Kimpe and Micha, 1974; Hogendoorn, 1984). The aquaculture industry in South Africa boomed as a result of Clarias culture but supply exceeded demand as competing cheap salted fish were made available, and the industry has since re-vamped its aquaculture industry to include a greater diversity of fish. Clarias and the Clarias ë Heterobranchus hybrid are major aquaculture species in Nigeria. Research on the artificial propagation of Clarias has provided a host of options for farmers, ranging from highly controlled induced spawning to many variations of partial control to the unpredictable but easy natural spawning. Simpler techniques using hypophysation and hapa spawning have been used at Sagana Fish Farm, in Central Kenya, whereas more intensive, electrical energy-dependent techniques of fry rearing have been developed over the years and are used in South Africa and in Europe.
Clarias have also become important to the Lake Victoria Nile Perch fishery, which is of enormous economic importance in Kenya, Uganda, and Tanzania because of its foreign currency earnings and the employment it provides for people near the lake. Trawling is practiced by some of the wealthier fishers but it is not legal on Winam Gulf, which is an important fishing zone in Kenya. Poorer fishermen have resorted to longline fishing and their favorite bait is fingerling-sized Clarias. Desired bait size varies but ranges from 5 to 20 grams per fish. A quick survey of fishermen gave hugely varied numbers of fishers and number of hooks per fisher but it seems that fishers bait from 100 to 1,000 hooks per boat per day, and assuming 50 boats this translates into a demand in Kenya between 5,000 and 50,000 Clarias fingerlings per day. At 300 fishing days per year, this equates to an annual demand of between 1.5 and 5 million fingerlings.
The present supply of fingerlings is intermittent and seems to be related to the extent of water hyacinth rafts drifting in near shore, with Clarias being numerous under the water hyacinth. Fishers sometimes use small-mesh beach seines and mosquito nets to catch fingerlings for bait, but beach seining is highly destructive of the spawning habitats of native cichlids and is illegal. Fishing with mosquito nets has also recently been banned by the government. Fishers find themselves in a difficult situation because they need the bait at an affordable price to be able to continue fishing. At the reported selling price of 5 to 8 KSh per fingerling and an estimated production cost of about 0.1 KSh per fingerling (Veverica, personal communication, 2001), farm-based production could be a highly profitable business for fish farmers.
Although a number of practical methods for the spawning of Clarias have been developed, survival of larvae to the fingerling stage remains problematic. A primary concern with respect to fingerling survival has been cannibalism. Type
one cannibalism occurs when the fingerlings are crowding in a dark corner and inadvertently eat each other. Type two cannibalism, in which the fingerlings actively hunt one another down, begins to develop as the fish reach about 3 grams. Altering habitat can help reduce the incidence of type one cannibalism and frequent grading can reduce type two cannibalism. Preliminary work at Sagana has indicated that frequent grading, daily feeding and weekly fertilizing, and use of corralled grasses as fertilizer and cover can increase the number of fingerlings per unit area harvested (Veverica, personal communication, 2001). Early observations were that compost piles along pond edges seemed to be the preferred habitat of the Clarias fry, so these enclosures were increased to cover 20% of the pond (from 5%) but using floating grasses instead of cut grass that sinks. The floating grasses are very fibrous and do not decompose fast enough to cause oxygen depletion. They do not take root in the pond and will not multiply in the pond and compete for nutrients as do water hyacinth and Nile cabbage or water lettuce (Pistia stratiotes).
Complete shading is preferred but limits the production of phytoplankton, which is needed to produce oxygen and as a base for the food web. In addition, partial harvesting to grade off the faster growing fingerlings is impeded by the presence of the grasses and many of the larger fingerlings can be left behind to cannibalize the others. A trial conducted in December 2000 gave very promising results, with survival over 50%.
Growth is density dependent; the higher the rearing density of larvae, the lower their growth rate. Therefore, there will be a need to determine optimum densities for stocking Clarias fry in the high altitude areas. Growth and survival of larvae fed on live feed also needs to be compared in order to evaluate the possibility of reducing farmersÕ dependency on costly formulated food.
The present set of studies was designed to follow up on preliminary work in order to develop a set of research-based extension recommendations for producers of Clarias fingerlings as bait fish. Partial enterprise budgets will be made for all treatments and a full enterprise budget for the "best" treatments will be reported. The specific objectives of these studies are to determine the optimal following conditions for Clarius fingerling production:
1) Shading regime.
2) Larval stocking density.
3) Larval grow-out period.
4) Larval live feed.
Methods and Materials
Four studies aimed at achieving the above objectives are be