The impact of invasive tunicates on the demand for phytoplankton in longline mussel farms

Abstract Mussels ( Mytilus edulis ) cultivated on the eastern coast of Prince Edward Island (PEI), Canada, must compete for food resources with three invasive tunicates ( Botrylloides violaceus , Ciona intestinalis and Styela clava ). Clearance rates determined for each species were used to calculate clearance rate per unit lease area ( CR Area ) for various fouling scenarios. CR Area for mussels alone was estimated at 338 ± 20 l h − 1  m − 2 . Surprisingly, the size of the mussels under cultivation had no significant effect on estimates of CR Area , due to self-thinning processes that gradually eliminated 89% of the seed mussels over the 2-year production cycle. CR Area was also unaffected by the presence of the fouling colonial tunicate B. violaceus . In contrast, the presence of the solitary tunicate C. intestinalis or S. clava significantly increased CR Area by 30–47% compared to non-infested scenarios, even with control/treatment measures in effect. Size fractionation of the available phytoplankton resources indicated that picophytoplankton (0.2–3.0 μm) accounted for a major proportion of the primary production (61.5 ± 4.0%) and standing stock biomass (75.8 ± 3.6%). M. edulis was much less efficient at retaining this picophytoplankton component (18.1 ± 2.2%) than were the invasive tunicates (59.0 ± 2.6%). Carrying capacity calculations indicated a balanced state of demand versus supply for nanophytoplankton (> 3.0 μm) when mussel leases occupy 33% of the spatial area of a bay. The presence of solitary tunicates effectively lowered this bay-scale occupancy threshold to 23–26% (treated sleeves) or 10–11% (non-treated sleeves).