The experiments described in this paper relate known target configurations under controlled conditions to acoustic characteristics of multiple moving fish. We wanted to further our understanding of the interactions between targets and the effects these interactions have on the measurement of the number of salmon migrating in rivers. Multiple targets in various configurations were passed through a horizontally oriented 4° × 10° beam from a split-beam echo sounder. The effects on measurements of target strength, detection probability and target location in the beam are presented. The observed target strength was not dependent on target velocity. There was a reduction in target detection due to the single-target selection criteria implemented by the hydroacoustic system. We mimicked the conditions in a river where a close range fish target may modify the beam geometry allowing detection of previously undetected targets. We demonstrated some of the effects resulting from moving targets into radial alignment and we demonstrated shadowing conditions that can cause extinction of target echoes.

Acoustic surveys of aquatic organisms are notorious for large data sets. Density distribution results from these surveys are traditionally graphed as two-dimensional plots. Increasing information content through wider acoustic frequency ranges or multiple angular perspectives has increased the amount and complexity of acoustic data. As humans are visually oriented, our ability to assimilate and understand information is limited until it is displayed. Computer visualization has extended acoustic data presentation beyond two dimensions but an ongoing challenge is to coherently summarize complex data. Our goal is to develop visualizations that portray frequency- and behavior-dependent backscatter of individual fish within aggregations. Incorporating individual fish behavior illustrates group dynamics and provides insight on the resulting acoustic backscatter. Object-oriented applications are used to visualize fish bodies and swimbladders, predicted Kirchhoff-ray mode (KRM) backscatter amplitudes, and fish swimming trajectories in three spatial dimensions over time. Through the visualization of empirical and simulated data, our goal is to understand how fish anatomy and behavior influence acoustic backscatter and to incorporate this information in acoustic data analyses.

The fish and invertebrate behavior of the Ubol Ratana Reservoir, Thailand, were monitored using up- and downlooking split beam sonar located at a fixed location. In the same area and period, ichthyoplankton nets and multimesh gillnets were used. The bulk of targets, recorded by acoustics and direct capture, consisted both of fish 3–4 cm long and insect larvae 0.2–1 cm long. Diurnal patterns of behavior were very distinct: during the daytime, invertebrates were hidden in the bottom and most fish stayed in compact shoals. Time course of acoustic fish biomass and abundance was very variable due to shoaling. Only the largest fish were recorded as solitary targets. At night, the whole acoustic range was filled with targets and the time course of fish biomass (5–15 kg ha–1) and abundance (20–45 thousand individuals ha–1) were more constant. The biomass increased mostly at surface layers. Fish appeared in the evening in the water column 1 h earlier and stayed there in the morning 1 h longer than invertebrates. Dawn and dusk are good periods for studying fish before invertebrates outnumber them. Apart from fish, according to the target strength, swimming speed and depth distribution, at least four groups of water invertebrates were distinguished acoustically, some with extremely fast vertical movement (7–9 cm s–1 vertical speed). Comparison of up- and downlooking observations gave comparable results in midwater layer outside the near-field of the transducer. The uplooking approach can be more suitable for night records; downlooking for the day.

When measuring the flux of migrating salmon in rivers there are often multiple fish in the beam simultaneously. To obtain accurate measurements of flux, an understanding of the effects of multiple targets is required. Multiple targets in various configurations were passed through a horizontally-oriented 4° × 10° beam from a split-beam echo sounder. The effects on target strength, detection probability, spatial and temporal patterns of echoes, and measurement of target position are presented. When multiple targets were in the beam at the same range, the target strength was positively biased, whereas, when they were in-line at different ranges, the target strength and detection probability were negatively biased for the shadowed targets. Targets at the same range, but separated in the vertical or horizontal direction, produced characteristic patterns in the X vs. ping and Y vs. X plots. Similar patterns were found in routine observations of migrating salmon, allowing the identification of some multiple-target events. Identification of these events can aid in the correct interpretation of migrating fish data for flux measurement.

Two independent surveys showed that aquatic insects can generate echoes comparable with the echoes of small fish. In the open water of two fishless mountain acidic waters of the Czech Republic, a very distinct population of well-defined single targets resembling fish (target strength –66 to –50 dB at 120 kHz) was discovered. These targets were found in the depth range of 5–7 m during the day (June–July 1997). At night, most targets were recorded by horizontal beaming (depth < 3 m). Towing of ichthyoplankton nets in these horizons revealed the presence of ‘water bugs’, Corixidae (Hemiptera), mostly genus Glaenocorisa. Target strengths of corixids was also verified by direct observations in an experimental tank, where it was possible to track side, dorsal and ventral aspects. In three Dutch reservoirs, Petrusplaat, Honderd-en-Dertig and De Gijster (area 100–300 ha, maximum depth 15–28 m, with fish presence) the targets of ascending dipteran pupae (Chironomidae, Procladius signatus) with target strength of –74 to –65 dB were observed by vertical beaming. The presence of pupae was verified by horizontally towed ichthyoplankton nets and by vertical plankton hauls inside the echosounder beam. The contribution of aquatic insects to the fish biomass was small but they can interfere in juvenile fish assessment.

A method is presented to estimate and diagnose the sources of uncertainty in acoustic survey measures of fish density. The method involves simultaneous Monte Carlo simulation of density and uncertainty resulting from imprecision in all terms in acoustic analyses. Known bias can also be considered. Results are presented from theoretical simulations based on assumed distributions of fish density and acoustic parameters at sample sizes from 10–1000, and on survey data for Atlantic cod and redfish. Uncertainty can be reduced by either increasing the sample rate or by decreasing the error in input variables (system and ocean parameters, backscatter, target strength, species identification, detectability). Uncertainty in inshore cod surveys in Newfoundland waters can be attributed for the most part to heterogeneous fish distribution and detectability variance (total R2 = 0.64). Uncertainty in offshore redfish surveys is attributed to heterogeneous fish distribution, and variance in target strength and species identification (total R2 = 0.82). Uncertainty can be reduced by survey design, not only by the classical methods of achieving less diverse measures of backscatter, but also by increasing precision in the input parameters to the density estimate, in particular, target strength, detectability, and species identification.

Under low signal-to-noise conditions, measurements of the acoustic backscattering cross-section σbs depend upon the position of the target in the beam. Split- and dual-beam measurements of σbs increase as the target moves away from the acoustic axis, leading to substantial overestimates of acoustic size for off-axis targets. We show how this phenomenon can be largely explained by measurement error in split-beam estimates of position, and by the effect of a minimum voltage threshold. We demonstrate a two-step correction to remove most of the bias. First, smoothed estimates of position are used to obtain an improved estimate of the beam directivity effect for each echo. Second, the statistical bias induced by the minimum voltage threshold is estimated and subtracted.

The side-aspect acoustic target strengths (TS) of 19 Atlantic salmons (Salmo salar), 16 brown trouts (Salmo trutta), 10 whitefish (Coregonus lavaretus) and 9 pikes (Esox lucius) were measured using a 200 kHz split-beam echosounder, in order to study the relationship between TS and fish size indices (length, weight and side area). The effect of side aspect angle on TS was also studied. Linear models between TS and the logarithm of the fish size indices were fitted with length being best for predicting TS. Typically, the standard error of estimate was 1.2–2.9 dB. The side-aspect TS measurements with specimens of known size showed that the linear relationship between full side-aspect TS and the logarithm of fish length for salmonid (Salmo salar + Salmo trutta) was on average 4.7 dB (SE = 0.7), lower than for whitefish and pike combined. The effect of side aspect angle on TS was modelled with cos3 (2α function. The differences in the TS between full side aspect and head/tail aspect were 17.4, 19.0 and 19.6 dB for salmonid, whitefish, and pike, respectively.

Acoustic surveys for determining fish abundance in shallow water present many challenges. One constraint to down-looking (or vertical) surveys is that the sampling volume is limited due to the short range and typically narrow beams of acoustic transducers. By utilizing more of the acoustic beam, into the side-lobes for example, the sampling volume may be increased. Although the echo energy contribution from the side-lobes is usually small, these echoes may influence target strength estimation and echo counting, and by extension affect abundance estimation either directly or indirectly. With the advent of wide dynamic range echosounders, it is possible to assess both small and large acoustic-size targets. In this case, it is possible to collect signals from large targets in the beam side-lobes, and therefore an algorithm to use side-lobe data is proposed. The smoothed Expectation Maximization method (EMS) is used to estimate densities and target strength of fish from only main-lobe and main-lobe with side-lobe observations in simulation, and suggestions are made for real data from surveys.

In order to manage the fish stock of the Senegal coastal zone it is necessary to carry out direct assessment surveys by acoustic methods. Acoustic surveys were carried out in this region from a sports-fishing boat, using a split-beam echo sounder in vertical mode, in areas with depth less than 15 m. On a short time scale, spatial structure of the fish population remained stable. The average acoustic biomass detected was variable, but we observed a relative stability of target strength distribution of single fish. The use of acoustic methods in these areas of shallow depths, the consequences of escape behaviour of the fish, and sampling strategies are discussed.