Gavin Partridge

This study evaluated the effects of replacing fishmeal with microbial single-cell proteins (SCPs) in diets for juvenile yellowtail kingfish (YTK, Seriola lalandi), assessing growth, feed efficiency, nutrient digestibility, gut histology and microbiome, blood biochemistry and gene expression. Eight experimental diets were formulated, using methanotrophic (SB) and hydrogenotrophic (LT) SCPs at inclusion levels of 4 %, 8 %, and 16 %, alongside a control diet (42.2 % fish meal) and a combination diet (4 % SB + 4 % LT). Fish were reared in 500 L tanks for 12 weeks, after which growth performance, feed intake, feed conversion ratio (FCR) and health indices were measured. Significant improvements in FCR were observed in the 8 % SB and 16 % SB groups (0.99 and 0.98, respectively), compared to the control diet (1.06). Protein and energy retention were also highest in the 8 % SB and 16 % SB diets, with respective retention efficiencies of 39.0 % and 37.8 %. Apparent digestibility was comparable across diets, with protein digestibility peaking at 87.9 % in the 4 % SB + 4 % LT diet. Histological analysis revealed no significant changes in intestinal or skin integrity. Blood biochemistry, hematology, gene expression profiles and microbiome were unaffected by SCP inclusion, except for a notable decrease in gut bacterial diversity in fish fed the 4 % SB + 4 % LT diet. Overall, these results demonstrate that SCPs, particularly SB at 8 % and 16 % inclusion levels, can effectively replace fishmeal in YTK diets without compromising health or growth, while enhancing feed efficiency and nutrient retention.

The present study evaluated the effectiveness of five dietary additives in improving growth performance, gut and skin health and disease resistance against Photobacterium damselae subsp. damselae (Pdd) in yellowtail kingfish (YTK) (Seriola lalandi). The additives were top-coated onto a commercial diet. The five additives evaluated were brewer’s yeast (BY), a yeast-based mannan oligosaccharide (MOS), a polyphenol product, a monoglyceride product and a combination of MOS and a superoxide dismutase (SOD) product. The monoglyceride and polyphenol products were trialled at both a low and high inclusion level. Diets were fed to YTK in triplicate tanks for 55 days. The results found that growth, feed utilisation and somatic indices were unaffected by additive inclusion. None of the additives affected the health of YTK liver, skin or the gut as determined by various histological metrics. These histological findings were supported by results of hindgut gene expression (TNF-α, IL-10, Hsp70, mucin 2 and i-mucin and SOD), all of which were unchanged across the dietary treatments. Similarly, the additives did not stimulate the skin mucus quantity and skin mucosa barrier measurements (epidermis thickness and mucus cells), consistent with the results of relevant skin health gene expression measurements (TNF-α, IL-8, calreticulin, Hsp70, mucin 2, CAT and SOD). However, the MOS and low inclusion polyphenol treatments increased survival against Pdd challenge relative to control treatment. The immuno-modulating capacity of the polyphenol product was indicated by the absence of Pdd in faeces following a mucosal (bathe) challenge at the high inclusion; however, this did not translate to enhanced survival under systemic infection. However, further studies are needed to understand if optimisation of the inclusion levels of each additive might more strongly influence the health of YTK.

Two additives, rosemary oil and a commercial herbal product (HP; AROTEC‐G) containing garlic essential oil, carvacrol and thymol were investigated for their potential to reduce the parasitic burden in yellowtail kingfish (YTK; Seriola lalandi ) infected with Zeuxapta seriolae . Naturally infected fish were fed diets supplemented with either 2.5 mL.kg −1 rosemary oil or two concentrations of HP (5 and 10 g.kg −1 ) for 20 days. At 10 and 20 days, the effect of the diets on parasite abundance was evaluated. Feed intake and growth were similar across treatments ( p > 0.05). By day 10, both rosemary oil and HP significantly reduced parasite burden compared to the control ( p = 0.03). Praziquantel (PZQ) baths on days 10 and 20 found that the supplements were more effective in reducing new parasite recruitment rather than eliminating adult parasites. The HP product, at both doses, significantly reduced reinfection by day 20. Rosemary oil reduced parasite load by 43% compared to the control by day 20. Liver and kidney function remained unaffected. This study concluded that rosemary oil and blended essential oils can assist in the management of Z. seriolae infection in YTK culture.

Background
With the global expansion of aquaculture and the increasing demand for fish meal, identifying appropriate and sustainable alternative protein sources for aquafeeds has become essential. Single-cell protein (SCP), derived from methanotrophic bacteria, presents a promising alternative by converting methane into protein, potentially addressing both the need for alternative protein sources and reducing industrial greenhouse gas emissions. This study aimed to evaluate the effects of different levels of SCP inclusion (0%, 25%, 50%, and 75% fish meal replacement) on the health, gene expression, and gut microbiome of yellowtail kingfish (YTK, Seriola lalandi) following a 35-day growth trial.

Results
The study found that SCP inclusion at the highest level of fishmeal replacement (75%) induced a mild inflammatory response in the hindgut of the fish. However, micromorphological assessments of the hindgut, serum biochemistry, and gene expression analyses revealed no significant detrimental effects from SCP replacement. Notably, there were indications of improved lipid digestibility with SCP. Furthermore, SCP inclusion significantly enhanced microbial richness and altered the composition of the gut microbiome, introducing beneficial bacterial taxa that may contribute to improved gut health and resilience.

Conclusions
This study highlights SCP as a viable and sustainable alternative to fish meal in YTK diets. The findings suggest that SCP can be included in YTK diets without adverse health effects at moderate levels and may even offer benefits in terms of lipid digestibility and gut microbiome diversity. These results contribute to the advancement of more sustainable aquaculture practices.

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.

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‰).

The authors regret that the printed version of the above article contained an error in our documentation of ethics approval for challenge infection of fish. The correct and final version follows. The authors would like to apologise for any inconvenience caused. Our experiment was conducted using methodology approved by the Animal Ethics Committee of the Department of Primary Industries and Regional Development (Permit number: 19-4-14), which follows the principles and guidelines of Australian Animal Care.

Praziquantel (PZQ) provides an effective treatment against monogenean parasitic infestations in finfish. However, its use as an in-feed treatment is challenging due to palatability issues. In this study, five formulations of PZQ beads (1–4 mm) were developed using marine-based polymers, with allicin added as a flavouring agent. All formulations attained PZQ loading rates ≥74% w/w, and the beads were successfully incorporated into fish feed pellets at an active dietary inclusion level of 10 g/kg. When tested for palatability and digestibility in small yellowtail kingfish, the PZQ-loaded beads produced with alginate-chitosan, alginate-Cremophor® RH40, and agar as carriers resulted in high consumption rates of 99–100% with no digesta or evidence of beads in the gastrointestinal tract (GIT) of fish fed with diets containing either formulation. Two formulations produced using chitosan-based carriers resulted in lower consumption rates of 68–75%, with undigested and partly digested beads found in the fish GIT 3 h post feeding. The PZQ-loaded alginate-chitosan and agar beads also showed good palatability in large (≥2 kg) yellowtail kingfish infected with gill parasites and were efficacious in removing the parasites from the fish, achieving >90% reduction in mean abundance relative to control fish (p < 0.001). The two effective formulations were stable upon storage at ambient temperature for up to 18 months, showing residual drug content >90% compared with baseline levels. Overall, the palatability, efficacy and stability data collected from this study suggest that these two PZQ particulate formulations have potential applications as in-feed anti-parasitic medications for the yellowtail kingfish farming industry.

Single-cell protein (SCP) derived from methanotrophic bacteria has significant potential as a fishmeal alternative in aquafeeds. However, SCP has known palatability issues, to overcome these issues tuna hydrolysate and garlic powder were tested as palatability enhancers against basal diets without these additives. This study tested the inclusion of SCP at four dietary levels of 0%, 10%, 20% and 30%, representing 0%, 25%, 50% and 75% fishmeal replacement in juvenile yellowtail kingfish (Seriola lalandi) (YTK) diets. YTK were fed these diets in triplicate over 35 days. Compared with the control diet, fish fed the SCP10% diet ate less feed, but had equal growth and subsequently an improved FCR. Feed intake decreased with increasing SCP inclusion, and the palatability enhancers were ineffective at improving intake. Despite the significant reduction in feed intake, FCR was equal across all SCP inclusion levels, demonstrating that the reduced growth performance at these higher inclusion levels was a function of only the reduced intake. Data showing equal protein retention efficiency and apparent digestibility coefficients across diets support this finding. This study showed that SCP derived from methanotrophic bacteria can replace 25% of fishmeal in a 400 g/kg FM diet and suggests if palatability issues can be overcome then higher inclusion of SCP could be achieved in YTK diets.