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

Abstract

Colonial epifauna are an important component of benthic communities, providing structural complexity at scales of millimetres to metres. Many are sessile, emergent and fragile — characteristics which render them vulnerable to disturbances associated with bottom fishing. Many also 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 effects of 3 yr of pulsed experimental otter trawling, following an asymmetrical before-after-control-impact design, on grab-sampled colonial epifauna. Our study site was on a cobble seabed on the Scotian Shelf in a formerly important fishing ground which had seen no disturbance by mobile fishing gears for 10 yr. The number of taxa, total biomass, the biomass of component major taxa (hydroids, bryo- zoans, sponges, tunicates, soft corals) and the community composition were analysed for single-year and cumulative effects. The study site had a rich colonial fauna containing at least 53 taxa, the major- ity of which were hydroids. The small vase sponge Scypha ciliata, the leafy bryozoans Dendrobeania spp. and the hydroids Symplectoscyphus spp. were the most frequent, occurring in >70% of the samples. 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 in community composition. Short-term effects of trawling were detected as decreases in the number of taxa per sample, total biomass and total hydroid biomass across the trawling events, although these trends were non-significant after Bonferroni adjustment. No cumulative effects from the pulsed trawling were detected, and colonial species assemblages on control and impacted lines were similar at the end of the experiment. While some of the tests for trawling effects were statistically weak, it is certain that any effects were small relative to natural inter-annual change.