Herbivorous fishes play important ecological roles in coral reefs by consuming algae that can otherwise outcompete corals, but we know little about the gut microbiota that facilitates this process. This study focussed on the gut microbiota of an ecologically important coral reef fish, the convict surgeonfish Acanthurus triostegus. We sought to understand how the microbiome of this species varies along its gastrointestinal tract and how it varies between juvenile and adult fish. Further, we examined if the bacteria associated with the diet consumed by juveniles contributes to the gut microbiota. 16S rRNA gene amplicon sequencing showed that bacterial communities associated with the midgut and hindgut regions were distinct between adults and juveniles, however, no significant differences were seen for gut wall samples. The microbiota associated with the epilithic algal food source was similar to that of the juvenile midgut and gut wall but differed from the microbiome of the hindgut. A core bacterial community including members of taxa Epulopiscium and Brevinemataceae was observed across all gastrointestinal and diet samples, suggesting that these bacterial symbionts can be acquired by juvenile convict surgeonfish horizontally via their diet and then are retained into adulthood.

Nile tilapia, Oreochromis niloticus is the third most commonly farmed finfish species in the world, accounting for nearly 5% of global aquaculture production. In the past few decades much of the success of this species has been attributed to the development and distribution of Genetically Improved Farmed Tilapia (GIFT). Despite the increasing availability of GIFT, the productivity of small-scale farming remains highly variable, particularly in developing nations. Commercial fish-feed pellets can increase fish farm productivity; however, many small-scale farmers rely on other means of feeding fish due to the high cost and limited availability of commercial fish feed pellets. Therefore, understanding how locally-sourced feeds affect the production of GIFT is an important step towards improving feeding practices, particularly for farmers with low financial capital. This study used stable isotope analysis (SIA) and 16S rRNA gene sequencing to compare the effects of a locally-sourced vegetablebased diet and commercial pellet-based diets on the relative condition, nutrient assimilation patterns and gastrointestinal microbiota of GIFT. GIFT fed a locally-sourced diet were smaller, and in a significantly poorer condition than those fed with commercial fish feeds. SIA showed no differences in dietary carbon between the two diets; however, δ 13 C, poor fish condition and the abundance of specific bacterial taxa (of such as Fusobacteria) were correlated. SIA revealed that GIFT fed locally-sourced diets that predominantly consisted of vegetables were significantly enriched in δ 15 N despite a perceived lack of dietary protein. This enrichment suggests that GIFT fed a locally-sourced diet may be supplementing their diet via cannibalism, a behaviour representative of poor farming practice. Overall this study highlights the need to increase the availability of suitable GIFT feeds in developing nations. The development a low-cost feed alternative could improve the success of small-scale GIFT farmers in PNG, increasing both food and income security within the region.

Freshwater fish farming is the largest sub-sector of the global aquaculture industry, with its continued success dependent on the industry's ability to be productive and sustainable. In recent years, microorganisms have been increasingly recognised for their role in the health and overall performance of aquatic organisms, providing the aquaculture industry with a promising avenue for further success. Here, we review the literature on freshwater fish microbiology to evaluate the potential for microbial research to enhance freshwater aquaculture. A systematic review revealed that the existing freshwater fish microbiology literature incorporates <1% of freshwater fish species, with most studies focussed on Cyprinidae, Salmonidae and to a lesser extent Cichlidae. Using social network analyses, this review identified 25 key research areas commonly addressed in the existing literature highlighting the narrow focus of existing studies. Microbial community structure was the most frequently researched area, followed by environment and diet. These studies often evaluated microbiota changes associated with diadromous migrations, new technologies and advancements in feed formulations. However, research areas, such as the role of microorganisms in fish fecundity, omega-3 content, sustainability and palatability, were underrepresented in the literature despite their influence on aquaculture success. These underrepresented research areas are examples of where additional investments in microbial research are needed to ensure the continued expansion and success of freshwater aquaculture.

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