Application of immunostimulants in larviculture: Feasibility and challenges

In this respect, fish are reliant on passive immunostimulation from maternal antibodies, and this mechanism has been shown in tilapias (Mor et al, 1990; Sin et al 1994). In any given species, size rather than age seems to be the most critical determinant of the development of immunity. For example, in salmonids, memory-dependent immunity has been achieved for sizes > 0.26g (8 weeks of age), whereas in carp it was acquired at (Eills,1988). It is therefore possible that during the stages when the lymphoid organs are developing, the main cellular defence is by the phagocyte populations within the integument (Ellis 1988). The nonspecific or innate immune system is regarded as the first line of defence in animals. Furthermore, it seem that the microbial problems in larviculture are most likely due to opportunistic bacteria rather than specific pathogens (Munro et al,1995).This emphasizes the importance of nonspecific defence system for larvae under intensive hatchery conditions, and the need for more knowledge of the ontogeny and functionality of the nonspecific immune system of larvae.