Finfish aquaculture production has increased dramatically in the past decades and has done so by utilizing wild captured finfish (reduction fisheries) as a primary protein source in fish feeds. At a global level, reduction fisheries appear to be fully exploited and the aquaculture industry now utilizes the majority of this natural resource. The aquafeed industry has been working to identify and utilize an expanding range of alternatives to fish meal but much work is needed to better understand the nutritional value, including the digestibility, of these ingredients. Insect larval meal, whereby insects are cultured on agricultural or food waste products, has gained attention as a potential sustainable protein source in aquafeeds. In this study, we evaluated the digestibility of two insect meals: 1) defatted mealworm meal (DMM) produced from Tenebrio molitor and 2) whole mealworm meal (WMM) produced from Alphitobius diaperinus.
Three diets were produced to evaluate the ingredient-specific apparent digestibility coefficients (ADCN) of the test ingredients. A basal mix was produced using a standardized formulation used for salmonid digestibility studies. Yttrium oxide (Y2O3; 0.1% of the diet) was used as an inert marker. The basal mix was used to produce the reference diet and was also used in a 70:30 mixture (basal mix: test ingredient) to produce the test diets. This approach allows for the estimation of the digestibility of the test ingredients, after accounting for the known digestibility of the reference mix and can be compared across studies using similar methodology. These diets were evaluated in a five-week digestibility trial with 500-700 g Atlantic salmon (Salmo salar) reared in a recirculating aquaculture system supplied with brackish (~3 ppt) well water. Crude analysis, amino acid composition and yttrium concentrations were measured in the feeds and feces and used to calculate ADCNs. For protein, ADCNingredient values were 90.3 ± 2.0 and 90.4 ± 3.9, for DMM and WMM, respectively, and were not significantly different between treatments. These values were in the upper ranges when compared to those previously reported for fish meals (77-94%) fed to Atlantic salmon. Overall, DMM and WMM appear to be highly digestible feed ingredients and, from this perspective, should be suitable alternatives to fish meal in the diets of Atlantic salmon.