Techniques for microbial control in the intensive rearing of marine larvae

Abstract

The intensive cultivation conditions for marine larvae may easily cause microbial problems, resulting in poor growth and mass mortality, and techniques for improvement by enhancing environmental and larval factors should be developed. Establishment of a beneficial, protective microflora of marine larvae can be obtained by use of microbial matured water and probiotics. Microbial maturation of the rearing water before use in the larval tanks, can be obtained by running the water through a maturation unit that selects for non-opportunistic microflora. A diverse bacterial flora established by non-opportunists is believed to inhibit proliferation of opportunistic pathogenic bacteria in the water and the larvae. When used in the earliest developmental stages of marine fish larvae, enhanced growth and survival can be obtained. Also, the introduction of probiotic bacteria may promote the defence of the gut flora against pathogenic bacteria. Probiotic bacteria can be added directly to the water or administered to the larvae via live food, such as rotifers and Artemia. Further developments in these techniques are needed for improved control of bacterial number, growth and colonisation in the larval gut, as well as the identification of bacteria with probiotic effect on the host. Disinfection of eggs is a positive factor for control of growth and transfer of bacteria in larviculture. For marine fish eggs glutaraldehyde has suitable properties as a disinfectant, which improves hatching, development and survival of larvae. Because marine fish larvae have no specific immune system at hatching, non-specific defence is very important during the first developmental stages, and suitable techniques for stimulation of the non-specific immune defence may become important in future marine larval rearing. Enhanced viability of marine fish larvae and juveniles, has been obtained by treatment with FMI, which is a mannuronic acid polymer. Routes of administration were either into the water or via live food. A technique for incorporating immunostimulants in rotifers and Artemia for controlled transfer of immunostimulants to marine larvae, is described.