This study was undertaken to quantify the ecological interactions between blue mussels (Mytilus edulis Linnaeus 1758) and vase tunicates (Ciona intestinalis Linnaeus 1767) in the context of mussel farming. To quantify the extent of competition for food resources at varying temperatures, clearance rates for both species were calculated using Tetraselmis striata (Butcher 1959). Between 4-13°C, mussel clearance rates were at least three times higher than those of tunicates. At 16°C and 19°C, the mussel clearance rates fell to the same level as the tunicates. Clearance rates were also examined using different sized algal species, and a substantial overlap in mussel and tunicate particle size utilization was observed. To determine the effects of tunicate density on mussel productivity, size and condition indices were measured in the field under varying tunicate density. Mussel size and condition decreased with increasing tunicate densities. In addition, up to 50% mussel mortality was observed under heavy tunicate fouling (~2 kg • m -1). Mussels and tunicates have the potential for substantial food resource competition, and tunicates were found to have a negative effect on mussel productivity. Heavy tunicate fouling was associated with higher mussel mortality, lower overall size and condition. The cost effectiveness of removing the vase tunicate from mussel lines is also discussed.