Sourcing and presenting polyunsaturated fatty acids in marine fish larval feeds.
Current procedures for optimising the presentation of dietary PUFA to marine fish larvae will be reviewed in relation to the advantages and disadvantages of using (a) single-cell, eukaryotic organisms or, (b) purified oils, as primary sources of these essential nutrients in larval production systems. For the former option (a), phototrophic and heterotrophic organisms can both be used to advantage and current knowledge of the origins and functions of PUFA in marine organisms can help to identify suitable organisms. For the latter option (b), control of PUFA peroxidation by various antioxidant systems is essential. Progress in this field may be limited more by lack of knowledge of larval requirements for antioxidant vitamins than by larval requirements for essential PUFA.
(NERC Unit of Aquatic Biochemistry, University of Stirling, Stirling, FK9 4LA, UK)
Histological and biochemical methods in nutrition studies with fish larvae.
Studies on the nutrition of fish larvae must consider two different aspects: a) quantitative as well as qualitative changes of nutritional requirements during development; b) ontogenesis of new structures and functions that alter the capabilities of fish larvae to ingest, digest, absorb and metabolize diets or nutrients. For a sound understanding of nutrient utilization in larval fish, classical growth experiments need to be supplemented by additional methodological approaches.
Histological and biochemical studies revealed common patterns in the ontogenesis of the digestive tract of both freshwater and marine larvae. Larval energy metabolism is dominated by aerobic fuelling, whereas glycolysis is of minor importance. Controversial findings exist on the relative importance of amino acids and fatty acids as substrates for aerobic catabolism. The hardware for endocrine regulation of metabolic and digestive processes is established early in larval development, although the functioning of endocrine regulation remains to be demonstrated. In addition to studies on the development of digestion and metabolism, histological and biochemical techniques have been successfully used to establish indicator parameters of larval quality and condition, e.g. RNA and protein concentrations or liver ultrastructural features. These parameters are of great value for a more comprehensive evaluation of the nutritional value of diets, however, they are primarily descriptive and they are not analytical, explanatory tools. The application of more specific histological and biochemical measures in larval nutrition studies remains technically difficult both with respect to feeding and rearing conditions (e.g. determination of food consumption) and with respect to the larvae (e.g. small body size, allometric organ growth). At present, the most suitable approach to define the physiological requirements of fish larvae for specific nutrients appears to be the use of manipulated live plankton.
(Department of Chemical Ecotoxicology, Centre for Environmental Research, Permoserstr. 15, D-04318 Leipzig, Germany)
The use of blastomere morphology as a predictive indicator of fish egg viability.
Fish hatcheries have a practical requirement for a simple and reliable predictive measure of egg viability in order to make efficient use of incubation facilities. A variety of characteristics have been adopted as egg viability criteria in fish, including buoyancy and fertilisation rate, but no single approach has been recognised as being widely applicable between fish species. The cells (blastomeres) of early cleavage stage fish embryos are easily visualised owing to their large size. Since it has been suggested that eggs which show a departure from the normal pattern of cell cleavage are less viable, the observation of blastomere morphology has been proposed as a tool for egg assessment. This technique has been employed for a number of fish species although its validity has not been rigorously tested. This paper reviews existing information on blastomere abnormalities in vertebrates and presents new data on the validity of the technique for Atlantic halibut (Hippoglossus hippoglossus L.). It is especially important to select viable egg batches for this species owing to the relatively long, complex incubation and yolk sac phases.
In field studies on pelagic fish eggs, morphological abnormalities in the blastomeres and embryos have been recorded. These have often been linked with chromosome aberrations, particularly in ecotoxicological surveys investigating the effects of certain marine pollutants. In studies on mammalian embryos, abnormalities occurring at the cleavage stages have been demonstrated to have a deleterious effect on subsequent development and ploidy. Disruptions to the function of the cytoskeleton have been implicated in these developmental defects. The possible causes of unusual cleavage patterns are discussed in the light of knowledge about the mechanisms of cell cleavage and its sensitivity to environmental extremes.
Evidence is presented of a correlation between blastomere characteristics and egg viability in Atlantic halibut. The practicalities of using these observations as a predictive indicator of viability are discussed.
(Seafish Industry Authority, Marine Farming Unit, Ardtoe, Acharacle, Argyll, PH36 4LD, Scotland, UK)
Development and regulation of intestinal function in larval teleost fishes.
The formulation of inexpensive artificial diets for larval fishes depends in part on an understanding of their digestive tracts and the mechanisms of uptake of critical components from live food. With sufficient knowledge of digestive processes, and especially the recognition that they may be very different from digestive processes in juveniles and adults, we might be able to formulate artificial diets that can truly replace rotifers and Artemia. The digestive tract of larvae changes in form and function relatively rapidly. At or shortly after hatching, the alimentary canal differentiates as the yolk sac is exhausted and the larva must begin exogenous feeding. The prevailing dogma is that lipid absorption occurs in the midgut and that protein macromolecules are taken up by pinocytic absorption and intracellular digestion in the hindgut. An analysis of macroprotein absorption by the larval intestine using antisera to lipovitellin from brine shrimp is presented.
With better understanding of normal developmental processes, we may also be able to accelerate development of the digestive tract to accept artificial diets. For example, in vertebrates generally, the development of the intestine and its subsequent functioning are clearly under the regulation of endocrine factors. In the human, there is a late gestational rise in cortisol coincident with increased intestinal activities. In the rat, there is good evidence that both thyroxine and cortisol coordinate the maturation of the gastrointestinal system with the transition of weaning. The strong possibility that intestinal development in teleosts resembles that of other vertebrates is only recently receiving attention. In the literature it has been reported that thyroid hormone stimulates gastric development in the flounder. This overview aims to bring forth evidence that hormones can regulate intestinal epithelium in the larval fish, to examine mechanisms of uptake of feed components, and to outline possible strategies for manipulating timing of weaning in culture situations.
(Department of Zoology, University of Rhode Island, Kingston, RI 02881, USA)
Production and use of copepods in marine fish larviculture.
Batch and continuous cultures of the harpacticoid copepod Tisbe holothuriae have been tried and the nauplii were used as food in preliminary trials with first-feeding turbot. The first stages of T. holothuriae are primarily planktonic and are an ideal size for first-feeding turbot or other species of marine fish which have small mouth sizes at first-feeding. The later stages are benthic and play an important role in maintaining hygiene in the larval rearing tanks.
A review of the species which have been used in laboratory cultivation, the size of the species and its naupliar stages and the size of culture and production potential are given. Finally, the potential benefits of the use of copepods as live food for marine fish larvae are discussed including the incentives important for the development of this field.
(Danish Institute for Fisheries Research, P.O. Box 101, DK-9850 Hirtshals, Denmark)
