Polysiphonia species are finely branched red algae that form dark reddish-brown tufts 10–50 mm tall on net panels, ropes, and cage frames. The thallus consists of a central axial filament surrounded by pericentral cells — a structure visible under low magnification. These algae reproduce rapidly through both spores and vegetative fragmentation, which means cleaning operations that tear rather than fully remove colonies can actually spread them. Polysiphonia is widespread from Scandinavia to the Mediterranean, with heaviest growth in nutrient-enriched waters near farm discharges.
Like other filamentous algae, Polysiphonia tufts trap fine sediment and organic particles, building up a mat that barnacle cyprids and mussel spat settle into preferentially. On net panels, this silt-trapping effect can reduce mesh openness by 20–40 % even before heavier foulers arrive. Polysiphonia also competes with cultured seaweed species for light and nutrients, making it a dual problem on multi-trophic aquaculture sites.
Net cleaning machines remove the tufts effectively, though fragments left behind regenerate within days. Air-drying for 12–24 hours kills the tissue, and the dried residue washes off on re-immersion. Timing net changes to coincide with the spring growth pulse reduces the algal load heading into peak fouling season. For details on controlling early-stage algal fouling — and preventing the secondary hard foulers it enables — see the methods comparison. Related algal species are listed in the organisms database.
Control Methods
Net cleaning machines Regular net changes Air-drying