Wrasse and lumpfish were introduced to salmon cages primarily for sea lice control, but their appetite for fouling organisms is proving to be a valuable secondary benefit.
Ballan wrasse (Labrus bergylta) and goldsinny wrasse (Ctenolabrus rupestris) are the main wrasse species used in Norwegian and Scottish salmon farming. Lumpfish (Cyclopterus lumpus) have become increasingly popular because they tolerate colder temperatures and are easier to breed in captivity. All three species naturally graze on invertebrates attached to hard surfaces — exactly the organisms that foul cage nets.
Farm operators report that cages stocked with cleaner fish at recommended densities (roughly 4-5% of salmon numbers) show visibly less fouling on the net walls closest to where the cleaner fish shelter. Hydroids and small mussels are particularly targeted. The cleaning effect is most pronounced during summer months when both fouling growth and cleaner fish feeding activity peak.
It is not a perfect system. Cleaner fish do not graze evenly across the entire net — they tend to concentrate around structures where they feel secure, such as kelp hides and artificial shelters placed inside the cage. Deep portions of the net and the cage bottom receive less attention. Hard-shelled adult barnacles are generally too well protected for cleaner fish to remove. And during winter, feeding activity drops sharply, offering little fouling control during the months when some fouling organisms (particularly mussels) continue to grow slowly.
Mortality is a real problem. Cleaner fish death rates remain high on many farms, and sourcing enough wild-caught wrasse has raised sustainability concerns that are driving investment in captive breeding programmes. Lumpfish hatcheries in Norway, Iceland, and Scotland have scaled up rapidly since 2015.
So cleaner fish are not going to replace your net cleaner. What they do is slow fouling growth and stretch the gap between mechanical cleaning runs. That saves money and means fewer disturbance events for the salmon. Farms that pair cleaner fish with regular mechanical cleaning and decent net coatings get the best results overall. For a detailed comparison of the two main species, see wrasse vs lumpfish.