This family-run mussel longline operation in Bantry Bay, southwest Ireland, produces approximately 300 tonnes of blue mussels annually from a 15-hectare licensed growing area. Since 2015, the farm had experienced increasing problems with invasive colonial sea squirts (Didemnum vexillum and Ciona intestinalis) overgrowing mussel ropes, smothering product, and adding up to 40 percent non-marketable weight at harvest. Manual removal was labour-intensive and ineffective, as tunicate fragments rapidly regenerated.
Beginning in spring 2019, the farm systematically tested air-drying protocols, lifting mussel ropes clear of the water for controlled periods ranging from 6 to 48 hours at intervals of 1 to 4 weeks during the May-to-October fouling season. The optimal protocol — 24-hour air exposure every 3 weeks — reduced tunicate coverage by 70 percent compared to untreated control ropes while causing less than 2 percent mussel mortality. Shorter exposures (6-12 hours) were insufficient to kill established tunicate colonies, while longer intervals between treatments (4+ weeks) allowed regrowth between cycles. Mussels from treated ropes reached market size approximately 6 weeks faster due to reduced competition for food and space.
The fouling organisms database provides identification guides for the tunicate species encountered at this site. Air-drying and other physical treatment methods are compared in the solutions comparison tool, and the biofouling cost calculator can help estimate the return on investment for implementing drying protocols. This site is plotted on the aquaculture farm map with local fouling pressure data.
Outcomes
Trialled air-drying regimes at different exposure intervals. Optimal protocol: 24-hour air exposure every 3 weeks during summer. Reduced sea squirt fouling by 70% with minimal impact on mussel growth.