The development of biofouling, particularly the hydroid Ectopleura larynx, on commercial salmon cage nets in Mid-Norway

The Norwegian fish fanning industry mainly uses copper-based coatings on nets combined with washing to reduce the amount of biofouling. This study quantified the net aperture occlusion at 1, 5, 10 and 15 m depth of two cages, which were irregularly washed in situ, at a commercial salmon farm in Mid-Norway from August to December 2008. Before in situ washing, the net aperture occlusion was the highest at the upper depths of the water column. Afterwards, the fouling community dynamics and distribution changed; the net aperture occlusion varied significantly with time for both cages, and also with depth for one cage, which had significantly higher net aperture occlusions at lower depths. Laboratory experiments quantified the regrowth of the dominant fouling organism, the hydroid Ectopleura larynx, because some damaged hydroids may remain on the nets after in situ washing and then regrow. Two experiments, determining the effects of different numbers of polyps cut off (all, half and none of the polyps cut off) and different time intervals between cuttings (all polyps cut off on days 0, 2, 4 and 6, on days 0 and 6, and not at all) on the regrowth of E. larynx, were conducted. Both experiments demonstrated that E. larynx can regrow its polyps. Five days after cutting, the hydroids with all of their polyps cut off had more polyps (115 +/- 9 polyps) than the hydroids with half of their polyps cut off (99 +/- 10 polyps) and the control hydroids (88 +/- 8 polyps). Similarly, six days after the final cutting, the hydroids repeatedly cut had more than twice the amount of polyps (90 +/- 8 polyps for the hydroids cut on days 0, 2, 4 and 6; and 101 +/- 6 polyps for the hydroids cut on days 0 and 6) than the control hydroids (41 +/- 3 polyps). The results of these experiments suggest that in situ washing of nets is only a temporary measure to control biofouling as E. larynx regrows and occludes the net apertures rapidly. (c) 2010 Elsevier B.V. All rights reserved.