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5.1 Fish Communities

There is a definite pattern in the distribution of fishes on the Nigerian continental shelf. The original description of the distribution of demersal fishes by Longhurst (1965) is still 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 silts) and the depths on the continental shelf, the slope of which is variable.

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. In the circumstances of a very long coastline (about 800 km), it is almost certain that many species form more than one stock. The distribution of demersal and pelagic fishes in the marine waters of Nigeria indicates discrete ecological fish communities, each of which are fairly homogeneous. However, there is also ecological and microgeographical heterogeneity of fish communities, whilst migration of species from the estuaries and creeks to the open shelf areas and vice versa is known to occur.

The following fish communities are exploited by the artisanal fishing units:

  1. the estuarine and creek sciaenid subcommunity;

  2. the offshore suprathermocline sciaenid subcommunity (on soft deposits);

  3. the shallow suprathermocline sparid subcommunity (on sandy, corally and rocky substrates);

  4. the deep subthermocline sparid subcommunity (on both hard and soft deposits).

Table 8
The composition and vessel characteristics of some inshore trawlers licensed to fish in Nigerian waters in 1984 (based on records of the Federal Department of Fisheries)
CompanyRange/Average GRTNumber of vessels
Adewunmi Industry123.06–133.24/129.363
Aiyetoro Bay Agency/ 29.391
Alfa-Sital Fish. Co./ 35.151
Atlantic Fish. and Shipping/137.271
Arawak Fish. Ltd.110.40–146.70/137.304
Banuso Fish. Ltd./137.152
Coastline Fish Co. Ltd/115.282
George/ 15.001
Xanthopoulos G.M. Epelle/ 9.001
Husania Fishing Industries/122.002
Mermaid Merdants15.95– 20.00/ 17.303
Nika Fishing Co./134.523
Ocean Fisheries/148.1517
Pan-Nikor Fish. Co./137.354
Republic Fisheries13.17–137.60/44.284
Sea-Horse fish. Co./113.591
Sobalaje Fish. Co./ 38.571
Sobat Ltd./113.591
West-Coast Fish. Ltd./126.434
Nats Food Inc./114.002

5.1.1 Estuarine and Creek (inshore) sciaenid subcommunity

The fish community inhabiting the estuaries, creeks and other coastal brackiswater 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. Thirdly, the family polynemidae (threadfin) 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 water. Additionally, other marine species in this sector include: Pteroscion peli (drum), which extends from the sea to the freshwater zone; Lutjanus dentatus (snapper); Cynoglossus (soles); Pomadasys jubelini (grunters); Penaeus notialis (Southern pink shrimp); the marine and estuarine Parapenaeopsis atlantica (Guinea shrimp); Palaemon sp. (white shrimp) and Macrobrachium sp. The other exploitable resources in the estuaries and creeks are: Periophthalmus koelreuteri (mudskipper), Chrisichthys nigrodigitatus (brackishwater catfish), Arius spp. (marine catfish), Trichiurus lepturus (hairtail/silver fish), Cybium tritor (spanish mackerel), Sardinella spp., Sphyraena spp. (barracuda) and tilapiine species.

Table 9
The number and characteristis of shrimp trawlers licensed to operate in
Nigerian waters in 1984 (based on records of the Federal
Department of Fisheries, Lagos)
CompanyAverage GRTNumber of shrimps
Eyibs Nutritional Food Enterprises113.002
Scott Fishing Ind.123.005
Nigerian Polish Fishing Ltd.141.731
Nigerian National Shrimp Co. Ltd.115.512
Osadjere Fishing Company127.468

5.1.2 Offshore suprathermocline sciaenid subcommunity

The dominant elements of this fish community are: Arius, Ilisha, Pseudotolithus spp., Drepane africana, Pomadasys jubelini, Pentanemussp., Galeoides, Cynoglossus, Polynemus and Pteroscion peli. The offshore suprathermocline community occurs along the entire Nigerian coast on or above the 40 m depth contour except in a few sandy and rocky bottom areas occupied by the sparid community.

5.1.3 Shallow water suprathermocline sparid subcommunity

The sandy and rocky bottom sparid community consists of Sparus caeruleostictus, Pagellus coupei, Dentex spp., Lutjanus sp., Epinephelus sp., Decapterus and Trachurus spp.

5.1.4 Deep water subthermocline sparid subcommunity

The deep water sparid community occurs on both sandy and muddy bottoms below the thermocline, down to the edge of the continental shelf. This fish community is comprised of Dentex congoensis, Dentex filosus, other Dentex spp., Pagellus coupei, Upeneus prayensis, Decapterus, Trachurus trachurus, T. trecae, Boops spp., Scomber japonicus, Trigla sp., etc. This fish community is not yet accessible to the artisanal fishermen because of the distance offshore and/or the need for mechanical handling of the more extensive fishing gear needed at depths over 40 m.

5.1.5 Eurybathic fish species

Those fish species having a large vertical range of distribution on the Nigerian continental shelf are: Cynoglossus, Vomer setepinnis, Brachydeuterus auritus, Trichiurus lepturus, Raja spp., shrimps and prawns, etc. It should be noted that this fish species group is harvested both by the artisanal fishery and by the industrial fleet. There is no proper documentation of migration patterns of species constituting this fish group and, besides, there is no reliable information concerning the age and length composition of the catch by the artisanal or the industrial fisheries. Proper documentation of migration patterns would require a rather long-lasting fish tagging programme. But information on size and age structure of eurybathic fish species can easily be obtained through biological sampling of landed fish catches.

5.2 Biology and Ecology of some Commercially Exploited Fish Species

5.2.1 Clupeidae

Most clupeid species are marine but some are anadromous (shads) and Ethmalosa fimbriata (bonga) are adopted to withstand low salinities particularly in the rainy season.

  1. Bonga (Ethmalosa fimbriata)

    Bonga is the most important clupeid species in the coastal inshore waters of Nigeria. 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 that are liable to have great variations in salinity. It prefers warm and turbid waters. Because of these ecological preferences, it tends to replace the flat sardinella, and even more clearly the round sardinella, in those sectors without upwelling but with strong surface desalination. Its biology and migrations seem small in extent and limited to estuaries and the adjacent coastal areas (Longhurst, 1960).

    Ethmalosa is a non-selective filter-feeder subsisting mainly on large diatoms and phytoplankton. The species migrates 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. Ethmalosa tends to be more abundant in Nigerian estuaries during the period October-April. Its migration is possibly due to spawning and feeding.

    Juveniles are definitely more abundant in rivers and in estuaries, while young spawners and adults can be found both in estuaries and at sea. This pelagic fish is a target species for the artisanal gillnet and beach seine fisheries.

  2. Shad (Ilisha africana)

    Shad is an anadromous clupeid inhabitinginshore waters, sand beaches and estuaries (in almost all fresh waters). Ilisha africana has a maximum length (L∞) of about 22 cm and it has a good preference for crustacea and small fishes (juveniles).

    It 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.

  3. Sardine (Sardinella spp.)

    The flat sardinella is found from Mauritania to Angola. It is coastal fish, more euryhaline, most often found to be abundant near the outlet of water courses. It prefers warmer waters with a temperature above 24°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 Baight of Biafra and a large portion of the Nigerian shelf.

    In Nigeria Sardinella spp. are caught by canoe fishermen using ringnets, castnets, gillnets, beach seines and also by trawlers. But Sardinella is not a target species of any of the main fisheries.

5.2.2 Carangidae

The following carangid species are fairly abundant in Nigerian waters: Caranx spp., Chloroscombrus chrysurus, Decapterus rhonchus amd Trachurus spp. There are mostly schooling species distributed on the continental shelf but some occur in brackish waters especially when young.

  1. Various jacks (Caranx spp.)

    Caranx spp. have 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 (over 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. This fish species group is caught in pelagic and bottom trawls, seines, set and ring gillnets and sometimes on line gear.

  2. Atlantic pumber (Chloroscombrus chrysurus)

    Chloroscombrus chrysurus occurs along the West African coast from Mauritania to Angola. This schooling pelagic species inhabits the Nigerian continental shelf at 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 seines as well as for the industrial fleets using trawls and operating in waters of 10–50 m depth.

  3. False scad (“Decapterus rhonchus” = Caranx rhonchus)

    This is a schooling carangid species inhabiting near bottom waters, mostly between 30 m and 50 m but can be located in waters over 200 m depth. It feeds on small fish and invertebrates.

    This species is mostly exploited by industrial fleets using trawls, but it can also be fished by artisanal motorized canoes using gillnets.

  4. Horse mackerel (Trachurus spp.)

    Horse mackerel occurs in schools in sandy bottom localities and usually at 100–200 m depth. Since the main fishing grounds are on the continental shelf, the species is not normally caught by artisanal fishermen. It is usually a target species of the offshore trawl and purse seine fisheries and sometimes it can be caught with longlines. It appears that the Nigerian industrial fisheries can exploit Trachurus capensis (Cape horse mackerel) and Trachurus trecae (Cunene horse mackerel).

5.2.3 Polynemidae

  1. Lesser African threadfin (Galeoides decadactylus)

    Galeoides decadactylus does not appear to penetrate below the thermolcline. It occurs in inshore waters adjacent to sandy beaches. The species is known to develop female gonads by passage through a nonfunctional hermaphroditic stage arising from a normal male (Longhurst, 1965). Understanding its reproductive and recruitment strategy appear to be vital in the managing 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.

    Galeoides decadactylus 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.

  2. 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 the species is also harvested offshore by the industrial fleet using trawls. Additionally Pentanemus can be caught with beach seines.

  3. Giant African threadfin (Polydactylus quadrifilis)

    The giant African threadfin (Polydactylus quadrifilis) can grow up to lengths 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 mangrove.

    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.

5.2.4 Sciaenidae

The croakers and drums are the important sciaenid species in Nigeria. 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.

  1. 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.

    They inhabit mud bottoms in coastal waters up to 50-m depth but also enter estuaries and coastal lagoons. This species, with maximum lenght of about 45 cm, moves further offshore to spawn during the rainy season. P. elongatus is jointly harvested by the artisanal and industrial fleets. It can be caught with bottom trawls, setnets, beach seines and longlines.

  2. Longneck croaker (Pseudotolithus (Pseudotolithus) typus)

    Pseudotolithus (Pseudotolithus) typus) grows to a larger size than P. elongatus. It attains a maximum length (L∞) of 100 cm and fish of 50-cm length are common in the catch. The main fishing ground for this species is from the Gulf of Guinea to the Congo. It is the most important commercial sciaenid species in Nigeria.

    Pseudotolithus (Pseudotolithus) typus inhabits mud and sandy bottoms up to a depth of 150 m but it is more abundant in waters of less than 60 m and temperature above 18°C. It also occurs in estuaries. Hence, it is fished by artisanal and industrial fleets using bottom trawls, bottom setnets and longlines.

  3. 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.

    This species is more accessible to the industrial fisheries using trawls and hook on line than to the artisanal fisheries using gillnets and beach seines.

5.2.5 Sparidae

The seabreams occur in fairly deep waters of the continental shelf and off the slope. The small young individuals do occur in shallow waters but mostly at a depth greater than 15 m, forming aggregations. The adult

seabreams are more solitary. The most common species are Dentex angolensis and Pagellus bellottii. The seabreams are mainly exploited by industrial fleets in Nigeria. Many species are hermaphroditic (having both male and female gametes). Sometimes the majority of individuals are male at first maturity and the females appear later (protandric hermaphroditisms). In some cases the females are more at first maturity and the males appear later (protogynic hermaphroditism). Since protogynic hermaphroditism is associated with efficient utilization of good resources and parental care, it appears to be a better strategy for exploited sparid species.

  1. Angola dentex (Dentex angolensis)

    Dentex angolensis occur along the West African coast from Morocco 33°N to Angola. It inhabits various bottoms on the continental shelf and the slope from about 15 m to about 300-m depth. It is a protogynic hermaphrodite with most individuals beginning as females and changing to males at a length 18–23 cm.

    Dentex angolensis is known to occur in Nigerian waters but the species is not an important element of the artisanal fisheries. It is caught by the trawl fishery but separate statistics are not available. Angola dentex is a carnivorous species feeding on crustacea, small fish, molluscs and other invertebrates. It can be caught in bottom trawls, bottom setnets and longlines.

  2. Red pandora (Pagellus bellottii)

    The geographical distribution of P. bellottii extends from the straits of Gibraltar to Angola and also around the Canary Islands.

    It is a protogynic hermaphrodite (the majority of individual are first females), then become males. Red pandora is omnivorous with a predominantly carnivorous diet consisting of crustacea, cephalopods, small fish and worms.

    This is one of the most abundant sparid species in the CECAF area but it is not a target species of artisanal fisheries in Nigeria. It is possibly caught by the trawl fishery but separate catch data are not reported.

5.2.6 Other exploited fish species

Some information concerning the biology and ecology of other exploited fish (e.g., Ariidae, Bagridae, Cynoglossidae, Pomadasyidae, Serranidae, etc.) plus species groups treated in Sections 5.2.1 and 5.2.4 also reveals interaction between the brackishwater and open-sea fisheries and possible competition between various sectors of any of the fisheries on certain species.

5.2.7 Penaeid shrimps

Three commercially important penaeid shrimps occur in Nigerian 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 brackish waters 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 between three weeks, and one month and thereafter in the coastal zones, in bays, estuaries, in mangrove swamps which are rich in food, or in submerged vegetation. As their development progresses the shrimps migrate toward greater and greater depths. When the areas of distribution of juveniles and adults are clearly separated geographically, a true migration seaward occurs after which spawning takes place.

Since the types of exploitation (and the operational zones of the various gears) are extremely diversified, there are in fact several successive recruitment phases:

  1. when the shrimps leave the nursery edges and become accessible to artisanal fisheries;

  2. when they reach the large bays where they are accessible to small trawlers;

  3. during migration, when they are caught by fixed nets;

  4. when they reach the sea and are caught by industrial trawlers.

The entry processus into the different fisheries 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. If the percentages are plotted against size a recruitment curve will be obtained.

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