Fish processing side streams are promising ingredients in diets for rainbow trout (Oncorhynchus mykiss) -Effects on growth physiology, appetite, and intestinal health.

Due to the growth of aquaculture and the finite supply of fishmeal and oil, alternative marine protein and lipid sources are highly sought after. Particularly promising is the use of side streams from the fish processing industry, allowing for the recovery and retention of otherwise lost nutrients in the food production chain. The aim of the present study was to evaluate the potential of three fish processing side streams as fish feed ingredients. The side streams originated from different stages of the production chain, were used without further processing, and included sprat trimmings (heads, frames, viscera), marinated herring (fillets), and mackerel in tomato sauce (fillets and sauce). The three side streams contained moderate levels of protein (28-32% dry matter) and high levels of lipid (34-43%). The sprat trimmings included ca. 29% ash and 1.5% phosphorous, which may add value due to the high level of essential minerals but needs to be considered in feed formulations. Three diets were formulated to include 50% of each side stream replacing all fishmeal and ca. 80% of the fish oil of the control diet, which contained 35% fishmeal and 10% fish oil. The diets were evaluated in a 12-week feeding trial using rainbow trout (Oncorhynchus mykiss). Fish fed the sprat diet displayed the highest feed intake and growth and showed no negative effects on the intestinal health. The mackerel side stream displayed a good digestibility but resulted in lower growth rates compared to the sprat trimmings. Fish fed the herring diet displayed the lowest performance regarding growth, feed intake, and digestibility. They further exhibited a reduction in nutrient uptake in both proximal and distal intestines, likely contributing to the observed lower digestibility and growth, and a reduction in plasma ghrelin levels. As part of a circular approach to increase marine lipid and protein production for fish feed, the tested sprat and mackerel side streams are promising raw materials; however, additional studies using more commercial-like feed formulations are encouraged.