By Mike Cappo, Australian Institute of Marine Science,
Townsville
For decades scientists have directly counted reef fish with
SCUBA in depths to 30 metres, but the deeper fauna has been
invisible -- and surveys there have been subject to the
selectivity inherent in trawl, trap and handline fishing
techniques. Meanwhile, the image quality of underwater
television and video has increased markedly, and costs and
size have decreased to such an extent that underwater video is
now an integral tool of marine research.
Researchers at AIMS have adapted cheap Hi8 "HandiCams"
in underwater housings made of PVC sewer pipe to launch a
fleet of Baited Remote Underwater Videos (BRUVS), to capture
the benefits of both underwater visual counts and trapping in
surveys of demersal fish. Tested to 100 metres, the BRUVS must
be robust and cheap enough to meet the high risk of loss due
to snagging, shark attack and strong currents.
Like traps, the BRUVS are baited with 1 kg of minced
pilchards in front of the camera, and deployed in a string of
six – each with rope for hauling and a string of surface
buoys. The cameras record for 90 minutes or 3 hours and are
retrieved with a pot hauler. Like direct underwater counts,
the immediate habitat type is visible when the tapes are
interrogated, and the timing and abundance of fish visits is
recorded using the time code stamped on the tape. Replay,
pause and zoom functions allow the researchers to closely
examine visitors to the BRUVS for identification, and – in
case of unknown species – still images can be
"grabbed" and emailed around the world to
taxonomists. The tapes offer a permanent record that can be
shown to the public and fishermen alike for their own
interpretation.
Applications to date have focussed on waters 30 – 90
metres deep, at the bases of reefs, on inter-reef shoals and
under the baitfish schools in the famous Cape Bowling green
billfish grounds. The most profitable approach has involved
habitat mapping and classification by towed video, by Dr
Roland Pitcher’s team from CSIRO Division of Marine
Research, Cleveland Laboratories, followed by deployment of
BRUVS in areas with or without specific features, such as
sponge and sea whip gardens. This approach has been used to
characterise the nature of specific "reds grounds"
located for us by Capt. Jim Dalling and other operators.
The range of fish, sharks, rays, sea snakes and other
animals sighted on tapes has been remarkable – over 300
species to date. From 3 centimetre leatherjackets to 3 metre
hammerhead sharks, the videos non-destructively record aspects
of the biodiversity present in previously unknown habitats.
Most fish are just passing the BRUVS, and are visibly
indifferent to the bait, but others vigorously attack the bait
canisters and create a berley plume that, in turn, attracts
other species. Deep sets to 91 m revealed a charted shoal on
the shelf edge off Townsville is, in fact, a huge mound (or
"bioherm") of dead Halimeda algae, overlain
by a veneer of living plants, and inhabited by rosy jobfish,
red emperor and iodine bream. Some of the productive
"reds grounds" have proven to be bare gravely sand,
pock-marked with large craters of unknown origin – perhaps
"wonky holes" in some cases. Sets under the baitfish
schools off Cape Bowling Green have revealed high densities of
juvenile red emperor associated with sparse patches of sponges
and sea whips, although the reasons for the baitfish
aggregations are still unknown.
Although approximate measurements can be made using single
cameras when fish swim past scale bars, developments are
underway to precisely and accurately measure the fish seen
anywhere in the field of view, by using the principles of
airborne topographic mapping. An informal consortium of
engineers and biologists, led by Dr Euan Harvey of the
University of Western Australia, has developed software to
measure length and other features using paired images from
stereo-video cameras. Field trials have shown errors to be
about 0.5%, or 5mm on the length of a 1000 mm fish. The next
step is full automation of tape analysis, where computer
software identifies targets and measures them.
These developments were the subject of an FRDC workshop on
"Video in Australian fisheries and mariculture
habitats", the proceedings of which can be viewed in full
on the AIMS website at:
Video-sensing
workshop
For more information,
contact
Mike Cappo or Pete Speare at the Australian
Institute of Marine Science
Telephone: 07 47534 444
(International +61
7 47 534 444)
Media release
'Big
Bruv' helps biologists keep watch on Barrier Reef
