Modelling the effect of swimbladder compression on the acoustic backscattering from herring at normal or near-normal dorsal incidences
Natalia Gorska
0
Egil Ona
0
0
N. Gorska; Institute of Oceanology of Polish Academy of Sciences
,
ul. Powstancow Warszawy 55, PL-81-712 Sopot
,
Poland.
E. Ona: Institute of Marine Research
,
PO Box 1870, Nordnes, N-5817 Bergen
,
Norway; tel:
1
47 55 23 85 00; fax:
Inaccuracy in herring target strength can be an important source of bias in the acoustic assessment of several important herring stocks. New acoustic data on herring target strength (Ona et al., 2001, submitted for publication; Ona, 2003) confirm previous suggestions and evidence on a possible reduction of the size of the herring swimbladder as a result of its compression with increasing water depth. Theoretical work for a better understanding of the acoustic scattering from herring over its entire depth distribution may therefore be essential for improving abundance estimation. This study supplements the analysis, conducted by Gorska and Ona (2003) for herring averaged-backscattering cross-section. The modalbased, deformed-cylinder model (MB-DCM) solutions, presented in that paper, are used. The sensitivity of the herring backscattering cross-section in case of normal or near-normal dorsal incidences is studied with respect to frequency, contraction factors of the swimbladder dimensions and some fish morphological parameters. The study is important for a better understanding of not only the backscattering by individual fish for the dorsal incidence, but also the depth- and frequency-dependencies of the mean-backscattering cross-section. The theoretical results have been applied in the interpretation of the actual measured target-strength data on adult herring.
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List of symbols
hsb csb=c
gsb qsb=q
-cross-section radius and major-axis length of swimbladder prolate spheroid;
-semi-minor-axis length of swimbladder prolate spheroid;
-cross-section radius and major-axis length of fish-body prolate spheroid;
-co-ordinate, along-major axis of prolate spheroid;
-depth;
-swimbladder sound-speed contrast;
-swimbladder-density contrast;
-gas density and sound speed of swimbladder, respectively;
-density and sound speed of surrounding seawater;
-body sound-speed contrast;
-body-density contrast;
-density and sound speed of fish body, respectively;
-wave number in surrounding seawater;
-echo-sounder frequency;
a and b
Introduction
The abundance of the Norwegian spring-spawning herring
stock has been carefully monitored by acoustic surveys
since its reappearance after collapsing in the late 1960s
(Dragesund et al., 1980; Rttingen et al., 1994;
Johannessen et al., 1995). The assessment of the stock is based on
the results from acoustic surveys of the spawning area, the
wintering area and the feeding grounds, which significantly
differ in fish behaviour, density and depth distribution.
Discrepancies in the acoustic estimates of biomass,
presumed to be similar, were observed for these areas. Several
sources of bias have been considered to explain the survey
results (Rttingen et al., 1994), among which vessel
avoidance and inaccuracy in target strength are considered to be
the most important.
New data on herring target strength (Ona et al., 2001,
submitted for publication; Ona, 2003) confirm earlier
speculations (Blaxter and Batty, 1984; Ona, 1990; Huse
and Ona, 1996) on herring target strength being depth- or
pressure-dependent. The correction of herring abundance
estimates by applying the new, pressure-dependent target
strength (Vab, 1999; Ona et al., submitted for publication)
may significantly decrease the discrepancies observed in the
acoustic-abundance estimates. It also encourages a deeper
theoretical and experimental study of the acoustic scattering
by herring with a depth-compressed swimbladder.
This was the main motivation for our earlier paper
(Gorska and Ona, 2003), in which the depth-dependent,
averaged-backscattering cross-section of herring was
studied. The main parameters controlling the backscattering
were discussed. The modelling results were compared with
the measured averaged-backscattering cross-section data.
The importance of the swimbladder compression in the
explanation of the measured, depth-dependent herring target
strength was demonstrated. The present paper supplements
the previous study. Here the depth-dependent,
backscattering cross-section of individual herring at normal or
nearnormal dorsal incidences is considered in detail. The terms
dorsal-aspect backscattering cross-section and dorsal
incidence will be used in case of normal or near-normal
dorsal incidences, for which the backscattering cross-section
reaches a maximum. The analysis of the dorsal-incidence
case for individual fish improves the understanding of the
main parametric dependencies obtained in the echo-average
case (Gorska and Ona, 2003). Moreover, the previous study
of mean-backscattering cross-section demonstrated, in its
comparison to the data, how the individual swimbladder
dimensions may vary with depth. The data for dorsal
incidence were also subtracted (...truncated)