Using brewing industry byproducts to create feed additives for aquaculture-raised rainbow trout under thermal stress

The aquaculture industry helps meet the seafood demands of a rapidly growing human population. The goal of this study was to determine the effects of feed additives, generated from byproducts of the beer brewing industry, on the growth rate, production parameters, and intestinal bacterial composition of aquaculture-raised rainbow trout ( Oncorhynchus mykiss ). Two prevalent brewery byproducts, brewer’s spent yeast (BSY) and brewer’s spent grain (BSG), were converted into high-value fish feed additives. To increase nutritional value of the byproducts, BSY was treated with exoenzymes, and BSG was used to cultivate black soldier fly (BSF; Hermetia illucens ) larvae, which was treated with proprietary chitinases. During a seven-week trial using a recirculating aquaculture system, 22.15 ± 0.24 g (mean ± standard error) fish were randomly separated with 12 fish/tank in quadruplicate for each of the six diets (48 fish per diet), and tanks were randomly assigned diets. Survival rates were recorded daily, and fish were weighed every two weeks to track growth rates, adjust feed amounts, and calculate feed efficiency at the tank level. Fish intestinal mucosa swabs were collected at the beginning and end of the trial to analyze changes in the bacterial communities. Due to a malfunction in the chiller that maintained cool water temperatures, temperature and dissolved oxygen became substantial chronic (two week) and then acute (12–24 hr) stress factors near the end of the trial. Therefore, the research focus shifted to understand the impacts of thermal and oxygen stress on fish fed these novel feed additives. Clear changes in the structure of the bacterial intestinal community were observed in the fish between the start and end of the study as evaluated by 16S rRNA gene analysis. Results suggest that fish fed three of the experimental diets benefitted by having higher survival rates and growth rates when subjected to the unplanned stress.