Background
Marine biofouling is the undesirable accumulation of organisms on submerged surfaces, which poses a major problem for the aquaculture industry. To reduce biofouling and its associated negative impacts on farm constructions, operations and fish welfare, the Norwegian fish farming industry mainly uses red-brown copper-based coatings on nets combined with regular underwater washing. Over the last decade, the hydroid Ectopleura larynx has become one of the most common fouling species in the Norwegian fish farming industry (Figure 1), dominating the fouling community on aquaculture nets in South- and Mid-Norway between July and November. During the peak of the biofouling season, the fish farmers need to clean their nets every 2 weeks, which is a resource-demanding task. Therefore, the aim of this project is to understand the settlement preferences, growth and feeding biology of the hydroid E. larynx using both laboratory and field experiments, and develop strategies to reduce, control and remove hydroids on aquaculture nets in a more efficient and sustainable way.

Figure 1. The hydroid Ectopleura larynx on nets.

Methods
To investigate the settlement preferences of the hydroid E. larynx with regards to surface colour and microtopography, we conducted settlement assays with E. larynx larvae in the laboratory. We also investigated the attachment of E. larynx on the fish cage nets and identified different strategies, which maintained their attachment to the nets. Furthermore, when the nets are washed underwater, small pieces of hydroids may remain on the nets, and we conducted two growth experiments in the laboratory to determine the effects of cutting hydroids on their ability to re-grow.

Finally, during field experiments at a commercial salmon farm near Hitra, we investigated the effects of colour and copper on the settlement and growth of biofouling. We also took underwater pictures of the biofouling community on cage nets at different depths (1, 5, 10 and 15 m) and over time (weekly to fortnightly sampling from August to December 2008). These pictures are currently being processed with an image analysis program to calculate the net aperture occlusion.

Results and discussion
The results of our laboratory study demonstrated that colour did not influence the settlement of E. larynx. While there was a statistically significant difference in the settlement of E. larynx between white and black surfaces in the single choice settlement assays, this difference was not biologically, or consequently economically, significant given high levels of settlement. Furthermore, when given a choice between these two colours in a multiple choice settlement assay, E. larynx did not preferentially settle on either colour. The field experiments with white and red nets also showed that there was no significant difference in the wet weight of biofouling after 10 weeks of immersion. In contrast to colour, cuprous oxide in coatings on nets effectively reduced the settlement of E. larynx, with no E. larynx settling on these nets within 10 weeks of immersion. However, given that discharges of copper from antifouling agents used in Norway are to be substantially reduced before 2010 and alternative copper-free biocidal coatings are currently lacking, novel solutions for the effective and environmentally sustainable control of biofouling on aquaculture constructions need to be developed.

This study also showed for the first time that hydroids maintain their attachment to the nets by three strategies: winding the hydrophyton around the net, growing between loose filaments and threads, and incorporating filaments into their chitinous perisarc. Therefore, current net constructions with multi-filament nylon threads provide a structure for maintaining the attachment of both juvenile and adult E. larynx on the nets and hinder their removal.

Finally, the growth experiments in the laboratory demonstrated that E. larynx, whose polyps had been cut off, could re-grow to complete adults within 5 days. Even when E. larynx were cut repeatedly every 2 days for 6 days, they could re-grow. These results suggest that hydroids need to be removed completely during the underwater washing procedure to delay their re-growth on the nets.

Future research in 2009 will focus on the effects of (1) nano-structured surfaces on the settlement of E. larynx larvae, (2) fish feed and faeces on the feeding of E. larynx, and (3) the underwater washing procedure on the net structure, coating and biofouling development on cage nets.