Overview
An investigation onboard MTS Xplorer, using Side scan sonar, Van Veen grabs and video recording equipment, conducted east of the mouth of the Fal estuary just outside the Special Area of Conservation (SAC). The investigation aim was to carry out a reconnaissance survey to produce a detailed habitat map of a small part of the Falmouth approach in order to add to a larger data set of several sampling sites within the area. This information was then included in a poster which could inform the local public about the benthic habitat distribution. The results showed a varied bed, both from a morphological and a biological perspective. Despite the survey being outside the SAC, maerl was still observed in surprisingly large quantities, making up a large proportion of the grab sample.
Meta data
Date: 24/06/2014
Time: 12.00-
Location: 50 08.78’N 004 59.64’W
Weather: 1/8 cloud cover, calm
Wind:7.4knots
High tide: 16:13 4.6m
Low Tide: 10:22 1.1m
Figure 1: Admiralty chart of Falmouth approach, the highlighted area represents the area sampled.
Figure 2: Diagram of side scan tow fish in operation.
Methods
Vessel: MTS Xplorer
Study area: The site of study was chosen as it is just outside the SAC zone making it a suitable site in order to investigate whether the zone needs to be extended.
Side scan: Side scan sonar is a method for mapping the bathymetry under a body of water. This is done by towing a side scan tow fish behind a boat, and the towfish emits pulses of sonar and then receives the backscatter from the seafloor and any irregularities. Different objects and habitats will cause different backscatter patterns thus altering the image produced. For this investigation the tow fish was pulled along four transects, each around 1km in length. The fish was deployed over the back of the boat and pulled along at a slow speed. A side scan plot was then produced, showing the backscatter patterns from the seafloor.
The side scan plot was printed and then cut into the four separate transects and pieced together in order to provide a detailed view of the benthos of the whole sampling area. Habitats were then identified by a change in backscatter pattern, each habitat was marked and several points around each habitat were marked with the time of passing. The points were then transferred onto a track plot with Eastings and Northings, producing a clear habitat map. By combining with the video feed it was possible to identify some of the habitats.
Van Veen Grab: Van Veen grab (Figure 4) can provide qualitative information on infaunal and epifaunal species in an area of soft sediment. The grab was deployed using a winch from the back of the vessel. The two halves of the grab are held open on descent and are closed on impact with the benthos. The grab is then recovered to the surface and opened carefully over a tray. Species can then be observed and identified as well as the substrate. The location of the grab can then be plotted on the habitat map in order to identify what that habitat substrate looks like.
Video feed: A fin with a video camera attached was lowered into the water using the winch until it was just above the seabed. The camera and fin were then towed at a very slow speed (drifting speed) and any species viewed were recorded from the video monitor. Video footage of the area allows confirmation of species when overlaid on the habitat map, this is known as ground truthing.
Figure 3: An example of a track plot obtained from a side scan transect.
Figure 4: Van Veen grab after deployment.
Figure 6: Photo of the video transect taken within the sample area, marked as video transect 1 on the habitat map.
Results and Discussion
The habitat map resulting from the side scan indicates that there are five habitat zones, as well as what appeared to be two rocky outcrops, within the sample area. However, it was not possible to identify all of the habitats as video footage was not obtained for each zone. Despite this, using video imaging, it was possible to identify the largest habitat zone, zone 5, as being a mixture of sand and maerl beds as seen in figure 4. From the side scan trace, zone 2 shows characteristics of a rocky outcrop with very definitive backscatter, however as ground truthing was not done in this zone it is not possible to define the habitat with full confidence. The Van Veen grab allowed further ground truthing of the main zone, the grab recovered confirming an extremely high abundance of maerl as can be seen in (figure 5) with few other species present. Such a high maerl abundance suggests that the SAC site needs to be extended in order to protect the new forming maerl beds which provide a habitat for many key ecological species. The SAC site caused some limitations to the investigation as it was not possible for the grab to be deployed within the area.
References
Figure 1: Available at: http://www.visitmyharbour.com/harbours/channel-
Accessed( 29/06/2014)