The replacement of fishmeal in trout feeds has been a perpetual challenge for the industry since the late 1990’s when the global supply of fishmeal fell from its peak, creating greater competition for use and increasing costs. As such, many alternative protein sources have been evaluated over the past 40 years. Leading the way has been soybean meal and soy concentrates, due to their favorable amino acid profile. Even so, dietary inclusion of soybean meal creates digestive challenges for trout and other salmonids. High concentrations of anti-nutritional factors and antigens result in enteritis of the distal intestine, and with it reduced nutrient utilization and fish growth and health. While soy is a staple in fish feeds today, higher prices coupled with soybean meal induced enteritis, has renewed the drive for novel protein ingredients. Furthermore, concerns over rainforest destruction and other sustainability challenges associated with soy have been at the forefront of more recent pushes for sustainable protein alternatives.
As a recent addition to the fish nutrition toolbox, insect meal provides an acceptable macronutrient and amino acid profile. Specifically, defatted, yellow mealworm (AKA Darkling Beetle; Tenebrio molitor) meal contains approximately 72% protein and 5% lipid and is only limiting in the amino acids methionine, threonine and lysine. Further, mealworm production is touted as sustainable, with the possibility of 365 harvests/year. Relative to soybean production, mealworm production capacity can achieve 5000x the acre yield of soy with half the power and 2% of the water requirements. As such, the objective of the current study was to evaluate defatted mealworm meal (DMW) fed to rainbow trout as a sustainable fishmeal alternative.
A digestibility and growth study were conducted. Apparent digestibility of DMW fed to trout was evaluated using standard practices, and the apparent digestibilities of dry matter, protein, lipid, energy, and phosphorus were 80, 87, 100, 84, and 90%, respectively. Growth was evaluated in juvenile trout fed over 13 weeks with five graded dietary treatments (0, 10, 20, 30, 40% as fed), substituting 1:1 for sardine fishmeal. No differences in fish performance or feed utilization were detected (P>0.05). Differences in body protein, lipid and energy composition were observed in fish fed the highest DMW treatment; however, no differences were observed for fillet composition. Survival, intestinal histology, and blood chemistry were similar among treatments. In total, DMW is highly digestible when fed to rainbow trout and can replace all the fishmeal in the diet, up to 40% as fed, while supporting fish performance, feed efficiency, fillet composition, and health. These results support the use of DMW as a novel and safe ingredient in rainbow trout feeds.