The significance of lipids at early stages of marine fish.

The quality and quantitative amount of lipids in the eggs, along the yolk sac stage, and in the first-feeding seem to be decisive for good growth and survival of most marine fish species. The present paper reviews the significance of lipids at these early stages of larval development.

Four marine species have been studied at different developmental stages: plaice (Pleuronectes platessa), cod (Gadus morhua), halibut (Hippoglossus hippoglossus) and turbot (Scophthalmus maximus).

Using different enrichment techniques, the importance of lipids in live feed (i.e. rotifers) is also discussed. Manipulation of the qualitative and quantitative content of essential fatty acids, such as docosahexaenoic acid and eicosapentaenoic acid in the main lipid fractions of rotifers is possible with adequate and controlled enrichment techniques. The individual content of these fatty acids and the possible interaction between them could be more important than the sum of highly unsaturated fatty acids of the n-3 series in some marine species.

(SINTEF Applied Chemistry, Center of Aquaculture, N-7034 Trondheim, Norway)

Nutritional effects of algae in marine fish larvae.

In first-feeding of marine fish larvae, e.g. turbot (Scophthalmus maximus) and halibut (Hippoglossus hippoglossus), microalgae are used both in the production and in short term enrichment of the rotifers (Brachionus plicatilis) in order to transfer essential nutrients from the algae to the live food. In addition algae are given directly to the larvae in the culture tank. The algae act as food for both the fish larvae and the live feed. The use of algae in the production of rotifers and directly to the larval tanks are considered to improve the nutritional conditions for the larvae, eventually resulting in improved growth and survival of the larvae. Special attention is given to the content of n-3 polyunsaturated fatty acids in the algae and the rotifers. The content of n-3 polyunsaturated fatty acids showed taxonomic conformity between algal classes, but the content varied with the extent of growth limitation of the algal cells. The lipid content and fatty acid composition of the rotifers reflected the corresponding composition of the algal diets, and the changes in fatty acid composition of the rotifers were fast. The algal species used may be an effective tool for the control of the fatty acid composition of the rotifers given to the fish larvae.

Yolk sac larvae of several marine fishes are shown to actively ingest microalgae in a period before they regularly start feeding on rotifers. In yolk sac larvae of halibut the ingested algal cells were only slightly assimilated, but even if the biomass contribution from the ingested microalgae was low, the microalgae may be nutritionally important for the further development of the larvae in the first-feeding stage.

Other effects of microalgae during the first feeding period, which can explain the enhanced early appetite, will be discussed.

(SINTEF Applied Chemistry, Center of Aquaculture, N-7034 Trondheim, Norway)

Status of the industrial scale larviculture of marine molluscs.

Seed sources for shellfish aquaculture are either hatchery based or come from collection of wild-produced stocks. The decision to use hatchery seed, naturally collected seed or a combination of the two is a fundamental decision for producing companies. The decision must be based on a careful evaluation of the reliability of the sources, the periodicity of production and the overall costs in relation to company goals.

The technology for hatchery seed production developed in the 1960's from simultaneous research programs in Europe and the United States, and by the 1970's a small number of commercial shellfish hatcheries were in operation. Now, worldwide, there is considerable hatchery production of molluscan seed, both by facilities whose final product is seed for sale, and by vertically integrated companies that grow an end-user food product. Significant developments in many areas of hatchery operations have occurred over the last 20 years. Especially notable are the advancing technologies in broodstock maintenance, genetics, larval physiology and nutrition, disease control, algal culture methods, biochemistry and systems monitoring.

This status report is based on an extensive survey of existing industrial-scale hatcheries that produce oysters, clams, scallops, mussels and abalone, and considers both private and public facilities. It includes information on hatchery start-up and operational costs, production results compared with production plans and the reliability of production. Important areas for future development are also addressed.

(Cultivos Marinos Internacionales SA, Casilla 30, Caldera (III), Chile)

Co-feeding marine fish larvae with inert diets.

Compared to live feeds, inert diets can more easily be balanced to meet the larvae's need for both macro- and micronutrients. Provided that the inert diet is digested and absorbed by the larvae, such diets will support a higher daily intake of nutrients and thus a higher growth compared to standard live feeds. On the negative side, use of inert diets requires careful husbandry to prevent deterioration of water quality which will have a negative impact on larval survival.

Results will be presented to show the positive effect of using inert diets as a co-feed together with live prey during startfeeding of marine fish. This effect could result from either increased nutrient intake per ingested particle that was achieved by feeding only standard live feeds such as rotifers and brine shrimp, or it could be due to an increase in the dietary supply of some specific nutrients. Of specific nutrients the long n-3 fatty acids seem to be of particular importance for growth and normal development in larval fish. For example, increasing the dietary supply of DHA has been found to have a positive effect on brain development and larval performance.

This presentation will discuss the potential of using inert diets as a co-feed during startfeeding in order to obtain an efficient production of high quality marine fish juveniles.

(NUTRECO Aquaculture Research Centre, P.O. Box 353, N-4033 Forus, Norway)