleet size. Consideration was given to harvesting and processing costs. The bio-economic simulation results show that the shrimp industry is operating close to the economic optimum as shown in Figure 6. A maximum profit of approximately 2.3 million dollars (excluding US$ 700 000 obtained from finfish landings) is realized with 18 shrimp trawlers and a total production of about 1 500 t of head-on shrimps. It should be noted that at present, 24 shrimp trawlers are operating in Sierra Leone waters. It would therefore appear that the available shrimp resources cannot continue to sustain an expanding shrimp trawler fleet.

It might be argued that an increase in fleet size will lead to larger landings of finfish by-catches. However, considering the reported conflicts between shrimp trawlers and canoe fishermen, higher fish by-catches alone may not counterbalance the losses from the artisanal sector. Besides, fish catches by the artisanal fishermen are more likely to reach the rural poor who are in greater need of protein intake.

Figure 6 Estimated gross revenue and profit/loss of the Sierra Leonean shrimp industry at different levels of fishing effort - bsed on Willman and Frielink (1985)

The fishery administrator should, therefore, take the necessary measures to protect the artisanal fishermen from harrassment by controlling the number and size of shrimpers and also to ensure that shrimp trawlers fly the national flag.