The use of earthworms as a feed for rainbow trout (Salmo gairdneri)
The nutritional and chemical characteristics of five earthworm species: Lumbricus terrestris; Allolobophora longa; Eisenia foetida; Dendrobaena veneta and Dendrodrilus subrubicundus were assessed on the basis of crude protein and amino acid composition, lipid and fatty acid composition, and ash and mineral composition. Analyses indicated that all the earthworm species evaluated possessed a high quality protein and lipid fraction suitable for use in fish feeds which was somewhat similar in composition to that of fish meal. During experimental feeding trials (50-84 days in duration) each earthworm species was nutritionally evaluated, on the basis of fish growth performance, feed utilization efficiency and gross carcass composition, as a complete feed (frozen slices of whole worm) for rainbow trout· (Salmo gairdneri) A dried 'earthw9rm meal' derived from each of the species E.foetida, D.veneta and D.subrubicundus was similarly evaluated as a potential replacement for fish meal in trout diets. Fish fed frozen slices of earthworm, with the exception of fish fed E.foetida, achieved growth rates and feed utilization efficiency comparable to. fish fed a control, fish meal based ration. Fish fed solely on frozen slices of E.foetida achieved little or no growth over the experimental period. The possible reasons for the reduced palatability of frozen E.foetida to the fish are discussed, pre-treatment processes applied and a significant improvement in the palatability of frozen E.foetida was achieved by blanching. High dietary inclusion levels of earthworm meal (replacing ~ 50% of the fish meal protein) resulted in depressed feed intake and growth of the fish. At reduced levels of inclusion, dried E.foetida meal (constituting 5-30% of a production salmonid diet) and dried D.subrubicundus meal (constituting 7-36% o~ a semi-synthetic trout diet) adequately replaced the dietary fish meal component without loss in fish growth and feed utilization efficiency. The possible uptake of potentially toxic trace elemeots (Fe, Zn, Mn, Pb, Cu, Cr, Ni, Co and Cd) into the fish carcass through the ingestion of contaminated earthworm diets was also investigated and the levels of certain elements, in particular Pb, were observed to increase in the carcass of fish fed high levels of earthworm in the diet. However, in no instance did the results indicate any harmful or toxic effect of including earthworms in the diets of rainbow trout.