J. Mar. Biol. Ass. U.K. (2000), 80, 365^366 Printed in the United Kingdom
Video-assisted grabbing: a minimally destructive method of sampling azooxanthellate coral banks P.B. Mortensen*, J.M. RobertsO and R.C. Sundt* *Institute of Marine Research, PO Box 1870 Nordnes, N-5024 Bergen, Norway. E-mail
[email protected]. O Scottish Association for Marine Science, Dunsta¡nage Marine Laboratory, PO Box 3, Oban, Argyll, PA34 4AD. E-mail
[email protected]
Traditional techniques used to sample azooxanthellate coral banks by dredge and trawl cause extensive impact to both the corals and surrounding seabed. Modern sampling techniques using submersibles and remotely operated vehicles cause very little or no damage, but are expensive and require specialized vessels. Here we describe a combination of video camera and benthic grab for sampling azooxanthellate corals and test this method on a Lophelia pertusa bank in Osterfjorden, western Norway. The video-assisted grab was successfully used both for locating and sampling L. pertusa. This method can largely replace the use of traditional, more destructive dredging and trawling techniques for sampling azooxanthellate corals.
The azooxanthellate scleractinian Lophelia pertusa (L.) has a wide geographical distribution, from around 568S to 718N (Dons, 1944; Cairns, 1994). Its depth distribution ranges from just 39 m in Trondheimsfjorden, mid-Norway (Jon-Arne Sneli, University of Trondheim, personal communication) to over 2000 m in the South Paci¢c ocean (Cairns, 1994). Other than the shallow occurrences in some Norwegian fjords, L. pertusa seems to be restricted to intermediate depths in oceanic water (Dons, 1944; Frederiksen et al., 1992). It forms colonies up to about 2 m high (Wilson, 1979), but on a time-scale of thousands of years can build much larger structures (Hovland et al., 1997), commonly called banks, or reefs. However, it often occurs as scattered patches of colonies, the so-called `coppices' (Wilson, 1979) which can be di¤cult to locate or sample. Previous investigations of L. pertusa and its associated fauna relied mainly on dredge sampling (Frederiksen et al., 1992). The easy accessibility of fjord sites means that many of these sites have been heavily sampled for many years. Some areas are now thought to be severely damaged by these destructive sampling methods. It was therefore of great interest to see whether a simple technique could be developed to minimize the impact of future sampling. Compared to a dredge or trawl that is typically towed for several hundred metres to sample a coral bank, a benthic grab with an area of 51 m2 causes minimal damage to the seabed. However, when sampling scattered coral colonies, grab sampling is very unlikely to be used successfully. Therefore it was necessary to develop a system which could not only sample coral colonies but also provide a means of locating them. To do this we used a video camera and light to show the seabed beneath the grab in real time on deck. Similar approaches have been used to target sampling on both a large scale to recover over one tonne of rock (Kenyon et al., 1998) and on a much smaller scale to sample the sediment photographed by a conventional ¢lm camera (Wigley & Emery, 1967). To test the e¡ectiveness of this method for sampling azooxanthellate corals, a location was chosen where L. pertusa occurs on a sill crossing Osterfjorden (60839.3'N 05843.9'E), western Norway, at a depth of between 70 and 80 m (Tambs-Lyche, 1958). The investigation was performed with the RV `Hans Journal of the Marine Biological Association of the United Kingdom (2000)
BrattstrÖm' (University of Bergen). The sill was located using the ship's echo sounder (Simrad EQ55) and its position recorded by using the di¡erential global positioning system (Magnavox MX200 DGPS receiver). A low-light-sensitive video camera (Remote Ocean Systems 20/20 SRC) and light were mounted 1.5 m above a van Veen grab (0.2 m2) so that when suspended, the grab was clearly visible hanging beneath the camera (Figure 1). A compass was positioned 75 cm from the camera to assist the navigation. To avoid magnetic disturbance to the compass, a nylon rope (diameter: 15 mm) was used between the camera and the grab. The camera system and grab were deployed using a deck crane and to transmit video to the surface, a conducting cable was simply attached to the wire with adhesive tape as it was lowered. During each deployment, the video-assisted grab was lowered above the sill until the seabed was seen (52 m above bottom). The vessel was then allowed to drift slowly over the sill. A structure, about 10 m high and 50 m long, was identi¢ed on the northern side of the sill at a depth of between 75 and 85 m. This appeared to consist of mainly dead L. pertusa supporting several live coral colonies of between 0.5 and 2 m in diameter and up to about 1m in height. When coral colonies were seen beneath the grab, it was lowered to sample them and then recovered to the surface (Figure 2). When the video-assisted grab was lifted from the bank, the sampled colony did not collapse, but a trace of the grab sample was seen as a darker area on the colony. Of ¢ve grab shots, four successfully sampled the intended target and only one failed because the angle of seabed slope triggered the grab prematurely. The time taken to deploy and recover the video-assisted grab varied from a 90 min initial deployment to just 5 min. The length of the ¢rst deployment re£ects the most time consuming part of the operation, namely the initial search for the coral colonies. There is increasing evidence that both benthic trawling and dredge sampling cause extensive impact to the seabed and to the benthic community (Jennings & Kaiser, 1998). However such sampling techniques are cheap, readily available and do not require specialized vessels which make sampling using submersible and remotely operated vehicles inaccessible and often prohibitively expensive to most research groups.
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SHORT COMMUNICATIONS
Figure 2. Video frames showing the grab above a colony of Lophelia pertusa in a water depth of 80 m (A), during coral sampling (B) and a photograph of the recovered colonies on deck (C).
We gratefully acknowledge help from the crew on RV `Hans BrattstrÖm'.
REFERENCES
Figure 1. Schematic outline of the video-assisted grab.
Video-assisted grabbing causes minimal impact to the seabed, and allows the user to survey the area and to put the recovered grab samples into a better spatial context. In contrast to dredging, grab sampling recovers samples from single spots of the seabed so allowing accurate positions to be recorded. The video pictures also give additional information about the samples such as the region of the colony they came from, its size and the distance to the nearest neighbouring colony. Such variables are relevant for faunistic studies, and understanding the ecology of these species. The sampled colonies were relatively large (up to 25 cm in diameter), were recovered intact and were not exposed to sediment and debris as they would have been in a dredge net. This is particularly important if live corals are required for subsequent laboratory study. In more challenging o¡shore conditions, simple re¢nements to this technique will allow coral banks to be sampled in deeper waters. While there is little doubt that this method will be limited to use in minimal sea swell, hydroacoustic positioning equipment to monitor the grab's position and a single wire with conducting cables will facilitate its use in deeper water. The great simplicity and cheapness of video-assisted grabbing should allow a move away from crude destructive techniques, to a more targeted method for sampling azooxanthellate coral banks.
Journal of the Marine Biological Association of the United Kingdom (2000)
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Submitted 18 June 1999. Accepted 6 January 2000.
