The effects of the invasive ascidian (Didemnum vexillum) on northern georges bank: From benthic macrofauna to demersal fish diet

Since the presence of the invasive tunicate Didemnum vexillum was documented on northern Georges Bank in the  Atlantic in 2002, concern has grown over its impact on marine benthic communities.  In particular, it may be that D. vexillum indirectly affects commercially-important fisheries resources.  To address these concerns, we quantified the differences in the benthic macroinvertebrate and demersal fish communities between sites with contrasting levels of D. vexillum occurrence for the Habitat Area of Particular Concern (HAPC) of Closed Area II on Georges Bank.  To address whether second-order effects were occurring, we examined fish diets across year and D. vexillum factors for a well-known benthivore: winter flounder (Pseudopleuronectes americanus).  Baseline benthic macrofaunal indices: species richness, species abundance (n/L), and species biomass (g/L) were generally greater at sites with D. vexillum present; however, an evenness index was higher where the ascidian was absent.  Interestingly, significant differences in fish feeding habits were detected and attributed to a D. vexillum effect.  For winter flounder, the consumption of D. vexillum did occur, although diet dissimilarity between sites was largely explained by other prey (e.g. Cancer spp. and Polychaeta) that were highly responsive to the presence of D. vexillum as seen in the benthic macroinvertebrate community.