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Disease is an enduring threat to aquaculture, with direct costs from outbreaks of more than US$6 billion a year. Efforts to reduce antibiotic use in aquaculture have resulted in a recent shift towards the use of immunostimulants that boost fish immune systems. Seaweeds are a potential source of unique immunostimulants because of their diverse taxonomy (11,353 species) and diverse bioactives, but there has been no synthesis of their effects on fish. A systematic review and meta‐analysis of 142 peer‐reviewed studies was conducted to evaluate the effects of seaweed on the innate immunity, resistance (to stress and pathogens) and growth of farmed fish. Overall, our review indicated that dietary supplementation with seaweeds or their extracts had substantial positive impacts on the health and growth of fish compared with control groups and were comparable to currently utilised immunostimulants. Dramatic effects on survival of fish during challenge experiments were detected, with an average of 33% higher survival of fish fed diets that included seaweeds or their extracts. This study provides broad evidence for the benefits of supplementing fish feed with the reviewed seaweeds or their bioactive extracts, including potential synergistic effects of supplements. However, the meta‐analysis also highlighted considerable variation among studies, and between fish and seaweed species, making it difficult to extrapolate results beyond individual experiments. Future studies should focus on developing synergistic combinations of ingredients and elucidating the mode of action through isolation of seaweed bioactives, together with standardisation of experimental conditions, to fast‐track the benefits of seaweed supplements for aquaculture.

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