Fruit Processing Residues as Alternative Feed Ingredients in Aquaculture: A Critical Narrative Review of Nutritional Value, Functional Effects and Circular-economy Constraints

Abstract

The rapid expansion of aquaculture has intensified demand for feed ingredients and renewed interest in low-cost, regionally available materials that reduce reliance on fishmeal and conventional plant proteins. Fruit processing generates large quantities of peels, pomace, seeds, kernels and reject whole fruit, and these residues carry fermentable carbohydrates, dietary fibre, pigments and polyphenolic compounds that have prompted their evaluation as aquafeed components. This review critically synthesises the evidence on fruit-derived residues in finfish and crustacean nutrition, organised around composition, antinutritional constraints, growth and feed-utilisation responses, functional effects on immunity and antioxidant status, pigmentation and product quality, and bioprocessing strategies that modify residue value. The available literature is dominated by short-term feeding trials in a small number of hardy freshwater species, most prominently Nile tilapia, common carp and rainbow trout, with grape pomace, citrus peel, apple pomace, pomegranate residues, banana peel, papaya, watermelon rind, mango kernel and olive cake among the residues examined. Across this evidence, a recurring dose-dependent pattern emerges in which modest dietary inclusion is often tolerated or associated with improved immune and antioxidant indices, whereas higher inclusion tends to depress growth and feed efficiency, an effect commonly attributed to fibre, tannins and other antinutritional factors. Processing interventions, particularly fermentation, are reported to lower antinutrient load and improve nutritional accessibility, but the supporting studies are heterogeneous and rarely standardised. Confidence in generalisable conclusions is limited by inconsistent characterisation of residues, narrow species coverage, an absence of long-term and grow-out data, and few economic or life-cycle assessments. The strongest defensible conclusion is that fruit residues are better positioned as functional or partial ingredients than as primary protein or energy sources. Priorities include standardised residue characterisation, dose-response and long-term trials, mechanistic work on polyphenol-microbiome interactions, and integrated sustainability assessment.