This heading covers all equipment lowered by cable from the deck of a ship, then towed over, or dropped to, the sea bottom before being hauled back aboard with a sample of the substratum and associated fauna. Brittlestar beds tend to occur on fairly hard grounds consisting of pebbles, gravel or coarse sediments, often strewn with boulders or rock outcrops. For this type of substratum, the Naturalist’s dredge (Holme, 1971) is the most commonly-used sampling device. This takes a superficial scrape from the sea bottom, and will not penetrate the substratum unless this is very soft. If towed over a dense brittlestar bed, the dredge will bring up enough animals to demonstrate the general nature of the community, but accurate estimation of densities is not possible. Quantitative sampling can be achieved using one of several types of grab (Holme, 1971), which sample a known area of substratum and its biota. Grab sampling will not generally be possible where the sea floor consists of anything firmer than shell gravel (eg. large cobbles, boulders or bedrock).

Remote sampling is useful for demonstrating the existence of a brittlestar bed if the presence of such a community is suspected. Much of the data reviewed by Holme (1984) was collected by grab and dredge sampling in the western English Channel. The spatial extent of an aggregation could be established by sampling at points along one or more transects, but this is likely to be very time-consuming. The use of a towed dredge also has the disadvantage that its passage may be highly destructive to the structural integrity of the brittlestar bed and may lead to the displacement of animals by water currents.

• Grab sampling allows precise measurement of faunal densities
• No depth or time limitations on sampling
• Field operation relatively simple
• Standard equipment widely available from marine laboratories and research institutes

• Use may be limited by bottom type (too hard) or topography (too irregular)
• Establishment of the spatial extent of beds may be very time-consuming. Abrupt boundaries will be very difficult to detect.
• Dredges may be destructive to brittlestar beds

SCUBA diving has been used increasingly since the 1970s for field studies of subtidal biotopes. Diving has also been the mainstay of the MNCR biotope surveys around the UK. The overwhelming advantage of the technique as applied to brittlestar beds is that it allows close-up observations and field experiments (eg. Warner, 1971; Broom, 1975; Aronson, 1989). Research dives can be carried out from small dories or inflatable boats, or if necessary from the shore, allowing access to shallow or enclosed inlets that larger boats cannot reach.

However, diving does have a number of important drawbacks. Using compressed air as a breathing gas entails strict depth and time limitations. For practical purposes, it is difficult to carry out detailed observations or experiments at depths below 30m, and most field studies of brittlestar beds have been conducted in much shallower water. The use of alternative breathing gases promises to extend the depth and time limits for diving studies, but these have not yet come into general use in UK scientific diving. Any form of diving entails exposure to physical hazards such as decompression sickness, and as a result the conduct of professional diving operations in the UK is strictly controlled by legislation. Standard training and operational requirements for scientists diving at work are enforced by the Health and Safety Executive.

Divers can examine the sea floor at a finer resolution than any photographic technique, but only relatively small areas can be covered on a single dive. The technique is therefore more suited to repeated monitoring of small fixed sites than to habitat mapping on a scale of hundreds of metres. Many brittlestar beds are in areas swept by strong tidal currents, which will severely constrain the time divers can spend on a site. The use of suction-samplers (air-lifts) by divers offers another method for sampling the benthos from brittlestar beds, particularly on rocky substrata where ship-borne grabs cannot be used. Suction samplers provide a fast and efficient means of collecting a quantitative sample of brittlestars and associated fauna. They can also be used in gravel, cobble and sandy areas.

• Allows first-hand behavioural observations and field experiments
• Allows quadrat measurements of animal densities
• Allows repeated monitoring of fixed study sites
• Equipment widely available, relatively inexpensive compared with ROVs or towed video
• Can be carried out from small boats or from shore

• Strict depth and time constraints. Also prevented by strong currents
• Has potential physical hazards (eg. decompression sickness)
• Operations subject to strict legislative controls
• Only possible to cover small areas on individual dives

Towed video provides a means to visually survey large expanses of sea floor without the depth or time constraints associated with diving. The basic apparatus involved is relatively simple, consisting of a low-light sensitive video camera mounted on a lightweight, runnered metal sledge, towed slowly over the sea bottom by a ship. A number of camera models suitable for this work are now available from commercial manufacturers. The camera is mounted on the sledge facing obliquely forwards, usually 70 - 100 cm above the substratum. One or two quartz-iodide lamps are positioned at the front of the sledge, pointing vertically or obliquely downwards to illuminate the sea bed within the camera’s field of view. The camera is connected to a video recorder on board ship by an umbilical cable loosely attached to the towing warp every few metres along its length.

For optimum picture quality, towing speed has to be carefully controlled and kept at 1 knot or below as far as possible. Positional information during the tow can be recorded using the ship’s navigational system (Decca or GPS). The visual field of the camera can be established prior to the survey by deploying the system with a calibration scale (graduated rule or marked string) fixed to the lower part of the sled within view of the camera. Analysis of the resulting videotapes usually consists of counting the features of interest within a strip of known width traversed by the moving camera sled. The frequency of counts or linear extent of the transect to be analyzed depends on the objectives of the survey and on the time available for the work (videotape analysis can be very time-consuming). A time-lapse still photographic camera is often mounted on a video sledge to provide an additional record (usually with better resolution than the video images) at intervals along the tow path.

Although the equipment required for towed video surveys is relatively simple, it is expensive and generally confined to large marine laboratories or academic institutions. The technique has been used to survey brittlestar aggregations in Strangford Lough (Magorrian et al., 1995) and the western English Channel (Wilson et al., 1977), but its application is limited to areas of level sea bottom without too many boulders or rock outcrops. Many brittlestar beds occur on grounds where towed video could not be used without serious risk to the equipment.

• Able to survey large expanses of sea floor quickly
• Allows precise density measurements of features of interest
• No depth or time constraints (in coastal waters)

• Equipment needs hard boat to operate. May be unable to access very shallow waters or enclosed inlets
• Equipment readily available but expensive
• Deployment may be constrained by sea bed topography

ROVs are video camera systems mounted in a compact submersible vehicle whose movements are controlled by a surface operator via an umbilical cable (Auster, 1993). The capacities of ROVs are in some respects intermediate between those of SCUBA diving and towed video. Operations are free from the depth and time constraints imposed on human divers, but have a radius of operation defined by the length of the umbilical cable. Surveying outside this radius is acheived by moving the support vessel. An ROV has the advantage over towed video of being able to hover over a selected point or ‘retrace its steps’, allowing the operator to closely examine a feature of interest. However, quantification of features on the sea bed is slightly more difficult than from a towed video recording, as an ROV will not always remain at a fixed distance from the substratum, and the field of view may therefore change. Some models of ROV have mechanical ‘arms’ controlled by the surface operator and so have some capacity to take benthic samples. ROV deployment may be restricted by strong water currents.

ROVs are used extensively in the offshore oil and gas industry but have not so far been widely employed in scientific studies in the UK. To date there are no published examples of their application in studies of the biotope complex discussed here.

• No time constraints. Depth range limited by length of umbilical but most models can access depths likely to be encountered in UK coastal waters
• Able to cover wide areas (relative to capacity of human divers)
• Mobility allows close-up examination of sea bed
• Deployment areas less restricted than towed video. Can be used in areas with submarine obstructions
• Some models able to collect benthic samples

• Equipment needs a hard boat to operate. May be unable to access very shallow waters or enclosed inlets
• Equipment very expensive and not widely available
• May be difficult to employ in areas with strong water currents
• Sampling is non-random, ie. areas for observation are selected by the operator, with consequent potential for bias in density estimations

Acoustic surveys using the recently-developed RoxAnn^TM system are becoming increasingly important in the large-scale mapping of benthic biotopes (Greenstreet et al., 1997). RoxAnn^TM is an electronic system connected to the transducer of a conventional echo-sounder in parallel with the existing display. The system functions by processing the first and second echoes returned from the sea bed to derive values for the roughness (ie. topographic irregularity) and hardness (ie. substratum type, rock/sand/mud etc.) of the sea floor. By plotting the roughness and hardness functions against each other and integrating this information with values for water depth, a detailed map of the distribution of substratum types in a survey area can be produced.

The great advantage of RoxAnn^TM is that information on substratum types over wide expanses of sea floor (ie. on a scale of tens of kilometres) can be gathered very rapidly, in far less time than it would take to collect and analyze grab samples over such an area (Greenstreet et al., 1997). In addition, the system is sensitive not only to the physical characteristics of the substratum, but also to certain biotic characteristics such as the presence of organisms projecting above the sea bed. The technique therefore clearly has enormous potential for rapid mapping of marine benthic habitats.

However, RoxAnn^TM data cannot be used in isolation. The substratum types distinguished by the system in its present form must be ‘ground-truthed’, ie. checked by analysis of grab samples, diver survey or photographic observations. In some cases the system distinguishes more sediment ‘types’ than can be recognized by traditional particle size analysis (Greenstreet et al., 1997). Although broad biotope categories can be identified, their precise species composition must still be determined by other means.

Because of its recent origins, RoxAnn^TM is only now coming into frequent use as a tool for benthic habitat mapping, and the capabilities and limitations of the system are still in the process of being defined. It has been used in surveys of several candidate SACs, including Strangford Lough (Magorrian et al., 1995), Loch nam Madadh (Entec, 1996), the Sound of Arisaig (Davies et al., 1996) and the Berwickshire/North Northumberland Coast (Foster-Smith et al., 1996). In all of these areas, brittlestar aggregations were identified and mapped. Acoustic survey is a rapidly-evolving field of marine technology and alternative systems with enhanced capabilities are likely to appear in the future.

Advantages and disadvantages of RoxAnn^TM

• No depth (within coastal waters) or time limitations
• Allows substrata to be mapped rapidly over large areas

• Equipment needs a hard boat to operate. May be unable to access very shallow waters or enclosed inlets
• Equipment expensive and not widely available
• Results need to be ‘ground-truthed’ by other methods (eg. grab sampling, towed video)
• Does not provide details of biological community composition or species abundance
• Not able to collect benthic samples

Table - A summary of capabilities of various monitoring techniques for subtidal brittlestar beds

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