Interactions between fish larvae and bacteria in marine aquaculture

Modern aquaculture provides effective means for intensive seafood production under "controllable" conditions. This rapidly growing industry, however, has experienced relatively severe disease problems owing to lack of control of the microbiota in rearing systems. Disease control is an inherent part of any intensive animal production system; however, in the aquatic environment, the intimate relationship between bacteria and their host and the frequent use of open production systems adds to this challenge. The use of antibiotics in aquatic ecosystems is presently kept to a minimum, and fortunately, vaccines and other health control means have so far kept most diseases under relative control. Various organisms, however, may not respond to vaccines, and new diseases or variants are a constant challenge to the industry. In aquaculture, eggs are kept in incubators with a microflora that differs considerably from that in the sea, and become heavily overgrown with bacteria within hours after fertilisation. Fish larvae ingest bacteria by drinking and are, thus, primed with antigens before active feeding commences. This may result in the formation of an indigenous larval microflora; however, at present, we know little about this process. The microflora of marine invertebrates may harbour bacteria that are pathogenic to other organisms and, thus, invertebrate co-inhabitants or food organisms in aquaculture may serve as vectors for transfection of fish pathogens. In intensive egg production and larviculture, the numbers of bacteria are kept low by various forms of water treatment and disinfection. These approaches, however, may disturb the balance between microbial communities, or favour proliferation of opportunistic bacteria or unpredictable development of bacterial communities. Thus, there is a need for better microbial control during intensive larval production. The use of probiotics has proven advantageous in domestic animal production, and microbial management may also have a potential in aquaculture. Better control of host-microbe interactions is a prerequisite for stable production of marine larvae in intensive systems.