Use of Underwater TV-survey to monitor trawl marks on Nephrops grounds

In this report trawl marks on Nephrops grounds are quantified. Records are from the first underwater TV survey in Icelandic waters carried out in 2016. Trawl marks or furrows identified on the videos were classified with a new procedure into 5 types, the depth of the furrow was measured and also the stage or age of it. All 85 stations from the survey were analysed and in total there were 609 furrows detected as an individual trawl mark. Type A, which is most likely caused by an otter board corresponded to more than half of the observed marks. The depth of the furrows was usually about 4 cm, but marks depicted from otter board or rockhoppers penetrated slightly deeper than other types. A positive correlation was found between the numbers of trawl marks and fishing effort from electronic logbooks. Trawling intensity varied, but on average it was estimated that the area was trawled with a Nephrops trawl 2.1 times over a 1.5 years period. Weak negative correlation was found between density of the sea pen Virgilaria mirabils and trawl marks, but the less common Pennatula spp. were found in higher abundance where fewer furrows were observed.