The effects of colour and copper on the settlement of the hydroid Ectopleura larynx on aquaculture nets in Norway

Abstract

In an attempt to find more effective and applied solutions to reduce the settlement of the hydroid Ectopleura larynx on aquaculture nets in Norway, this study determined the settlement preferences of E. larynx with regards to colour and the presence of copper in antifouling coatings. Settlement assays showed that actinulae settled significantly less on white (67.0 ± 5.4%) than on black surfaces (87.0 ± 2.6%), whereas there were no significant differences between the remaining colours yellow, red and blue. However, when given a choice between (1) white and black, (2) red and black, and (3) red and white surfaces, actinulae did not preferentially settle on any colour. A field experiment at a salmon farm in Mid-Norway tested the effects of colour (red and white) and the presence of copper (with and without cuprous oxide) in the coating of net panels on the accumulation of fouling. After 10 weeks of immersion, the only macro-fouling species on the nets was E. larynx. The presence of cuprous oxide significantly reduced the wet weight of fouling, while colour did not have a significant effect. The mean wet weights of fouling on net panels with cuprous oxide coatings (347 ± 25 g m − 2 and 330 ± 19 g m − 2 for red and white nets, respectively) were approximately half of the mean wet weights of fouling on the net panels without cuprous oxide (695 ± 43 g m − 2 and 695 ± 45 g m − 2 for red and white nets, respectively). There were no macro-fouling species present on nets with cuprous oxide coatings, and the wet weights of these nets were only due to absorbed water and sediment.