Effects of Commercial Otter Trawling on Benthic Communities in the Southeastern Bering Sea
Eloise J. Brown, Bruce Finney, Sue Hills, and Michaela Dommisse
Abstract. The effects of commercial bottom trawling for yellowfin sole Limanda aspera on benthic communities were investigated in a sandy habitat exposed to high wave and tidal disturbance at 20–30 m depth in the southeastern Bering Sea. We compared an area that has been closed to commercial trawling for 10 years with an adjacent area that is now open to commercial trawling. In addition, we examined the immediate effects of experimental trawling on benthic community structure in the area closed to trawling. The fished area was characterized by reduced macrofauna density, biomass, and richness relative to the closed (unfished) area, but diversity was not different. Interannual variability of macrofauna assemblages was high in the system, yet assemblages in the two areas were distinguished using multivariate analyses and dominant taxa. After 10 years, sessile taxa (e.g., Maldanidae polychaetes) were prevalent in the closed area, and mobile scavengers (e.g., Lysianassidae amphipods) were more common in the fished area. Immediate responses of macrofauna to experimental trawling were subtle (i.e., reduced richness, absence of rare taxa, and patchy changes in assemblage biomass), but no differences were detected relative to controls for density, diversity, or total biomass. Fragile, structure-forming megafauna were rare. However, when trawled, they appeared mostly unaffected. Though we could not completely rule out other factors such as food supply from water column primary production, our results indicate that trawling altered macrofauna communities. Our findings also suggest that individual taxa respond differently to trawling but that commonly used summary measures such as total abundance do not capture these changes. Based on the functional attributes of individual taxa, hypotheses explaining different macrofaunal assemblages include bottom-up shifts caused by physical disturbance from trawling, altered benthic food webs from discards and processing waste, and top-down shifts caused by altered predator–prey interactions between Asterias amurensis and yellowfin sole. Stomach contents of yellowfin sole indicated that this flatfish did not target specific prey taxa; however, fishprocessing waste was prominent in its diet. More ecological and bioenergetic data are required to determine how changes in benthic communities are linked specifically to the productivity of yellowfin sole.