Carbon, nitrogen and phosphorus loadings from tilapia fish cages in Lake Malawi and factors influencing their magnitude

Lake Malawi is a great lake not only because of its large size (30,800 km2); but also because it hosts between 700 and 1000 fish species, mostly endemic cichlids. The fish are most abundant and more diverse in the littoral zone. Cage aquaculture in Lake Malawi started in 2004 in the nearshore waters of the southeast arm. Approximately 50 cages were operational by 2009 harvesting about 20 tons fresh fish/cage/year. In 2007, data were aggregated from cage feed and production records to produce a mass balance for carbon (C), nitrogen (N) and phosphorus (P) added to cages in the form of feed and juvenile fish for 22 production cycles. Nutrient losses from cages to the surrounding environment accounted for between 81 and 91% for C, 59 and 80% for N and 85 and 92% for P. Production strategies such as stocking density and feeding rates were also analysed to assess their effect on fish harvests and nutrient loads from the fish cages. The study showed that nutrient loads from the cages were significant and can be exacerbated by poor feed quality, overstocking, stocking of premature fish and use of lower than recommended feeding rates to grow the fish to target sizes. The inefficient production strategies necessitated longer production cycles (376 ± 42 days) and more feed usage to achieve market sizes of ≥ 300 g/fish than if generally recommended stocking and feeding rates were followed. If cage aquaculture continues to expand, it will become an important new source of nutrients, not only locally, but on a lake-wide basis.