Mass mortality of Jesogammarus annandalei, an amphipod species endemic in Lake Biwa, has been recently observed. J. annandalei is useful in fisheries; therefore, analyzing its occurrence is important for conserving the species and maintaining biodiversity. The acoustic method can be used in monitoring the distribution and abundance of J. annandalei. Therefore, along with measuring total body length (TL), we measured the acoustic factor target strength (TS), density contrast g, and sound speed contrast h necessary for the application of the theoretical model (distorted-wave Born approximation model) of J. annandalei. The results indicated g = 1.0725 and h = 1.003, and the measured TS values agreed well with the estimated ones. The relationships between the average TS and TL at 38, 120, and 200 kHz were TS = 60.4logTL- 165.6; TS = 53.4logTL- 141.4; and TS = 46.5logTL- 129.0, respectively. Overall, this study advances our understanding of the acoustic properties of J. annandalei, potentially aiding in future acoustic monitoring, particularly for identifying the cause of similar mass mortality events.

Target strength (TS) is essential for acoustic fish biomass estimation. Traditional narrowband surveys, which are constrained by single-frequency TS measurements, limit species identification. In contrast, broadband technology, which utilises continuous frequency ranges, enhances differentiation through TS frequency response. The aim of this study was to investigate whether the structural differences in swim bladders between physostomous and physoclistous fish produce distinguishable TS characteristics. Pacific herring (Clupea pallasii) and Pacific chub mackerel (Scomber japonicus) were selected to represent physostomous and physoclistous species, respectively. TS measurements were conducted using the tether method in freshwater and seawater tanks, employing calibrated split-beam quantitative echo sounders (Simrad EK80 scientific echosounder system) within the range of 45-260 kHz. After the acoustic measurements, the theoretical TS estimated by the Kirchhoff ray mode model was compared with the measured results. The maximum TScm increased with increasing L/lambda (ratio of length (L) to wavelength (lambda)) for both species. The tilt angle distribution was assumed to be represented by a normal distribution (mean value: -5 degrees, standard deviation: 15 degrees) regarding average TScm. Pacific herring showed an overall decreasing trend, whereas Pacific chub mackerel showed no significant trend. At L/lambda < 4.5, significant differences were observed in maximum and average TScm and L/lambda characteristics, with Pacific herring exhibiting a marked decrease and Pacific chub mackerel showing a slight increase. These results may improve the accuracy of fishery resource assessments of these two species, assist in their identification, and provide a reference for differentiating between physostomous and physoclistous fish.

Broadband backscattering measurements of Pacific mackerel (Scomber japonicus) can improve acoustic surveys of the species for the management of its fisheries throughout the Pacific Ocean. The determination of its target strength (TS), the logarithmic form of the backscattering cross-section, is the aim of this work. It was measured for fourteen individual specimens, eight in a freshwater tank and six in a seawater tank, using calibrated broadband echosounders spanning the frequency band 24–84 kHz. The TS is expressed as a function of frequency and tilt angle, with fish length as a parameter. The individual broadband TS patterns with the tilt angle of fish showed size and frequency dependencies. The fish length-normalized TS of mackerel decreased with increasing fish length-to-acoustic wavelength ratio (l/λ) in the small l/λ range (approximately 2–6) but was flat in the larger l/λ range (>6). This variation in the normalized TS indicates that a pair of regression equations is necessary to span the range of commercially important mackerel relative to the acoustic wavelength. The relative l/λ characteristic of the normalized TS showed constant values with tilt-angle distributions over a large l/λ range and can be used as a characteristic of acoustic backscattering for discrimination among species.

Measurement of target strength (TS) is important for estimating the abundance of species using fisheries acoustics. However, most researchers have only used a limited number of representative frequencies for acoustic measurements of fish without a swim bladder (bladderless fish). Here, we measured the broadband TS of three bladderless fish species, arabesque greenling (Pleurogrammus azonus), Pacific sand lance (Ammodytes personatus), and pointhead flounder (Cleisthenes pinetorum), using two broadband echosounders. TS measurements were conducted in a seawater tank over frequency ranges of 45-90 and 80-120 kHz using a tether method. Higher TS and directivity were observed at higher frequencies than at lower frequencies for pointhead flounder and arabesque greenling. However, the TS for Pacific sand lance was relatively flat over the measured frequency spectra. Additionally, the TS of pointhead flounder and Pacific sand lance could be expressed as a function of body length and the TS of arabesque greenling could be expressed as a function of body length and frequency, which could be used in fish species discriminations and size estimations.

Measurements of the broadband acoustic backscattering from fish should improve acoustic discrimination between species. The pulse compression processing of broadband systems can be used to measure acoustic backscattering with high range resolution and improve signal-to-noise ratio. This may increase opportunities for in situ target strength (TS) measurements, the preferred method of collecting TS data. To evaluate the availability of TS spectra for acoustic discrimination, three Simrad EK80 wideband transceivers and split-beam transducers of 70, 120, and 200 kHz were used to collect in situ frequency responses of TS from age-0 juvenile walleye pollock and pointhead flounder, a swim-bladderless flat-fish, distributed in and around Funka Bay, Hokkaido, Japan. The single echoes were extracted from backscattering data, and the TS spectra of the two species were obtained. However, processing of the broadband acoustic data is under discussion. To ensure the reliability of our data, the TS spectra of standard targets were also calculated. The measured TS spectra of the standard targets were close to the theoretical TS spectra. The individual TS spectra of two species included complex frequency response; however, the mean TS spectrum was relatively stable. The different characteristics of the frequency responses observed for the two species were in good agreement with reported observations. The results available for the acoustic discrimination were similar to the multi-frequency method, which uses two or more single frequencies. The raw TS spectra should aid in not only discriminating between species but also estimating the size of the fish.

The acoustic characteristics of bladderless fishes were examined by measuring the target strength (TS) of three species, pointhead flounder (flat-shaped), Arabesque greenling (spindle-shaped) and sandeel (cylinder-shaped). TS measurements were collected in a seawater tank (10 × 6 × 5 m) using the EK80 echo sounder (Simrad, Norway) and ES70-7C transducer (Simrad) over a frequency range of 45–90 kHz with a tether method. After TS measurement of the whole fish, we separated the flesh from the body (for pointhead flounder and arabesque greenling) and measured the TS of the head and bones. Pitch angle characteristics were measured from a head-down orientation (−30 deg) to a head-up orientation (30 deg), and TS was processed using Echoview 9 software (Echoview, Australia). TS patterns were examined with respect to orientation using the distorted-wave Born approximation model. The standard length, width, and height of fish were inputted into the model. The three bladderless fishes tested have different body shapes, meaning different areas are exposed to sound waves at the same body length. Therefore, the relationship between TS and the cross-section of the dorsal aspect of bladderless fish is discussed. [Work supported by the Sasakawa Scientific Research Grant from The Japan Science Society.]

When using acoustic methods to investigate fish biomass, it is important to distinguish the target species from other organisms. Previous limitations to acoustic information in surveys meant it was not possible to discriminate juvenile walleye pollock from their predator, pointhead flounder, in Funka Bay, Japan, possibly leading to overestimates in population size. Thus, here, an acoustic method was developed to distinguish the two species based on relative frequency response. This method was used to survey juvenile walleye pollock to evaluate capture potential around Funka Bay, Japan, where pointhead flounder is reportedly its most abundant predator. Volume backscattering strength of pointhead flounder was highest at 200 kHz, while that of juvenile walleye pollock was highest at 38 kHz. Therefore, the relative frequency response at 38 kHz and 200 kHz was successfully used to distinguish between the two species. This method could be more extensively applied to other species in other aquatic environments globally.

There are only a few acoustic surveys of the bladderless fish. As is known by all, Atlantic mackerel was surveyed these years by echo-sounder and showed the high target strength at high frequency. In northern Japan, an interesting fish attracted the attention these years which called pointhead flounder, Cleisthenes pinetorum, without a bladder. It is a flatfish but captures prey in the middle water column. And the body shape is not the normal spindle-shaped, but a flat shape, which may cause different acoustic characteristics with Atlantic mackerel. We did the surveys in Funka Bay, northern Japan and keep the samples to the tank to measure the acoustic characteristics of live flounder. The acoustic characteristics of pointhead flounder measured both using the tether method (38, 120 kHz) and free swimming method (120 kHz). During the measurements of free swimming method, we also observed the swimming actions of pointhead flounder and calculated the swimming angle at the same time. We also discussed the characteristics of TS of the swimming angle.

This study addresses the methods for distinguishing the pointhead flounder Cleisthenes pinetorum from the juvenile walleye pollock Gadus chalcogrammus in and near Funka Bay, Hokkaido, Japan. Acoustics data of th Gadus chalcogrammus e fish were monitored using a Simrad EK60 (38, 120, 200 kHz) split-beam echo sounder, and biological samples were collected using a rod for the pointhead flounder and a frame-type midwater trawl for the juvenile walleye pollock. The pointhead flounder schools presented a patch shaped echo on the echograms and showed strong scattering at all three frequencies, whereas the distribution patterns of the juvenile walleye pollock schools were layered. The volume backscattering strength (SV) of the target schools extracted from the echograms showed that the pointhead flounder presented a higher SV at high frequency, which is consistent with the early surveys of other bladderless fish. In contrast, the juvenile walleye pollock showed higher SV at low frequency, which also agreed with the early surveys.

In this study, the acoustic differences between the pointhead flounder and the juvenile walleye pollock were examined using a quantitative echo-sounder around the Funka Bay, Japan and the acoustic characteristics of the pointhead flounder have been surveyed. Acoustics data of the fish were monitored using a Simrad EK60 (38, 120, 200 kHz) split-beam echo sounder in the field. The target strength (TS) and swimming angle of free swimming pointhead flounders were measured in a seawater tank (length: 10 m, width: 5 m, and height: 6 m). As the result, pointhead flounder schools presented a patch shaped echo on the echograms, whereas the distribution patterns of the juvenile walleye pollock schools were layered. The volume backscattering strength (SV) of the target schools extracted from the echograms showed that the pointhead flounder presented a higher SV at high frequency. In contrast, the juvenile walleye pollock showed higher SV at a low frequency. For pointhead flounder, the distribution of pitch angle was measured both by camera and echo-sounder at the experiments in the tank and shown the same distribution pattern. The TS of pointhead flounder is large bigger than other bladderless fish.