Larval release and attachment modes of the hydroid Ectopleura larynx on aquaculture nets in Norway

Abstract

[Extract] In the finfish industry, clean cage nets are essential for the health of the stock (Braithwaite & McEvoy 2005). The accumulation of biofouling, the unwanted attachment of marine organisms on submerged surfaces, decreases the water flow through nets. Subsequently, the water quality within sea cages is reduced, which may affect fish health negatively (Cronin, Cheshire, Clarke & Melville 1999; de Nys & Guenther 2009). Over the last decade, the hydroid Ectopleura larynx (syn. Tubularia larynx) has become one of the most common fouling organisms in the Norwegian fish farming industry, causing increasing problems for farmers (Guenther, Carl & Sunde 2009). The rapid growth of E. larynx on aquaculture nets requires fish farmers to clean their nets regularly, often on a fortnightly basis during the peak of the biofouling season between July and November (Guenther et al. 2009). To reduce biofouling, the majority of Norwegian salmon farmers use copper-based coatings on nets, combined with regular underwater high-pressure washing (200–300 bar) with rotating discs (Olafsen 2006). Anecdotal observations suggest that hydroids grow back faster once the first washing has taken place (B. Jensen, salmon farm manager, pers. comm.).