Ascorbic acid (vitamin C) function in fish gametogenesis and fertility.
Ascorbic acid in high concentrations has long been associated with fish ovaries although its major function in female reproduction and egg fertility has not been elucidated. Dietary deficiency of vitamin C during vitellogenesis resulted in a depletion of ascorbic acid in tilapia, rainbow trout, cod and yellow perch, whereas the effects on embryo viability were detected only in tilapia and rainbow trout. A significant decrease in rainbow trout fecundity and egg production were documented in one study whereas a reduction in yolk materials deposition during vitellogenesis was found in fish fed ascorbate-deficient diet in another study. The first would suggest that vitamin C deficiency may interfere at the moment of oocyte selection long before vitellogenesis. This result of oocyte atresia in fish of low vitamin C status would contrast with the general effect of repeated stress which resulted in reduced egg size. The increased mortality was observed mostly in the progeny from ascorbate-deficient mothers at the eyed-stage although the eyed-hatching ratio was between 0.5-0.85. Recent data also suggest that concentrations of ascorbate in the seminal plasma of several teleost and acipenserid fish are much higher than in the blood plasma. However, the levels of ascorbate in testis were significantly lower than in the ovaries. In rainbow trout, seminal plasma ascorbate concentrations declined with a decrease of dietary source of vitamin C. Sperm concentrations and motility were reduced in fish which obtained ascorbate-deficient diets. The evidence of a higher frequency of abnormal and/or haploid embryos of rainbow trout of fathers maintained on diets deficient in vitamin C suggested that endogenous oxidative DNA damage might affect sperm quality and increase the frequency of genetic defects. These results indicate that the mechanism causing mortalities of the progeny in ascorbate-deficient male and female teleost fish might involve chromosomal errors. These results are significant because they demonstrate that the nutrient requirement in gametes producing fish are considerably different (higher in case of ascorbate) than that for optimum somatic growth.
(School of Natural Resources, The Ohio State University, Columbus, OH 43210, USA)
Mesocosm approach in marine fish larviculture: present status and potential.
Following an overview of the organisation and status of Mediterranean finfish mariculture industry, the authors analyse the economical, technical, human and biological causes of the actual limitations of intensive economic hatchery techniques. They also point out the long term negative effects of dependence on competition for fry provision.
An innovative approach for larviculture will be presented: the mesocosm technology based on extensive larval rearing strategies. The basics of production and their specific variants, the needed infrastructure, the organisation and staff competence, as well as the cost of production are described and compared to those of intensive strategies. The data are discussed in relation with the market adaptation. This technology is regarded as a viable alternative for small hatchery operators.
(Institute of Marine Biology of Crete, GR-71003 Iraklion, Crete, Greece)
Microbial management in marine fish larviculture.
A microbial management has been developed for larval culture of the red drum Sciaenops ocellatus. The procedure consists in the elimination of microbial contaminants and, their replacement by a beneficial microflora.
Culture tanks are covered with lids or plastic bags. All culture systems including airlines are chlorinated and rinsed. Air is filtered through 0.2 m filters. Seawater used for culturing algae, rotifers and fish larvae is disinfected by filtration and chlorination. Bacterial probiotics are added immediately following the neutralization of chlorine with sodium thiosulfate. This is referred as treated seawater.
Axenic algae are cultured in 1 l Erlenmeyers, then successively transferred to 20 and 150 l tanks filled with treated seawater. Bacteria-free rotifers (Brachionus rotundiformis) are obtained by disinfection of cysts with sodium hypochlorite. Rotifer populations are built by successive transfers into exponentially grown algal cultures. Fish eggs are disinfected with hydrogen peroxide and, after hatching, transferred to 150 l tanks filled with treated seawater. On the third day of culture, internal biological filters are inoculated with selected nitrifying bacteria. Larvae are fed daily with rotifers.
Integration in this management of diverse laboratory-grown single-bacterial strains, mixtures of strains, 'selected microbial communities' or commercial bacterial products leads to significantly different results. With the use of selected probiotics, this microbial management significantly reduces crashes of algal, rotifer and fish larvae cultures. It also improves production while reducing the coefficients of variation between replicates. This management can be easily applied in commercial hatcheries.
(Marine Science Institute, The University of Texas at Austin, P.O. Box 1267, Port Aransas, TX 78373, USA)
