Biofouling of fish-cage netting: efficacy and problems of in situ cleaning

Abstract

The rapid biofouling of fish-cage netting in Tasmania, Australia, necessitates the frequent changing of nets for on-shore cleaning. To reduce the cost associated with this capital- and labour-intensive process, a prototype underwater net cleaner was designed and constructed for the Tasmanian Atlantic salmon industry. This study describes trials aimed to determine the efficacy of this prototype, and to identify areas for improvement. The reduction in open area of netting mesh due to fouling growth, together with subsequent increases in open area after cleaning, were monitored by image analysis of underwater photographs. The preferential removal of larger fouling species and the occurrence of resistant species were documented. The design of the cleaning head and displacement of netting away from the cleaner reduced the severity of scrubbing, limiting fouling removal. However, with increased contact between the brushes and netting the cleaner prevented significant fouling development over a 10-week period. Scanning electron microscopy was used to identify the extent of residual fouling after cleaning, and the difficulty of removing fouling from crevices in the netting surface and from the sides of netting bars. The system can be highly effective, but the current design for presentation of the brushes greatly limits cleaning efficacy. The research has identified areas for design improvement, the problems associated with residual fouling and regrowth and the requirements for effective in situ net cleaning.