Impacts of otter-trawling on colonial epifaunal assemblages on a cobble bottom ecosystem on Western Bank (northwest Atlantic)

Colonial epifauna are an important component of benthic communities, providingstructural complexity at scales of millimetres to metres. Many are sessile, emergent and fragile —characteristics which render them vulnerable to disturbances associated with bottom fishing. Manyalso have impressive abilities to rapidly regenerate both sexually and asexually and, consequently,the ultimate results of impacts of physical disturbance are difficult to predict. We analysed the effectsof 3 yr of pulsed experimental otter trawling, following an asymmetrical before-after-control-impactdesign, on grab-sampled colonial epifauna. Our study site was on a cobble seabed on the ScotianShelf in a formerly important fishing ground which had seen no disturbance by mobile fishing gearsfor 10 yr. The number of taxa, total biomass, the biomass of component major taxa (hydroids, bryozoans,sponges, tunicates, soft corals) and the community composition were analysed for single-yearand cumulative effects. The study site had a rich colonial fauna containing at least 53 taxa, the majorityof which were hydroids. The small vase sponge Scypha ciliata, the leafy bryozoans Dendrobeaniaspp. and the hydroids Symplectoscyphus spp. were the most frequent, occurring in >70% of thesamples. Significant inter-annual differences at control sites were observed. The number of taxa,total biomass and hydroid biomass increased over the study period, with associated changes incommunity composition. Short-term effects of trawling were detected as decreases in the number oftaxa per sample, total biomass and total hydroid biomass across the trawling events, although thesetrends were non-significant after Bonferroni adjustment. No cumulative effects from the pulsedtrawling were detected, and colonial species assemblages on control and impacted lines were similarat the end of the experiment. While some of the tests for trawling effects were statistically weak, it iscertain that any effects were small relative to natural inter-annual change.