Health management of marine fish larvae in a microbial world

We review the literature on microbial management in larval rearing and give recommendations for future research. Based on a range of different approaches, we show that, as for many farmed animals, detrimental host/microbe interactions are a main reason for low viability of young stages of farmed fish, crustacea and shellfish. The composition of the microbiota of larval stages is determined by both selection and stochastic processes, which makes it possible to steer the colonization of larval fish by controlling the abundance and the inventory of species of the microbes present in aquaculture facilities. The microbiota evolves rapidly during larval development, leading to need for a continuous management. It is important to emphasize that it is not which species, but which functions that the microbiota provide that is important for the viability of the larvae. These functions are difficult to quantify, and we have limited knowledge. We discuss a range of microbial management methods that have been proposed and evaluate current knowledge and use in the industry. We group these methods as 1) targeted and non-targeted decimation, 2) targeted enhancement, and 3) stimulation of the immune system. Many different methods within these three groups are discussed. There has been considerable progress with some methods, but few are well studied. Only a few methods are implemented in the industry. To develop a research program that aims to establish the scientific knowledge-based needed for development of microbial management methods we call for a concerted action with participants from the industry, the research community, and national and international funding agencies. The research should be based on a consensus understanding of the current problems and be more diverse than now, as complex problems need complex solutions.