Greenshell TM mussel (Perna canaliculus) culture is the primary aquaculture industry in New Zealand. However, our knowledge of biofouling on Greenshell TM mussel farms, and its contribution to farm ecotrophic effects, is poor. We conducted a preliminary study of biofouling accumulation at two Greenshell TM mussel farms during Intermediate and Final seed on-growing stages (each of 6 months duration) with sampling of mussel ropes at 0, 3, 5 and 6 months during each on-growing stage. A diverse range of biofouling organisms (71 distinct taxa) accumulated on mussel ropes, with biofouling biomass dominated by suspension-feeding organisms (*88% of biofouling biomass) such as other bivalves, ascidians and, to a lesser degree, bryozoans. Biofouling biomass increased with culture time, varied between farms and was generally greater at 2 m than at 8 m depth. After 6 months, biofouling organisms on average comprised 54% of the total rope biomass. The reseeding of ropes between Intermediate and Final seed crops reduced the amount of non-Greenshell TM mussel biofouling. However, after 6 months, non-Greenshell TM mussel biofouling on average still comprised 15% of the total rope biomass. To evaluate potential ecotrophic effects of biofouling on Greenshell TM mussel farms, we compare the clearance rates of Greenshell TM mussel longlines based on Greenshell TM mussels alone and when combined with the two dominant biofouling species observed in our study (the mussel Mytilus galloprovincialis and the ascidian Ciona intestinalis). Our study shows that accumulated biofouling biomass on Greenshell TM mussel ropes can be significant and recommends further investigation as to actual ecotrophic effects of biofouling to ensure sustainable mussel farm practices.