| Summary: | Nutritional aspects of three species of marine fish larvae, herring Clupea harengus L., plaice Pleuronectes platessa L. and turbot, Scophthalmus maximus L. were examined. Larvae were fed three different diets all of which were derived from rotifers Brachionus plicatilis (O.F. Mtlller). These rotifers had been mass cultivated on baker's yeast and then enriched with baker's yeast, or with one of two unicellular algae, Isochrysis galbana Park or Nannochloropsis (Nannochloris) oculata (Droop) Hibberd. The biochemical changes that these enrichments conferred on the rotifer were examined. There was little difference in proximate and amino acid compositions. Dry weights, and calorific contents of the rotifers fluctuated according to the type, temperature and duration of enrichment, with the yeast enriched rotifers tending to weigh less (200-320 ng) than their algal counterparts (250-370ng). Major differences were found in the total fatty acid profiles of the rotifers. Yeast-enriched rotifers had no 18:3n-3 or 20:5n-3 and only trace amounts of 22:6n-3 fatty acids. The Nanrinochioropsis-enriched rotifers had substantial amounts of n-3 fatty acids though only of 20:5n-3 (11-14%), while rotifers enriched with Isochrysis had only trace amounts of 20:5n-3 (2-4%). All three species of fish responded similarly to the yeast enriched rotifer diet. Their growth, in terms of length and weight, was minimal and they never developed any fin ray elements. Both flatfishes developed a looped gut in the short growth duration on this diet and some turbot (20%) inflated their swim bladder. This retardation of growth was attributed to the lack of long chain highly unsaturated n-3 fatty acids (n-3 HUFA) in the diet. During one month feeding trials it was shown that herring and plaice were able to exploit both algal-enriched rotifer diets equally well. Turbot, on the other hand grew better, over 14 days, on rotifers enriched with lsochrysis compared with Nannochioropsis. The reaction of the different species was linked to the fatty acid profiles of the diet. Hence, turbot require a dietary input of 22:6n-3 while herring and plaice thrived equally well on diets containing only 20:n-3. Juvenile plaice consistently exhibited improved pigmentation when Isochrysis compared to Nannochloropsis were used as the rotifer enrichment. The effect was attributed to specific nutrients particular to algal chloroplasts, which need to be present in the diet of the larvae up to metamorphosis. Assessment of daily rations, employing the rotifer enriched with Isochrysis diet only, showed herring to reduce their food intake from 46 to 19% body dry weight/d between days 10-21 post hatch while turbot consumed between 34-169% body dry weight/d with no apparent age related effects. The gross growth efficiencies (Ki, Brett & Groves, 1979) for both species were seemingly unaffected by age related processes and fluctuated between 15-50% in turbot and 20-61% in herring according to fluctuations in daily food intake. Ingestion is further described for both species with a linear model that related food intake to larval weight. The weight-specific ingestion was found to be a constant 13% and 43% of body dry weight in herring and turbot respectively. The weight-specific ingestion and specific growth rate estimates were combined with other data available from the literature, revealing that larvae tend to consume food (in dry weight) at a level equivalent to approximately twice their daily dry weight increase.
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