Introduction
Climate change and geopolitical instability are contributing to growing uncertainty regarding the availability of raw materials for animal feed, particularly protein sources used in aquaculture. This underscores the importance of continued research into alternative protein ingredients. In recent years, considerable attention has focused on assessing the nutritional value of insect-based products for farmed fish, especially carnivorous species such as salmonids (1). However, with ongoing advancements in the industrial processing of insects into protein meals, further evaluation is needed to determine the nutritional quality of these next-generation ingredients. This study aimed to assess the impact of partially and fully replacing fishmeal-derived proteins with insect-derived proteins in diets formulated for rainbow trout (Oncorhynchus mykiss). The insect proteins were sourced from three by-products of black soldier fly (BSF) processing: a crude insect meal (CIM), a refined insect meal (RIM) with reduced chitin content, and a protein extract derived from the aqueous fraction separated during the wet processing of BSF larvae into protein meal and oil , and commonly referred to as stickwater (SW). Due to its high content of water-soluble compounds, stickwater is particularly rich in low molecular weight proteins, peptides (including bioactive peptides), and micronutrients, notably minerals.
Materials and methods
A digestibility trial and a 12-week growth experiment were conducted on juvenile rainbow trout to evaluate seven experimental diets. The control diet included 17% fishmeal (FM). Two diets involved complete replacement of fishmeal protein with either crude (CIM100) or refined (RIM100) insect meal. Four additional diets replaced 50% of fishmeal protein with CIM (CIM50), RIM (RIM50), or a blend of 70% CIM or RIM and 30% stickwater (CIM-SW, RIM-SW). Diets, faeces, and whole fish were analyzed for proximate composition to calculate apparent digestibility coefficients (ADCs) and assess nutrient retention. Transcriptomic analysis of the distal intestine was performed to evaluate molecular responses to dietary treatments. Data were analyzed using a two-step approach: a two-way ANOVA assessed the effects of insect meal type and substitution level among the six insect-based diets, followed by a one-way ANOVA comparing all diets, including the control.
Results
Partial or total replacement of fishmeal with crude (CIM) or refined (RIM) insect meal reduced the apparent digestibility coefficients (ADCs) for protein, lipid, and energy compared to the fishmeal (FM) control, with the lowest values observed in the CIM-100 diet. However, supplementing 50% replacement diets with stickwater (SW) restored digestibility to control levels. Growth performance was not significantly affected by diet. Absolute feed intake increased with SW inclusion, and, when normalized to metabolic body weight, was also higher in CIM-50 and RIM-100 diets. Feed efficiency declined with full fishmeal replacement by CIM compared to control FM diet. Insect-based diets increased digestible nitrogen, fat, and energy intake, especially with SW. Fecal nutrient losses were also higher, particularly under full replacement, but were significantly reduced by SW in the 50% substitution diets. Nitrogen, fat, and energy gains and retentions were not significantly different among diets. Transcriptomic analysis of the distal intestine revealed limited changes between 50% and 100% substitution levels (2–3 differentially expressed genes, DEGs, per insect meal type). Comparing RIM-based diets revealed 27 DEGs, versus only 2 in CIM-based diets. Relative to the FM control, RIM diets showed 27 DEGs, and CIM diets 7, many associated with immune and inflammatory responses. Ladderlectin a and b were upregulated with insect diets, while samd9, an antiviral gene, was downregulated in CIM-100-fed fish.
Discussion
Diets with partial or total replacement of fishmeal by insect proteins of varying quality supported growth performance comparable to that of the control diet containing 17% fishmeal. Chitin content did not appear to limit growth, but feed efficiency declined with insect-based diets. These insect-based diets led to higher nutrient intake and fecal losses, resulting in similar nutrient gains and retention. This potentially could reduced economic and environmental sustainability , particularly with crude insect meal, which showed the poorest efficiency when used in full replacement. Although stickwater improved digestibility in insect-based diets, its growth-promoting effect, as observed with stickwater issued from fishmeal production, in salmon (2), was less evident here. This may be attributed to the less balanced amino acid profile of insect meal compared to fishmeal . Overall, refined insect meal exerted a more pronounced effect on the intestinal transcriptome than crude insect meal . Notably, ladderlectin a and b , key components of fish innate immunity involved in defense against bacterial infections (3) , were upregulated in trout fed insect-based proteins. This response may reflect the ability of ladderlectins, which possess sugar-binding domains, to recognize and bind chitin, a long-chain polymer of N-acetylglucosamine present in insect meals. In contrast, samd9 , a gene linked to antiviral defense and potentially acting as a pattern-recognition receptor for microbial double-stranded DNA or RNA (4 ), was downregulated in trout fed the diet with full crude insect meal replacement. These immune-related transcriptomic shifts highlight the need for pathogen challenge trials to assess the health robustness of trout fed insect-based diets.
References