PE EIE[R-Rg RESEARCH ON - HJ Andrews Experimental Forest

Proceedings-Research on Coniferous Forest Ecosystems-A symposium .
Bellingham, Washington-March 23-24, 197 2
Hydroacoustic assessment of
limnetic-feeding fishes
Richard E . Thorn e
Fisheries Research Institut e
University of Washingto n
Seattle, Washington 9819 5
Abstract
Hydroacoustic techniques have been applied at the University of Washington to determine the number and
biomass of limnetic fishes in order to evaluate their role in the productivity of lake systems. The lakes are
surveyed with high frequency, high resolution portable echo sounders . The echo signals are recorded o n
magnetic tape and analyzed by a special computer program. Information on size and species composition is
obtained primarily by net sampling, but acoustic determination of size appears feasible in some cases .
Introduction
Determination of the numbers or biomas s
of fishes has been a continual problem in
fishery management . Traditional techniques
based on catch-per-unit-effort and tagging
experiments have many inadequacies. Th e
problem of assessing fish populations in lake
systems is further compounded by the fac t
that fisheries in lake systems are generally
limited, highly selective, or nonexistent, s o
that catch statistics are of little value .
Determination of the numbers and biomas s
of limnetic-feeding fishes in lake systems i s
essential to assess recruitment, growth, mortality
rates, distribution patterns and inter -
actions with other trophic levels . As part of
International Biological Program studies o f
the Coniferous Forest Biome at the Universit y
of Washington, this problem has been
attacked through the application of hydroacoustic
assessment techniques .
History of
Acoustic Assessment
of Fish Populations
Echo sounders have been used since th e
mid-1930's to study the distribution and relative
abundance of fish populations, especiall y
in marine environments . However, prior t o
about 1960, hydroacoustic studies of fis h
populations were essentially dependent on
subjective interpretation of echogram records .
During the last decade, a variety of electroni c
devices for automated signal processing has
been developed. These advances, combined
with improved data acquisition systems an d
increased understanding of acoustic principles,
have resulted in a number of successfu l
applications of acoustic techniques to fish
population estimations both in marine and
fresh waters (Truskanov and Scherbino 1966 ;
Cushing 1968; Thorne 1970 ; Thorne and
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