Ulva and Gracilaria in finfish aquafeeds: standardisation, health effects and evidence for bacterial disease resistance

Abstract

Bacterial diseases remain a major constraint in intensive aquaculture and have increased interest in functional feed ingredients that can support fish health while reducing dependence on antimicrobials. The present review critically examined Ulva and Gracilaria derived meals, extracts and enriched fractions as functional aquafeed ingredients, with focus on the links between biomass origin, cultivation, processing, chemical composition and disease-related outcomes in finfish. Both genera provided nutritional components together with sulfated polysaccharides, phenolics, pigments, fatty acids and sterols that may act through direct antibacterial effects, redox regulation, innate immune modulation, maintenance of mucosal and intestinal barriers, and changes in the gut microbiota. However, the available evidence showed that efficacy was strongly product specific. Responses varied with seaweed species, production environment, extraction method, dietary dose, fish host and pathogen model. Studies in commercially important fishes, including seabass and tilapia, indicated that improved growth, immune responses or gut microbiota profiles did not necessarily demonstrate resistance to infection. Studies combining a chemically characterised product with dietary exposure and a controlled bacterial challenge provided stronger evidence. Major drawbacks include incomplete taxonomic authentication, poor reporting of extraction and feed-manufacturing conditions, limited chemical standardisation, short feeding periods and uneven pathogen coverage, particularly for gram-positive bacteria. Controlled cultivation and tissue culture may improve traceability and biomass consistency, but their functional advantages remain to be demonstrated. Progress therefore depends on converting variable seaweed preparations into safe, reproducible and chemically defined products whose efficacy is validated in the intended fish-pathogen system.