The Menace of Microplastics and Their Physiological Implications in Aquaculture

The rapid expansion of aquaculture has increased its reliance on plastic-based infrastructure and materials, creating opportunities for microplastic (MP) contamination from culture systems, surrounding waters, and formulated feeds. This review synthesises current evidence on MP sources, exposure pathways, biological fates, and toxicological effects in farmed fish, with emphasis on their interconnected implications for fish health and aquaculture performance. MPs enter aquaculture systems through degraded equipment, contaminated water, atmospheric deposition, and feed ingredients, including fishmeal derived from MP-exposed fish. Ingestion represents the principal exposure route, with particle size and shape, polymer composition and concentration, exposure duration, life stage, and species influencing retention, translocation, and biological responses. Although gastrointestinal accumulation is common, smaller particles may translocate to internal organs, while evidence for trophic transfer does not yet establish consistent biomagnification. Across studies, MP exposure is associated with intestinal and gill damage, altered digestive enzyme activity, impaired feeding and feed utilisation, metabolic disturbances, haematological and histopathological alterations, and reduced growth. Oxidative stress emerges as a central, but not exclusive, mechanistic pathway, linking reactive oxygen species imbalance and altered antioxidant defences with inflammation, immunotoxicity, neurotoxicity, behavioural alterations, and endocrine and reproductive disturbances. These responses are highly heterogeneous, reflecting differences in species, polymer type, particle characteristics, exposure concentration, duration, life stage, environmental conditions, and experimental design. Interactions between MPs and co-occurring contaminants may further modify their biological effects. Overall, the evidence indicates that MP exposure represents a multidimensional stressor capable of connecting environmental contamination with physiological dysfunction and impaired aquaculture performance. Understanding these interconnected responses is essential for strengthening MP risk assessment and supporting effective feed and farm management, fish health protection, food safety, and sustainable aquaculture.