Milling behaviour is problematic when using hydroacoustics to estimate the number of migrating fish in rivers. Milling behaviour was observed for adult sockeye salmon migrating upstream in the Wannock River, to their spawning grounds in the tributaries of Owikeno Lake in the central coast area of British Columbia, Canada. We classified the acoustic salmon tracks to separate the milling fish from the actively migrating fish in an attempt to obtain an estimate of sockeye salmon flux as they migrate to their spawning grounds. We used discriminant function analysis and found that three variables measured on each track were sufficient for the classification of the acoustic tracks into milling and non-milling categories with an approximate classification accuracy of 98%. The method we present can also be used to separate tracks of targets of interest from noise or debris tracks that occur in the acoustic data, if discrete track characteristics are ascertained.

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.

The Gambia Estuary is one of the last estuaries in West Africa not to have been affected by strong environmental changes and adverse human disturbances. In order to describe the spatio-temporal change in fish biomass in this estuary, and the seasonal changes in fish size classes, five research surveys have been conducted at characteristic periods in the river's water regime and meteorological seasons. The fish assemblages were sampled by two vertical echosounding protocols: mobile surveys (zigzag transects) in the estuary and moored stations. The two sampling methods produced a similar image of the fish assemblages in terms of the distribution of total biomass and target strength distributions. Large fish aggregations were detected in the most marine area at the end of the wet season, and fish biomass increased through the dry season in the upstream zone. Fish were smaller at the beginning of the wet season and they were always larger in downstream areas.

A hydroacoustic monitoring technique to quantify and assess the ecological requirements for migration of the anadromous clupeid, Alosa fallax fallax (twaite shad) was developed, and its effectiveness studied, on the River Wye in Wales. The acoustic monitoring technique was a side aspect application, with two transducers fixed permanently to the riverbank and the acoustic beam from each aimed horizontally across the river towards the opposite bank, perpendicular to flow. Two split-beam echo sounders and transducers were deployed, each operating at different frequencies (200 and 420 kHz). Using a combination of these two frequencies it was possible to demonstrate that shad show bold avoidance behaviour to sound transmitted at 200 kHz and would not pass the monitoring site when sound was transmitted at this frequency. They remained unaffected by sound transmitted at 420 kHz and were observed migrating upstream in large, loosely aggregated shoals. From visual observations above and below the water, shoals were estimated to comprise of many hundreds of individuals, covering a size range of between 30 and 45 cm. Only a few individuals could be resolved by the acoustic system operating at 420 kHz, and it was therefore, not possible to obtain a count of fish by "target tracking" single shad. However, by transmitting 200 kHz sound pulses on a 50% duty cycle the seasonal and daily patterns of shad migration were derived from the analysis of data gathered by the acoustic system operating at 420 kHz.

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.

Split-beam positional estimates of fish detected in a river at close range often do not correspond to the actual position of the target. These inaccuracies create problems in determining whether a fish is moving upstream or downstream. We hypothesize that these positional estimates are degraded by two factors: size of target relative to beam diameter, and the complex scattering of the fish. These parameters create a near-field effect, within which the phase measurements of the returning echoes are corrupted. Examples of fish tracks from near and far range fish detected by a split-beam echo sounder are provided to illustrate these inaccuracies. Experimental data from tethered spheres and complex targets show increasing distortion with target complexity and proximity to the transducer.

Experiments conducted at the Qualark Creek acoustic site on the Fraser River have shown unexpected systematic errors in the split-beam angle measurement. A tethered dead salmon with a target strength of approximately –30 dB was used as a target at 3.7 m range. A signal-to-noise ratio of ~12 dB was observed. This target strength and signal-to-noise ratio are typical in some rivers where migrating adult salmon are observed. The target's location was measured both from the frame used to position the target in the beam and from the acoustic data. Comparison of these two sets of measurements demonstrated a bias that increased with the target's distance off the beam axis. The acoustically measured target locations were closer to the beam axis than the actual locations. Each acoustic estimate represented the mean location obtained after 3 000 pings. Results from a simulation model that includes the effects of random noise on the split-beam angle measurement reproduce the systematic underestimate of these angles. This bias increases rapidly with increasing off-axis target location and decreasing signal-to-noise ratio. The bias has been observed for circular and elliptical beams and the model accurately predicts the magnitude and direction of the systematic part of the angle measurement error in both cases. Results for a circular transducer are used here to illustrate the situation. The presence of this bias can affect measurements of fish density and migration speed. At the low signal-to-noise ratios often obtained in riverine environments, those who use a split-beam system for observation or estimation of fish should be aware of this phenomenon.

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.