This article reports theoretical values of target strength (TS) for mesopelagic lanternfishes based on morphological measurements of their swimbladders. Three species of lanternfishes, Diaphus theta (26.9–77.4 mm standard length (SL)), Symbolophorus californiensis (85.0–108.4 mm SL), and Notoscopelus japonicus (126.0–133.2 mm SL), were examined. After external morphological measurement of the fish body, a specialized “soft X-ray” imaging system was used to map the swimbladders and obtain their morphological parameters. The swimbladder was inflated in D. theta, uninflated in S. californiensis, and was absent in N. japonicus. For D. theta, the swimbladder length does not increase in proportion to the body length, suggesting that the contribution of the swimbladder to acoustic reflection is reduced with growth in this fish. Based on the morphological measurements, the theoretical TS of the fish at 38 kHz was calculated using the approximate deformed-cylinder model (DCM) and the general prolate-spheroid model (PSM). For all three species, the calculations showed about 3 dB difference between the TS indicated by the DCM and PSM. Given that the description of body shape is poor in PSM, the DCM results were adopted for fish without a swimbladder or an empty one. The intercept b20 in the standard formula TS = 20 log SL + b20 was −85.7 dB (DCM) for S. californiensis and −86.7 dB (DCM) for N. japonicus. On the other hand, the PSM model was adopted for D. theta since its swimbladder has too small an aspect ratio to apply the DCM. For D. theta, the relationship between SL and TS is best expressed by TS = 11.8 log SL − 63.5, which implies that its scattering cross-section is not proportional to the square of the body length.
Yasuma, H., Sawada, K., Takao, Y., Miyashita, K., and Aoki, I. 2010. Swimbladder condition and target strength of myctophid fish in the temperate zone of the Northwest Pacific. – ICES Journal of Marine Science, 67: 135–144. We report theoretical values of the target strength (TS) of four myctophid fish (Ceratoscopelus warmingii, Myctophum asperum, Diaphus garmani, and Diaphus chrysorhynchus) based on morphometry of the swimbladder. None of the D. chrysorhynchus had an inflated swimbladder, but the other species had both inflated and non-inflated swimbladders, depending on body size. The relationships between swimbladder and body length showed that once gas production started, the swimbladders grew faster than the rest of the body (positive allometric growth). However, M. asperum showed regression of the swimbladder after positive allometric growth, so larger specimens had non-inflated swimbladders. Based on the measurements of swimbladder and body length, the theoretical TS values at 38 and 120 kHz were calculated using existing sound-scattering models. In fish with inflated swimbladders, TS values were relatively low (less than −67 dB, reduced TScm) at both frequencies. Regression slopes on TS–body length (log) plots were >20, suggesting that their scattering cross sections were not proportional to the square of the body length. In contrast, the TS values of M. asperum decreased with growth in large fish (60–80 mm long) through swimbladder regression. Scattering cross sections of fish without swimbladders were not proportional to the square of the body length over the whole size range.
The acoustic characteristics of biological backscattering in the western North Pacific were studied to verify expert knowledge on species composition in the echosigns. The survey was conducted in the KuroshioOyashio inter-frontal zone and the subarctic waters in April 2003. The species composition of backscatterings was identified by using midwater trawl, Isaacs-Kidd midwater trawl and Bongo net. The differences of mean volume backscattering strengths between 120 and 38 kHz (DMVBS 120)38 ) were calculated for the backscatterings. Six types of backscatterings were classified based on the results of net samples and the DMVBS 120)38 . The DMVBS 120)38 of each group was as follows: copepods 13.7-17.3 dB, krill 11.6-15.3 dB, Japanese anchovy )1.6 to 1.1 dB, a myctophid, Diaphus theta )0.8 dB, sand lance 6.1 dB and larvae and juveniles of pelagic and mesopelagic fish )9.6 to )4.0 dB. The results suggest that biological backscatterings in the Kuroshio-Oyashio inter-frontal zone and subarctic waters of the western North Pacific in spring can be characterized by using DMVBS.
The relationship of target strength (TS), tilt angle and the effect of swimbladder resonance were examined using a theoretical scattering model for Maurolicus japonicus (30.4-46.8 mm standard length (SL)) at 38 kHz and 120 kHz, based on swimbladder measurements. The effect of tilt angle on TS was seen at 120 kHz, but was minimal at 38 kHz. The effect of resonance on TS was clear at 38 kHz, but was minimal at 120 kHz. This was due to the small size (average 11 % of SL) and oval shape with a relatively high aspect ratio (average 0.48) of the swimbladder. Considering the effect of resonance, the TS-SL relationships at 38 kHz were estimated: TS = 10.0 log 10 SL − 61.3 at 200 m depth. Considering the effect of tilt angle, the average TS (TS avg) −SL relationship at 120 kHz was estimated: TS avg = 16.5 log 10 SL − 75.1.
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
Sound-speed and density contrasts (h and g, respectively), important acoustic material properties, of Japanese sandeel Ammodytes personatus were measured to estimate theoretical target strength (TS). The measured sound-speed contrast of adult fish varied between 1.016 and 1.023 (mean, 1.020), which showed temperature dependence. The measured density contrast differed significantly between juvenile and adult. The density contrast of juvenile varied between 1.017 and 1.024 (1.021), and that of adult varied between 1.026 and 1.038 (1.032). Using these results, TS at 38 and 120 kHz in the fishing season were estimated by an empirical sound scattering model. TS of an individual fish varied significantly with change of tilt angle. TS of near dorsal aspect (TS max ) and tilt-averaged TS (TS ave ) differed up to 7 dB. At both frequencies, two different TS ave -length relationships (TS ave = a log L + b) were obtained for adult and juvenile. The coefficients of log L of adult were close to 20, suggested that backscattering strength was proportional to the square of body length. These values were larger in juvenile (34.0 at 120 kHz, 56.5 at 38 kHz), suggested that backscattering strength varied drastically with the cube or fifth power of body length.
Sustainable management of the kelp forests of the Shiretoko Peninsula, a World Natural Heritage site, is necessary due to kelp's ecological and economic importance. The objectives of this study were to estimate the area of kelp forests and to clarify their spatial characteristics in coastal waters of the Shiretoko Peninsula. Data on the presence/absence and thickness of kelp forests were collected via acoustic observation on transects over about 80 km using a echosounder at 200 kHz. Acoustic data were geostatistically interpolated, and the areas covered by kelp forests were estimated. Differences in kelp distribution between the eastern and western sides of the peninsula were compared. The total area of kelp forest was 3.88 km 2 (eastern area: 3.49 km 2 ; western area: 0.39 km 2 ). The range of thickness of the kelp forests was 34-91 cm. Many kelp forests in the eastern area were thick (>78 cm) and distributed continuously, while kelp forests in the western area were sparsely distributed.
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