Valorization of Mushroom By-Products From Agaricus bisporus and Pleurotus ostreatus as Novel Feed Ingredients for Rainbow Trout (Oncorhynchus mykiss) Diets.

A 60-day feeding trial evaluated meals derived from discarded mushroom biomass, including stems and nonmarketable mushrooms, generated during commercial cultivation and processing of Agaricus bisporus (AB) and Pleurotus ostreatus (PO) as lower-impact alternatives to soybean meal (SBM) in rainbow trout (Oncorhynchus mykiss, BWi: 22.1 ± 0.3 g) diets. Five isoproteic (47%) and isolipidic (23%) diets were tested: a control diet and four diets replacing 25%-50% of SBM with AB or PO meals (AB25, AB50, PO25, PO50). Fish fed the AB25 diet showed growth performance, feed intake (FI), and nutrient utilization comparable to the control (p > 0.05), whereas AB50 reduced growth (p < 0.05) without affecting FI or nutrient utilization (p > 0.05). In contrast, PO inclusion caused a progressive decline in these parameters, more pronounced at PO50, accompanied by increased hepatosomatic index (HSI) and reduced perivisceral fat index (PFI) (p < 0.05). Apparent protein digestibility decreased in the AB50 diet (p < 0.05). In PO-fed fish, digestive enzyme activities reflected adaptive responses to reduced FI, including lower pepsin (PEP) and higher total alkaline proteases (TAPs), trypsin (TRY), lipase (LIP), aminopeptidase-N (APN), and maltase (MAL) activities. Hematological and plasma markers remained within normal ranges, and liver histological organization was similar among treatments (p > 0.05). Hepatic glutathione reductase (GR) activity increased in PO diets (p < 0.05), whereas AB diets remained comparable to the control group (p > 0.05). Overall, mushroom by-product meals were physiologically safe for O. mykiss, while AB meal emerged as a promising circular alternative to SBM, supporting up to 25% replacement without compromising growth performance, feed efficiency, or nutrient utilization.