Test of an antifouling treatment on tuna fish-cages in Boston Bay, Port Lincoln, South Australia

Abstract

A test of the antifouling properties of Wattyl NetClear® was conducted on fish farms containing southern bluefin tuna (Thunnus maccoyii) during a 6-month period (February-July 2002). Wattyl NetClear® is a water-based synthetic latex-based coating where the active antifouling agent is a mixture of two isothiazolinones likely to affect both biochemical conditioning and bacterial colonisation. The development of fouling was monitored using underwater stereo-photogrammetry on 3 treated and 3 untreated net cages with the main factors treatment, depth and cage nested within treatment. A significant treatment and depth effect was found but variable through time and in some instances with significant cage effects. By the end of the study, the difference in fouling load between treated and untreated nets was 14.7%. In total 72% of all free-space data points were on treated nets. The dominating fouling organisms were Enteromorpha sp. and sponges with low settlement of blue mussel and paper oysters. Passive deposition of tuna faeces contributed significantly to the cover. The largest cover of fouling was observed in April-May with a dominance of sponges in June-July. Enteromorpha sp. dominated shallow depths while sponges dominated at deeper levels. Tuna faeces were distributed independent of depth but varied with time. The results showed that fouling of fish cages consisted of both active settlement and passive deposition, the latter independent on antifouling treatment.

Keywords:

• Sea-cage
• fouling community
• tuna
• aquaculture
• antifoulant
• stereo-photography

Acknowledgements

This study was supported by the Tuna Boat Owners Association of Australia (TBOAA), Fisheries Research and Development Council (FRDC) and the Aquafin CRC. We are grateful to Professor G. Quinn and Dr J. Tanner for providing statistical advice and valuable comments, and to the SARDI crew of RV Breakwater Bay for assistance. An anonymous reviewer improved the manuscript.