Obelia dichotoma is a colonial hydroid that produces delicate, branching stems 10–30 mm tall, each bearing cup-shaped hydranths along its length. The colonies appear as a feathery white or pale brown fuzz on net surfaces, ropes, and floats. Obelia is widely distributed across the North Atlantic and into the Bay of Biscay, tolerating temperatures from 5 °C to 22 °C. It is one of the fastest macrofouling colonisers — appearing on clean nets within 10–14 days of deployment, often alongside or shortly after Ectopleura.
As an early-succession fouler, Obelia matters less for its own biomass (which is light) than for its role in enabling secondary fouling. The hydroid’s branching structure traps fine particles and increases surface complexity, creating micro-habitats that mussel spat, barnacle cyprids, and amphipod larvae preferentially settle into. On Norwegian salmon farms, nets with heavy Obelia coverage accumulate 2–3 times more secondary fouling over the following month than cleaner nets nearby.
In-situ net cleaning removes Obelia colonies easily, but regrowth takes only 1–2 weeks during summer, so the practical benefit is short-lived. Anti-fouling coatings — particularly copper-based paints — suppress hydroid settlement for the first several months of net deployment. Regular net changes aligned with the early fouling season (May–June in Scandinavia) can interrupt the succession before heavy foulers establish. For guidance on managing the full fouling sequence, see the methods comparison or browse related hydroid and cnidarian species in the organisms database.
Control Methods
In-situ net cleaning Anti-fouling coatings Regular net changes