Critical Success Factors for Marine Fish Farming: Kill the Pathogens or Live with Them!

Alain Michel, Aquaculture Consultant, France

In the farming of several species of finfish, the industry has mastered its husbandry at the hatchery, nursery and grow-out stages. In grow-out in open cages or in indoor facilities, there is a degree of control and sophisticated feed delivery systems. Selective breeding programs answer the need for doubling growth rates within a targeted number of generations.

However, the consequence of crowding a large number of fish in small volumes of water, in ponds or in small bays, is the story of aquaculture. With this comes a permanent fight against existing and emerging pathogens. This is true even in the farming of marine tropical fish, where we often have to deal with many different species. In addition, the equivalent of the salmon model is still in its infancy. Production of several marine fish species is often segmented. While the numerous backyard hatcheries are still able to produce consistently, industrial farms using technological advances are failing, mainly because they are not able to control the pathogens. Asian seabass, grouper, snappers, pompano, cobia and amberjacks are good candidates for farming. However, the farming of these species continues to suffer from disease outbreaks, usually with no known treatments.

In the 1980s, outbreaks of viral nervous necrosis (VNN) were common among the seabass. Mass mortalities in the nursery stages were attributed to “unknown diseases.” From 1998 to 2014, when working on a fish farm in Indonesia, this situation became the field reality, batch after batch at the production scale for over 16 years. The diseases were identified as:

  • two viruses (VNN and one Iridovirus RSIV-ISKNV type);
  • four bacteria, i.e. big belly bacteria – an intracellular vibrio, Streptococcus iniae – a bacteria found everywhere, Tenecibaculum marinum that causes saddle back disease, and an external bacteria (Cytophaga sp) that affects the gills and kills the fish by anoxia; and
  • one parasite known as Benedenia sp.

This presentation will discuss field experiences that confronted these diseases, and scientific knowledge on mitigation measures. The aim is to share information and advance further explanations on aquaculture problems. The focus is based on the mantra: follow the scientific way to try and transform doubts in approaching the comprehension of reality.

At the farm, we found that temperature shocks could prevent viral replication and are always boosting both the innate and adaptive immune system to increase the whole metabolism. The objective is to maintain, in a controlled way, the pathogens at a low level in the culture systems to allow the immune system to learn about them and induce an auto live vaccination. This strategy strengthens juveniles, irrespective of the different bacterial serotypes or virus mutants present. It is clearly a relation host-pathogen relationship and answers the question: how do we reinforce the normal defense of the organisms to give them more power to deal with the different pathogens? Our new paradigm is not to kill the pathogens, but to learn to live with and control them.