Fish size, distribution, age, biomass—although fisheries
scientists are interested in all those things, they also
want to know more about fish behavior: how fish interact
with each other and other organisms, how they use habitat.
“The question has always been,” said Kevin Boswell,
“what are they doing there?”
Boswell, a Ph.D. student, and his colleagues in the Department
of Oceanography & Coastal Sciences (DOCS) are excited
about a new piece of equipment that promises to revolutionize
the way they look at fish.
The DIDSON, manufactured by Sound Metrics, is an acoustics
system with a difference—it can produce near-video
quality images. The Applied Physics Department at the University
of Washington developed the DIDSON for military use, but
recently fisheries scientists recognized that the DIDSON
could change the way they do fisheries research.
Using acoustics in fisheries work is nothing new. Boswell’s
doctoral research, which is nearing completion, focuses
on the use of acoustics to quantify fish abundance and biomass.
What is new is the DIDSON technology that turns acoustic
data into images.
Boswell explained that whereas the traditional acoustics
system he uses is a dual-beam or split-beam system operating
at 120 or 420 kilohertz, the DIDSON has 96 beams and operates
at a very high frequency of 1.2 megahertz. The multiple
beams are what gives the unit the ability to image. A novelty
is that it utilizes a series of acoustic lenses that concentrate
the acoustic beam, and the last lens is filled with an acoustically
defined media, the composition of which is proprietary.
The lenses can be focused as the distance changes.
“With this system, we can image habitat, fish, and
behavioral interactions between organisms. We can count,
size, and track fish. The technology is in its infancy,
and the possibilities are limitless,” said Boswell.
“The system is the function of the merging of creativity
and physics.”
Bowell learned about the DIDSON from an article by Debbie
Burwen with the Alaska Department of Fish and Game, who
is using multiple DIDSON units for an acoustic study of
salmon in shallow water. “After talking to her,”
Boswell said, “I was sold. I contacted Sound Metrics,
and they arranged for me to use a unit for a week.”
Using acoustics in shallow water, which is also what Boswell
does, has physical challenges. Because of the depth limitation,
soundings must be taken horizontally instead of vertically.
Burwen used the DIDSON in two feet of water. Although Boswell
isn’t often confronted with the need to work in water
that shallow, Louisiana coastal waters present another challenge—turbidity.
Often the water is so murky that traditional underwater
photography is not an option. The DIDSON acoustic system
isn’t hindered by murky water, nor by lack of light,
which means data can be gathered by night as well as day.
A major limitation to the system is that it collects approximately
1.2 gigabytes of data an hour. Powered by either two car
batteries or an A/C line, it can run indefinitely. The acoustic
data collected on a computer is later converted to image
data. The DIDSON is limited in range. After about 12 meters,
it loses some fine-scale resolution. Sound Metrics is working
to enhance the unit’s usefulness in shallow water
at longer ranges.
Boswell tested the DIDSON in a tidal mangrove at Fouchon,
where he set up the DIDSON a few meters off the bank and
imaged fish that were using the edge habitat in a small
tidal channel. He also took it to the LSU Aquaculture Facility
at Ben Hur during catfish feeding time, and used it in Barataria
Bay. Boswell and Andy Fisher have set up a page on the Coastal
Fisheries Institute (CFI) Web site, www.cfi.lsu.edu/hydroacoustics/DIDSON,
where images taken during the week-long testing of the DIDSON
system can be seen.
Other CFI and DOCS faculty are interested in using the DIDSON
technology in applications as diverse as artificial reef
fish surveys and monitoring the behavior of invertebrates.
Dr. Jim Cowan expects that CFI will obtain at least one
DIDSON unit by the end of the summer.
“It’s great technology,” said Boswell.
“If used properly, it can be a very powerful tool.”
