Botryllus schlosseri (Subphylum Tunicata: Class Ascidiacea) is a nonindigenous ascidian species of global and national interest, which has extensive populations along the south coast of insular Newfoundland. Economically, this species has been of concern to industry, management, and policymakers because non-indigenous ascidian species have been a severe and costly nuisance for bivalve aquaculture. Ecologically, the presence of this temperate-adapted species in Newfoundland represents an expansion of its global range into subarctic waters. Thus, I aimed to describe the population dynamics of B. schlosseri in Arnold's Cove, Placentia Bay, Newfoundland and Labrador, Canada, by determining the temporal and spatial patterns of recruitment and the seasonal cycle of colony abundance. In addition, I aimed to compile a checklist of extant indigenous and non-indigenous ascidian species of eastern Canada with an emphasis on species from Newfoundland and Labrador. -- Artificial plates were used to determine recruitment rates among three sites, depths (1.0, 2.5, and 4.0 m from the water surface), and substrate types (aluminum, PVC, and wood in 2010; only PVC in 2011), in Arnold's Cove. Concurrently, density and cover of colonies were determined from the analysis of high-definition video surveys of a belt transect of wharf pilings. Seasonal biomass production was estimated from carbon to nitrogen (C:N) ratios and dry weight per unit area of dissected tissue subsamples. -- The seasonal window for recruitment was from early August to mid-October. Recruitment rates were greater near the water surface than at other depths, and on PVC in comparison to aluminum and wood substrates. Maximum recruitment rates on PVC at 1.0 m were 29.3 and 43.5 m⁻²d⁻¹ in September of 2010 and 2011, respectively, coincident with maximum seasonal seawater temperatures of 16-17°C. Colonies were present year-round on pilings. In the upper subtidal zone, monthly mean cover ranged from an annual minimum of 0.6% in May to a maximum of 2.8% in October. Colony size and biomass, though not C:N ratios, had a significant seasonal signal. -- These findings suggest that recruitment was predominantly constrained by seawater temperature within the short productive season, and that the population was sustained from one year to the next because of high cover of overwintering colonies. The efficacy of utilising PVC to track recruitment of Botryllus schlosseri, and perhaps other closely related ascidian species is supported by my data. Future management of B. schlosseri should target mitigation efforts before the annual onset of sexual reproduction and recruitment in July and within the upper 3-4 m of the water column
