Introduction

The Falmouth and Helford area is a designated Special Area of Conservation created under the Habitat Directive (JNCC). One of the main reasons for this is the presence of the largest Maerl beds in the South West of the United Kingdom.  Maerl beds are an important habitat for other marine species, especially as a nursery ground for fish and shellfish.  Habitat mapping is used to identify changes in community structure and sediment type within the Falmouth Bay which may be affected by scour from heavy shipping, turbidity, tidal influences and the prospective dredging plans.


The aim of the geophysical survey was to use a combination of side-scan sonar and video camera equipment to produce a habitat map, which would show our interpretation and conclusions of the sediment types and community structure within the surveyed area.  Van Veen grabs were to be used alongside the camera as another ground truthing method; however, this was prohibited due to the presence of live Maerl beds.


Method and Equipment

The RV Viking was used on the 26/06/2013 from 1305 until 1603 GMT to tow the side-scan sonar fish along six tracks (Figure.1.) in order to produce an image of the sea floor using backscatter with a swath of 150 m. A drop camera was towed across the six transects  for ground truthing.

Zones were then created to produce a habitat map by calculating the location of predetermined boundary lines relative to the position of the tow fish. This calculation followed a series of steps:

1. Distance between fish and sea bed (Hf cm) was determined by the distance in cm between the centre of the tow fish line and the first dark area next to it. This is then multiplied by 75 m (the distance to the edge of the track from the centre) and divided by 12.8 (the distance to the edge of the track from the centre in cm).

2. Slant range (Rs) was defined by measuring the distance from the fish to the object and then multiplied by Hf(m)/Hf(cm).

3. True horizontal distance (Rh) was calculated using Pythagoras to complete the triangle (Figure.2)












Conditions

Disclaimer: Views and opinions are not representative of the University of Southampton or the National Oceanography Centre Southampton

Figure 1: Location map of transects. Hover over image to enlarge.

Figure 2: Pythagoras Theory

Hover over the images on the habitat map below (figure.3) to enlarge video screen shots of sea floor.


Figure 3: Habitat map for the transects.



Conclusion


The entrance to the Fal estuary is a busy area and although the site surveyed was not within the main channel, the biota and substrate type can still be influenced by shipping activity. The study is particularly significant given that according to the BBC (2012), there are efforts to attempt to dredge the entrance to the Fal estuary and build a new cruise terminal to accommodate larger more modern vessels. Given that maerl is a protected species under the EU Habitats Directive (1992) and that the entire area is classified as a Special Area of Conservation (SAC), there are concerns from Seasearch (2012) that any such development could damage the maerl communities through both silting and mechanical disturbances. Maerl is known to provide an important habitat for both fish and marine plants; Bosence & Wilson (2001) observed that Maerl in Cornwall grows incredibly slowly, at rates of 0.8 – 1.4m every 1000 years, which means that should the communities in the Fal estuary be disturbed they will take many years to return to their current state. The silting resulting from the dredging process may also bury the maerl and as such, the Port of Falmouth has proposed a translocation of the maerl to the north whilst a study is ongoing and regularly reported to Natural England.


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