Quantification of biofouling on nets: a comparison of wet weight measurements and optical (image analysis) methods

Biofouling has been shown to have a number of negative impacts on marine aquaculture. Studies aiming to characterize, inhibit or remove biofouling typically require the quantification of biofouling assemblages on cage nets. Several methods have been used for this whose results are not easily comparable. This study compares three widely used biofouling quantification methods: measurements of (1) wet weight and the (2) analysis of images taken of nets under water and (3) in air to determine percent net aperture occlusion. Comparisons were made for two common types of net biofouling, hydroids and ascidians. Both image analysis methods correlate linearly for both biofouling types, but percent net aperture occlusion calculated from images taken under water (PNOUW) was generally higher in comparison with images taken in air (PNOair). PNOUW was about 50 % larger than PNOair for hydroids, which collapsed when exposed to air and about 10 % larger for ascidians, which retained much of their volume when lifted from the water. Both PNO measures correlate linearly with wet weight for both biofouling types, but for hydroids this study found an upper limit for this correlation at PNOair of around 80 % and a wet weight of about 2 kg m−2. The upper limit for a linear correlation was given by complete net aperture occlusion from wet weight of about 1 kg m−2. Transfer functions were derived between PNOUW, PNOair and wet weight. Such transfer functions may allow the comparison of studies using different quantification methods, but similar fouling organisms.