The diet of farmed fish has evolved rapidly over the past 40 years to cope with demand and key limiting resources such as fish meal. Growth and food conversion ratio were the main criteria when developing aquafeed, but recently the fish immune system and its microbiome are emerging important considerations. Seaweeds and their natural products are also emerging as a alternative source of functional ingredients in aquaculture. Here we evaluated the effects of 11 species of seaweed from three taxonomic groups (Phaeophyceae, Chlorophyceae and Rhodophyta), two commercial ingredients (sodium alginate and Hilyses®) and two microalgae previously used as functional ingredients (spirulina and Haematococcus pluvialis) on the innate immunity and the gut microbiome of the rabbitfish Siganus fuscescens.
Each functional ingredient was included at 3% by weight in a commercial fish feed and fed to the rabbitfish for 2 weeks. All 15 treatments altered the immune response and hindgut microbiome compared to the control diet. At least one seaweed from each taxonomic group led to a doubling in the activity of one immune parameter (the alternative complement system) compared to the control. Bacteria of the genus Ruminococcus sp. and Akkermansia sp. were lowest in abundance in control fish but increased with all treatments, up to 6- and 9- times respectively. Understanding how dietary inclusions shape the gut microbiome of fish and whether, in turn, the microbiome influences fish health, will be crucial for developing tailored fish feed.