Spatial distribution of otter trawl effort in Icelandic waters: comparison of measures of effort and implications for benthic community effects of trawling activities

Ragnarsson, S. A´., and Steingrimsson, S. A. 2003. Spatial distribution of otter trawl effort inIcelandic waters: comparison of measures of effort and implications for benthic communityeffects of trawling activities. – ICES Journal of Marine Science, 60:1200–1215.We examined the spatial distribution of trawling effort from logbook data from all Icelandicvessels fishing for demersal fish between 1991 and 1997 with a spatial resolution of 1degree of latitude and 1 degree of longitude. The trawling effort was widely distributed butwas intensive only in small and localised areas. Three measures of effort were compared;tow frequency, tow duration and separate estimates of swept area for otter boards andtrawls. In each year, the area swept with otter trawl was 1.7 times greater than the total areain which fishing occurred over the 7 year period. In contrast, the area swept with otterboards was 4% of the total fishing area. Most of the fishing effort was confined to depthsshallower than 400 m. With increasing depth, the size of trawls became larger andaccordingly, also the area swept per haul. Calculations assuming no variation in the size ofthe trawl in relation to depth, produced inaccurate swept area estimates. Furthermore, sweptarea estimates based on depth corrected door spreads were greater than estimates where nosuch correction was made. Swept area was considered to be a more appropriate measure ofeffort than tow frequency and tow duration as long as variation in the size of the gear (e.g. inrelation to depth) was taken into account. Effort within Icelandic waters was compared infive depth strata within seven zones. Effort was highest off the south and NW coasts andlowest off the north and east coasts. Effort was most intensive at the 100–500 m depth in allzones but in some areas (such as off NW Iceland), effort extended to deeper waters.Knowledge of the distribution of fishing effort is important for predicting larger scale effectsof fishing gears on benthic communities.