Fish trophic engineering: Ecological effects of the invasive ascidian Didemnum vexillum (Georges Bank, northwestern Atlantic)

In the northwest Atlantic, concerns for the benthic communities of Georges Bank have evolved following the widespread detection of the invasive ascidian Didemnum vexillum in 2002. One question is whether D. vexillum affects the feeding of fishes, particularly commercially-important species. The major objectives were to examine the diets of five demersal fishes and the benthic epifauna (prey field) across contrasting levels of D. vexillum occurrence from 2004 to 2008 in and around northern Closed Area II of Georges Bank (42.0°N, 67.3°W). The fishes examined were winter skate (Leucoraja ocellata), little skate (Leucoraja erinacea), haddock (Melanogrammus aeglefinus), winter flounder (Pseudopleuronectes americanus), and longhorn sculpin (Myoxocephalus octodecemspinosus). Cumulative fish trophic diversity was often higher at sites where D. vexillum was present as measured by Shannon's H′. Diets were significantly different across levels of D. vexillum for the five fishes, and feeding by haddock was positively correlated with the benthic epifauna within rather than across sites, indicating site-specific feeding. For many fishes, prey that contributed to the diet dissimilarity between sites were benthic epifauna strongly associated with the presence of D. vexillum (Class Polychaeta: Nereis zonata, Harmothoe extenuata, and Lepidonotus squamatus; and Order Decapoda: Cancer irroratus) and absence of D. vexillum (Order Decapoda: Crangon septemspinosa). These feeding alterations are not necessarily negative, as fish diets in this region regularly contain prey positively associated with D. vexillum. However, with the momentum to incorporate habitat science into fish stock assessments, managers should not consider protected fish habitat to be static in the presence of habitat modifiers such as invasive ascidians. From influencing the trophic ecology of demersal fishes to driving substrate homogeneity, long-term monitoring and invasive ascidian management for this continental shelf region is recommended.