Nigeria is situated entirely within the tropical zone and is located between latitudes 4° and 14° North of the Equator and longitudes 3° and 15° East. It is bounded on the west by the Republic of Benin, to the north by the Republic of Niger, to the east by the Republic of Cameroon and to the south is bathed by the Atlantic Ocean.
It has a total land mass of about 924,000km2 and a population of 88.5 million (Anon., 1992). Physiographically the country consists of several extensive plateaux. The major plateaux surfaces are the Jos Plateau, Udi Plateau and the north central high plains. The coastal areas are usually covered by soft rocks which are prominent along the Niger Delta, Niger-Benue trough and Lake Chad Basin. The high plateaux are underlain by basement complex and volcanic rocks. Examples of the volcanic hills are the remains of extinct volcanoes as seen in Jos, Biu plateaux and the Eastern borderlands. The craters created by these volcanoes are well preserved and several of them contain crater lakes. The Mount Cameroon volcano is still active on the eastern border of Nigeria; activity during which several lives were lost was recorded in 1989. The lowland areas are composed of sedimentary rock and cover the Sokoto plains, Chad Basin, Niger-Benue trough, western areas of Nigeria, south-eastern Nigeria and coastal margins and swamps. The major rivers of Nigeria are found in these lowland areas.
Nigeria has two major rivers, the Niger, after which the country is named, and the Benue. They meet at the Lokoja confluence and enter the Gulf of Guinea through a network of creeks and distributaries which form the Niger Delta. There are, however, a few other tributary rivers which drain into the Niger-Benue trough and Lake Chad. These include the Sokoto-Rima, Kaduna, Anambra, Gongola, Hadejia, Jama'are and Yobe rivers. The basins of these major rivers and their tributaries constitute the drainage pattern of the entire country. Other major rivers e.g. Cross, Imo, Ogun, Osun, Benin, Qua Iboe etc. empty directly into the Atlantic Ocean. The majority of small rivers are seasonal.
The climate, determining the different ecological zones, is influenced by two wind systems, the south-westerly that brings rain and the north-westerly from the Sahara Desert that brings the dry and dusty harmattan wind. According to Garnier (1967) and Iloeje (1980), Nigeria, and indeed all the West African countries which experience similar weather conditions, can be said to have four main climatic zones.
The Equatorial Climate which extends from the coast to about 150km inland. Rainfall is between 1500 and 3000mm per annum, with an average temperature range of 17–24°C and relative humidity ranges between 60–90%. It has two seasons, the wet season March to October, and dry season November to March. Both Port-Harcourt and Lagos are in this zone.
Tropical Hinterland, about 150–240km northwards from the coast, with 1000 to 1500mm rainfall, temperature range of 21–25°C and relative humidity range of 50–80%. It has a longer dry season, of 4–5 months, compared with the equatorial zone which lasts from October to April. Examples are Ibadan and Enugu.
Tropical Continental which falls into the Sudano-Sahelian vegetation zone with rainfall of 250–1000mm, temperature of 25–30°C (but with lower night temperatures especially during the harmattan) and low relative humidity of 20–40%. The characteristic dry hot, harmattan wind may last from October to May. Examples are Sokoto, Kano, Maiduguri and Yola.
Montane or Plateau type climate is limited to the highland areas, with a high annual rainfall of 1400–4000mm, relatively low temperatures of 5–20°C and high humidity of 30–90%. Example is Jos.
In general, rainfall, temperature and humidity have the following trends. The temperature is usually high, with an average of about 25°C and increases as one moves northwards although variations are influenced by season and latitude, while the rainfall and humidity increase towards the coastal areas.
Recent years have seen a general trend of increasing drought conditions compared with the weather conditions of the 1930s to late 1940s (Oguntoyinbo, 1983). There was a notable drought period between 1968–1973 and this led to an upsurge of water conservation strategies. These included the construction of several dams, boreholes, irrigation projects and the formation of small water bodies both for domestic use and for migrating animals (Illiasu & Alsop, 1987; Satia, 1990) plus the creation of several River Basin Authorities. Another change is in the harmattan which has now extended its frontiers into the tropical hinterland, and sometimes as far as the equatorial climatic areas. This is readily observed by obvious harmattan hazes and dry dusty winds.
The severity of the harmattan has been attributed to the encroaching desert due to deforestation as a result of human activities, bush burning, human settlement and development, logging and felling for firewood. There is also usually a drop in humidity causing dryness, coldness, and a dusty and hazy atmosphere. The unpleasant experiences which result from a severe harmattan include thick deposits of dust on buildings and furniture and increased incidence of conjunctivitis and cracking of human lips due to dryness.
Climate (particularly rainfall) has an important influence on the distribution of vegetation in Nigeria. There are ten main vegetation zones (Udo, 1970): the Sahel, Sudan and Northern Guinea zones. Jos Plateau, Montane forest and grassland, Rain forest, Oil palm bush, Southern Guinea zone, Swamp and Mangrove forest.
These major zones have different vegetation types which can be further subdivided into coastal forest and mangrove, deltaic swamp forest, swamp forest and wooded savanna, secondary forest, mixed leguminous wooded savanna. Isoberlinia savanna, Afzelia savanna and semi-deciduous forest, plateau grass savanna, mixed Combretaceous woodland, wooded savanna, mixed wooded savanna, floodplain complex, Sorghum grass savanna, Burkeo africana savanna, wooded tropical steppe and moist lowland forest.
An estimated 68 million hectares of land is utilized for farming with an average of two hectares per farming family. Because of the differing vegetation and climatic conditions and socio-cultural base, each ecological zone has some degree of specialization in farming system, type of crops and animals reared.
The crop farming system is mainly the traditional rain fed agriculture, contributing about 95% of the farming activity, while mechanized and irrigated agriculture uses only 5% of the cultivated land. There are six main farming patterns: shifting cultivation, sedentary and permanent cultivation, terrace agriculture, irrigated agriculture and mixed farming. In animal husbandry, there are four main systems, free range, sedentary, migratory and intensive animal husbandry.
In the forest zone more tubers, such as cassava, yam, cocoyam and forest related crops such as cocoa, palm tree, coconut, banana, pineapple, orange and mango, are grown while in the savanna and arid zones of the Sahel, Sudan and Guinea vegetation more cereals such as sorghum, maize, rice, beans, soyabean, guineacorn and vegetables (including pepper, carrots, garden eggs, potatoes and some other vegetables) are grown. A higher population of livestock is raised in the savanna region because of the limiting effects of livestock diseases and the high humidity of the southern vegetation zones. Dwarf goats, sheep and Ndama cattle are more abundant under sedentary conditions in the southern vegetation zones.
The recent population figures from the National Population Commission indicate that Nigeria is a highly populated nation and as might be expected, the use of resources depends on the population and socio-economic structure. Areas with a high population have greater pressures on the utilization of both terrestrial and aquatic resources. The available resource commodities that are utilized would in turn influence the economic activities of such communities.
The abundance of water in a given area under natural conditions is partly a function of soil, types of geological formation, vegetation and other environmental factors. This is because the ability of any soil to retain moisture and its water-holding capacity determine the extent and formation of pools of available surface water.
Figure 1. Hydrological map of Nigeria.
Nigeria's fragile soils are susceptible to erosion, the various causes of which are both natural, and man-made due to abuse of the environment. However, the natural phenomena are over-shadowed by the effects of human activities. While some of the causes of erosion are limited most have very wide distribution. Depending on the type, soil erosion is of concern because it can devastate human settlements, agricultural grounds and recreational development, can disrupt and destroy industrial structures and facilities and severely upset the hydrological regime. The deformation of hydrological regimes usually leads to increased rates of siltation. Hydrological disruption may also affect the pattern of water discharges, hence affecting the normal pattern of aquatic life.
Nigeria is blessed with abundant inland water resources (Fig. 1). There are 149,919 km2 of inland waters made up of major lakes, rivers, ponds, floodplains, mining and stagnant pools (Ita et.al., 1985). In the 1980s there were 347 reservoirs and lakes, 839 floodplains and rivers, 5000 fish ponds (Satia, 1990), 89 cattle drinking ports, and many earth wells and boreholes. There are several abandoned mine pits particularly in Plateau, Anambra, and Enugu States that hold a considerable amount of water all year round, ranging from 0.2–0.7 hectares. Also excavation ponds of abandoned sand and stone quarries associated with road construction sites are common along major highways. The determination to solve the problems of drought resulted in the development of lakes and reservoirs which are now abundant in the northern parts of the country most affected by drought, such as Kano, Jigawa and Katsina States.
The sources of these waters are both surface and underground (Ayoade, 1981) with rainfall as the primary contributor. Rainfall varies from 10 months in the coastal areas to about 2–3 months in the Sahelian part of the country. The rainfall range therefore varies from 3000mm on the coast to 250mm at the farthest north.
The country is less well endowed with groundwater resources because half of it is underlain by Basement Complex rock which is very impermeable except in areas cleaved or fissured. The southern half of Nigeria, towards the coast, is covered with sedimentary rocks and is therefore richer in underground water. The yield in these areas sometimes exceeds 20,000 litres per hour and the water table is usually low, about 30m but near the coast there is usually infiltration of sea and brackish water. It is estimated that Nigeria can utilize about 70% of its underground water resources without depleting surface reserves from rivers, lakes and similar water bodies and that the total annual quantity of groundwater recharge is about 9.5 × 1012 litres (Mitchelle-Thome, 1961). There is therefore no doubt that the country has a considerable water resource potential.
Although water is reasonably abundant there is always the problem of drought and the wasteful discharge of surplus water from the hinterlands of Nigeria to the Atlantic Ocean. It is estimated that about 7000m3 sec-1 is discharged every year through the Niger Delta and its distributaries (Ayeni, 1991). This surplus could have been rediverted to recharge the soil and aquifers where water is not always abundant. There is a need for such water conservation which can be achieved by damming, channelization, pumping and extension of some small perennial lakes. There is a high demand for water due to increasing population, expanding areas of irrigated agriculture and urbanization. There is therefore a need to retain as much water as possible on land in dams and reservoirs and to optimize water use throughout the country. The recharging of the existing aquifer is possible with the proper management and conservation of the catchment areas.
There are three main watersheds in Nigeria, the Western Highlands, the Udi Plateau and Jos Plateau. The main rivers draining from these watersheds are the River Niger and its tributaries, both seasonal and perennial, the River Benue and its tributaries, and other large river systems which include the Hadejia-Jama' are - Yobe Rivers, the Cross River and its tributaries, the Kaduna River, the Gongola River and its tributaries, the Sokoto-Rima River complexes and the Osun, Ogun and Imo Rivers. The drainage systems of these large rivers have characteristic hydrological patterns which can be grouped into three categories. The first consists of short, relatively swift rivers which are found mainly in the coastal regions and drain directly into the Atlantic ocean. These rivers have double peak flow and flood regimes, corresponding to the two peaks of rainfall each year associated with the coastal areas. To the second category belong long rivers such as the Sokoto-Rima, the Gongola, Kaduna and Hadejia. These are rivers mainly of the plateau and north which usually have one main peak flood and a flow pattern corresponding to the single maximum rainfall season common in the northern part of the country. The third hydrological pattern is found in the catchments of the Rivers Niger and Benue. These very long rivers, each with several tributaries, have a complex flow pattern. There are usually two floods (the white and black flood) which depend on rainfall outside Nigeria.
In Nigeria there are eight main river basins, i.e. the Benue, Delta and Cross Rivers, the Imo-Anambra, Hadejia-Chad, Sokoto-Rima, Niger, Owena and Ogun, and Osun Basins. There have been a number of major engineering developments in these basins, beginning with the Kainji Dam impoundment on the River Niger in 1968 (Oyedipe, 1983). Several similar efforts to conserve and manage water resources have followed (Ita, 1993). New habitats have been created, while some have been destroyed. These changes have had some major effects on the aquatic fauna and flora.
The quality of water, unlike the very obvious physical changes that take place during the development of water resources, is an attribute that affects the biodiversity (flora and fauna) of aquatic systems. The effects are usually subtle and before any obvious changes are noticeable extensive damage would have been done. Most Nigerian rivers are generally turbid with a high concentration of suspended silt, particularly during the rainy season (Ita et.al., 1985). Nigerian freshwaters are also generally very productive at the primary (algae), secondary (zooplankton) and tertiary (fish and other aquatic vertebrates) levels. However, in industrial areas and urban centres there is some pollution with high levels of faecal coliforms (Ogbondeminu, 1986), heavy metals and industrial wastes which constitute public health hazards (Oluwande et.al., 1983). Also, in some areas of the country, both rural and urban, where water is in short supply the incidence of water borne diseases such as typhoid, cholera, dysentery and gastroenteritis is high due to contamination of the water supplies. Although water quality is to some extent an index of water pollution, the indices presently used in Nigeria are inadequate to indicate the damage that is done by heavy metals, metalloids, organic and inorganic compounds and blue green algae. The common indicators for assessing water quality in Nigeria (Oluwande et.al., 1983) are temperature, pH, biological oxygen demand, turbidity, dissolved oxygen, ammonia, nitrogen and coliform counts.
Studies of heavy metals in Jebba and Kainji Lakes, Kano State (Adeniji & Mbagwu, 1990) have revealed that organochlorine compounds, lead, iron, zinc, copper and manganese in these water bodies are above acceptable and tolerable levels. However, how these compounds affect the fish and other aquatic resources of Nigeria remains to be investigated. Another major contribution to the pollution of aquatic systems comes from industrial pollutants which originate from industries requiring large volumes of water for their operations. These include textile producers, sugar production, paper mills, sawmills and logging mills, petro-chemical works and hydro-electric power stations. They are situated along the Rivers Niger and Kaduna and in the coastal areas of Lagos and Port-Harcourt. For example, Momoh (1988) listed the materials used daily in the manufacture of paper at Jebba paper mill. Amongst those listed, those known to be toxic to aquatic animals and released to the River Niger included alum, resin, sodium silicate, sodium sulphite, cyclohexamins, sulphuric acid, sodium sulphate and fuel oil.
In Lake Kainji several litres of fuel oil, grease and transformer oil spill into the reservoir every day. How all these pollutants affect the fish and other aquatic fauna has yet to be studied.
Information on water chemistry and the productivity of Nigerian lakes and rivers is scanty and the parameters which have been studied are varied and dependent on the needs and facilities available to the investigators. There is a need for a more consistent and systematic study of the chemical components and productivity of major natural and man-made lakes in Nigeria, for a better understanding of the productivity of these water bodies.
Table 1 shows the limited information gathered from the available literature. Conductivity varied in the major lakes between 30 – 260μS cm-1. The highest conductivity was recorded in the reservoir located at the International Institute for Tropical Agriculture (IITA). The factors responsible for the high conductivity of this reservoir and its correspondingly high fish productivity are discussed by Ita (1993). Among the reservoirs, the lowest conductivity was recorded in Kainji (33μS cm-1), while that in Jebba, located immediately downstream, was higher (44 – 250μS cm-1). Jebba reservoir is a long, narrow lake compared with Kainji and still retains some riverine characteristics. Ita et.al. (1982) observed a general trend for conductivity in the rivers to be lower than in the impoundments of those rivers. The same goes for Total Dissolved Solids (TDS), which have a close similarity to conductivity. The lowest TDS of 29.4mg l-1 was recorded for the Shiroro Reservoir impounded on the River Kaduna. The range observed for the stagnant pools of the River Kaduna was 32.2 – 33.6mg l-1 compared with the higher, but overlapping, range (29.4 – 43.4 mg l-1) for Shiroro Reservoir. Alkalinity showed the same trend as conductivity and TDS. Both alkalinity and conductivity can be converted to TDS depending on which information is available.
pH in the reservoirs ranged from 6.3 – 8.4. It was lowest in Jebba Reservoir and highest in Kainji Reservoir. pH in the rivers studied ranged from 6.4 – 8.5. The lowest pH was observed in the River Kaduna and the highest in the Asa River.
Dissolved oxygen in the reservoirs ranged from 0.0mg l-1 in Kainji Reservoir during oxygen stratification to the highest 10.04mg l-1 in Kainji Reservoir when the lake was fully mixed during the flood season. In the rivers, dissolved oxygen varied from 3.7mg l-1 in the River Kaduna to 9.6mg l-1 in the River Eku, a fast flowing river which empties into Jebba Reservoir.
Limited information is available on the phosphate, nitrate and potassium concentrations of the different water bodies in Nigeria and Table 1 presents the scanty information from all available sources. These data are not consistent and could not be used in extrapolating to the productivity of the different water bodies.
Brief summaries of the major characteristics of five Nigerian reservoirs are given below.
Physical/morphometric features
The important morphometric features of the reservoir are extracted from Halstead (1975) and Imevbore (1970).
The reservoir is situated on the River Niger and lies, at an altitude of 108m asl, between Yelwa (latitude 10° 53'N:
longitude 4°45'E) and Kainji (9°50'N: 4°35'E). The surface area is 1250km2, and at full capacity the volume is
15km3. The maximum length, maximum width, maximum and mean depths are 136.8km, 24.1km, 60m and 11m
respectively.
The chemical characteristics (Table 1) of Lake Kainji have been extensively studied both immediately before (Imevbore, 1970) and at various times since impoundment (Karlman, 1973; Henderson, 1973; Imevbore, 1975a; Adeniji, 1975; Adeniyi, 1978). Imevbore (1975a) noted that both salinity and ionic composition showed no exceptional peculiarities during the first year of this reservoir's existence.
Adeniyi (1978) contended that the chemistry of the reservoir had remained more or less the same as that of the Niger before its impoundment. The water is rather dilute when compared with other reservoirs in Nigeria (Table 1), as is clearly borne out by its low conductivity, total dissolved solids, salinity and total ionic content (Adeniyi, 1978). It belongs to the group of large African lakes and reservoirs with the lowest dissolved solids concentrations (Balon & Coche, 1974). The Kainji chemistry has characteristics typical of African waters in general, such as comparatively high values of the ratios HCO3/Cl+SO4 and Cl/SO4, Mg/Ca (Talling & Talling, 1965).
Primary and secondary productivity
Studies on the primary and secondary productivity of the reservoir include the works of Karlman (1973); Imevbore
(1975a); Adeniji (1975); and Adeniyi (1978). Imevbore (1975a) estimated the phytoplankton productivity of Lake
Kainji at 500mg m-3. Adeniji (1975) estimated the production at 2628 algae/ml of water. Adeniyi (1978)
mentioned the abundance of Melosira spp. and the profusion of desmids.
Impact of limnological characteristics on fish production
Chemical changes recorded by Imevbore (1975a) soon after impoundment which were thought to be at higher
values, were related to the biological developments that followed inundation rather than the effects of leaching
from the newly inundated soils. Although several mass mortalities of fish and low concentrations of zooplankton
were recorded (Imevbore, op.cit.), generally the new conditions encouraged high fish production in the reservoir
from the outset.
The seasonal variation in the water temperatures results in a thermally stratified water column and this was found to affect fish distribution and fish catch (Adeniji, 1975). The low level of dissolved oxygen recorded at the deepest part of the lake was found to affect fish catch. Lelek (1972) noted that bottom gillnets which were set when Lake Kainji was stratified, and hence deoxygenated at the bottom, caught little fish whereas similar bottom gillnets caught more fish than those at the surface when the lake was not stratified.
Henderson (1970) estimated the potential fish catch using the morpho-edaphic index which uses values of conductivity against the mean depth. The fish production of the lake was also estimated by Karlman (1973) using the trophodynamic method considering mainly the levels of primary and secondary productivity.
Physical/morphometric features
Bakolori Reservoir is located in the Talata-Mafara area of Sokoto State between latitude 12° 25'N to 12° 35'N
and longitude 6° 10'E to 6° 17'E within the Sokoto River basin. The reservoir is 19km long and has a surface area
of 80km2, with a storage capacity of 0.45km3. It's mean depth is 5.6m and the length of the shoreline 107km.
Chemical characteristics (Table 1)
Pre-impoundment studies showed that the high air temperature influenced the water temperature which was also
high (Adeniji, 1980).
The lowest pH value of 6.7 was measured at the deepest point (15m) and a very low value of dissolved oxygen (1.8mg l-1) was also recorded at 15m (Adeniji, 1980). The drop in dissolved oxygen content was the result of water stratification, and corresponded to the water temperature drop from 27.4°C at 10 m depth to 26.3°C at 15m.
The conductivity of the water was 58 – 70μS cm-1 and the concentration of suspended solids was high, resulting in a high level of turbidity.
Primary and secondary productivity
The pelagic primary productivity was a very low 0.125mgC l-1 day-1 (Adeniji, 1980). The zooplankton was
dominated by copepods.
Impact of limnological characteristics on fish production
The low oxygen content near the bottom during stratification is likely to affect fish distribution and catch at that
depth. Some authors have shown that a dissolved oxygen concentration of 3mg l-1 is still too low for most fish.
The total dissolved solids of Bakolori Reservoir, estimated from conductivity measurements, gave a range of 47 to 58mg l-1. This, with the mean depth of the lake (5.6m), was used by Adeniji (1980) to predict a potential fish production of about 25 to 40kg ha-1 or 200 – 300 tons per annum.
Physical/morphometric features
Tiga Reservoir was constructed by the Water Resources and Engineering Construction Agency (W.R.E.C.A.) in
1974 for irrigation and hydroelectric functions. It is located at Tiga on the Kano River (about 80km from Kano
town) in Kano State, longitude 8° 16' to 8° 38'E and latitude 11° 15' to 11° 29'N.
The reservoir has a maximum length of 40.4km, width 24.4km and depth 40m, mean depth 13m, surface area 178 km2 and total storage capacity 1978.49 × 106 m3.
Chemical characteristics (Table 1)
Adeniji & Ita (1977) reported that the lake water transparency decreased from 1.6m to 0.15m from the dam site
to the inflow. Thermal stratification was observed in the deepest part of the reservoir. The slightly acidic condition
of the hypolimnion at about 15m was found to be the result of the thermal and dissolved oxygen stratifications.
Primary and secondary productivity
Blue-green algae dominate (91.5%), represented mostly by Anacystis and Anabaena spp.. Diatoms 8.2% and
dinoflagellates 0.3% follow. Zooplankton consist of Copepoda (54.5%); Cladocera (44.5%) and Rotifera (1.0%)
(Adeniji & Ita, 1977). There are no quantitative estimates of primary and secondary productivity.
Impact of limnological characteristics on fish production
The inshore waters of Tiga Reservoir are rich in fish, which feed on the rich invertebrate fauna there. However,
the presence of large-sized zooplankton, such as the Daphnidae and Diaptomidae indicates the lack of predation
by small pelagic fishes such as clupeids, which are absent from this reservoir. The total standing crop for the
reservoir is estimated to be 2107.4 metric tonnes.
Chemical characteristics (Table 1)
In comparison with the others, this reservoir has high values of conductivity, alkalinity, hardness and phosphates,
but is low in dissolved oxygen.
Primary and secondary productivity.
This reservoir is relatively rich in phytoplankton, zooplankton and zoobenthos. The phytoplankton standing crop
was 788 – 2408 cells/ml, that of zooplankton 29.32 – 62.66 organisms/litre and of zoobenthos 100 – 20700
organisms/m2.
Impact of limnological characteristics on fish production
The high conductivity and shallow depth of this reservoir (mean depth 4m) result in a high morpho-edaphic index
of 62.5, giving an estimate of total annual fish production of between 50 and 75kg/ha. This is two or three times
higher than that for Kainji Reservoir. Thus, the low dissolved oxygen concentration does not seem to have a
negative impact on the total biological production in this reservoir.
Physical/Morphometric Features
Jebba Reservoir is located between latitudes 9° 10'N to 9° 55'N and longitudes 4° 30'E to 5° 00'E at 76m above
sea level (a.s.l). The reservoir started to form in August 1983 in the valley of the River Niger. It has a maximum
length of 100km, maximum depth of 32.5m, maximum width of 10km and a mean depth of 3.3m. The surface
area is 350km2 (Ita, 1993) and maximum volume of 1000 × 106 m3.
Chemical characteristics (Table 1)
There are marked seasonal variations in the physico-chemical and biological characteristics of the reservoir. Most
of the physico-chemical factors such as temperature, conductivity, total alkalinity, turbidity, nitrite, nitrate,
phosphate, sulphate and silica generally have higher values between September and November with lower values
at other times of the year.
Primary production
Six major groups of phytoplankton, the Chlorophyta, Euglenophyta, Chrysophyta, Pyrrophyta, Myxophyta and
Cyanophyta are present in the reservoir (Adeniji, 1991). Most show seasonal variations in abundance. For
example, Myxophyta and Chrysophyta are well represented with peak abundance of 1,843,450 cells per 100ml
of water in the dry season and 67,850 cells per ml of water in the rainy season. The maximum daily gross pelagic
primary production was 4.7mg 02 l-1d-1.
Impact of limnological characteristics on fish production
Fish production estimated from the total gross pelagic primary production ranges from 9.4 – 10,411kg/ha, with
a mean fish yield of 2,373.49 – 16,372.61 mt per annum. Results obtained from experimental fish catch and catch
assessment surveys (Ita et.al., 1984) projected 2,901–13,291 and 1,007 – 4,612mt per annum respectively. These
are both within the fish yield estimates based on the pelagic production.
