Development of methods for monitoring subtidal biotope extent using remote video: A Report for the UK Marine SACs Project 2001

Abstract

This report describes an assessment of the use of remote video, collected by ROV, towed sledge or drop-down video methods, to monitor seabed biotope extent in a marine SAC. Data and video footage for the assessment were taken from the results of ROV trials carried out in the Sound of Arisaig in February 1998 and drop-down (ground-truth) video taken during the AGDS (acoustic ground discrimination system) trials in Loch Maddy in 1998 and August 1999.

The first stage of the project was to examine remote video footage collected during the Arisaig and Loch Maddy surveys, and the biotope data derived from that video footage. Some inconsistencies in the biotope recording from the video were noted, particularly where the seabed was dominated by kelp. Some basic rules for amalgamating biotopes that could not be consistently identified into larger biotope groupings that could be consistently identified were formulated. These rules were applied to the Arisaig and Loch Maddy datasets, which were then imported into a MapInfo GIS. A methodology for creating biotope maps from the point source data was then devised, using hand-drawn polygons based on knowledge of the relevant biotopes and bathymetric information from an Admiralty chart backdrop. The resulting polygons were labelled and coloured using a standard colour chart to distinguish the biotopes.

The biotopes maps generated by this methodology were then compared with biotope maps previously generated by AGDS. There were significant differences in the distribution and extent of the biotopes in the two sets of maps, due primarily to the inadequate sample site density of the video data. In the inner parts of Loch Maddy and nearshore parts of the Sound of Arisaig the heterogeneity of the seabed biotopes was too great for the sample site density. The remote video data used were not initially collected for the purposes of this project, but the report shows that biotope maps could be compiled with these methods if the site layout was designed to suit the seabed heterogeneity.