There is a definite pattern in the distribution of fish species on the continental shelf of Sierra Leone (Longhurst, 1965). 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 bottom deposits (sand and silts), and the depths on the continental shelf, the slope of which is variable. There are discrete ecological fish communities, each of which is fairly homogeneous. However, there is also ecological and microgeographical heterogeneity of fish communities. Besides, 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:
the estuarine and creek sciaenid sub-community,
the offshore suprathermoclinal sciaenid sub-community (on soft deposits),
the sparid sub-community (on sandy and harder substrates),
the eurybathic fish species,
the pelagic fish species.
The sciaenid sub-community inhabiting the estuaries and creeks consists of Pseudotolithus elongatus, Polydactylus quadrifilis, Pomadasys jubelini, Drepane africana, Arius spp., Cynoglossus, Ilisha africana, Ethmalosa fimbriata, Penaeus duorarum notialis, Parapenaeopsis atlantica, Trichiurus lepturus, Sphyraena, etc.
The dominant elements of this fish sub-community are: Pseudotolithus senegalensis and P. typus (Sciaenidae); Galeoides decadactylus and Pentanemus quinquarius (Polynemidae); Pomadasys jubelini (Pomadasyidae); Drepane africana (Drepanidae); Arius spp. (Ariidae); Cynoglossus spp. (Cynoglossidae); Ilisha africana and Ethmalosa (Clupeidae).
Pagrus ehrenbergi, (= Sparus caeruleostictus), Pagellus coupei, Decapterus punctatus, Dentex spp. and Epinephelus are common on the sand and rocky bottms in the inshore suprathermocline areas.
The deep-water sparid community which occurs on both sandy and muddy bottoms below the thermocline, down to the edge of the continental shelf includes: Dentex angolensis (= Dentex congoensis), Sparus caeruleostictus (Pagrus ehrenbergi), Pagellus coupei, Boops boops, Epinephelus and Arioma ledanoisi, Balistes, etc.
The fish species with a large vertical range of distribution on the continental shelf of Sierra Leone area: Cynoglossus, Vomer setepinnis, Brachydeuterus auritus, Trichiurus lepturus, Raja spp., shrimps and prawns, etc. It should be noted that this eurybathic fish species group is harvested both by the artisanal fishermen and the industrial fleet. But there is no proper documentation on migration patterns of species constituting this fish group.
There is a somewhat diverse pelagic fishery resource (Okera, 1976; Longhurst, 1983). The coastal pelagic fish species include Caranx, Sphyraena, Cybium, Trichiurus, Sardinella, Ethmalosa, Chloroscombrus, Vomer, Ilisha africana, etc.
Most clupeid species are marine but some anadromous shads (Ilisha) and bonga (Ethmalosa) are adapted to withstand low salinities particularly in the rainy season.
(a) Ethmalosa fimbriata (= Ethmalosa dorsalis) - Bonga
Ethmalosa (bonga) is an important clupeid species in the inshore waters of Sierra Leone. Bonga rarely goes below 20 m. It is more euryhaline than the flat sardinella and it occurs in estuaries and on the continental shelf of estuaries as well as in lagoons. This species prefers warm turbid waters. It is dominant in inshore areas without upwelling but with strong surface desalination. Its migrations seem small in extent and limited to estuaries and adjacent coastal areas (Longhurst, 1960).
Ethmalosa is a non-selective filter feeder subsisting mainly on large diatoms and phytoplankton.
Most of the information available comes from Salzen (1958) who examined the bonga fishery and some aspects of the biology of the species in the Sierra Leone River estuary.
Three size groups can be distinguished. These are the immature fish (‘awefu’), ‘bonga’, and ‘bonji’. Awefu are less than 21 cm long and are restricted to depths of about 2–4 m in estuary. They move further upstream than the bigger fishes. The bonga have lengths ranging between 22–32 cm. The largest size group (bonji) consists of fish with lengths above 33 cm. They are found in areas of the estuary where the water is deeper than 6 fath (11 m). The modal length for awefu in the estuary is 12–15 cm whereas bonga (medium-sized) has a modal length of 28 cm. The largest size groups ‘bonji’ has a modal length of 36 cm. In the estuary, awefu and bonga are abundant while bonji are few. Salzen (1958) suggested that the species after growing into adults (bonji) leaves the estuary for the sea.
Maturity is attained at sizes between 22 and 25 cm. Awefu are caught in the estuary all through the year indicating that breeding occurs all through the year. Spawning is highest the beginning of the dry season in November. It then declines progressively but increases again when the wet season sets in. Salzen (1958) suggested that the decline in observed spawning is due to the spawning area shifting further upstream as the dry season progresses while the increase during the wet season is due to the spawning area moving toward the mouth of the river as the fish return to the sea due to reduced salinity in the estuary.
Migration of bonga into and out of the estuaries is associated with seasonal changes in salinity as well as with abundance of plankton in the estuary during the dry season. For example, Ethmalosa is most abundant in the Sierra Leone River during November-April. As the dry season progresses, the species moves further up the river and then during the wet season (May-October) the species again moves down the estuary toward the sea. It is possible that the migration is due to spawning or feeding needs.
(b) Sardinella aurita (round sardine)
Sardinella aurita occurs almost throughout the CECAF area. It is a cold water species extending to the shoreline. The species shoals near the surface during upwelling and retreats below the thermocline up to a depth of 300 m during the hot season (ATLANTNIRO, 1980). In Sierra Leone the largest aggregations of round sardine are recorded in the northern sector at a depth of 15–20 m during the period March-September. Concentrations of Sardinella aurita consists of individuals of length 10–24 cm.
Northward migrations from Sierra Leone begin September-October and are completed in February-March. Sexual maturity is attained at a length (Lm) of about 13 cm. Spawning occurs at a depth of 20–40 m with two spawning peaks: (a) August-October and (b) February-April. Sardinella aurita feeds on phytoplankton, copepods, euphausids, etc.
(c) Sardinella eba
Sardinella eba prefers warm waters with a temperature above 24°C and appears to avoid turbid waters. Spawning is more or less continuous throughout the year but there is a period of maximum reproduction which should be during the hot season in Sierra Leone, Liberia, and Ivory Coast.
The young fish are concentrated in the coastal waters from where they gradually move off as they grow older; but the stratification toward the open sea with age is never so clear cut as it is for the Sardinella aurita (Boely, 1979), and the great majority of the adults remain confined over the shallow half of the continental shelf.
(d) Shad (Ilisha africana)
Shad is an anadromous clupeid species inhabiting inshore waters, sand beaches and estuaries (in almost fresh waters). Ilisha africana feeds mostly on crustacea and small juvenile fish.
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. It is common in the CECAF coastal areas between Sierra Leone and Congo.
The following carangids occur in the waters of Sierra Leone: jacks, crevalles, scads, bumper, pompanos and blue runners. There are mostly schooling species distributed on the continental shelf but some occur in brackishwaters especially when young.
(a) Caranx spp. (various jacks)
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 small fish, shrimps, some crabs and other invertebrates. This fish species group is caught in pelagic and bottom trawls, seines, set gillnets and ring nets and sometimes on line gear.
(b) Chloroscombrus chrysurus (Atlantic bumper)
Chloroscombrus chrysurus occurs along the West African coast from Mauritania to Angola. This schooling pelagic species inhabits the Sierra Leone continental shelf at depths of 10–15 m. It also occurs in estuaries and mangrove fringed lagoons and brackishwater areas. Atlantic bumper can be a target species for the artisanal fleets using set gillnets and seines as well as for the industrial fleets using trawls.
Results of the joint USSR-Sierra Leone fishery research in 1978–79 indicate this species has a maximum length (Linfin) of about 27 cm and a maximum weight (Winfin) of 280–300 g. First maturity is attained at a length (Lm) of about 13 cm and at an age (tm) of one year.
Fishing grounds for this species are located on the continental shelf at a depth of 10–50 m. It can be caught with trawls, seines and set nets.
(c) Decapterus rhonchus (= Caranx rhonchus) - False scad
Decapterus rhonchus is a schooling carangid inhabiting near-bottom waters mostly between 30 m and 50 m but can also be located in waters over 200 m depth. This species makes seasonal migrations. Results of the joint USSR-Sierra Leone fishery investigations conducted in 1982 indicate denser aggregations of Decapterus during the period September-March and in areas with water temperature 15°–22°c. Incidentally, during the same period massive spawning occurs at depths 10–30 m.
Main concentrations of false scad were located in the northern area of the continental shelf between 8°N–9°N. Its population is less dense in the southern sector but the species is fairly abundant around the Sherbro Island and latitude 7°N.
The length at first maturity (Lm) is 17–20 cm whereas spawning females are 21–35 cm long.
This species is mostly exploited by industrial fleets using trawls, but it can also be fished by artisanal fishermen using motorized canoes and gillnets.
(d) Trachurus trecae - Cunene horse mackerel
Trachurus trecae occurs in the Eastern Atlantic along the African Coast from Mauritania to Southern Angola. It is a shoaling species occuring in near-bottom waters of 20–100 m depths; and sometimes pelagic, shoaling near surface.
There is a small local population of T. trecae making no lengthy migration in Sierra Leone waters. The distribution of this species appears to be greatly influenced by the hydrological regime and mostly by the location of the upwelling zones at the periphery of which the species shoals. The largest common concentrations are recorded in the dry season when upwelling is greatest.
Cunene horse mackerel attains a maximum length (L∞) of about 25 cm and attains first maturity at a length (Lm) of about 14 cm. Spawning occurs at a depth of 30–75 m during the dry season (December-February).
The fishing grounds for this species are mainly over the continental shelf. It can be a target species for both pelagic and bottom trawls as well as purse seines.
The croakers, drums and meagres are the most important sciaenid species in Sierra Leone. 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.
(a) Pseudotolithus (P.) senegalensis (Cassava croaker)
Pseudotolithus (pseudotolithus) senegalensis occurs along the West African coast between Morocco and Angola but the species is rare north of Senegal. It inhabits mud, sandy and rocky bottoms from the shoreline to 70-m depth; the smaller and younger ones prefer shallow waters and move to midwaters when bottom temperature falls below 18°C. Pseudotolithus senegalensis occurs in similar habitats as Pseudotolithus (fonticulus) elongatus. The two species are jointly harvested by artisanal and industrial fisheries, using set gillnets, beach seines, longlines and bottom trawls.
(b) Pteroscion peli (Boe drum)
Pteroscion peli occurs only along the West coast of Africa, from Senegal to Angola, where it is found in midwaters as well as on mud and sandy mud bottoms. Its depth distribution extends from the shoreline to 200 m but the species prefers waters of less than 50 m. According to ATLANTNIRO (1983) Pteroscion peli and Pseudotolithus senegalensis are equally important in the trawl catches south of the Sherbro Island.
(a) Galeoides decadactylus - Lesser African threadfin (local name - Sulema)
Galeoides decadactylus does not appear to penetrate below the thermocline. 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). Hence understanding its reproductive and recruitment strategy appears 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. There are two spawning seasons: (a) a smaller portion of the stock spawns between March and June (spring-summer period); and (b) greater aggregations of this species spawn between June and December (rainy season) when temperatures and salinities are lower. Galeoides decadactylus attains a maximum length (L∞) of about 45 cm. The males mature earlier at a length of 12–13 cm whereas the females attain first maturity at about 15–16 cm.
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.
(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 near shore sandy bottoms but the species 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) could grow up to lengths from 150 cm to 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 and industrial fleets. Its attractive marketable size has made it extremely vulnerable to the gillnet and beach seine fisheries.
The seabreams inhabit the continental shelf and slope. The small and young individuals occur in shallower waters forming aggregations whereas the adults are solitary. 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 hermaphroditism). In some cases the females are more at first maturity and more males appear later (protogynic hermaphroditism). Since protogynic hermaphroditism is associated with efficient utilization of food resources and parental care, it might be a better strategy for exploited sparids.
(a) Dentex angolensis (Angola dentex)
Dentex angolensis occurs 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 of 18–23 cm.
Fairly dense concentrations of this species have been reported on the continental shelf south of Sherbro Island. Angola dentex is a carnivorous species feeding on crustacea, small fish, molluscs and other invertebrates. It can be caught in bottom trawls, bottom set nets and longlines.
(b) Pagellus bellottii (= Pagellus coupei) - Red pandora
The geographical distribution of P. bellottii extends from the Straits of Gibraltar to Angola and also around the Canary Islands. P. bellottii inhabits hard and sandy bottoms. It is mostly caught in bottom trawls, hooks on line and traps.
(c) Sparus caeruleostrictus (= Pagrus ehrenbergii) - Bluespotted seabream
Sparus caeruleostrictus inhabits hard bottoms with rocks and rubble up to 150 m depth. The bigger and older individuals prefer the deeper waters whereas the younger fish live much more inshore. The species attain sexual maturity at about 2 years. The species makes spawning migrations parallel to the coastline.
The bluespotted seabream is carnivorous and feeds on bivalves, crustacea and fish. In Sierra Leone, the species has been observed in association with flying gurnards (Dactylopterus) and the driftfish (Arioma) (ATLANTNIRO, 1983). Sparus ehrenbergii can be caught with bottom travels, seines and traps.
(a) Balistes capriscus (Grey triggerfish)
The grey triggerfish (Balistes capriscus) appears to be one of the most abundant near-bottom pelagic species occuring in the Sierra Leone waters. According to ATLANTNIRO (1980) this species can be located all over the shelf at depths 15–100 m but the preferred depth range is 30–50 m. Results of the joint USSR-Sierra Leone fishery research in 1978–79 indicated three major concentrations: (a) northern sector delimited by 8°15'N and 8°45'N; (b) central area between 7°45'N and 8°15'N; and (c) southern sector between 7°20'N and 7°45'N.
In summer during the rainy season, aggregation of B.capriscus occur north of Sherbro Island. In the dry season (winter) the species migrates southward and forms a mosaic structure of individual schools.
Balistes capriscus can attain a maximum length (Linfin) of 45 cm, but the size range in the Soviet trawls catches is 13–38 cm. Sexual maturity is attained at an average length (Lm) of 20–21 cm. Spawning takes place in dry season (November-February), Balistes capriscus is a benthophage, feeding mostly on crustacea, molluscs, echinoderms polychaetes, corals.
Fishing grounds are located throughout its depth range on the continental shelf. Triggerfish is gaining more commercial importance in West Africa. It can be finished with bottom trawls, traps, beach seines and handlines.
Three commercially important penaeid shrimps occur in Sierra Leone waters. Penaeus notialis (the pink shrimp) is by far the most dominant species. Penaeus notialis inhabits the same areas in which Penaeus notialis (Caramote prawn) occurs. Parapenaeus longirostris is less abundant and occurs in deeper water. The deepwater roseshrimp fishery has not yet been developed in Sierra Leone.
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 (during the juvenile and sub-adult stage). 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. There are several successive recruitment phases:
when 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;
when they reach the sea and are caught by industrial trawlers.
The entry processes into the different fisheries are 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 (at sea and in the lagoons) that is present in that area. If the percentages are plotted against size a recruitment curve will be obtained.
(a) Penaeus (Farfantepenaeus) notialis - Southern pink shrimp
The southern pink shrimp is known to exist from Mauritania (Cape Blanc 21°N) to Angola. It inhabits mud or muddy sand bottoms between 10-m and 100-m depth. It prefers waters with temperature of 18–24°C around river mouths and lagoons inlets.
Data on the life cycle, growth rate and natural mortality of Penaeus notialis are given by Garcia (1977), and Garcia and Lhomme (1980). Total mortality was estimated from 1982 and 1983 monthly catch figures by commercial size categories and data on fishing effort (Willmann and Frielink, 1985).
Penaeus notialis of three or four months migrates from the lagoons and estuaries into offshore waters where they become available to the trawl fishery. At this age the shrimp have a carapace length of 16–17 mm and weight on an average about 5 to 6 grams. In Sierra Leone recruitment into the offshore fishery coincides with the rainy season lasting from February/March to September/October. Clear recruitment peaks are in the months of February, March and April before the onset of the main rainfall. The best season for shrimp trawlers is between April and September, during which about two-thirds of the total catch is taken.
The pink shrimp fishery is intensive off the Freetown Peninsular, especially around Banana Island. Penaeus notialis can be caught in lagoons with traps, stow-nets, conical nets, beach seines, trawls towed by two persons and double-rigged bottom trawls.
(b) Penaeus kerathurus (Caramote prawn)
Penaeus kerathurus prefers muddy bottoms, sandy bottoms with detritus and seagrass beds, between 5-m and 50-m depth. It spawns during the period May-November and its postlarvae are found around river mouths and in lagoon areas. In Sierra Leone the species appears to be more abundant north of the Sierra Leone River. It is mostly caught in bottom trawls.
(c) Parapenaeus longirostris (deepwater rose shrimp)
This species inhabits sandy mud bottoms between 100 m and 400 m in waters with temperature 8°–15°C. It is more abundant off the coastal sector between Morocco and Senegal. It is less abundant off the coast of Sierra Leone and Liberia. A deepwater shrimp fishery is not yet developed.
