Effects of mussel fouling on oyster feeding performance and retention of essential polyunsaturated fatty acids

Mussel fouling is a major issue in oyster aquaculture along the northeastern coasts of Japan. We conducted a two-month field experiment rearing Pacific oysters with intentionally attached mussels at varying densities to examine how mussel fouling affects oysters' feeding functions and retention of essential fatty acids. As results, oyster soft-tissue dry weight declined with increasing mussel density, whereas the ratio of arachidonic acid to total fatty acids (ARA/TFA), a stress indicator, increased. Soft-tissue dry weight and condition index were inversely related to ARA/TFA, indicating that mussel fouling impairs oyster growth and condition. Elevated ARA/TFA was accompanied by higher ratios of docosahexaenoic acid to eicosapentaenoic acid (DHA/EPA) in oyster digestive glands, suggesting that stress at high mussel densities is associated with reduced dietary reliance on diatoms. In contrast, neither feeding rates nor Gill/Palp area ratios of oysters were related to mussel density, providing no evidence for morphological adjustment to mussel fouling. Mussels exhibited higher consumption rates of EPA and DHA but retained lower levels of these fatty acids in their tissues than oysters, implying that mussel fouling diverts EPA and DHA away from oysters and reduces their recovery for human nutrition, and hindering oyster growth. Furthermore, biodeposition rates per unit biomass were significantly higher for mussels than for oysters, suggesting that mussel fouling increases biodeposit loading on the seafloor and may exacerbate local environmental impacts. Overall, mussel fouling imposes multiple nutritional, production, and environmental costs on oyster aquaculture, and its control is therefore crucial for efficient and sustainable farm management.