The growing demand for sustainable aquaculture requires the exploration of alternative protein sources for fish diets. However, some of these alternatives can have adverse health effects, prompting research into functional feed ingredients to mitigate these issues. This study investigated pistachio shell powder (PSP), a readily available by-product rich in antioxidants, as a potential functional feed ingredient in rainbow trout diets. The effects of PSP inclusion (0%, 0.5%, 1%, and 2%) on the growth performance, intestinal health, and gut microbiota of rainbow trout were evaluated through administration in either a traditional fishmeal (FM) or an ultra-modern plant-meal (PM) based diet.
In a 12-week feeding trial, rainbow trout fingerlings were assigned to experimental diets with varying PSP levels. Growth performance, intestinal morphology, gene expression, total antioxidant capacity (TAC), total phenolic compound (TPC), and gut microbiota composition were assessed.
PSP significantly (P<0.05) improved average weight gain and daily growth index at the 1% inclusion level in FM treatments but did not affect growth in PM treatments. No significant (P>0.05) effects were observed on other growth parameters, intestinal morphology, oxidative stress, or inflammatory gene expression, although a trend toward down-regulation was noted in PM treatments at 2% PSP. Serum antioxidant capacity and phenolic content were unaffected by PSP inclusion, but TPC increased with higher PSP levels in PM treatments. PSP inclusion did not significantly (P>0.05) impact gut microbiota alpha diversity but significantly (P<0.05) altered beta diversity in FM treatments at the 0.5% inclusion level. Although no significant differences were observed in the alpha and beta diversity of gut microbiota of PM treatments, subtle shifts in community composition were observed. Differential abundance analysis revealed taxa-specific responses to PSP, particularly the genus Candidatus arthromitus increasing in relative abundance with PSP inclusion in both plant meal and fishmeal-based diets.
Overall, PSP inclusion up to 2% did not negatively impact growth, intestinal health, or antioxidant status. The lack of pronounced effects on gut microbiota and physiological parameters may be attributed to the limited reliance of carnivorous fish on fiber fermentation and complex microbial interactions for energy utilization.