Abstract

The replacement of fishmeal with plant-based proteins in aquafeeds is key for sustainable aquaculture but may compromise filet pigmentation in rainbow trout (Oncorhynchus mykiss), a quality trait dependent on astaxanthin (Ax) deposition and metabolism. This study aimed to assess the effects of graded fishmeal replacement with a plant protein blend on Ax retention, pigmentation, lipid metabolism, and gut microbiota composition. Rainbow trout were fed three isoenergetic diets containing 60%, 36%, or 12% fishmeal, each supplemented with equal amounts of natural Ax from Haematococcus pluvialis, for 12 weeks. Ax retention, pigmentation, plasma metabolites, lipid digestibility, and distal intestinal microbiota were evaluated. The high plant protein diet (12% fishmeal) significantly reduced Ax concentrations in filet and plasma and decreased dorsal and belly pigmentation scores (p < 0.05). It also lowered plasma cholesterol and bile acid levels by 18–30%, reduced di-esterified Ax digestibility by 15%, and lipid absorption efficiency by 12%. The gut microbiota shifted significantly, with a marked reduction in Bacillaceae, positively correlated with Ax retention and pigmentation. Fish fed High Plant Meal diets exhibited impaired performance parameters, along with reduced lipid accumulation in the liver. High plant meal compromises Ax bioavailability by altering cholesterol–bile acid metabolism and gut microbiota, impairing Ax absorption. However, moderate inclusion of plant meal preserved pigmentation, underscoring the need for dietary cholesterol management and microbiota modulation in plant-based aquafeeds. © 2025 by the authors.