Integrity of the Bed

Indications of this should be gained from the acoustic surveys of the bed with RoxAnn and side-scan sonar. However the level of detail really needed will probably require the running of video and or photographic transects across the bed. If the bed is sufficiently shallow this could be done by diver held cameras, but it is more likely that remote cameras will be used.

Using Differential GPS it should be possible to position the start of video transects with a measure of replicability. Undertaking tows across the bed always in the same direction will be much more difficult as in some cases it will be difficult to achieve much more than a drift transect as influenced by tide and wind conditions. With a sledge mounted camera system, in good weather, working over slack water on a neap tide and with a vessel equipped for slow running and with an acoustic doppler system to measure speed over the ground, it should be possible to run along transits that a repeatable within about 30 metres.

At present there is no information about the level of damage done to a horse mussel bed by towing a camera sledge over it. It is inevitable that some of the epifauna will be damaged or dislodged, but whether a trail of broken large mussels is left is not known. Intuitively it is suggested that camera sledge tows done at intervals of several years would not cause unacceptable damage, but it would be unwise to undertake replicated tows several times a year.

For the purposes of measuring exactly the amount of ground covered by mussels it is likely that still camera images will be more suitable. In this case the camera should be pointing directly downwards and the light should be at an angle of around 60 . This arrangement provides a constant known size quadrat field of view and the minimum amount of back-scatter from particles in the water. If as sometimes happens, the front of the sledge lifts as it is being towed, the pattern of lighting will usually allow the change in orientation to be detected. Some experimentation will be needed to achieve the optimum length of towing cable and speed over the ground. As a first approximation we suggest that the towing cable needs to be about twice the water depth. On vessels where towing is off a trawl winch it is recommended that a braided nylon or other synthetic rope about 20- 30 metres long is used as a leader to minimise the risk of the heavier wire touching the bottom ahead of the sledge. Use of a wire, rather than relying solely on a rope has advantages as the sag of the wire in the water helps to keep the gear down, particularly for deeper deployments. Ball swivels are recommended both where the rope is attached to the bridles on the sledge and between the leader rope and the towing wire. A float is needed to counteract the weight of the swivel and shackles. Normally a tail rope is attached at the back of the sledge, with a surface float. This serves as a safety recovery mechanism, it helps indicate where the sledge is relative to the boat and it's drag helps to stabilise the direction of travel of the sledge. It is best to arrange to the sledge against the tide. This has the advantage of allowing the towing vessel to maintain steerage at speeds of less than 1 knot over the ground and it means that any turbid material stirred up by the sledge runners does not form a cloud in the field of view.

Where conditions permit, consideration might be given to establishing permanent dive transects across part of a bed. How relocatable marks can be established in an offshore mussel bed has yet to be addressed, though in some cases there may be identifiable large boulders protruding from the bed which could have marks attached to them.

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