Assessment of environmental impacts of cage aquaculture on Lake Volta of Ghana

Abstract In recent years, tilapia cage farming has become important aspect of commercial fish production in Lake Volta and the number of the cage culture establishments (now 467) continues to expand in the multipurpose lake. Cage farms release nutrients that can cause pollution to water and sediment quality. However, there is paucity of information on the impact of cage culture on the lake’s environment. Consequently, a study was conducted from August 2013 to April 2015 at two fish farms in the Lake Volta to assess the potential impact of cage culture on the environment of the lake. The study utilized four methods. The first was through structured questionnaire to gather information on the cage farms. The second method used physico-chemical analyses of water and sediment following standard procedures. The third method considered mass balance model to estimate the amount of nutrients discharged from the cage farms into the environment. The fourth method used Dillon Rigler phosphorus mass balance model for the prediction of ecological carrying capacities of two selected farm areas in the lake. The results from the questionnaire survey on the cage farms revealed that only large scale farmers have adequate environmental awareness and therefore monitor the water and sediment quality in their farm areas. The study on the nutrient waste emission from the cages indicated that 64.8–68.1 % of C, 72.0–75.8 % of N, and 81.0–84.7 % of P of the total feed input were released into the lake’s environment for each tonne of fish produced, and only 31.9–35.0 % of C, 24.2–28.3 % of N and 15.3–19.2 % of P were harvested as fish biomass. Despite the large discharges of particulate and dissolved nutrients estimated by the mass balance models, physico-chemical water quality parameters including dissolved inorganic nutrients (NO3-N, NO2-N, NH4-N and PO4-P), chlorophyll-a varied temporally and showed no significant differences between the fish farm sites and the reference sites (ANOVA, p > 0.05). The low impact of the cage aquaculture could be attributed to the nutrient losses through the outflow at the dam which was 43.56 km3 per annum (i.e about one-third of the lake’s volume); dispersion of the wastes by water currents, consumption of waste by schools of wild fish species found around the fish cages and also by dilution. Sediment analysis revealed the sediment texture of all monitoring sites to be sandy clay loam with moderate impact on sediment quality with respect to organic matter. The total organic carbon (TOC), total organic matter (TOM), and total nitrogen (TN) content in the sediment under the cages were significantly lower than those of the reference sites (ANOVA, p<0.05). The results showed that concentrations of heavy metals in the water column and sediments were low and within the range of tolerable levels for the lake ecosystem. The estimated ecological capacities were from 3,697 to 4,621 ty-1 for Farm A and from 28,322 to 33,042 ty-1 for Farm B zones. The estimated ranges of values are higher than the present production in the zones. This suggests that more cage culture of tilapia could be established in the lake without compromising the water quality and the ecosystem. However, water quality monitoring should be undertaken periodically to accurately determine the current state of the lake in order to both confirm and refine predictions, and ensure sustainable cage culture development in Lake Volta. Overall, the water quality indices (CCME-QI and GWQI) carried out indicted that the lake water is good and suitable for tilapia production and other ecosystem uses such as irrigation, recreation and water supply.

Source: KNUST