Characterising biofouling communities on mussel farms along an environmental gradient: a step towards improved risk management

Biofouling pests can have significant economic impacts on aquaculture operations, including increased processing and production costs. An important first step towards improved biofouling management is understanding the density and distribution of the biofouling species within a growing region. In this study, biofouling communities were sampled from 73 commercial mussel farms within New Zealand’s main mussel growing region, Pelorus Sound. At each farm, photoquadrats (0.08 m2, n = 6) of biofouling organisms were obtained at 2 depth ranges (3 per range) from suspended long-line droppers, both at the surface (0 to 3 m of the dropper) and bottom (9 to 24 m, depending on dropper length and water depth). Biomass samples and visual estimates of biofouling biomass were also obtained. Strong spatial variation in the structure of biofouling communities was evident, with increasing dissimilarity between communities along Pelorus Sound. Problematic taxa (e.g. the brown alga Undaria pinnatifida and calcareous tubeworm Pomatoceros sp.) were dominant near the entrance to the Sound, where annual temperature cycles are often reduced and salinity concentrations are higher. Generally, biofouling cover decreased with increasing water depth. A large proportion (48%) of biofouling biomass scores were categorised as high, equating to 121.2 ± 20 g m-2 (or 16%) of long-line for a heavily fouled farm, or 10 t for a typical 3 ha farm. Distributional patterns, such as those identified in this study, could be used by aquaculture industries to better inform the timing and placement of susceptible crop species and production stages (e.g. mussel spat). Refined monitoring methods may also facilitate industry participation in collecting long-term biofouling records