Acoustic ground discrimination survey

Admiralty charts

Aerial surveys

At Newcastle University, the BioMar project (Davies et al., 1997) has developed a survey protocol for mapping the sea floor using acoustic techniques, validated by biological sampling, with the data stored and analysed using geographic information systems (GIS). A RoxAnnã processor was used for acoustic mapping. Based on the video samples, grab samples, diver surveys and previous detailed records of biological surveys in the Sound of Arisaig study area (Howson, 1990; Howson et al., 1994), a map of the sea-bed showing the predicted distribution of a total of 23 biotopes was constructed. Acoustic mapping using a RoxAnnã system provided data on the physical nature of the sea-bed (depth, smooth/rough, soft/hard), and biological information was then added to the acoustic data. It was not found practicable to relate each biologically based biotope classification to a particular acoustic pattern. Instead, the biotopes determined from a biological approach had to be grouped into 15 much broader categories in which the species component was generally lost The acoustic signatures for maerl, maerl-derived life forms, gravel and coarse sand were all very similar. RoxAnnã methodology has also been used in the Fal.

Although no statistical estimates of the probable accuracy of the group of biotopes predicted from a set of RoxAnnã data have been presented, it is probable that the development of the RoxAnnã method will allow mapping of maerl beds in areas where they are known to be present. An important consideration is the density of the ship tracking, which will affect the accuracy of the resulting maps.

Large-scale features such as plains, ripples and megaripples can be monitored by remote devices, such as side-scan sonar or by direct observation (Hall-Spencer, 1995a). Methods of seabed survey at present in use for geological and archaeological survey work can be adapted to monitor the topology of maerl biotopes.

The notation ACrl as used on the hydrographic charts produced by European nations generally designates deposits of coralline algae (Minchin, 1997). These details were recorded around the coasts of the British Isles during the 19th century as invaluable information to the shipping trade for navigating, beaching and anchoring of sailing vessels. The nature of the bottom was determined by "swinging the lead", where a hollowed lead cylinder was plugged with tallow and dropped to the bottom. The depth of water was noted and the tallow trapped evidence of the nature of the bottom. This is a minimal technology method and a rapid and extremely accurate way of determining the nature of (especially) soft bottoms. Detailed historical charts of European waters are readily available from which could be derived possible locations of maerl beds in the past. Modern hydrographic mapping techniques do not generally provide this detailed information on the nature of the benthos.

However, caution must be used in interpreting the information on charts, as other calcareous sediments not obviously made up of mollusc shells were also sometimes called Crl. Hall-Spencer (1995a) noted that in the Clyde some reports were based on true coral (Lophelia), Sabellaria alveolata reefs, or piles of bryozoan/hydroid tests.

Minchin (1997) has found that maerl beds, even when subtidal down to several metres depth, can be identified from aerial photographs. This method can be used to make a very broad-scale, rapid assessment of the extent of maerl deposits where there is no excessive water turbidity. This applies to most of western Ireland, the Western Isles of Scotland and other island groups.

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