Journal Article

Target strength of the lanternfish, <i>Stenobrachius leucopsarus</i> (family Myctophidae), a fish without an airbladder, measured in the Bering Sea

Hiroki Yasuma, Yoshimi Takao, Kouichi Sawada, Kazushi Miyashita and Ichiro Aoki

in ICES Journal of Marine Science

Published on behalf of ICES/CIEM

Volume 63, issue 4, pages 683-692
Published in print January 2006 | ISSN: 1054-3139
Published online January 2006 | e-ISSN: 1095-9289 | DOI: http://dx.doi.org/10.1016/j.icesjms.2005.02.016
Target strength of the lanternfish, Stenobrachius leucopsarus (family Myctophidae), a fish without an airbladder, measured in the Bering Sea

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This paper reports theoretical values of target strength (TS) for the lanternfish Stenobrachius leucopsarus, a fish without an airbladder, which dominates the Subarctic marine mesopelagic fish community. Two models for liquid-like slender bodies, the general prolate-spheroid model (PSM) and the deformed-cylinder model (DCM), were used to compute the TS of the fish relative to its orientation. The relative mass density g and the sound speed h in seawater were measured and used in both models. To confirm the appropriateness of the models, tethered experimental measurements were carried out at 38 kHz for five specimens. The value of g measured by the density-bottle method was very low (1.002–1.009) compared with that of marine fish in general. The value of h measured by the time-average approach was 1.032–1.039 at the water temperature at which S. leucopsarus is found. TS-fluctuation patterns against fish orientation (the TS pattern) estimated from the DCM and PSM were in good agreement in the area of their main lobes. Both models reproduced the main lobes of the measured TS patterns in near-horizontal orientation (<±20°), and they were considered to be effective in measuring the TS of S. leucopsarus in a horizontal (swimming) position. After these comparative experiments, we computed the TS of 57 fish (27.8–106.9 mm) at 38, 70, 120, and 200 kHz, using the DCM. A plot of body length (in log scale) against TS showed a non-linear relationship at all frequencies. S. leucopsarus had a very low TS (<−85 dB, TScm), suggesting that acoustic assessment would be highly sensitive, especially when the proportion of small fish is high (e.g. L/λ < 2), and an appropriate frequency should be considered that takes into account both the length composition and the depth of occurrence.

Keywords: body-mass density; body sound speed; deformed-cylinder model; spheroid model; Stenobrachius leucopsarus; target strength

Journal Article.  5246 words.  Illustrated.

Subjects: Environmental Science ; Marine and Estuarine Biology

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