Incorporation of composted and dried water hyacinth in pelleted feed for the tilapia Oreochromis niloticus (Peters)

. Eight diets were formulated using either composted or dried water hyacinth, Eichornia crassipes (Mart.) Solms., incorporated into a conventional pelleted tilapia feed in a simple displacement procedure: 100. 75, 50 and 25% composted water hyacinth: 100, 75 and 25% dried water hyacinth; and a control diet with 0% displacement of the conventional feed. The experimental diets were fed in duplicate treatments to Oreochromis niloticus in a series of 16 outdoor 4‐m 3 tanks stocked with eight fish/tank for 84 days. Good growth and feed utilization efficiencies were obtained with diets containing up to 75% composted water hyacinth with no significant reduction in fish performance compared to the control diet. Fish performed better with diets containing composted than with diets containing dried water hyacinth, except for the 100% composted water hyacinth diet in which the pellet was exceptionally hard. The better performance with diets containing composted compared to dried water hyacinth may have been due to the lower levels of crude fibre in the former diets, although significant fish growth was achieved even with the diet containing 100% dried water hyacinth. Some nutrition must have been obtained by the fish indirectly from the plankton in the static water experimental system but it was concluded that the fish also obtained a significant amount of their nutrition directly from the diets, including those with a high percentage of composted water hyacinth which in some treatments had relatively low mean phytoplankton biomass levels in the water. An experimental protocol is recommended for the evaluation of the nutritional value of non‐conventional fish feeds in which determination of fish growth without concern for underlying mechanisms precedes studies to assess the relative contribution of the direct and indirect nutritional value of the feed.