Chronic Bottom Fishing, the Invasive Ascidian Didemnum Vexillum, and Demersal Fish Feeding: The "Pros" and "Cons" of Habitat Alteration on Northern Georges Bank

On northern Georges Bank in the Northwest Atlantic, the chronic use of mobile bottom fishing gear and the invasive presence of Didemnum vexillum have altered the compositions of benthic macroinvertebrate communities.  However, whether these factors negatively affect the feeding habits of commercially-important fisheries resources is poorly known.  Here, we quantified the differences in the benthic macroinvertebrate assemblages both in the environment and in demersal fish stomach contents.  Study sites with contrasting levels of bottom fishing disturbance and D. vexillum occurrence were selected within the Habitat Area of Particular Concern (HAPC) region of Closed Area II.  To address impacts on fish feeding habits, we examined the diets of winter skate (Leucoraja ocellata), little skate (Leucoraja erinacea), haddock (Melanogrammus aeglefinus), winter flounder (Pseudopleuronectes americanus), and longhorn sculpin (Myoxocephalus octodecemspinosus).  Baseline benthic macroinvertebrate abundance (n/L), biomass (g/L), and species richness were generally higher in the non-fished and D. vexillum present areas, whereas an evenness index was greatest in areas disturbed by bottom fishing and devoid of D. vexillum.  Interestingly, significant differences in fish feeding habits were detected between contrasts, and greater association between fish diet and baseline benthos was shown within a sampling site as opposed to across sites using rank correlation methods.  Further results suggest D. vexillum has a stronger influence on benthic macroinvertebrate community composition and fish feeding habits in comparison to chronic bottom fishing disturbance for this region.  For a few benthic macroinvertebrate species, the presence of D. vexillum was shown to be favorable, and this artifact was also visible in demersal fish diets.