Cage aquaculture and environment in Lake Malawi: an assessment of water quality, food web shifts, and development of a decision support tool for sustainable aquaculture

Abstract

The impact of cage aquaculture on the water quality and native fish community was studied between November 2011 and September 2012 in the South East Arm of Lake Malawi. This ancient African Great Lake has the greatest number of fish species of any lake in the world, and the fishery is a major source of animal protein in Malawi. However, the decline of the capture fishery stocks has forced the Government of Malawi to promote cage aquaculture. The Maldeco Aquaculture Limited is the first cage aquaculture operation initiated in 2004 to farm endemic and native fish species in Lake Malawi at commercial scale. Unfortunately there is no legal framework for sustainable cage aquaculture development. The study found that cage aquaculture attracts wild fish populations and also changes their community structure. However, the diversity was not significantly affected by the cage farm despite the increased abundance of fish, especially smaller fish. Water temperature and dissolved oxygen were minimal between April and June, a period that cage aquaculture farms should consider as critical in their operation. Chlorophyll a had a single seasonal peak in April concurrent with the minimum in transparency. There was no significant difference in water quality parameters between the aquaculture site and sites 5 km away. Stable isotope signatures of carbon (δ13C) and nitrogen (δ15N) of the wild fish revealed shifts in small particle feeding planktivorous fish and possibly in zooplanktivorous fish, but not in benthivorous, molluscivorous, and zoobenthivorous fishes which maintained their natural diets even at the farm site. A dissolved oxygen model indicated that the average carrying capacity of the farm is 1,870,000±96, kg. A mean stocking density of 23,000 ±440kg per cage is recommended to allow adequate fish growth rates and attainment of desirable marketable size within a short cycle without significant impact on oxygen concentration. Un-regulated expansion of cage aquaculture activities at Maldeco Aquaculture farm or increase in the number of cage farms has the potential of increasing changes in wild fish community structure, modifying food webs, and causing conflicts with local fishermen.