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

We examined the spatial distribution of trawling effort from logbook data from all Icelandic vessels fishing for demersal fish between 1991 and 1997 with a spatial resolution of 1 degree of latitude and 1 degree of longitude. The trawling effort was widely distributed but was 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 and trawls. In each year, the area swept with otter trawl was 1.7 times greater than the total area in which fishing occurred over the 7 year period. In contrast, the area swept with otter boards was 4% of the total fishing area. Most of the fishing effort was confined to depths shallower than 400 m. With increasing depth, the size of trawls became larger and also the area swept per haul. Calculations assuming no variation in the size of the trawl in relation to depth, produced inaccurate swept area estimates, Furthermore, swept area estimates based on depth corrected door spreads were greater than estimates where no much correction was made. Swept area was considered to be a more appropriate measure of effort than tow frequency and tow duration as long as variation in the size of the gear (e.g. in relation to depth) was taken into account. Effort within Icelandic waters was compared in five depth strata within seven zones. Effort was highest off the south and NW coasts and lowest off the north and east coasts. Effort was most intensive at the 100-500 in depth in all zones 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 effects of fishing gears on benthic communities. (C) 2003 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.