Mariculture throughout the world is hampered by the lack of reliable rearing systems and practical diets for larvae. For example, high yields of red drum (Sciaenops ocellatus) depend upon feeding the larvae live prey for one to several weeks after hatching. The production of live food is costly, unreliable and high mortalities are seen when larvae are weaned to artificial diets. In addition, live prey and artificial diets traditionally offered to first feeding larvae might not provide adequate amounts of lipids, vitamins, or other nutrients. Nutrition and feeding studies here are focused on increasing the quality of both live and artificial food sources, and expanding our knowledge of the nutrient requirements of marine fish larvae. The goal is to determine essential nutrients for larvae and the best means for supplying those nutrients in order to enhance the growth and survival of marine fish reared in closed systems.
Red drum are an important commercial and recreational fish found along the Atlantic and Gulf of Mexico coastlines. They were first spawned in captivity in 1978 by Dr. Connie Arnold here at the Fisheries and Mariculture Laboratory. Since this time, numerous state, commercial, and university related hatcheries have based the spawning and production of red drum on these techniques.These fish are also commonly referred to as redfish or channel bass. |
Red drum (Sciaenops ocellatus) |
Red drum larvae
one day after hatching
ten days after hatching |
One method used to determine the specific nutritional requirements of fish larvae is through the use of artificial diets comprised of varying levels of lipids, proteins, vitamins, etc. Recent nutritional studies utilizing this method have indicated a high dietary lipid requirement for red drum larvae. For example, larval fish fed diets containing high ratios of docosahexaenoic acid [22:6(n-3) or DHA] to eicosapentaenoic acid [20:5(n-3) or EPA] exhibit greater resistance to stress than larvae fed low ratios of DHA to EPA. DHA-deficient diets have been associated with decreased visual acuity and rate of prey capture attempts at low light levels in laboratory reared juvenile herring. The purpose of this study is to further investigate the response of red drum larvae to dietary changes in total lipid and fatty acid composition. Semi-purified, microparticulate diets have been supplemented with varying levels of ICES Reference Oils to control the percentages of (n-3) highly unsaturated fatty acids and ratios of EPA to DHA in each of six diets. The growth, survival, and stress resistance of red drum larvae reared from first feeding to 20 days after hatching will be assessed for each diet and compared to a commercially available artificial diet. |
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12 day old comet larvae feeding on Artemia
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Rotifers and Artemia nauplii are currently the standard initial foods in the culture of many species of marine fish. However, neither of these organisms is naturally rich in PUFAs, which are known to be important in fish nutrition. In an attempt to increase the nutritional value of rotifers and Artemia, they are typically fed PUFA-rich diets prior to being fed to larval fish. However, this practice further increases the production cost and effort of rearing marine fish. Culturing a prey species more able to meet larval fish nutritional requirements would improve the efficiency of fish hatcheries currently using rotifers and Artemia. Two possible alternatives to using rotifers as live food are copepod nauplii and dinoflagellates. Several species of these organisms possess high PUFA levels and are important prey items for larval fish in the wild. This project is aimed at culturing both copepods and dinoflagellates, and assessing their nutritional value as first foods for larval fish by measurements of larval survival, growth and stress resistance. Of particular interest are certain copepod species that produce resting eggs, which may allow the development of a storable larval food similar to Artemia cysts. This is a collaboration with Dr. Nancy Marcus (Florida State University) whose lab produces the copepod eggs for use in this study. |
These fiber glass tanks hold 150-L of seawater and are equipped with a biological filter to maintain water quality. Approximately 2,000 red drum larvae are placed in each tank during experiments. |
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Several species of algae are grown in 40-L cylinders by supplying them with continues light and nutrients such as nitrogen and vitamins. Algae are harvested from these cylinders and used to raise rotifers or added directly to larval rearing tanks. |
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Studies in our lab have shown that the addition of microalgae to rearing tanks improves the growth and survival of red drum larvae. A number of hypotheses have been proposed as possible explanations for these improvements and will be investigated over the course of this project. One hypothesis is that microalgae may be increasing the nutritional condition of the larvae by maintaining a high nutritional quality of the rotifers supplied to the rearing tanks. Also, the passive uptake of algae through drinking may provide additional nutrients to the larvae, stimulate development of the digestive tract, or stimulate the production/secretion of important digestive enzymes. Algae may increase the feeding rate of larvae by adding contrast to the water, thereby increasing the visibility of food items, or by chemically stimulating the foraging activity of the larvae. Finally, the addition of algae to tanks may improve water quality by controlling bacterial populations and aiding in the early establishment of the gut flora. A second objective of this project is to improve current rearing methods by manipulating the amount and type of algae, live food, and dry food added to rearing tanks. For example, a recent study conducted in our lab suggests that the addition of Isochrysis galbana to rearing tanks will allow us to decrease the number of days red drum larvae are fed live food without compromising growth and survival. |
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