The topography and hydrographic regimes of the West African continental shelf affect the distribution and constituents of the benthic animal communities. Long hurst (1958) considered the absence of reef-building corals of many tropical waters and accounted for the predominance of sand, silt and shells in the bottom deposits. Previous oceanographic surveys also indicate that the prevailing hydrographic conditions are of biological significance to fisheries. Hence, this report gives a summary of topographic and hydrographic conditions influencing fish productivity along the Nigerian continental shelf.
The Nigerian coastline is characterized by a mixture of sand and mud deposits consisting of Pleistocene/Holocene sands and recent muddy silts (Allen and Wells, 1962). These bottom deposits influence fish distribution and fishing operations.
At the axis of the Niger Delta, there is more burying of the sands by silts. The occurrence of sand and mud deposits is related to the river outflows. The silt muds are deeper at the axis of the Niger Delta off the Kulama River. These muds grow progressively shallower westward, and west of the Mahin flats (Ondo State) sand deposits are dominant. East of the Niger Delta the prevailing pattern is one of alternating sand and mud deposits, since the estuaries of the Opobo, Imo and Cross Rivers provide recent muds on the continental shelf of this geographic sector. The width of the continental shelf is variable along the Nigerian coast as shown in Table 1 and Figure 1.
The 22 fath (40 m) contour is considered a fairly reliable boundary of the thermocline separating the upper layer oceanic currents from the lower oceanic currents (Scott, 1966). The thermocline tends to limit the extent of the distribution of the demersal fish stocks, hence, it is this zone between the highwater mark on shore and the 40 m depth contour running parallel to the coast at an average distance of 37 km (20 mi) from the shore which contains the demersal resources which provide opportunities for further development of the coastal inshore artisanal fisheries. But this same inshore zone has had increasing fishing activities of trawlers and shrimpers in more recent years. The concentration of artisanal and industrial fishing fleets in this coastal sector might be due to its higher productivity.
The extent of the Nigerian continental shelf and the boundary of the thermocline with the 40 m depth contour are shown in Figure 2. Moses (1980 and 1981) determined the length of the coastline of Cross River State to be about 129 km and also gave estimates of fish biomass. Hence, the coastal inshore fishery of Cross River operates in an area of about 4 700 km2 (up to 40 m contour). As for the Niger Delta (River State coastal area) the activities of the artisanal coastal inshore fisheries cover an area of about 8 200 km2 (based on the work of Scott, 1966). For most of Lagos, Ogun and Ondo States, the continental shelf is narrower: off Lagos, the 40 m depth contour marking the thermocline is located at a distance of 6 nautical miles (11 km) offshore. Therefore, for Lagos State, the area available for most demersal stocks (located between the shore and thermocline) is about 1 800 km2.
The continental shelf are between the shoreline and the 10 fath (18 m) contour is supposed to be exclusively fished by the artisanal fishermen but both the artisanal and industrial fisheries exploit the continental shelf sector between 10 fath (18 m) and 22 fath (40 m). Estimates of continental shelf area for the sector with depth range (a) 0-18 m and (b) 18-91 m for the coastal States are given in Table 2. Using indices of biological productivity of the Nigerian continental shelf and the area of various sectors, rough estimates of possible yield magnitudes can be derived.
The brackishwater sector consists of estuaries, beach ridges, intertidal mangrove swamps, intersecting rivers and winding saline creeks. The exact area of the brackishwater sector for the entire Nigerian coastline is unknown except for the Niger Delta portion the area of which was estimated by Scott (1966) as follows: the permanent saline creeks form an area of about 1 000 km2, whereas the intertidal mangrove swamps comprise an area of about 5 048 km2. Additionally, the intersecting rivers and the estuaries plus beach ridges comprise an area of about 679 km2 and 688 km2, respectively. Thus, the total brackishwater area for the Niger Delta is about 7 415 km2 (Scott, 1966). The inland extent of the mangrove swamp fairly delimits the brackishwater area. The brackishwater area of the coastal zone of Nigeria between the Benin River and Cross River has been delineated in detail by Nduaguba (1983). The coastal features (forest swamps, mangrove, marsh, beach ridges, estuaries and creeks) between Benin River and Cross River are illustrated in Figures 3 to 6. These features are of significance to location of fishing settlements and influence fishery development activities.
| State | Area off | Distance offshore | Depth of edge of shelf (metres) |
|---|---|---|---|
| Lagos | Lagos | 15 | 120 |
| Bendel | Escravos R. | 31 | 270 |
| Rivers | Dodo R. | 36 | 220 |
| Rivers | Nun R. | 39 | 150 |
| Rivers | St. Bartholomeo | 49 | 280 |
| Rivers | Opobo R. | 44 | 200 |
| Cross River | Calabar R. | 40 | 90 |
| Coastal sectors | Continental shelf area | ||
|---|---|---|---|
| 0 - 18 m | 18 - 91 m | Total area | |
| Cross River State | 2 810 | 3 590 | 6 400 |
| Rivers State | 5 200 | 10 890 | 16 090 |
| Bendel State | 1 200 | 1 470 | 2 670 |
| Ondo/Ogun/Lagos States | 2 570 | 5 730 | 8 300 |
| Total area | 11 780 | 21 680 | 33 460 |
An estuary can be defined in various ways. But, from the point of view of hydrography and fisheries, an estuary is an area of interaction between fresh and sea water. This report adopts the definition by Came ron and Pritchard (1963), that is, an estuary is a semi-closed coastal body of water which has free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage. The Nigerian coast is characterized by bar-built estuaries. Recent sedimentation has kept pace with inundation. On this depositional coast some estuaries with restricted cross sectional areas (e.g., Cross Rivers, New Calabar, Benin and Forcados), the current is strong at the mouth but further inland the current velocity is much less. The Nigerian estuaries are generally shallow (a few metres deep), have extensive lagoons and shallow intersecting rivers and winding creeks inside the mouths, and exhibit the dynamic properties of bathymetrically arrested estuaries. Estimates of the area of some estuaries and river sectors delimited by the mangrove plus adjoining creeks are given in Table 3. The Nigerian estuaries and neighbouring shelf areas are good fishing grounds for bonga and shrimp.
The surface water of the Gulf of Guinea is basically warm (temperature greater than 24°C) and with low salinity of less than 35°/00. The sector between Cape Palmas (Liberia) and Cotonou (Benin) experiences seasonal upwelling when cold, high salinity water comes to the surface between June and October.
The most significant biological features of the hydrographic regime in the Nigerian waters are: the relatively stable thermocline, the steep temperature gradient and stable oceanographic conditions below the mixed layer throughout the year. The Eastern Tropical Zone (ETZ) of the Gulf of Guine a from Cotonou (Benin) to Cape Lopez (Gabon) is not affected by seasonal replacement of surface water. But even here, the surface water temperature fluctuates between 26° and 30° (Williams, 1968). The hydrographic conditions in the coastal area are greatly affected by the effluent rivers. The effect of the effluents on the oceanographic conditions depends on the average annual discharge. It is greatest in the Niger Delta area, where the total annual discharge is about 21 800 m3. The annual discharge is greater for rivers west of the delta axis and west of the Brass River.
Berrit (1958, 1959 and 1973) gives the general oceanographic situation of the Niger Delta area between the mouth of the Escravos River (5°30'N, 5°5'E) and the mouth of the Bonny River (4°23'N, 7°8'E) for the sector extending seaward to 200 m depth. Off the Escravos River an inshore belt of water of low salinity of 15–10°/00 has been observed. This is due to the dilution of sea water by much more effluent from thils river. But the 20°/00 isohaline is located 31 km (19 mi) off the Brass River, whereas no such belt of low salinity is observed off rivers east of the Bonny river. The annual discharge over the various outlets of the Niger Delta is mainly through rivers west of Brass.
An estuary contains waters in a mixing series, from river water with an average dissolved salt content of 120 mg 1-1, to sea water with a salinity of 35°/00 (Livingstone, 1963). Proceeding from the mouth of a coastal estuary towards the head, the salinity decreases from about 30°/00 to about 0.1°/00. Beyond the salinity of 0.1°/00, chlorinity drops from 0.06°/oo to about 0.01°/oo and ratio of major dissolved constituents changes rapidly. This point marks the upper limit of the estuary (Jhingran, 1982). Upstream of this point conductivity and chlorinity are more or less constant but the tidal currents may still persist although there may be no flood flow of the tide.
| Brackiswater system | State | Brackishwater area in km2 | |
|---|---|---|---|
| Based on federal survey mapsa | Based onradar mosaicsb | ||
| Epe Lagoon | Lagos | 460 | - |
| Lagos Lagoon | Lagos | 460 | - |
| Lekki Lagoon | Lagos | - | - |
| Ogun Lagoon | Ogun | 26 | - |
| Benin River | Bendel | 150 | 109 |
| Escravos River | Bendel | 150 | 160 |
| Forcados River | Bendel | 120 | 201 |
| Ramos River | Rivers | 50 | 46 |
| Dodo River | Rivers | - | - |
| Pennington River | Rivers | 21 | 17 |
| Kulama River | Rivers | 10 | 12 |
| Fishtown River | Rivers | 5 | 5 |
| Sangana River | Rivers | 37 | - |
| Nun River | Rivers | 67 | 52 |
| Brass River | Rivers | 94 | 91 |
| St. Nicholas River | Rivers | 44 | 21 |
| San Barbara River | Rivers | 49 | 48 |
| San Bartholomeo River | Rivers | 84 | 81 |
| Sombreiro River | Rivers | 117 | 132 |
| New Calabar River | Rivers | 92 | 163 |
| Andoni River | Rivers | 160 | 117 |
| Imo River | Rivers | - | 51 |
| Kwa Ibo River | Cross River | - | 7 |
| Cross River | Cross River | 750 | 510 |
a Ssentongo, Ajayi and Ukpe (1983) determined areas using Federal survey maps
b Nduaguba (1983) estimated areas using Side-Looking Airborne Radar Mosaics
The dry season extending from November to April is generally characterized by increased fishing activity and higher fish catches; but variation in fishing performance could be limited by cyclic changes in oceanographic conditions. During the dry, warm season (November-April) the southwesterly winds (southwest monsoon) are weak in the Gulf of Guinea. In this period, the equatorial undercurrent (Lomonosov current) often attains maximum velocity and penetrates the Bights of Benin and Biafra. The equatorial undercurrent weakens during the wet season (April-June) when the southwesterly winds are reinforced. There is a second short “high-sun” season, extending from September to November, with sufficient occasional rains, after which is the onset of the southwest monsoon.
At the end of the rainy season a more stable oceanographic situation exists in the Niger Delta area. There is a warm superficial layer of Tropical Surface Waters (TSW) with a temperature range of 25° to 27°c and a salinity range of 14°/oo to 31°/oo. The thermocline is characterized by a temperature range of 18° to 25°c and this occurs between a depth of 30 to 50 m. The temperature gradient is lower off the Escravos River, where the thermocline is encountered at a depth of 40 to 50 m and 42 km(26 mi) offshore. But, in the sector off Brass River the thermocline occurs at about 27 km (17 mi) offshore and between 35 to 40 m depth.
In the Nigerian sector of the Gulf of Guinea, between 4°N and 6°N, the southwest monsoon may occur at anytime, but this phenomenon is not naturally typical. These reversing winds influencing the oceanographic conditions are partly maintained by seasonal thermal contrasts between land and sea surfaces. These winds are also an equilibrating flow between two regional pressure patterns, whilst they could equally be triggered by an anomaly in the planetary air circulation.
Other factors influencing the extent of the fishing season along the Nigerian coastline are: the warm Guine a current which flows eastward but finally swings around in the Bight of Biafra; existence of moving oceanic fronts which alter the characteristics of surface water within a short period (a few days); and possible extension of the coastal upwelling phenomenon to the coastal sector of the Bights of Benin and Biafra.
Although there is sufficient information on the oceanographic conditions of the Nigerian coastal waters, satisfactory answers on how the southwest monsoon, the equatorial undercurrent (Lomonosov current), the warm Guine a Current and various oceanic fronts affect fish availability and fishing performance, have still not been given by the Nigerian Institute of Oceanography and Marine Research (NIOMR).
Previous studies showed that, during and after September, at the end of the rainy season, the inshore waters of the Niger Delta were more saline to the east of the Bonny River than to the west. This factor also affects the distribution and seasonal pattern of estuarine species.
The distribution of oxygen and other dissolved gases in the sea is mostly controlled by temperature, salinity, biological activity, currents and mixing processes. Turbidity of Nigerian coastal waters is accounted for by the presence of suspended organic matter in the effluent, silt and sand and also by the quantity of plankton. It has been noted that the coastal waters east of the Bonny River are less turbid than the Benin, Escravos and Forcados Rivers. The intensity of wave turblence is shown by a 15 m transparency line which is located at about 29 km (18 mi) off the Bonny River, whereas the same line is located 48 km (30 mi) off the Escravos River (Eziuzo, 1965). The presence of more suspended organic matter in estuaries with lower salinity, e.g., Escravos, Forcados, Cross River, appears to be an important factor determining the abundance of shrimp (particularly, Penaeus notialis).
The higher turbidity of estuarine and coastal waters between the Ramos River and the Ogun State coastline appears to favour more abundance of filter feeders, e.g., bonga (Ethmalosa fimbriata), and shrimps and also appears to limit predatory fish species preferring more clear waters far-off the Niger Delta axis.
