Íslenskar landbúnaðarrannsóknir - 01.09.1978, Page 137

ÍSL. LANDBÚN.
j. agr. res. icel. 1978 10,2: 135-140
A new portable hydroacoustic
data acquistition system
Torfinn Lindem
University of Oslo
* and
Edmund P. Nunnallee
College of Fisheries, University of Washington,
Seattle, Washington
ABSTRACT
An echosounder is described which meets the needs for stock assessments of the biologist and the fisheries
manager. The unit incorporates calibration features which allow processing of the signals on a digital
computer after they have been recorded on magnetic tape. Alternatively, signals can be analyzed on an
oscilloscope. Weight of the sounder and transducer mounted in a fin is less than 15 kg. The power
consumption is 24W at 12V. The unit is adapted for freshwater lakes and rivers but can equally well be
deployed in the coastal and estuarine marine areas.
INTRODUCTION
The last decade has seen great progress
toward quantiíication of hydroacoustic
signals for assessment ofpelagic fish. Most
systems, however, have been developed
for use in the open sea; generally both
their cost and weight prohibit a general
use for assessment in freshwater lakes.
However, a small, portable unit, de-
veloped at the College of Fisheries at the
University of Washington in Seattle
(THORNEet al. 1972), has been deployed
with great success in a number of sockeye
salmon-producing lakes along the
perimeter of the eastern part of the North
Pacifíc Ocean, as well as in estuaries, large
rivers, and other coastal areas.
Basically the unit consists of: 1) a trans-
ducer mounted in a surface towing veh-
icle, 2) a battery operated echosounder, 3)
a magnetic tape-or cassette recorder for
storage of received signals, and 4) an os-
cilloscope to monitor signals in the field
(Fig. 1.)
The towing vehicle for the transducer
can be either of plywood, plastic, or
aluminum. Experience has indicated that
a narrow beam transducer with about a
10° angle down to the — 3 dB point works
well for assessment of freshwater fish
populations. In this particular case a Ross
narrow-beam transducer was used. The
first order of sidelobes is down — 17 dB
and can be ignored. However, the unit has
some serious limitations, such as no accu-
rate TVG function, insuflficient dynamic
range and a high power consumption.