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

• Clearance rates of mussels and fouling tunicates were determined.
• Estimates were scaled up to represent tunicate-infested mussel leases.
• Solitary but not colonial tunicates increased lease-scale clearance rates.
• Solitary tunicates effectively reduce the carrying capacity for mussel culture.

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 (CRArea) for various fouling scenarios. CRArea 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 CRArea, due to self-thinning processes that gradually eliminated 89% of the seed mussels over the 2-year production cycle. CRArea 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 CRArea 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).