| One
herring, two herring
Counting fish is difficult.
But in the future
laser technology may
make the task fast and
efficient.
 |
COUNTING
FISH: The new laser is mounted in the belly of a
plane that flies just 300 metres over the surface
of the sea. Laser counting technology isn’t yet fully
developed for commercial use, but the prototype
is already cheaper and quicker than current tallying techniques.
Contact: Jens Hjelmstad, Department
of Electronics and Telecommunications, NTNU
Tel: +47 452 49 613
Email: jens.hjelmstad@iet.ntnu.no |
| Photo: Scanpix. Graphics: Department
of Electronics and Telecommunications |
Today, the size of Norwegian fish stocks
is estimated with the help of sound devices such as echo sounders
and sonars installed in large research vessels. These measuring
techniques are steadily improving, but are costly and time-consuming.
Alternative methods, using aeroplanes and lidar technology are now
under evaluation.
Lidar (light detection and ranging)
is a kind of radar that makes use of laser light instead of radio
waves. If you’ve ever been caught speeding, you may have already
encountered lidar: the laser meter used by the police is a lidar,
but it is not as advanced as the one now being developed by scientists
at NTNU and the Institute of Marine Research.
MILITARY ORIGINS
Lidar technology was originally developed by the American armed
forces to detect foreign submarines. Professor Jens Hjelmstad and
PhD student Eirik Tenningen at NTNU have been working for two years
to develop the technology and related methods to help fisheries
researchers and managers survey fish stocks in various areas.
This is crucial to allow sound management
of the world’s fisheries. Mackerel and herring are particularly
difficult to count because they are a type of fish that swim in
schools in the upper layers of the ocean. They are easily scared
off by ships, and counting the fish is further complicated by the
fact that echo sounders only register fish 10 to 12 meters below
the ship’s own position. In contrast, planes don’t frighten
fish away, and the lidar is able to survey everything from the ocean
surface down to depths of 40 to 50 meters.
AFFORDABLE AND EFFECTIVE
During cruises last summer, the lidar only registered whether or
not there were fish and whether they were in large or small quantities.
The aim is to be able to distinguish one fish from another, as well
as to determine fish type, but these results lie just over the horizon,
researchers say.
“We will need about three or
four more years,” estimates professor Hjelmstad. Nevertheless,
the method is already far quicker and more affordable than current
methods. In just 30 hours, a plane can cover an area that two ships
take two weeks to map. The estimated cost is a tenth of what the
operation costs today.
3D
The lidar laser is mounted in the belly of a plane that flies at
an altitude of just 300 meters above the ocean’s surface.
Every second, thirty pulses of green or blue-green light are shot
into the water. Blue-green light is the colour that penetrates the
deepest before it is filtered out. When the light hits the water,
it is reflected back to a receiver in the plane. The information
that is returned is projected as images onto a system of co-ordinates,
with the depth and extent on the two axes corresponding with echograms
in acoustics. Fish are projected as small lines.
The closer together the lines, the
more fish there are. By using a greater number of colour frequencies
and receivers, the image will be able to provide information about
the type of fish the light hits: different kinds of fish reflect
light differently. The aim is to make 3D images that accurately
determine the amount and type of fish. Once this level of resolution
has been achieved, the technology could also be applied to other
fields of research, such as detecting oil pollution and constructing
topographical maps of the ocean floor.
Christian Fossen |
