Ulva species produce broad, bright-green, sheet-like thalli that can reach 300–400 mm across. The tissue is just two cell layers thick, making it translucent and papery when dry. Ulva is an opportunistic genus — it flourishes wherever dissolved nitrogen and phosphorus are elevated, which makes aquaculture sites ideal habitat. Nutrient run-off from feed waste and fish excretion fuels explosive summer growth. The genus is found in all European marine waters, from the Baltic to the Mediterranean, and tolerates salinities as low as 5 ppt.
During peak growth, Ulva can cover 100 % of a net panel in 2–3 weeks, forming a green curtain that blocks water exchange almost entirely. Oxygen levels inside heavily fouled cages can drop below safe thresholds within hours during warm, calm weather — a situation that triggers emergency cleaning. On shellfish lines, Ulva sheets drape over mussels and oysters, reducing their access to planktonic food and slowing growth. The economic toll of repeated algal fouling adds up fast; the cost calculator can model cleaning frequency against production losses.
Mechanical net cleaning removes Ulva easily, but fragments regenerate rapidly — sometimes visibly within 48 hours. Regular net changes on a 2–3 week cycle during the summer algal season are more effective than reactive cleaning alone. Reducing nutrient loading through better feed management also lowers Ulva growth pressure, though this is harder to control at open-water sites. For a broader view of algal foulers and integrated control strategies, see the methods comparison and the organisms database.
Control Methods
Net cleaning machines Regular net changes Manual removal