There is a definite pattern in the distribution of fishes on the Cameroonian continental shelf. The original description of the distribution of demersal fishes by Longhurst (1965) is fairly valid. The available data indicate that the distribution of a number of species is limited by the depth of the thermocline and is influenced by the type of deposits (sand and silt) and the depths on the continental shelf, the slope of which is known to be quite variable. The generalized distribution patterns of species assemblages are shown in Figure 3 (based on Crosnier, 1964) and Figure 6 (based on Villegas and Garcia, 1983). Additional information on the general distribution of species assemblages on the shelf is given in Table 3.
Though the broad distribution of the commercially exploited fish species groups is known, there is no adequate information on the composition of communities or on temporal and spatial distribution of stocks. Given the length of coastline, it is almost certain that many species form more than one stock; and the demersals and pelagics are distributed to form more than one stock; the distribution of demersal and pelagic fishes in the national EEZ indicates discrete ecological fish communities, each of which are fairly homogeneous. However, there is also ecological and micro-geographical heterogeneity of fish communities, while migrations of species from the estuaries and creeks to the open shelf areas and vice versa are known to occur. The distribution of exploited fish communities on the Cameroonian shelf is illustrated in Table 3 and Figures 3 and 6. Useful detailed information on the ecology, biology and bionomics of marine fish species constituting species communities in West Africa is given by Longhurst (1958, 1960, 1963, 1964, 1965, 1965a, 1969 and 1969a). Njock (1979, 1985, 1985a, 1985b) describes the state of exploited fish stocks.
The following fish communities are exploited by both artisanal and industrial fishing fleets:
the coastal (suprathermoclinal) sciaenid community (on soft deposit) this community presents a particular estuarine facies very close inshore and in the creeks
the shallow water (superathermoclinal)sparid community (on more sandy, corally and rocky substrates) in the southern sector of Cameroon
the deep water (subthermoclinal) sparid sub-community (on both hard and soft deposits) on and off the slope.
The fish community inhabiting the estuaries, creeks and other coastal brackishwater consists of both freshwater and marine fish species. The estuarine Sciaenidae (croakers) are dominated by Pseudotolithus elongatus whose bathymetric distribution extends to 20 m depth, but P. senegalensis and P. typus (which are a common element in the catch of the coastal open waters) also occur in the estuaries. The family Clupeidae constitutes an important element of the estuarine fish community. Ethmalosa fimbriata (bonga) and Ilisha africana (shad) are both caught in the shallow open waters and in the brackish water. Third, the family Polynemidae (threadfins) contributes significantly to estuarine and creek fisheries, but it is not yet possible to determine the exact magnitude of Galeoides decadactylus, Polynemus quadrifilis and Pentanemus quinquarius, which are harvested from brackish waters. Additionally, other marine species in this sector include: Pteroscion peli (drum), which extends from the sea to the freshwater zone; Lutjanus (snapper); Cynoglossus (soles); (Pomadasys jubelini (sompat) grunt; Penaeus notialis (southern pink shrimp); the marine and estuarine Parapenaeopsis atlantica (Guinea shrimp) and Palaemon (white shrimp). The other significant exploitable resources in the estuaries and creeks are: Chrisichthys nigrodigitatus (brackishwater catfish), Arius spp. (marine catfish), Trichiurus lepturus (hairtail/silver fish), Cybium tritor (Spanish mackerel), Sardinella and Sphyraena (barracuda).
The dominant elements of this fish community are: Arius, Ilisha, Pseudotolithus spp., Drepane africana, Pomadasys jubelini, Pentanemus, Galeoides, Cynoglossus, Polynemus and Pteroscion peli. The offshore suprathermocline community occurs along the Cameroon coast on or above the 40 m depth contour in a few sandy and rocky bottom areas occupied by the sparid community in the southern sector towards the border of Equatorial Guinea.
The sandy and rocky bottom sparid communityconsists of Sparus caeruleostictus, Pagellus coupei, Lutjanus sp., Epinephelus sp. and Decapterus. The Sparidae are not well represented on the continental shelf.
The deep water sparid community occurs on both sandy and muddy bottoms below the thermocline, down to the continental shelf. This fish community is comprised of Dentex congoensis, D. filosus, Pseudupeneus prayensis, Paracubiceps, Decapterus and Trigla sp. It is not yet accessible to the artisanal fishermen because of the long distances to be covered and the fishing gear needed at depths over 40 m. Also because of their low abundance, the deepwater sparid community is not a target species for the industrial fleet.
Fish species with a wide vertical range of distribution on the continental shelf are: Cynoglossus, Vomer setapinnis, Brachydeuterus auritus, Trichiurus lepturus, Raja, shrimps and prawns. It should be noted that this species group is harvested by both the artisanal and industrial fleets. There is no documentation of migration patterns of species constituting this fish group and no reliable information concerning the age and length composition of the catch by the artisanal or the industrial fisheries. Documentation of migration patterns would necessitate an extensive fish tagging programme. However, information on size and age structure of eurybathic fish species can easily be obtained through biological sampling of fish catches landed along the coast.
Most clupeids are marine species but some are anadromous, e.g., Ilisha and Ethmalosa fimbriata. They are able to withstand low salinities, particularly in the rainy season.
(a) Bonga (Ethmalosa fimbriata)
Bonga is the most important clupeid species in the coastal inshore waters. This species rarely goes below 20 m. It is more euryhaline than the flat Sardinella and it is found in estuaries, the sea, lagoons and also in places liable to have a variation in salinity. It prefers warm and turbid water. Because of these ecological preferences, it tends to replace the flat Sardinella, and even more evidently the round Sardinella in those sectors without upwelling but with strong surface desalination. Its biology and migrations seem small in extent and are limited to estuaries and the adjacent coastal areas (Longhurst, 1960) and Salzen (1958).
Ethmalosa is a non-selective filter-feeder subsisting mainly on large diatoms and phytoplankton. The species migrate into and out of the estuaries following seasonal changes in salinity as well as with the abundance of plankton in the estuaries during the dry season. Bonga tends to be more abundant in the estuaries during November-April. Its migration is possibly due to spawning and feeding needs. The seasonal fishery for bonga varies according to its migration route.
Juveniles are definitely more abundant in rivers and in estuaries whereas the young spawners and adults can be found both in estuaries and at sea. Bonga is a target species for the artisanal gillnets, beach seines and more recently the artisanal purse seines. Bonga is rarely caught in trawls and if so, it is usually discarded together with shad (Ilisha africana).
(b) Shad (Ilisha africana)
Shad is an anadromous clupeid inhabiting inshore waters, sandy beaches and estuaries (in almost all fresh waters). Ilisha africana has a maximum length (Loo) of about 22 cm. Its preferred diet is Crustacea and small fishes (juveniles).
Shad may be caught at the surface or near the bottom down to about 25 m. Hence, it can be a target species for beach seine, gillnet, purse seine and inshore trawl fisheries. In Cameroon, Ilisha is caught in the thermocline zone (25–50 m) by trawlers but all of it is discarded.
(c) Sardine (Sardinella spp.)
The flat Sardinella is found from Mauritania to Angola. It is a coastal species, euryhaline, most often found to be abundant near the outlet of water courses. It prefers warmer waters with a temperature above 25°C and seems to avoid waters that are not clear. It is not very abundant in areas without upwelling where the warm and low saline superficial layer is permanently present as in the Bight of Biafra.
In Cameroon, however, Sardinella is fairly abundant and ranks second to bonga as a target species of the artisanal pelagic fishery. The Sardinella stock is possibly part of a similar southern sub-tropical stock that extends from Lobito (Angola) 13°S to Mayumba (Gabon) 3°30S. Sardinella maderensis is caught by canoe fishermen using beach seines, strong konda nets (meshes of which are smaller than bonga nets), drift nets, artisanal purse seine and cast nets. The species is not an important element of catches of the industrial fleets.
The following carangid species are fairly abundant in Cameroonian waters: Caranx spp., Chloroscombrus chrysurus and Decapterus rhonchus. These are mostly schooling species distributed on the continental shelf but some occur in brackish waters especially when young. It should be noted that carangids are not abundant on the continental shelf of Cameroon. They are not a target group for the industrial vessels and artisanal fleets.
(a) Various jacks (Caranx spp.)
Caranx spp. have a wide distribution along the West African coast from Senegal to Angola. Some species inhabit inshore waters and estuaries and the others are located in deeper waters (more than 100 m depth). Hence, this fish group can be vulnerable to both artisanal and industrial fleets. Caranx spp. feed mainly on fish but also on shrimps, some crabs and invertebrates. They are caught in pelagic and bottom trawls, seines, set gillnets and ring nets and some times on line gear.
(b) Atlantic bumper (Chloroscombrus chrysurus)
Chloroscombrus chrysurus occurs along the West African coast from Mauritania to Angola. This schooling pelagic species inhabits the Nigerian continental shelfat depths of 10–50 m. It also occurs in estuaries and the mangrove fringed lagoons and brackishwater areas. Its juveniles are sometimes located offshore in association with jellyfish.
Atlantic bumper can be a target species of the artisanal fleet using set gillnets and artisanal seine nets as well as for the industrial fleets using trawls and operating in waters of 10–50 m depth, particularly in the thermocline zone.
(c) False scad (Decapterus rhonchus = Caranx rhonchus)
This is a schooling species inhabiting near bottom waters, mostly between 30 m and 50 m but can be located in waters over 200 m depth. It feeds mainly on small fish and invertebrates.
It is mostly exploited by industrial fleets using trawls, but it can also be fished by artisanal motorized canoes using gillnets in deeper waters in the thermocline zone.
(a) Lesser African threadfin (Galeoides decadactylus)
Galeoides decadactylus does not appear to penetrate below the thermocline. It occurs in inshore water adjacent to sandy beaches. The species is known to develop female gonads by passing through a non-functional hermaphroditic stage arising from a normal male (Longhurst, 1965). An understanding of its reproductive and recruitment strategy is essential in the management of this fish species.
Galeoides prefers silty and sand-silty bottoms. It is a semi-diadromous fish with spawning migration into estuaries and lower reaches of rivers.
It feeds on benthic organisms such as Crustacea and Polychaetes. It is a target species for the artisanal fishery using gillnets and beach seines as well as the industrial fleets employing trawls in the inshore areas.
(b) Royal threadfin (Pentanemus quinquarius)
Pentanemus quinquarius has a normal reproductive cycle. It occurs on sandy bottoms down to a depth of 50 m.
It is caught by the artisanal gillnet fishery on nearshore sandy bottoms but is also harvested offshore by the industrial fleet using trawls. Additionally, Pentanemus can be caught with beach-seines.
(c) Giant African threadfin (Polydactylus quadrifilis)
The giant African threadfin (Polydactylus quadrifilis) can grow up to lengths of 150–200 cm. The species inhabits inshore and offshore sandy bottoms up to a depth of 50 m. It also occurs in estuaries and lagoons fringed by mangroves.
This fish species is jointly harvested by the artisanal fishermen and industrial fleets. Its attractive size has made it extremely vulnerable to gillnet and beach-seine fisheries.
Croakers and drums are important sciaenid species in Cameroon. This fish species group is primarily marine but also occurs seasonally in brackishwater areas. Most of the species inhabit sandy and muddy bottoms in coastal areas with large river flows. Longhurst (1969) gives a useful synopsis of biological data on West African croakers.
(a) Bobo croaker (Pseudotolithus (Fonticulus) elongatus)
Pseudotolithus (Fonticulus) elongatus prefers surroundings that are less saline. In fact, commercial concentrations correspond to the great estuaries in the Gulf of Guinea where the species can be caught in large quantities in certain seasons.
Bobo croaker inhabits mud bottoms in coastal waters up to 50 m depth but also enters estuaries and coastal lagoons. This species, with a maximum length of about 45 cm, moves further offshore to spawn during the rainy season. Le Guen (1971) reports on the dynamics of bobo croaker. It is jointly harvested by the artisanal and industrial fleets. It can be caught with bottom trawls, gillnets, beach-seines and longlines; and is a target species of bottom set gillnets and also trawlers.
(b) Longneck croaker (Pseudotolithus (Pseudotolithus) typus)
Pseudotolithus (Pseudotolithus) typus grows to a larger size than Pseudotolithus elongatus. It attains a maximum length (Loo) of 100 cm; and fish of 50 cm length are common in the catch. The main fishing grounds for this species are in the Gulf of Guinea and off Congo. It is the most important commercial sciaenid species for the Cameroonian trawl fishery.
Pseudotolithus (Pseudotolithus) typus inhabits mud and sandy bottoms up to a depth of 150 m but is more abundant in waters of less than 60 cm and temperatures above 18°C. It also occurs in estuaries. Hence, it is fished by longlines. Pseudotolithus typus is a target species of the trawlers.
(c) Boe drum (Pteroscion peli)
Pteroscion peli occurs along the west coast of Africa, from Senegal to Angola. It inhabits mud and sandy mud bottoms in coastal waters extending to 200 m depth, but it is most common in waters of less than 50 m depth. Exploratory fishing and catch sampling results show that Pteroscion is not abundant in the waters of Cameroon.
This species is more accessible to the industrial fisheries using trawls than to the artisanal fisheries using gillnets and beach-seines.
Table 5 also gives information concerning the biology and ecology of other exploited fish (e.g., Ariidae, Bagridae, Cynoglossidae, Lutjanidae, Sparidae, Serranidae, Sphyraenidae, Sphyrnidae, Haemulidae (= Pomadasyidae), Rajidae and Dasyatidae). The distribution pattern of these species also reveals interaction between the brackishwater and open-sea fisheries and competition between various species.
Three commercially important penaeid shrimps occur in Cameroon waters. Penaeus notialis (the pink shrimp) is by far the most dominant species. It occurs in the lagoons, estuaries, creeks and open sea. Parapenaeopsis atlantica (Guinea shrimp) is also fairly abundant in the open sea depth (10–16 m). The estuarine “white shrimp” (Palaemon hastatus) occurring in brackishwater and open sea is mainly exploited by artisanal fishermen.
The coastal penaeid shrimps have interesting recruitment features, details of which are given by Garcia and Le Reste (1981). The first phase in the life of coastal penaeid shrimps takes place at sea and within three weeks the post-larvae move inshore in mangrove swamps which are rich in food, or in submerged vegetation. As their development progresses the shrimps move to greater and greater depths. When the areas of distribution of juveniles and adults are clearly separated geographically, a seaward migration occurs after which spawning takes place. Detailed information on life-history pattern of Penaeus notialis is given by Garcia (1977) and Garcia and Lhomme (1980).
Since the types of exploitation(and the operational zones of the various gear) are extremely diversified, there are in fact several successive recruitment phases for shrimps:
when the shrimps leave the nursery edges and become accessible to artisanal fisheries;
when they reach the large bays where they are accessible to small trawlers;
during migration, when they are caught by fixed nets (e.g., the ngoto), and;
when they reach the sea and are caught by industrial fleets (shrimpers/trawlers).
Recruitment to the different fishereies is associated with the development stage of the shrimps. If recruitment is defined as “the probability of a shrimp of a given size to be found in the fishing area”, this probability can be expressed for shrimps of each size as the percentage of shrimps at that size in the total population that is present in that area.
Additional information on the biology and ecology of exploited species is given in Table 6.
