The Janus Face of Aspergillus in Fish Aquaculture: From Pathogenic Threat to Functional Feed Additive

The dramatic rise in global aquaculture has brought renewed focus on filamentous fungi impacting fish health and aquafeeds. The fungal genus Aspergillus has a Janus-like role in aquaculture. On the pathogenic side, opportunistic pathogens like A. flavus, A. niger and A. fumigatus cause outbreaks of aspergillosis, with granulomatous lesions and significant mortality, especially in fish with impaired immunity and/or poor environmental conditions. Toxigenic strains of A. flavus and A. parasiticus also contaminate aquafeeds with aflatoxins, some of the most toxic natural hepatotoxins and carcinogens, which have been shown to affect growth, immunity and aflatoxin residues in edible fish fillets. On the positive side, non-toxigenic strains of A. niger, A. oryzae and A. awamori are increasingly used as fungal probiotics, solid-state fermenters to upgrade plant protein feed supplements, and as sources of industrially useful hydrolytic enzymes, secondary metabolites and antifungal compounds. Synthesizing evidence from over 90 peer-reviewed studies, this narrative review elucidates how species, strain, and rearing context jointly determine whether Aspergillus behaves as a pathogen or as a functional additive. We highlight knowledge gaps, offer an interpretative model and suggest practical strategies to limit risks and leverage the biotechnological uses of Aspergillus for sustainable aquaculture. The novelty of this review lies in integrating, within a single finfish-focused framework, the three usually separate faces of Aspergillus-pathogen, aflatoxin producer, and functional feed additive-and in translating this dual nature into practical, strain-level guidance for aquaculture.