Fish silage has been used as a feed supplement for various livestock and poultry animals and results have generally shown that it has good nutritional quality. The biological value of its protein was also comparable with that of fish meal protein. However, only recently has its potential in aquaculture diets been recognised, hence few studies have assessed their suitability. Generally, fish silage has been compared with fish meal and its suitability (or otherwise) assessed by fish growth responses, protein utilization and digestibility. Conflicting results have been reported on fish silage as fish meal replacer (either partially or totally) in fish diets. Moist acid silage has been fed to carps, salmonids, eels, catfish, sea bass and tilapias with satisfactory results but few comparable results are available for fish fed fermented silage. Fagbenro (1994) showed that O. niloticus and C. gariepinus fed with moist diets containing autolysed protein from fermented tilapia silage stored for 15-60 days showed good growth performance and protein utilization. There were no differences in body (carcass) composition and hepatosomatic index in C. gariepinus fed increasing dietary levels of autolysed protein from fermented fish silage and no morphological deformities were observed (Fagbenro, 1994).
Liquid fish silage is viscous, bulky and difficult to transport, stir or store, and can only be fed to pigs directly. There are no solids present to make into presscake; hence water removal by evaporation is necessary. Because of the low solids concentration, it is difficult to dry alone. Several methods of removing the water content of silages include spray drying, vacuum evaporation or drum drying. Alternatively, filler can be added and then dried together, after which the co-dried product can be used as protein supplement for poultry or fish. The nutrient content of the dried product is easily altered by the type and amount of filler material used, such as wheat offal, palm kernel cake, cassava flour, rice bran, maize flour, whey, potato flour, soybean-feather meal mixture, soybean meal, poultry by-product meal, meat and bone meal, feather meal (Akande, 1990; Fagbenro, 1994), the choice of which is determined by cost and local availability. Ayinla and Akande (1988) reported that dietary inclusion of acidulated tilapia silage at 410 g/kg for C. gariepinus resulted in a better weight gain than diets containing 40 g/kg fish meal. Fermented tilapia silage co-dried with soybean meal replaced up to 75% of fish meal component in dry diets for O. niloticus and C. gariepinus while total replacement gave inferior growth responses, feed conversion and protein utilization, caused by reduced palatability of diets or reduced appetite. No differences occurred in the hepatosomatic indices of O. niloticus and C. gariepinus fed increasing dietary levels of co-dried fermented fish silage: soybean blend and no morphological deformities were observed (Fagbenro, 1994).