Incorporating mushroom production by-product in mealworm (Tenebrio molitor) rearing: Implications for rainbow trout (Oncorhynchus mykiss) growth performance and nutrient utilization

The yellow mealworm ( Tenebrio molitor ) has gained attention as a sustainable alternative in aquafeed formulation due to its favourable nutritional composition and reduced environmental footprint. Nevertheless, high production cost remains a key constraint. This study investigated the use of mushroom spent substrate (SMS), an agro-industrial byproduct, as a partial replacement for conventional mealworm larvae rearing substrates, with the goal to increase the sustainability and reduce the production cost of mealworm meal (MM), when used as fishmeal and vegetable oil replacer in aquafeeds for juvenile rainbow trout ( Oncorhynchus mykiss ). Five experimental diets were formulated for juvenile rainbow trout to be iso-proteic (45 %) and iso-lipidic (22 %). A control diet (CTRL) was designed to reflect a standard commercial formulation. Four additional diets, formulated based on the CTRL diet, included 12.5 or 25 % of mealworm meal (replacing 50 and 100 % of the fishmeal, respectively) produced from larvae reared either on a commercial substrate (CC-MM) or partially reared on SMS (SMS-MM) (CC12.5, CC25, SMS12.5, and SMS25). An eight-week growth trial and a digestibility trial were conducted to evaluate growth performance, apparent digestibility coefficients (ADCs), nutrient retention, digestive enzyme activities and economic indicators. Mealworm meal partially produced on SMS (SMS-MM) exhibit a lower lipid content compared to the mealworm meal reared conventionally (CC-MM). No significant differences on growth performance were observed across diets; however, the 25 % inclusion of both mealworm meals resulted in reduced voluntary feed intake. Rainbow trout juveniles fed SMS-MM, especially at a 25 % dietary inclusion, exhibited higher apparent digestibility coefficients for dry matter, protein, ash, phosphorus, and energy. Additionally, fish fed the SMS25 diet showed higher trypsin activity than those fed the CTRL diet, while fish fed the CC25 showed improved feed efficiency, reduced hepatosomatic index, and higher digestible energy retention relative to the CTRL diet. Despite these benefits, the SMS25 and CC25 diets showed higher economic conversion ratios. Overall, these findings show that MM derived from larvae partially reared on SMS is nutritionally comparable to conventional MM and fishmeal in rainbow trout diets, with no adverse effects on zootechnical performance. The use of SMS as a rearing substrate represents a valuable approach to valorise an agro-industrial by-product. Thereby contributing to the circular economy and supporting a more resource-efficient and sustainable aquafeed production. • Mealworm larvae partially reared on spent mushroom substrate (SMS-MM) were leaner than larvae reared on cereals (CC-MM). • Rainbow trout juveniles fed SMS-MM based feeds showed improved nutrient and energy digestibility. • Fish fed the CC25 diet showed improved feed efficiency, reduced hepatosomatic index and higher digestible energy retention. • Fully replacing fishmeal with either MM type led to increased economic conversion ratio.