"Large Scale" description of the seafloor at the study site by visual and other means.
Source: Davies et al. (2001)
Video or still photography of the seafloor, along a transect (from the fish/shellfish farm toward the reference station) is extremely desirable as it will provide large-scale information on macrophytobenthos, epibenthic macrofauna, seafloor status, etc. which core or grab sampling cannot do. Video can be used for interpreting and ground truthing data from an acoustic survey or as a primary survey technique for habitat mapping. If video is limited by turbidity good timing of the survey may improve the quality of a video survey considerably, e.g. in tidal areas water clearance is often best around slack tide. Good synchronization between video and GPS data is a prerequisite for mapping habitats.
Video cameras can be towed above the seafloor. In areas where obstacles (rocks, wrecks etc.) can be expected, the camera system should be protected by a frame. The frames can also be used for close-up inspections when placed on the seafloor. The resolution of still pictures from photo cameras is much better than the resolution of video footage. Thus, an additional photo camera facilitates the interpretation of videos considerably. The frame-camera should be towed at a constant distance above the ground. A weighted rope of known length within the view of the camera is a simple but very helpful way to achieve this goal. Alternatively, the video camera and accessory equipment (lamps, still photo) can be mounted on a sledge. Comprehensive guidelines for identifying biotopes using video techniques and in situ survey of sublittoral epibiota using towed sledge video and still photography have been published in the Marine Monitoring Handbook (Procedural Guideline No. 3-5, pages 241-251, and No. 3-14, pages 331-337). If a frame or sledge based camera system is not available and depth does not exceed ca. 30 m and only a small area needs to be surveyed for ground-truthing a hand-held video- or photo camera can be employed (see Marine Monitoring Handbook, Procedural Guideline No. 3-13, 327-330) or habitat classifications can be conducted by scientific divers who are familiar with habitats and species typical for the region (Marine Monitoring Handbook, Procedural Guideline No. 3-3, pages 233-239).
Transect Photography. For monitoring the impact of an existing mariculture facility a photographic documentation along a transect line has been shown to be a valuable tool. Necessary equipment are standard diving equipment, a compass, a good visible transect line (e.g. yellow, minimum length 50 m, diameter ca. 3 mm) on a seawater-resistant reel, an underwater photo-camera (focal length should be not longer than 28 mm for 35 mm film) with an external strobe.
A lead weight (1 to 2 kg) is attached to the beginning of the line and the line marked with tape every 5 m, for distances longer than 50 m every 10 m and the distance written on the tape. Even if the number is not always legible on the photos, it is helpful for evaluation and orientation of the divers. The weight is placed at the point of maximum impact, e.g. in the centre under a fish cage. From there, the diver swims towards the desired direction, preferably with the current, thereby unrolling the transect line. While swimming back, the diver exposes a horizontal photo at each mark (with the mark visible in the picture) for qualitative overview pictures. If quantitative pictures are desired (e.g. for counting the number of feed pellet) vertical pictures should be taken with a ruler within the picture or a photo camera with a frame should be used.
Acoustic ground discrimination systems (seabed classification systems) allow for mapping bathymetry, sediment type and some types of habitats (vegetation, mussel beds) in a range of a few kilometres around the planned location of aquaculture facility. For a proper sampling design, the desired spatial resolution of the map should be defined in advance, e.g. an area of a certain seabed type and a maximum diameter of 10 m should be identified with a probability of at least 90 %. This can be achieved by choosing the distance between tracks scanned by the echosounder appropriately. Depending on the system in use a more or less extensive ground truthing (diver observations, photo, video or grab samples) is necessary to link sea bed types with habitat types.
Standard Operational Procedures vary with the technique, hard- and software in use, thus a thorough training of staff is necessary. Systems based on multibeam technology or sidescan sonar allow for a faster and more accurate mapping but are more expensive and thus less widely available. Comprehensive guidelines on seabed mapping using acoustic ground discrimination interpreted with ground truthing have been published in the Marine Monitoring Handbook (Procedural Guideline No. 1-3, pages 183-197, No. 1-4, pages 199-209).
References
Davies J, Baxter J, Bradley M, Connor D, Khan J, Murray E, Sanderson W, Turnbull C and Vincent M (2001) (eds.) Marine Monitoring Handbook. http://www.jncc.gov.uk/page-2430, ISBN 1 86107 5243.
Rumohr H (1995) Monitoring the marine environment. Sci. Mar. 59 (Suppl.1) 129-138.
Smith C, Rumohr H (2003) Imaging Methods. In: AD McIntyre, A Eleftheriou (eds) Methods for Study of Marine Benthos. Blackwell.
