Visual survey methods

Visual monitoring can be carried out either by divers or by remote means (towed video, ROV). Which method is used will depend on local circumstances (water depth may preclude diving) and on logistic factors (the availability of research vessels and video systems). Diving-based surveys can re-visit precisely located study areas and provide information on changes at a very small spatial scale. Study sites can be buoyed, then simple rope transect lines or grids (composed of negatively-buoyant line) can be pegged out on the sea floor to give a standardized counting area. Atkinson (1989) used a rope transect 200 m x 2 m in size to map megafaunal burrows in Loch Sween. Deployment of a transect of this size can be achieved by slowly paying it out from a small boat, then pinning the corners to the substratum. Shorter transect lines (eg. 20m long) can be taken down by a diver wrapped around a central post, pegged at one end, then paid out as the diver swims slowly over the bottom.

For repeated monitoring, rope grids or transect lines can be left in position indefinitely, but if visits are to be infrequent (eg. only one or a few occasions per year) it is probably best to remove them after each visit. Structures left on the sea bed will rapidly become fouled by attached organisms. Rope grids will also tend to trap loose seaweed, which will decompose and alter the sediment characteristics by localized organic enrichment of the study area. Fixed structures are also vulnerable to damage by fishing gear or boat anchors, and close liaison with the local community will be required to avoid this if grids, transects or camera systems are to be left in place for extended periods.

The length and duration of towed video surveys will probably be determined by the available ship-time, but it is quite possible to survey virtually the full linear extent of lochs as large as Fyne and Sween in a few days (Atkinson, 1989; Howson & Davies, 1991). Analysis of the resulting videotapes will usually take far longer than the time spent in the field, and this factor should be borne in mind when planning a towed video (or ROV) survey. It is very easy to generate more videotape than there is time to analyze, and some selectivity is usually necessary. For example, Atkinson (1989) examined in detail video records of stretches of sea bed 200 m long by 1 m wide. The sections for analysis can be taken at regular intervals along the ship’s path, but in practice, the occurrence of stretches of poor visibility may disrupt the regular spacing of sampling areas. The area of sea floor surveyed needs to be large enough to take account of the patchy distributions of many megafaunal burrowers. A high-resolution still camera used in conjunction with the towed video will provide information on subtle sediment features and small burrow openings.

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