bedford institute of oceanography 2001 in review - Région des ...

SCIENCE ACTIVITIES / Marine Environmental Sciencesassociated with winds and waves. Wave dissipation takes energy from thespectrum and puts it into surface currents. During a rapidly developingstorm, surface currents may be enhanced as much as 30-40% by wavedissipation. During quiescent periods when waves are small or are notrapidly developing, this surface current enhancement effect is small.Thus, waves interact with the atmospheric boundary layer thatforms and drives them. Through wave-induced roughness, sea sprayand other processes, the ocean exchanges mass, momentum, and heatwith the atmosphere. These effects are caused by the marine winds,and also have impact on the processes that create and determine thesewinds. The waves are also coupled to surface currents. These, in turn,affect the wave growth. Wave growth leads to wave breaking whichadds new momentum to these currents. It is only by working with acoupled upper ocean and atmospheric boundary layer that one canhope to arrive at an improved understanding of marine storms andtheir impacts on human activities in Atlantic Canada.Marine Environmental SciencesThe Grand Banks Otter Trawling Experiment– Donald C. Gordon Jr., Kent D. Gilkinson, Ellen L.R. Kenchington, Cynthia Bourbonnais,Kevin MacIsaac, David L. McKeown, and W. Peter VassMobile fishing gear such as beam trawls, otter trawls, scallop rakes,and clam dredges is widely used around the world to harvest fisheryresources. Concerns have been raised about their environmentaleffects. Understanding the impacts on benthic habitats and communitiesis difficult and expensive, especially for offshore fishing banks.Benthic habitats and communities display considerable natural variability,both spatially and temporally, which must be factored into thedesign of research programs.Over the past 50 years, otter trawls have been widely used inAtlantic Canada to capture bottom dwelling species such as cod,haddock, plaice, flounder, and shrimp. From 1993-1995, weconducted an experiment to examine the effects of intensive ottertrawling on a sandy bottom ecosystem at a depth of 120-146 m on theGrand Banks off Newfoundland. Analysis of historical effort dataindicated that the study site had not been trawled for at least 13 yearsand was therefore undisturbed. Three, 13 km long experimentalcorridors were trawled twelve times each year, with an Engel 145 ottertrawl equipped with rockhopper footgear. A broad array of imagingand sampling instrumentation was used to survey trawled and nearbyreference corridors both before and after experimental trawling overdifferent spatial scales. This instrumentation included sidescan sonar,an epibenthic sled, and a video-equipped grab.Location of study site on the Grand Banks and orientation of trawled (T) and reference(R) corridors (300 meters apart). All three corridors were trawled while theepibenthic sled and videograb samples were only collected in corridors A and B.The most pronounced impacts were the immediate physicaleffects on habitat. Sediment was resuspended by all parts of the ottertrawl, furrows and berms were formed by the doors, biological sedimentstructures were either modified or destroyed, and organicdetritus was dispersed. However, these physical effects appeared to berelatively short-lived. The available evidence suggested that thehabitat recovered in about a year or less.The most immediate biological effect was the removal of largeepibenthic organisms from the seabed by the otter trawl. Except forsnow crabs and basket stars, removal appears to have an insignificanteffect at the population level. This is because of the low efficiency ofthe otter trawl in catching epibenthic organisms.Less obvious, but more significant, was the immediate damagedone to epibenthic organisms that were left behind on the seabed.Some were killed outright, while others were damaged or exposed.Most susceptible were the larger epibenthic organisms that live on thesediment surface such as snow crabs, basket stars, sand dollars, brittlestars, sea urchins, and soft corals. The net effect was a 24% reductionin mean biomass, within a few days after trawling. This reduction inbiomass was due to removal by the otter trawl, predation by scavengingorganisms, displacement outside the disturbed area, andperhaps burial by resuspended sediment.Both the immediate and long-term impacts of otter trawling onbenthic infauna, which live within the sandy seabed, appeared to beminor. For the most part, significant effects were seen in a limitednumber of polychaetes and were restricted to one year of the experiment(1994). All available evidence suggested that the biologicalcommunity recovered from the trawling disturbance in less than ayear. Most of the affected epibenthic species have some power oflocomotion, and migration into the disturbed area from thesurrounding undisturbed area was highly probable. Effects wereobserved on only 25 species out of over 250 taxa, so the majority ofspecies present (including all the molluscs) were not affected by ottertrawling. No significant effects could be seen on the benthic communityafter three years of trawling. The habitat and biological communityat the experimental site were naturally dynamic and exhibitedmarked changes irrespective of trawling activity. The effects of24 / BIO-2001 IN REVIEW