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Methanotrophic bacteria represent an appealing opportunity to convert methane, a potent greenhouse gas, into a highly nutritious animal feed ingredient, single-cell protein (SCP). SCP has a comparable or superior nutritional profile that to most conventional protein sources and can be produced within a lower environmental footprint. The present study investigated the effect of replacing fishmeal (FM) with methanotrophic SCP in diets for barramundi (Lates calcarifer), a carnivorous fish with a high demand for dietary protein and energy. Dietary inclusion levels of 0 %, 10 %, 20 % and 30 % SCP (representing 0, 25, 50 and 75 % FM replacement) were tested, with and without additives. Triplicate groups of juvenile barramundi were fed the diets over 31 days. The inclusion of SCP significantly improved weight gain and feed conversion efficiency (FCE). Dietary SCP inclusion supported good gut health, with decreasing trends of hepatosomatic index, improved plasma biochemistry, and no adverse histopathological changes. Barramundi fed the SCP diets showed an intact intestinal barrier and a significant improvement in villi and lamina propria area when fed the additive supplemented SCP diets. This study demonstrates that this SCP is highly palatable to barramundi (even without dietary additives) and can replace up to 75 % FM with significant improvements in growth and FCE.

Rotifers are used as the first feed for marine fish larvae and are grown in large cultures that have high loads of organic matter and heterotrophic bacteria; these bacteria are passed on to the developing fish larvae and can potentially lead to bacterial infections. A modified minimum inhibitory concentration (MIC) protocol for antimicrobial peptides was used to determine the potency of ten antimicrobial peptides (AMPs) in artificial seawater relevant to a rotifer culture (salinity of 25‰) against common marine pathogens. All of the AMPs had antimicrobial activity against the bacterial isolates when the salt concentration was approximately zero. However, in high salt concentrations, the majority of the AMPs had an MIC value greater than 65 µg mL−1 in artificial seawater (25‰). The only exceptions were 2009 (32.5 µg mL−1) and 3002 (32.5 µg mL−1) against Vibrio rotiferianus and Tenacibaculum discolor, respectively. The selected synthetic AMPs were not effective at reducing the bacterial load in brackish salt concentrations of a typical commercial rotifer culture (25‰).

Infectious diseases have been estimated to cost the global aquaculture industry billions of dollars annually1,2. With concerns over emerging resistance and residues of antibiotics in food3 many such chemicals are now being banned and environmentally friendly alternatives are being sought. Probiotics influence the composition of the gut microbiota and confer health benefits to their host4,5 and are one of several alternative approaches gaining significant popularity in aquaculture. Whilst primarily used to manage bacterial disease, there is also some evidence that probiotics can provide protection against parasites4 and viruses6. Probiotics can inhibit the growth of pathogens in the gut through the excretion of antagonistic substances including bacterocins6,7; prevent pathogen adhesion in the gut through competition of space and nutrients3 and by modulating the immune system5. Some probiotics have been reported to improve growth and feed utilisation efficiency5 and others can also improve water quality2,8,9, which confers indirect benefits to host healthsup>6.