The use of immunostimulation in marine larviculture: possibilities and challenges

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Inadequate microbial conditions are one of the main problems in rearing marine larvae. It is therefore an ultimate goal to develop methods for establishing microbial control at all stages of the cultivation process. In addition to measures improving environmental conditions, methods of improving the resistance of the larvae to bacterial infections need to be developed. One possibility is immunostimulation, which includes methods of enhancing the capacities of the specific and nonspecific immune systems. Although the information is limited, one must conclude that larval fish have a poorly developed immune system and that they primarily rely on nonspecific immune system. For specific defence, larvae have to rely on maternal immunity, which lasts only for a short period. Experiments have shown that maternal immunity may be manipulated by immunisation of the broodstock and that increased resistance to infections may be obtained. Direct immunostimulation of larvae must be aimed at the nonspecific part of the immune system, and several substances are known to have this ability. Experiments on nonspecific immunostimulation of fish suggest that the method has considerable potential for reducing losses in aquaculture, both during larval and on-growing stages. Reports on immunostimulation of larvae are, however, very limited. Further development of this method for larviculture will require the establishment of methods for the administration of the stimulant, and the adaptation of methods for detecting the response of the immune system. This last point is a particular challenge due to the small size and fragility of larvae. It is hypothesised that immunostimulation, together with other methods of achieving microbial control, will help to reduce the probability of microbial problems in larviculture. As a result, increased and more stable survival and growth are anticipated; resulting in the production of high-quality juveniles for the on-growth period.