Habitat Mapping

Aims and Objectives

The aim of this exercise is to produce a working habitat map of the area surveyed in Plymouth Sound.

Using side-scan sonar, underwater cameras and Grabs, a working habitat map can be produced for a surveyed area of the Plymouth Sound.

Side Scan Sonar

Although habitat maps are an important and valuable tool to many scientists and conservationists they receive relatively little funding due to the time consuming and expensive methodology. There are two main approaches to habitat mapping. Firstly a preliminary study could be conducted on a site that has not been mapped before, this method is called prospecting. These are the most costly and time consuming (often take up to a month or more) due to the level of detail required to produce a good quality baseline for the area. Secondly an area that has already been mapped can be revisited and new maps can be compared to older examples to see how the environment has changed. Ideally mapping would take place once a month in special interest zones. However, the seagrass beds in Plymouth sound which are protected haven’t been surveyed since 2012.

Methodology

There are 3 major components to our habitat map. Side scan sonar is used to image the sea bed, this is a multi-beam sonar with a 100 kHz frequency and tracks 65 on either side of the towfish. There are a couple caveats to look for on the side scan trace, boat wakes will stir up bubbles in the column, they are very reflective and hard to read. Shoals of fish, boat sonar's and fixed structures like moorings will also cause unusual disturbances on the trace. A marine mammal watcher is also needed due to the acoustic nature of this work to ensure we do not disturb the local wildlife. If any mammals are spotted all work must cease. Grab samples are also taken in a process called ground truthing, which goes some way to confirming the habitat type. Video is also used to aid ground truthing and to help identify the areas flora and fauna in their natural environment. Furthermore, the video is deployed first to assess whether the environment is suitable for a grab sample. Our video method was a fly over transect rather than drop sites, this allowed us to cover a wider area but the image was of lower quality, with no scale or lighting on board.

In this practical samples were taken on the west side of Plymouth sound as it had not yet been mapped by students of other groups, and was likely a soft sediment environment based on instructors previous knowledge. 3 x 2km transects were taken 100m apart. As well as 2 grab samples using van Veen grab equipment and one video sample.

Home Boatwork Habitat Mapping Gallery References

Top image shows the side-scan data plotted with time stamps. The middle and bottom images show two different angles of the data plotted in 3D with contour depths plotted on to help visualise the data.

Navigation

HYDROpro Navigation Software was used to plot the transect lines needed by the crew in order to conduct the side scan sonar survey. The coordinates for the survey location were decided using a chart and the Skipper’s mapping software. Once these coordinates were decided they had to be converted from Latitude and Longitude to Northings and Eastings. This is because the HYRDOpro Navigation Software doesn’t use Lat and Long; it requires a planar view, hence the need for a conversion from latitude and longitude. Below are converted coordinates of the start and end points of the transect in eastings and northings.

 

Start: 51,538.997m Northings, 246,393.057m Eastings

End: 50,520.513m Northings, 244,248.216m Eastings

 

Configuration settings for HYDROpro Navigation Software:

Configure -> Guidance options (GO) -> Enter group name -> Add GO -> Enter converted coordinates.

There was an issue with the crew’s system due to 2 lines being plotted either side of the original line; one line was in an area of very shallow water. This created a complication on the skipper’s system, and hence one of the lines was omitted (line 7) from the transect.

X----------------------------------------------------------------------------------X Gr3_04

X----------------------------------------------------------------------------------X Gr3_05

X----------------------------------------------------------------------------------X Gr3_06

^ Visual representation of the lines plotted by HYRDOpro. 

The transect begun on Gr3_06 (Line 6).

Click image to the left to enlarge

The views and opinions expressed are those of the individuals and not representative of the University of Southampton or the National Oceanography Centre