D. Nanton, J. Castell
Abstract:
Live food organisms containing relatively high concentrations of n-3 highly unsaturated fatty acids (HUFA) are required for the first-feeding of cold-water marine fish larvae. In particular, long-chain fatty acids such as 20:5n-3 and 22:6n-3 are essential. Two species of harpacticoid copepods isolated from the Atlantic Ocean near Halifax, NS, Tisbe sp. and Amonardia sp., demonstrated the ability to elongate and desaturate 18:3n-3 to 20:5n-3 and 22:6n-3 when fed a diet deficient in these essential fatty acids (EFA). Decreasing the culture temperature from 20 to 6 C increased the proportion of long-chain EPA (20:5n-3, 22:6n-3 and 20:4n-6) in de copepods. Both harpacticoid copepod species maintained a 22:6n-3 tot 20:5n-3 ratio of greater than two for all dietary (the algae Chaetocerus calcitrans, Dunaliella tertiolecta, Isochrysis galbana and baker's yeast) and temperature (6, 15 and 20 C) regimens, suggesting their usefulness as a live food for marine fish larvae. For comparison, trochophores of the common mussel Mytilus edulis and wild zooplankton obtained from St. Andrews, NB were examined for use as potential live food and were also found to have a high percentage of long-chain EFAs.
The relative amounts of free amino acids (FAA) in Tisbe remained fairly consistent despite large differences in the amino acid composition of the diet. The proportion of essential free amino acids was nearly doubled in Tisbe fed I. galbana (clone T-iso) compared with Tisbe fed yeast.. The harpacticoid copepod Tisbe was mass cultured in a system which produced nearly 10^6 individuals in a relatively small tank volume of 32 L.
Preliminary feeding trials comparing growth and survival of marine fish larvae (American plaice and haddock) fed rotifers or Tisbe were conducted. The plaice and haddock larvae were observed feeding on the harpacticoid copepods. Tisbe, when compared with the rotifers, gave superior growth but poorer survival of the haddock larvae.
(Institute for Marine Biosciences, National research Council, 1411 Oxford Street, Halifax, NS B3H 3Z1, Canada)