6. POTENTIAL RESOURCES AND PRESENT STATE OF EXPLOITED STOCKS

6.1 Nominal Catch Data

The total demersal finfish and shrimp catch by artisanal and industrial fleets account for about 70% of the total national catch from brackish waters and the sea. The croakers (Pseudotolithus), threadfins (Polynemidae), soles (Cynoglossidae), grunters (Pomadasyidae) and sharks and rays are the dominant fish species landed. The pelagic species (Clupeidae and Carangidae) account for about 30% of the “marine” fish catch.

Although sampling coverage in some coastal sectors (particularly the Niger delta area) is incomplete, the total catch of demersal and pelagic fish species by the artisanal fishing units was reported to increase from 199 600 t in 1971 to about 370 000 t in 1983 and suddenly dropped to about 227 600 t in 1984 due to certain economic factors encountered by the fishing industry. The total catch trends of various fishery sectors are given in Tables 11 and 14.

Similarly the demersal finfish catch of inshore fish trawlers increased from 5 700 t in 1974 to 12 400 t in 1982 and then to 22 300 t in 1984 as shown in table 11. The catch and effort trends of the inshore trawlers based at lagos are shown in Table 12.

The total annual shrimp catch by shrimpers increased during the period 1972–84 (that is, 1 200 t in 1972; 1 700 t in 1976; 1 900 t in 1979; 3 500 t in 1982 and 2 300 t in 1984 (see Table 11).

There are at present insufficient data to enable reliable assessment of the shrimp resources accessible to the artisanal coastal inshore fishermen. Shrimps occur in an inshore strip along the entire Nigerian coast. The total exploitable area is put at 6 475 km² (2 500 mi²). The most important stock is located in the Nigerian Delta area. Exact catches by fishing grounds of penaeid shrimp species, even those of Penaeus notialis are unknown. There are also no separate data on the exact catch of Palaemon.

It is shown in Table 11 that the artisanal fishery has contributed 90 to 98% of the annual marine and brackishwater catch for the last 14 years. It noted that the catch of the artisanal fishery increased by about 18% in 1980–81. There are no obvious factors or fishing innovations to account for such an increase and, therefore, it is most probable that the annual catch magnitudes for the period 1981–83 were overestimated. There are differences in fish production in the six coastal States as shown in Table 13.

It is noted in Table 10 that Trachurus spp. (horse mackerel) is not accessible to the artisanal canoe fishery. Therefore, the carangid catch of the artisanal fishery mostly consists of jacks (Caranx spp.) and Atlantic bumper (Chloroscombrus). Considering the predominance of bonga monofilament gillnets in several fishing villages in Cross River, Rivers, Bendel and Ondo States, and also the abundance of processed bonga in Nigerian fish markets, the catch of this species is possibly underestimated. But it is also possible that part of the bonga catch is included in the category referred to as “other marine fishes”.

No distinction is made between the catches of different species of croakers (Sciaenidae) but available information on bathymetric distribution of sciaenids implies that the Bobo croaker (P. (F.) elongatus) and the longneck croaker (P. typus) dominate the catch of the artisanal fishery. The catch of threadfins (Polynemidae) is significant in the coastal inshore waters. The other fish species that are easily accessible to the artisanal canoe fishery are: tonguesole (Cynoglossus browni), Guinea tonguesole (Cynoglossus monodi), African red snapper (Lutjanus agennes), African striped grunt (Parapristinoma octolineatum), Sompat grunt (Pomadasys jubelini), stingray (Dasyatis pastinaca); estuarine white shrimp (Palaemon) and pink shrimp (Penaeus notialis).

6.2 Effort and Catch per Unit Effort (CPUE)

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 missing. There are no data on rates of discards from shrimp trawlers or other major gears operated by the industrial fleets but these are likely to be considerable. Reliable data on separate fishing effort in the creeks, estuaries, coastal lagoons and shallow inshore waters are still lacking. However, general trends in fishing effort exerted by the artisanal fishery are given in Table 4.

Available data also indicate an increase in number of inshore fish trawlers and shrimpers operating on the Nigerian continental shelf during the period 1971–84 (see Table 7) the rapid expansion of the industrial fleet in the early 1980s is due to the Government supplying Polish inshore trawlers to artisanal fishermen cooperatives under the 50% subsidy scheme.

Past trends in the industrial fisheries (trawlers/shrimpers) is given in Table 7. Since there is no information on number of days fished for the entire national industrial fleet, the determination of equilibrium yield could not be attempted. Available catch and effort data from the Nigerian National Shrimping Company Ltd., and from Osadjere Fishing Company Ltd., are shown in Tables 15 and 16 respectively. The data, however, indicate a decline in total catch and cpue between 1982 and 1984. Additionally, trends in catch rates of trawlers owned by Obelawo Farcha are given in Table 17. It is shown that the catch rate (tons/trawler/month) have generally declined since 1979.

6.3 Migration Patterns of Exploited Species

Some of the exploited fish species, e.g., bonga, croakers, sardinella, snappers, threadfins, pink shrimp and barracuda, make seasonal migrations from the sea into the creeks and back to sea. The mechanism of migrations and their significance to the coastal inshore fisheries has still to be evaluated. The extent of lateral migrations along the coast should also be studied, since such migrations are likely to affect the movement of fishing units along the coast. Besides, the inshore/offshore and lateral migrations mean that a number of stocks are harvested by both artisanal and industrial fleets.

6.4 Population Structure and Parameters

Information 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 very scanty, nor is there data on selectivity of mesh sizes (range 50.8–76.2 mm) used in the creeks and the shallow inshore waters, thus the length/age structure of fish caught in the relatively small mesh sizes is unknown. Since the creeks and the very shallow coastal waters are nursery grounds for certain species, it is difficult to assert that the small mesh size gillnets/castnets, etc., used by the artisanal and coastal inshore fishermen harvest mostly mature individuals.

Available data on length distribution of catch of the artisanal fishermen indicate slight variations in length composition for given species along the Nigerian coast. For example, in Cross River State (Iko/Okoroete village) the length of bonga (E. fimbriata) caught by the artisanal fishermen ranged from 14 to 30 cm and the modal length was 23 cm, whereas in Ondo State (Ajegnule village) the size length was 23 to 31 cm and the modal length was 27 cm. In January 1982 a sample of croaker (P. typus) taken at Oyorokoto in Rivers State has a size range of 31 to 44 cm, with a mode of 38 cm, compared with a size range of 17 to 43 cm and a mode of 25 cm for a sample of the same species taken at Ibuno in Cross River State. Even for the West African shad, the exploited size ranges vary along the coast. For a sample catch of shad taken near Kampa on the Kwa Ibo Creek adjoining Kwa Ibo River in Cross River, the length ranges 14 to 18 cm, with a mode of 15 cm and mean weight of about 41 g, whereas, for the same species in Ondo State (Orioke-Iwamimo village), the size range was 12 to 25 cm, with a modal length of 18 cm and mean weight (W) of 51 g. These differences in size composition can be accounted for by spatial variation in factors affecting the growth of fish and by differences in selection, in that various fishing units exploit different parts of a fish population.

There is already valuable information on some aspects of the biology of a number of commercially important fish species but the main limitation to 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, fecundity, etc., has so far mostly focused on catch of industrial fleets. Therefore, the management picture is made more difficult by the fact that the artisanal coastal inshore fishermen fish the same fish stocks but most probably take 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. Some of the useful parameters that have been estimated for a few species are given in Table 18. The control of mesh size is meant to ensure capture of fish at a size greater than length at first maturity so that recruits are given a chance to reproduce.

6.5 Yield Potential of the Nigerian Continental Shelf

The available data are not adequate to enable the use of conventional management models in assessing the yield potential of exploited species of the Nigerian continental shelf, estuaries and creeks but preliminary estimates of potential yield made in the 1960s indicate that the magnitudes are modest. Longhurst (1965) advanced estimates for the potential of the demersal fishery resources of the Nigerian continental shelf and the same estimates were verified by Troadec and Garcia (1980), viz.

1. trawler fishery in coastal strip (0–50 m) 11 000 t (400 kg/km²) but part of this potential yield is also accessible to the artisanal fishery;

2. deepwater strip (50–200 m) 4 000 t (250 kg/km²), however, this sector is not easily accessible to the artisanal fishery;

3. artisanal fishery in the shallow coastal strip 25 000 t, this coastal strip was supposedly not accessible to the coastal trawlers and shrimpers in the past but in more recent years industrial fishing vessels have encroached more and more on this sector which is also exploited by artisanal fishermen. This has enhanced conflicts between the artisanal and industrial sectors.

The yield potential estimates made by Longhurst (1965) and verified by Troadec and Garcia (1980) appear modest. It should be noted, however, that the above estimates made for the artisanal fishery exclude the potential of the fishery resources in the creeks, coastal mangrove swamps and estuaries.

6.5.1 Yield potential of coastal lagoons and brackishwater coastal systems fringed by the mangrove.

The estimated brackishwater area of some estuaries, lower river sectors fringed by the mangrove and adjoining creeks is given in Table 3. The precise productivity of Nigerian estuaries and creeks is still unknown, but on the basis of comparative productivity of African floodplain systems, the coastal lagoons and the “lagoon-like” estuaries (which are bar-built) should have a productivity range of 50–60 kg/ha or 5–6 t/km² (Welcomme, personal communication). But estuaries with more nutrients derived from the hinterland would have typical productivity values of 100–200 kg/ha or 10–20 t/km² (Kapetsky, personal communication). On the basis of this last comparative information, the following potential yield estimates for only finfish and crustacea are considered reasonable :

1. Lagos and Ogun States lagoons (about 500 km²) - estimated average yield potential is between 5 000–1 000 t;

2. Bendel State, Benin, Escravos and Forcados Rivers (area about 300 km²) - estimated average yield potential is between 3 000–6 000 t;

3. River State, estuaries and lower river sectors between Ramos River and Imo River (about 1 000 km²) - estimated average yield potential is between 6 000–12 000 t.

4. Cross Rivers State, Kwa Ibo River and Cross River (about 6 000 km²) - estimated average yield potential is between 6 000–12 000 t.

Hence, the estimated average yield potential for the entire national brackish water sector is 24 000 to 48 000 t. There appears no reasonable basis to suppose that the average yield potential of the coastal brackishwater sector could greatly exceed this magnitude.

6.5.2 Yield potential of the offshore marine sect for the depth range 0–91 m

Estimates of the area of the continental shelf in the depth 0–91 m are given in Table 2. In the Gulf of Guinea, and intensive survey of the demersal resources was carried out by the Guinean Trawling Survey (Williams, 1968). A more systematic survey was made off Nigeria by Longhurst (1965) but no estimates of absolute abundance were derived from catch rates. However, Longhurst (1965) used the catch rates and estimated the potential of trawl catches to be 15 000 t. It is possible that the estimated potential might have been biased by variability in density of demersal fish between muddy and sandy bottoms. But this bias must have been compensated by the lower natural mortality rates of 0.1–0.3 which were applied.

In 1976 a trawl survey was conducted by the FAO/USSR vessel FIOLENT. The commercial demersal fish stocks for the surveyed area was estimated at 28 600 t (Robertson, 1977). The finfish trawl catch in Nigeria for 1984 was estimated to be about 22 300 t. This catch magnitude is believed to be close to the maximum sustainable potential (Ajayi, personal communication). A summary of trawl and acoustic surveys carried out in Nigerian waters is given in Table 19.

The yield potential in the very inshore waters (0–18 m depth) was not estimated as a separate sector during the 1960s. Since the very inshore waters are adjoining the estuaries and creeks, one can suppose that the productivity of the two sectors do not differ much. In these circumstances, the inshore waters up to 15 m depth could possibly have a much higher potential than the deeper waters studied, with a finfish and crustacea density range of 100–200 kg/ha (or 10–20 t/km²) similar to estuaries and creeks. On the basis of these values the following estimates are made :

1. Cross River State area (for the sector 0–18 m) is about 2 800 km² ; the estimated yield potential is of the order of 2 800–5 600 t;

2. River State area (for the sector 0–18 m) is about 5 200 km², the estimated yield potential is of the order of 5 200–10 400 t;

3. Bendel State area (for the sector 0–18 m) is about 1 200 km², the estimated yield potential is of the order of 1 200–2 400 t;

4. Ondo, Ogun and Lagos States area (for the sector 0–18 m) is about 2 500 km², the estimated yield potential is of the order of 2 500–5 000 t.

Therefore the potential from Nigerian continental shelf area from the shoreline up to 18-m depth is put at 11 700–23 400 t. It is obvious that the artisanal fishermen, operating mostly in shallow waters (0–18 m depth), but also capable of extending to 40-m depth, interact with the industrial trawl fisheries. Hence, the combined yield potential for finfish and crustancea, including 15 000 t for the trawl fishery, operating in the offshore marine waters 18–50 m deep, is of the order of 26 700–38 400 t.

Finally, the coastal brackishwater sector and the inshore zone (0–18 m depth), which are mainly exploited by artisanal fishermen, appear to have a yield potential of 35 700–71 400 t whereas the trawlers have a potential yield of about 15 000 t. Considering productivity figures of similar neighbouring and highly productive coastal systems one could derive a maximum potential yield of 100 000 t for the artisanal sector. The finfish catch of trawlers in 1984 was about 22 300 t compared to an estimated potential of 15 000 t. It is a fact that data from trawl and acoustic surveys are subject to great variability. Even then, the maximum potential yield accessible to industrial fishery is not likely to exceed 50 000 t. This does not mean that the Nigerian trawl fishery has still good prospects for expansion because it is possible that the catches of the industrial fishery have often been under-reported. There might also have been transshipment of part of the trawl catches to more lucrative markets. In these circumstances, there are no reliable data to justify a maximum potential yield of greater than 150 000 t for the combined artisanal and industrial marine fisheries.

Nigeria has adequate potential for penaeid shrimp (Penaeus notialis and Parapenaeopsis atlantica) as well as the estuarine white shrimp (Palaemon hastatus) global production models (Schaefer and Fox models) have been applied to assess the combined shrimp resources of Cameroon and Nigeria (CECAF, 1985). The area of muddy biotopes of Nigerian continental shelf is six to seven times greater than that of Cameroon (CECAF, 1985). Hence, the pink shrimp stocks in Nigerian waters are more abundant than those on the continental shelf of Cameroon. Standardized effort data for CRECAM Shrimping Company (based in Cameroon) are given by Njock (in press) and are indicated in Table 20. FAO (1979) gives the combined shrimp resources of Cameroon and Nigeria as 3 370 t. The Schaefer and Fox models lead to an estimated potential of 3 600 t and 3 500 t respectively (CECAF, 1985). Considering previous studies made by FAO (1979), Garcia and Lhomme (1980) and also considering the shelf areas of Nigeria and Cameroon where shrimp occur; the shrimp resources of Nigeria have a potential of 2 500–3 000 t.