60 DECEMBER 2014 • WORLD AQUACULTURE • WWW.WAS.ORG Potential Effects of Cultural Eutrophication on Cage Culture in Lakes and Reservoirs in Nigeria Moshood Mustapha Freshwater cage culture is an aquaculture production system where fish are grown from fry to table size in cages or enclosures that are fixed, floating or submerged in lakes, reservoirs, rivers or streams. Cages are usually enclosed on all sides with mesh netting and the complete system often includes materials such as PVC pipes, bamboo, wood, used tyres, plastic or steel drums, weights and anchors (mooring) and ropes. Cage fish culture involves simple technology in operation and management and can use locally available and cheap materials for construction. Water exchange occurs between the water body and the cages. Cage culture has existed for many centuries in several countries, especially in Asia (Beveridge 2004), but is now becoming widespread in other countries, especially in sub-Saharan African countries like Nigeria, where large bodies of fresh water abound. The expanion of cage culture can be attributed to the high demand for fish and increased competition for available resources faced by the existing aquaculture sector (Foley et al. 2005). There are numerous advantages of cage culture. Cages use limited space in existing water bodies and thus eliminate the need to buy land. Compared to pond culture, the investment or capital needed to construct facilities is relatively low. Production capacity is high arising from high stocking densities; production in cages can be as much as 20 times greater than in pond culture (Das et al. 2009). Unwanted recruitment, especially in tilapia culture, can be controlled. Eliminating losses from predation, simple methods of harvesting, observation and sampling of fish, and quarantine and disease treatment is rapid and easy. The system is viable, economical and conserves the fishery of the water body in which it is sited. Above all, cage culture can contribute to the livelihoods of people through employment, income generation, poverty alleviation and provision of low-cost fish protein, ensuring food security. With these advantages, the success of freshwater cage culture depends largely on the water quality of the water body in which the system is sited. Water quality includes all physical, chemical and biological factors of water that influence the beneficial use of that water for various purposes. Thus, water quality dynamics must be taken into account to conform with the requirements of the species cultured. Cage culture leaves the fish susceptible to prevailing physicochemical and biological conditions in the water body. One of the challenges of freshwater cage culture in Nigeria is deteriorating water quality stemming from cultural eutrophication of lakes and reservoirs. Eutrophication from high nutrient loading is one of the most important causes of water quality deterioration and the consequent decline and collapse of fish populations and production in lakes and reservoirs (Allan et al. 2005, Jones-Lee and Lee 2005, Mustapha 2008, 2011). The focus of this article is to examine the potential effects of cultural eutrophication on the emergence of cage culture in lakes and reservoirs of Nigeria and to offer suggestions to mitigate possible effects. The Threat of Cultural Eutrophication Cultural eutrophication is the anthropogenic increase in loadings of nutrients, especially phosphate and nitrate, into water bodies. It also occurs through human alteration of the physical and biogeochemical conditions of the watershed of a lake or reservoir. Phosphate and nitrate limit the growth of phytoplankton and aquatic macrophytes and thus have significant impact on the trophic status and productivity of lakes and reservoirs. Freshwater lakes are more vulnerable to ecological changes caused by inputs of phosphorus than nitrogen (Rojas and Wadsworth 2007). When nutrient concentration increases from external loading, excessive phytoplankton and macrophyte production often results, leading to water quality problems. Human activities that lead to cultural eutrophication of Nigerian lakes and reservoirs include bank erosion, urban runoff, agricultural runoff of fertilizers, washing and bathing with phosphate-based detergents and soaps, and runoff from concentrated livestock operations, all regarded as non-point sources. Point sources include discharges from wastewater treatment and industrial facilities. Nonpoints source nutrient inputs from the watershed are the leading causes of cultural eutrophication and water quality problems in lakes and reservoirs (Carpenter et al. 1998, Mustapha 2009). The rainy season often exacerbates cultural eutrophication from nonpoint sources during the rainy season in many Nigerian lakes and reservoirs. Effects of Cultural Eutrophication on Lake and Reservoir Water Quality that Affect Fish in Cages There are numerous potential effects of cultural eutrophication on water quality of lakes and reservoirs where fish cages are sited and that could negatively impact fish growth and production. Eutrophication increases the risk of dissolved oxygen depletion, potentially leading to severe mortality events in cages due to confinement at high densities. Eutrophication can increase biofouling of cage nets, restricting water exchange and oxygen supply. Biofouling can cause the weight of nets to double, reducing cage buoyancy (Piccolotti and Lovatelli 2003). Erosion in the watershed can bring excessive suspended inorganic (mineral) matter, causing gill irritation to fish, causing stress that can lead to disease outbreaks in caged fish. High turbidity can lead to stunting of cultured fish populations (Lee and Jones-Lee 1991). High turbidity also reduces the ability of sight feeders, such as many carnivorous species, to locate feed. Some algae, especially blue-green algae, produce metabolites that cause off-flavor in cultured fish. These off-flavors can cause economic losses because harvested fish are not acceptable by consumers. Although rare, some blue-green algae excrete toxins that can kill fish (Jones-Lee and Lee 2005).
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