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During movement, the body angles of ribbonfish change, including a unique vertical swimming motion that influences their target strength (TS). In this study, we observed body angle changes in ribbonfish using an acceleration data logger and video recordings. Based on these data, variations in echo intensity in response to body angle changes were revealed using radiograph images of the swimbladder and the respective TS values. The frequency of a body angle >60° was 57.78% peak at 77°. The swimbladders were found to be slender ellipses inclined 3–4° upward from the body axis. The TS peaked (about −30 dB for the average pre-anal length of 223.9 ± 21.8 mm) when the body angle was horizontal or inclined 2–3° downward. The TS decreased substantially when the body angle was vertically inclined. From about 10° upward inclination to vertical, the TS fluctuated between −50 and −60 dB. In a comparison of the average TS values among body angle categories, the TS values for body angles grouped as ‘vertical’ (between 60° and 90°), showing static mode activity patterns in video recordings, were low (−55.7 ± 3.9 dB, Mean ± SD), whereas those for the horizontal body angle (between 0° and 30°) category, comprising the active mode, were high (−47.0 ± 8.7 dB) (P-value < 0.01). In the natural habitat, ribbonfish are known to gather at the sea floor during the daytime. In contrast, at night they disperse and move actively while foraging. Consequently, their echo intensities in natural habitats change substantially in response to their diel changes in behavioural ecology. Additional measurements or theoretical models considering the influence of ribbonfish behaviour should be undertaken as a result of this study.

Fin whales (Balaenoptera physalus) undergo seasonal migration in the Arctic Sea. Because their migration and distribution is likely affected by changes in global climate, we aimed to examine the migration timing of fin whales, and the relationship with prey availability within the oceanographic environment of the Arctic Sea, using passive and active acoustic monitoring methods. Automatic Underwater Sound Monitoring Systems were deployed in the southern Chukchi Sea from July 2012 to 2014 to determine the acoustic presence of fin whales. Furthermore, water temperature and salinity were recorded by a fixed data logger. An Acoustic Zooplankton Fish Profiler was additionally deployed to estimate prey abundance through backscattering strength. Sea ice concentrations were obtained by remote sensing data. Fin whale calls were automatically detected using a custom-made software, and the per cent of half-hours containing calls were counted. Fin whale calls were detected from 4 August to 20 October 2012 (78 d) and 25 July to 1 November 2013 (100 d). The extended period of acoustic presence of fin whales during 2013 when compared with 2012 is likely related to a longer ice-free period during 2013. Furthermore, generalized linear model analyses showed that half-hour periods containing calls increased with a rise in water temperature and zooplankton abundance during the initial call presence period, while they decreased with a decrease in water temperature and salinity during the end of the call presence period. Our results suggest that the rise in water temperature and zooplankton abundance affect the timing of migration of fin whales in a way that is consistent with the expansion of their suitable habitats and the extension of their presence in the Arctic Sea.

Yasuma, H., Takao, Y., Sawada, K., Miyashita, K., and Aoki, I. 2006. Target strength of the lanternfish, Stenobrachius leucopsarus (family Myctophidae), a fish without an airbladder, measured in the Bering Sea. e ICES Journal of Marine Science, 63: 683e692. 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 (1.002e1.009) compared with that of marine fish in general. The value of h measured by the time-average approach was 1.032e1.039 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 (

It is well known that flatfish species such as plaice can utilize the selective tidal stream to conduct vertical movements. However, detailed description of actual swimming behaviour is lacking, principally as a result of the difficulties encountered in monitoring the behaviour of flatfish in the open sea. The present study describes the use of a newly developed data-logger in obtaining simultaneous recordings of the swimming speed, depth, tailbeat, and body angle of free-ranging Japanese flounder (Paralichthys olivaceus) in the open sea. Our data indicate that Japanese flounders adopt a tailbeat-and-glide behaviour. They are found to glide downward without tailbeats for propulsion, and only during the ascent phase are tailbeats conducted. Flounders move horizontally at speeds of 0.59–1.23 km d−1 and at a maximum speed of 0.70–0.82 km h−1 in the open sea. Modal flounder swimming speeds are 30–40 cm s−1 (0.57–0.76 and 0.58–0.77 BL s−1), i.e. sometimes lower than the threshold of the speed sensor. In most cases, however, tailbeat oscillations occur at frequencies of 1.2–1.4 Hz. Moreover, flounders travel at a significantly steeper angle during the ascent phase than during the descent phase. In both cases it is believed that flounder optimize the energetic costs of migration, as has been shown for tuna, sharks, and seals.

Diurnal changes in the acoustic-frequency characteristics of ''shirasu'', a post-larval stage of Japanese anchovy (Engraulis japonicus), were analysed based on theoretical scattering models. Since post-larval swimbladders contain gas at night but not during the day, the distorted-wave Born approximation (DWBA), based on the deformed-cylinder model, which assumes there is no gas in the swimbladder, was applied to daytime observations, and the fish-bladder resonance model, which assumes there is gas in the swimbladder, to those taken at night. The two sets of acoustic frequencies employed were 50 and 200 kHz, which are those used for shirasu commercial fishing, and 38 and 120 kHz, which are usually employed for acoustic surveys using a quantitative echosounder. During the daytime, differences in shirasu target strengths between the two frequencies were large, especially between 38 and 120 kHz, but at night there was little difference with frequency, suggesting that acoustic surveys for identifying and estimating the abundance of shirasu should be done during daytime, when differences occur between frequencies and the shirasu form dense schools. 2003 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.

Yasuma, H., Sawada, K., Ohshima, T., Miyashita, K., and Aoki, I. 2003. Target strength of mesopelagic lanternfishes (family Myctophidae) based on swimbladder morphology. � ICES Journal of Marine Science, 60: 584� 591. 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 ¼ 20 log SL þ 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 ¼ 11:8 log SL � 63:5, which implies that its scattering cross-section is not proportional to the square of the body length. 2003 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights

Kang, M., Furusawa, M., and Miyashita, K. 2002. Effective and accurate use ofdifference in mean volume backscattering strength to identify fish and plankton. –ICES Journal of Marine Science, 59: 794–804.Acoustic species identification is very important for fisheries’ operations and surveys.One of the most promising methods for identification is to utilize the difference ofmean volume backscattering strengths ( MVBS) among frequencies. Improvement ofthis technique is the aim of this study. The MVBS must be obtained for a commonobservation range among frequencies so that the difference can be attributed solely tofrequency characteristics of the sound scattering of targets organisms. We derived thecommon observation range of at least up to 150 m for our quantitative echosounderoperating at 38 and 120 kHz with the same beam widths of 8.5 . We related MVBSdata obtained off northeastern Japan to specific marine organisms combined with theswimming depth and water temperature information. The echoes with small MVBS( 1dB< MVBS

The swimming angle of isada krill (Euphausia pacifica Hansen) was measured in a tank and the target strength (TS) values were calculated using a theoretical scattering model. The average swimming angle was 30.4° (s.d.=19.9°), which was about 15° less than that reported for Antarctic krill (Euphausia superba). Parameters for the swimming angle distribution were substituted into the straight cylinder model and the corresponding TS values were determined. The mean TS values of 16.4 mm isada krill for hovering animals were lower than the maximum TS; differences were 7.8-5.0 dB, 14.8-8.6 dB, and 18.1-10.8 dB at 50, 120, and 200 kHz, respectively. These differences will lead to a 1/6-1/3, 1/30-1/7, and 1/65-1/12 proportional difference, respectively, in estimating abundances compared to estimates based on the maximum TS. Measurements carried out at higher frequencies are more influenced by swimming angle than those at lower frequencies. However, the TS is less at lower frequencies and it is more difficult to filter out noise. Thus, medium frequencies are suggested as the best for krill surveys. Additionally, when conducting resource surveys by echosounding, it is necessary to assign the appropriate swimming angles for different targets. Swimming angles need to be investigated further under a variety of conditions.