7. THE IMPACT OF AGRICULTURAL AND INDUSTRIAL DEVELOPMENT ON WATER QUALITY AND INLAND FISHERIES

At the traditional level, agricultural activities in river basins are known to have little effect on water quality and fisheries and, in certain cases, may be regarded as a non-competing joint-use of the resource. However, with increasing human population and the need for more crop production, agricultural methods have become more advanced, with water retained in large dams for use in sophisticated irrigation systems. This naturally results in desiccation of downstream farmlands to the detriment of traditional agriculture and loss of fishery resources as a result of habitat loss (Welcomme, 1983).

The natural cycle of flooding generates organic nutrients in the flooded plains and the subsequent phase of flood recession leads to deposition of these nutrients in the drawdown zones, thus giving rise to natural fertilization of the soil. The alternation of anaerobic and aerobic phases in the soil of the plain increases the availabiliy of phosphates and potassium which are essential elements for plant growth. Once this natural process of fertilization is stopped as a result of drought or flood control, the soils of the plain rapidly become depleted of nutrients thus necessitating the application of artificial fertilizers. Such inorganic fertilizers are washed down by rain into stagnant ponds and pools of water on the plain and also into storage reservoirs. The sudden increase in nutrients in the water generates excessive growth of phytoplankton. Such blooms could be both advantageous and disadvantageous to the fishery, in the sense that if the water contains fish that feed on algae (e.g. tilapia, Labeo etc.), the algae would be exploited to the benefit of the fishery. Excessive blooms of algae, however, lead to high biological oxygen demands thus resulting in aquatic pollution as a result of drastic oxygen depletion of the water, causing much stress to the great majority of fish species, which gradually die off. Hardy species like Clarias and tilapia can tolerate such polluted waters and therefore become overpopulated, with Clarias feeding on tilapia fingerlings and tilapia feeding on the algal bloom.

Many eutrophic reservoirs have been identified in Kano State including the Jakara and Warwade Reservoirs. The former, in addition to pollution from fertilizers, also receives effluents from the industrial city of Kano. High levels of toxic heavy metals have been detected in this reservoir (Butt, 1985; Adeniji and Mbagwu, 1990). These authors also detected high levels of copper, chromium, zinc, iron and manganese in crops irrigated with the reservoir water. Butt (1985) detected high levels of arsenic, lead, copper, zinc, iron and chromium in fish from the reservoir. The Ministry of Health, Kano State had prohibited the consumption of fish from the reservoir but, because of lack of enforcement, the local fishermen continue to exploit and process the fish by sun-drying, prior to distribution to city centres and other States. Warwade Reservoir is also polluted and overpopulated with Tilapia and Clarias. The tilapias are exploited, sundried, packed in sacks and distributed to towns and villages both within and outside the state.

The application of inorganic fertilizers to the soil of the floodplains, with a low water table and lack of flooding, can lead to the build up of salts in the soil. If this process is allowed to continue, the soils of the wetlands become saline and the plains become useless for agriculture (Welcomme, 1983). The excess dissolved salts in the groundwater could also affect the health of the local fishermen and farmers who utilize such water for domestic purposes.

7.1. IMPACT OF PESTICIDES ON FISH AND FISHERIES

Intensive dry season agriculture not only requires the application of inorganic fertilizers, but the green patches of farmlands adjacent to dry, abandoned upland farms attract numerous crop pests to the green fields, thus necessitating the application of pesticides for increased yield. Pesticides used in Nigeria include certain chemicals that for environmental reasons, have been partially or completely banned in developed countries, but for which effective cheap substitutes have yet to be evolved. Such chemicals continue to find their way into Nigeria for pest control. Pesticide use is known to cause serious environmental problems, especially in the dry season, because during this period the dilution capacity of the water systems is low, thus increasing the risk of high concentrations of toxic chemicals. Moreover, the dry season is often the critical period for many animals, especially fish and birds. Fish stocks suffer from natural mortality and high fishing pressure at the end of the dry season. Contamination of water by pesticides either directly or indirectly can lead to fish kills, reduced fish productivity and elevated concentrations of undesirable chemicals in edible fish tissue which can affect the health of humans eating these fish.

In spite of the upsurge in the use of pesticides in Nigerian agriculture, there has been little or no awareness among the users of the hazard to the environment. Information on their use, distribution and environmental impacts is scanty in Nigeria. Until the recent publication of the “Guidelines and Standards for Environmental Pollution Control in Nigeria” (1991) by the Federal Environmental Protection Agency (FEPA), there had been no government regulation or control of the use of pesticides and other toxic chemicals in the country.

Herbicides and pesticides in common use in Nigeria are tabulated in Tables 34 & 35 together with World Health Organisation (WHO) classification for toxicity and EPA criteria for protection of aquatic life in freshwaters. Fenthion (an insecticide) is effective also as an avicide and is used in areas such as the northern states of Kano, Borno and Bauchi which are prone to bird pests. DDT and gammalin 20 which have rodenticide properties, although outlawed, are still being used illegally in some parts of the country. These insecticides are classified as moderately to highly hazardous by the WHO and as harmful to extremely harmful to fish. Monochrotophos and Phosamidion are slightly harmful to fish.

Table 36 presents the interim effluent limitation guidelines in Nigeria for all categories of industries published by FEPA (1991). In the table, pesticides are not classified, but the limit of less than 0.01 mg/l applies to all types of pesticides. The absence of listing shows the gap in knowledge of the toxicity of the existing pesticides in current use in Nigeria. FEPA plans to update the information with time. There is an urgent need to commission such a study in Nigeria in view of the widespread use of pesticides in this country.

There is presently no information on the lethal limits of pesticide residues in Nigerian fish species. Table 37 however, shows the lethal limits of some organochlorine insecticides in some temperate fish. There is a need for such bioassay studies for different fish species in Nigeria since there are different degrees of accumulation in different fish species, with higher concentrations in carnivorous fishes and birds.

Alabaster (1981) reviewed the state of aquatic pollution of East African inland waters. His investigations confirm that pesticides are being used increasingly in the countries studied. They have caused unwanted fish kills and have been found in the tissues of fish, sometimes at concentrations which have given cause for concern because they might be approaching values having longterm and sub-lethal adverse effects. Few tests have been carried out to measure the toxicity of pesticides to African species of fish and no long term tests, or regular monitoring programmes for concentrations in fish, have yet been carried out, although several proposals are in the pipeline. Some of the countries (but not all) have a system for screening pesticides for safety and for issuing advice on safe use, including information on toxicity to fish.

The application of pesticides which have caused fish kills has aroused concern about possible long-term and sub-lethal effects on fish, including the accumulation of chemicals in edible fish tissue with resultant adverse effects on humans.

Table 34. Trade names, active ingredients and properties of pesticides commonly used in Nigeria

Table 35. Other herbicides and insecticides produced and distributed by CIBA GEIGY company in Nigeria.

Table 36. Interim effluent limitation guidelines in Nigeria for all categories of industries. Units in mg/l unless otherwise stated.

Source: FEPA (1991)

Table 37. Lethal limits of organochlorine insecticides to temperate fish.