Fish Target Strength Research Articles

Diel dynamics of acoustic measurements of Baltic fish

Although acoustic methods play an important role in monitoring vertical and horizontal fish distribution, for example for fish stock assessment, the fish community as an acoustic target is not precisely specified and is seldom related to behavioural reactions. This paper describes a new type of approach to enhance the possibilities of analysing the time-depth diel variability of the acoustic parameter, s v , which characterizes the volume density of fish echoes, and is closely related to the backscattering cross-section of fish.Multi-channel integration of echoes was applied to estimate the average diel models of s v -dependence on the time of day in separate depth layers. The final characteristics were calculated by computer as a vectoral field of gradients, expressing quasi-continuous diel transformation of the time-depth distribution of s v values. Examples based on data collected during RV “Baltica” research cruises carried out in October in 1994-1996 are given and discussed. A high level of variability in s v values with time and depth was found; diel phases of fish behaviour were characterized. Similarity among the cruises indicated a highly repeatable pattern of fish behaviour. The differences were mostly correlated to water temperature distribution. Analysis of the results indicates the necessity of further research on fish target strength, as a group behavioural phenomenon, within separate periods of the diel cycle to define biomass conversion factors more accurately. The method presented could significantly enrich detail and lead to more systematic physiological studies on fish behaviour.

Inverse problem solution techniques as applied to indirect in situ estimation of fish target strength

In situ indirect methods of fish target strength (TS) estimation are analyzed in terms of the inverse techniques recently applied to the problem in question. The solution of this problem requires finding the unknown probability density function (pdf) of fish target strength from acoustic echoes, which can be estimated by solving the integral equation, relating pdf's of echo variable, target strength, and beam pattern of the echosounder transducer. In the first part of the paper the review of existing indirect in situ TS-estimation methods is presented. The second part introduces the novel TS-estimation methods, viz.: Expectation, Maximization, and Smoothing (EMS), Windowed Singular Value Decomposition (WSVD), Regularization and Wavelet Decomposition, which are compared using simulations as well as actual data from acoustic surveys. The survey data, acquired by the dual-beam digital echosounder, were thoroughly analyzed by numerical algorithms and the target strength and acoustical backscattering length pdf's estimates were calculated from fish echoes received in the narrow beam channel of the echosounder. Simultaneously, the estimates obtained directly from the dual-beam system were used as a reference for comparison of the estimates calculated by the newly introduced inverse techniques. The TS estimates analyzed in the paper are superior to those obtained from deconvolution or other conventional techniques, as the newly introduced methods partly avoid the problem of ill-conditioned equations and matrix inversion.

Developments in Indirect Methods for Estimating Fish Target Strength

Developments in Indirect Methods for Estimating Fish Target Strength

Wavelet decompositions versus singular value decomposition (SVD) in a fish target strength estimation

The fish target strength estimation when using acoustic echoes from a single-beam echosounder is possible after solving ill-conditioned equations where probability density functions (PDFs) of echo level, target strength, and beam pattern are involved. The modified singular value decomposition (SVD) algorithms are typically used in such cases. Unfortunately, this technique often leads to artifactual modes in solution due to Fourier nature of underlying eigenfunctions. The expectation maximization smoothing (EMS) method is used to improve smoothness of solution but by the cost of increasing the time of computation of estimates. D. L. Donoho (1995) proposed the wavelet-vaguelette decomposition (WVD) as being well suited for solving ill-conditioned equations with inhomogeneous functions. This concept was extended to fish target strength estimation and obtained results were compared to SVD performances.

Searching for species identifiers in multifrequency target strength variability

Acoustic research has shown that the species of some schooling fish can be assigned based solely on measurement parameters from acoustic returns. For single targets, this process is complicated by the high variability of in-situ target strength (TS) measurements. Attempts at speciation based on the patterns of TS variability were confounded by interactions between acoustic frequency and fish directivity. Initial attempts at defining speciation parameters based on multifrequency target strength measures look promising. This paper reports on the results of simultaneously measuring the in-situ target strength of fish at 38-, 120-, and 420-kHz acoustic frequencies, and proposes the definition of several speciation parameters.

A re-evaluation of relationships between fish size, acoustic frequency, and target strength

There are many measurements of dorsal aspect target strength of fish, but relatively few studies compare estimation methods or attempt to combine data from different studies into general relations between fish size, acoustic frequency and target strength (or acoustic cross-section). We edited and updated earlier summary regressions relating fish size, acoustic frequency and acoustic cross-section. Experimental measurements on marine fish were separated into swimbladder and non-swimbladder species, model results compared with experimental data, target strengths of marine, and freshwater fish were contrasted, and previous and new target strengths of commercial New Zealand fish species were placed in the context of the new regressions. Analysis of variance was used to show significant effects of species, freshwater vs. marine, swimbladder vs. non-swimbladder fish, model vs. experimental, and dead vs. alive fish on the relationship between maximum dorsal aspect target strength and fish length. For experimental results on gadoids we found significant species and live vs. dead (or stunned) fish effects on the target strength to length relationship. The quadratic dependence of target strength on fish length was also tested and found to be the exception rather than the rule. Existing pitch tilt angle data was compiled and a normal distribution with standard deviation of at least 15° was required to describe most distributions.

Application of dual-beam and split-beam target tracking in fisheries acoustics

Dual-beam and split-beam acoustical techniques were originally developed to provide direct in situ measurements of the target strength of individual fish and plankton. The amount of information that can be extracted from these acoustic systems can be significantly increased when the techniques are used in conjunction with ping-to-ping target tracking. For example, target tracking can be used to isolate multiple target-strength measurements from an individual fish or plankton. These measurements can then be averaged to provide a lower variance estimate of the target strength. The measurement of the angular location of the target provided by split-beam systems further enhances the usefulness of tracking. The angular data measured on subsequent pings can be used to resolve returns from single and multiple targets and estimate the direction and speed of the fish as they pass through the beam. Information on fish speed and direction is particularly useful for fixed location acoustic studies. This paper uses a combination of actual data, simulation results, and analysis to describe the various applications and expected performance of acoustic systems that combine dual-beam and split-beam techniques with target tracking.

Depth dependence of target strength of live kokanee salmon in accordance with Boyle's law

The target strength (TS) offish varies with a number of factors, especially the presence or absence of a swimbladder. This study investigated the eVect of the swimbladder on the TS of live kokanee (Oncorhynchus nerka) in relation to depth. Experiments were conducted at Lake Kuttara, Hokkaido, Japan, which is abundant with kokanee. Fish samples were caught with a fishing net. The live fish were suspended on the sound beam axis, and tilted from 50) (head-down aspect) to +50) (head-up aspect). The dorsal aspect TS functions were measured at 50 kHz. Measurements were performed when the fish were forced to change depth rapidly. The TS values were measured as a function of tilt angle and depth, and the maximum dorsal aspect TS (TSmax) values and the averaged dorsal aspect TS (TSavg) values were calculated and compared. TSavg values were calculated with respect tofish tilt-angle distribution, which was assumed to have a mean value of 5) and a standard deviation of 15). Both TSmax and TSavg decreased with depth in accordance with Boyle’s law, i.e. a reduction rate of 6.7 dB per 10 atm was observed. ? 1996 International Council for the Exploration of the Sea

Modeling of target strength of swimbladder fish at high frequencies.

An aim of this paper is to compare a proposed simple model for the target strength (TS) of swimbladder fish with existing models. Through the comparison we verify the validity of the simple model. Particularly, two more sophisticated models are compared with: namely, the exactly solvable soft spheroid model and the ray-Kirchhoff approximate model. The comparisons show that at high frequencies or intermediate frequencies the present model can yield results in reasonable agreement with that from the other two models respectively. Indirect comparison with experimental data is also briefly made. It is shown that the model may facilitate general calculation of the target strength of swimbladder fish.

Simple model on the relationship between fish acoustical target strength and aspect for high-frequency sonar in shallow waters

Summary The relationship between fish target strength and the orientation of fish in horizontal (yawing) and vertical (pitching) planes was modelled by the equation TS = m. cos 2 α+ b (also with cos3, cos5 and cos7). The cos3 model gave the best fit of data for three fish species: brown trout, roach and rudd. The model can be used for 1) estimation of the most probable aspect in the horizontal plane; 2) estimation of the mean all-aspect target strength of fish and 3) calculation of probability of recording of a fish of given size when a known noise threshold is applied.

Determination of Composition and Vertical Structure of Fish Communities Using in situ Measurements of Acoustic Target Strength

A split-beam echo-sounder was used to estimate in situ acoustic target strengths (TS) of fish from a number of different fish communities on the South African continental shelf. The TS and size distributions (obtained by aimed trawling) were used to describe the size structure and vertical distribution of these communities. TS distributions obtained from several monospecific populations of juvenile Cape hake (Merluccius capensis), the dominant species present, were self-consistent, and there was good correspondence between modes in the TS and length distributions of juvenile hake, adult round herring (Etrumeus whiteheadi), anchovy (Engraulis capensis) recruits, pelagic goby (Sufflogobius bibarbatus), horse mackerel (Trachurus trachurus capensis), and ribbon fish (Lepidopus caudatus). Average TS values for all these species, both per individual and normalized by weight, are presented and compared with published values. The use of TS information in studies of the small-scale community structure and dynamics of fish populations is discussed. It is concluded that the method can be effective on low-density, multispecific assemblages such as those in our study, avoiding many of the pitfalls of conventional net sampling.

東シナ海で漁獲された魚の背方向ターゲットストレングスとうきぶくろとの関係

The target strength (TS) of fish varies with acoustic frequency, fish size, fish activity, orientation, swimbladder and so on. Among these, the swimbladder is recognized as having the most important effect on TS. The dorsal aspect TS as a function of tilt angle of fish caught by bottom trawling in the East China Sea was measured at two frequencies (25 and 100 kHz). The swimbladder sizes (length and height) and inclination angle of the swimbladder to the lateral line of the fish were measured by X-ray photographs. TS and swimbladder data were then compared and analyzed, and the following results were obtained: (1) The eight species of fish, except the silver pomfret, have swimbladders, but the angle and size of the swimbladder varies. (2) For bladder fish, the angle of the maximum dorsal aspect TS agrees with the broadside direction of the swimbladder. (3) The dorsal aspect TS of the silver pomfret is clearly lower than that of bladder fish at the two frequencies. (4) The maximum and averaged dorsal aspect TS values of the silver pomfret, were lower by about 7-8 dB, 8-10 dB at 25 kHz, and by 8-10 dB, 9-11 dB at 100 kHz than two other bladder fish (yellow sea bream and whitefin crevalle).

水産資源の音響調査手法の現状と展望 ターゲットストレングスの推定法とその応用 統計的手法を用いた間接推定法

水産資源の音響調査手法の現状と展望 ターゲットストレングスの推定法とその応用 統計的手法を用いた間接推定法

Insitudetermination of the target strength of roach(Rutilus rutilus)in lake Bourget with a single beam sounder

There is an increasing use of acoustic methods in freshwaters. Information on the size of the fish stock, combined with information from non-acoustic studies, improve previous stock estimates. Fish target strength (TS) is essential to express echo integrator measurements in terms of fish biomass (Foote, 1987), and to estimate the proportion of the different length classes in the fish population, i.e. the different cohorts, especially the yearling fish population.

Conditions for the precise measurement of fish target strength in situ.

Conditions for precise measurement of in situ fish target strength (TS) are empirically studied and two indexes are introduced for this purpose. One is the number of fish in the effective reverberation volume which contributes echo formation at a certain instant and the other is the percentage of the multiple echoes which is derived from a residual of the single echo extraction. With the decrease of both indexes measured target strength approach a certain asymptotic value which is admitted as reliable from the past study. This shows the existence of some threshold values and below these threshold values TS measurement will be successful. The effectiveness of both indexes is confirmed by the data set obtained from one large same fish school in the eastern shelf of Bering sea during the intership calibration between Japanese and U.S. vessels on 15 and 16 August 1991.

Summary of methods for determining fish target strength at ultrasonic frequencies

Summary of methods for determining fish target strength at ultrasonic frequencies

Determination of fish number density by a statistical analysis of backscattered sound

A theory is presented for estimating the density of fish aggregations from the second normalized moment of intensity of overlapping backscattered acoustic pulses. The chief advantage over the normal echo-integration method is that no knowledge of fish target strength is required. The model assumes that the probability distribution function (PDF) of backscattered amplitudes from the individual fish is Rician, and that within regions of constant density the fish have a Poisson volume distribution. Problems arising from variations in density can largely be overcome by grouping together intensity values from regions of like density and estimating the density for each region separately. It is shown theoretically that the error in estimated density increases with the number of scatterers per resolution cell, which implies that the method is most useful at low fish densities. The theory has been tested at sea on 38-kHz backscattered pulses from two nonhomogeneous layers of small pelagic fish. The results are encouraging judged by their self-consistency and by comparison with estimates obtained by echo integration of the same signals. The method is considered to be a potential alternative or supplement to echo integration in low-density regions.

Application of a dual beam echo sounder to measuring fish length.

This paper discusses the application of a dual beam echo sounder to the estimation of fish length. The dual beam method is typically used for measurement of the in situ target strength of fish. The in situ target strength is an important parameter, not only for obtaining the echo integrator scaling factor but also for estimating the fish length that is related to target strength. A prototype dual beam echo sounder system with computer technology was developed and used in the acoustic survey conducted at Lake Kuttara. Kokanee Oncorhynchus nerka is abundant in this lake. Many individual echoes were recorded using the dual beam echo sounder and target strength data were collected. Converting target strength, TS, into body length, l, was done using the following relational formula: TS(dB)=20logl(cm)-66.0 Results of the analysis indicated that both the distributions of target strength and body length which were estimated by the dual beam method agreed to the catch data obtained from net sampling.

Estimation of dorsal aspect target strength of deep-water fish using a simple model of swimbladder backscattering

A practical model for estimating target strength of gas-filled swimbladdered fish is described using the scattering function of a finite cylinder. Two equivalent cylinders are derived from measurements of swimbladder length, surface area, and volume. The scattering functions of both equivalent cylinders were tested using data available for gadoids, then applied in the estimation of target strength of deep-water hoki (Macruronus novaezelandie). Molds of 23 hoki swimbladders were constructed for measurements of their internal dimensions. Estimates of target strength using the scattering function of the equivalent surface area cylinder agree closely with estimates by other methods for gadoids. Estimates using the scattering function of the equivalent volume cylinder are lower, but approach the former estimates when the swimbladders are fully inflated. The advantage of using the latter method is the simplicity in measuring the swimbladder volume. The dorsal aspect target strength of hoki were estimated at four frequencies, 38, 49, 70, and 120 kHz for several distribution functions of fish tilt angle.

Physiological factors causing natural variations in acoustic target strength of fish

The swimbladder is recognized as responsible for a major part of the acoustic backscattering from fish. In most fishes it has the function of a buoyancy regulator but in others its main function is rather unclear. Based on methods for exact mapping of the swimbladder shape, observations of deviations from normal appearance and shape are discussed in relation to possible effects on target strength. Evidence for both periodic variations, as from uncompensated vertical migrations, and seasonal variations, caused by the fat cycle and gonad development, are presented.