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.

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.

Reducing fishmeal (FM) in aquaculture diets is essential for improving sustainability and reducing reliance on marine resources. Black soldier fly (BSF; Hermetia illucens ) larvae meal is a promising alternative protein source. This study evaluated the effects of replacing FM with BSF meal on the growth and health of juvenile yellowtail kingfish (YTK) ( Seriola lalandi , initial weight ~22 g). Fish were reared in 24 tanks (three replicates per treatment) and fed for 33 days under controlled conditions. Eight diets were tested: a control (40% FM) and seven diets with BSF meal replacing FM at 25%, 50%, or 75%, with or without garlic and tuna hydrolysate additives. Fish fed 25% and 50% BSF diets showed growth and feed conversion comparable to the control, while 75% BSF significantly reduced growth due to decreased feed intake. Additives did not improve feed intake. Histological analysis indicated good gut health and nutrient absorption. Serum cholesterol decreased with BSF inclusion, and urea levels varied. No significant changes in gene expression were observed in the gut, liver, or brain. Microbiome analysis showed increased diversity and compositional shifts at higher BSF levels. These results support the use of BSF meal as a sustainable FM replacement at moderate inclusion levels, with further research needed to address palatability at higher levels.

Yellowtail kingfish (Seriola lalandi, YTK) has many characteristics suitable for aquaculture, but infectious diseases have the ability to hinder the development of the industry. Photobacterium damselae subsp. damselae (Pdd) is an emerging pathogen for S. lalandi and other cultured fish species, causing severe economic losses. A better understanding of the immune response of YTK to Pdd infection is crucial for the development of effective approaches to disease management. In the present work, we evaluated the immunopathological response of YTK to Pdd infection and examined differences between those fish that survived or succumbed to infection. Mortality of juvenile fish (36-38 g) increased with increasing infectious dose and the LD50 was estimated to be 8.0×104 CFU/fish. Infection with Pdd induced both innate and adaptive immune responses in YTK. Compared to control (non-challenged) fish, the tissues of infected fish showed major histological changes, including infiltration of inflammatory cells, macrophage aggregations, and cell necrosis. Challenge infection induced significant increases in the expression of both pro-inflammatory and anti-inflammatory cytokine genes in the blood after three days. Fish which did not survive infection had a higher expression of tissue cytokine genes than those which survived up to eight days.