Improved techniques for studying the temporal and spatialbehavior of fish in a fixed location

John E. Ehrenberg and Tracey W. Steig

Ehrenberg, J. E., and Steig, T. W. 2003. Improved techniques for studying the temporal andspatial behavior of fish in a fixed location. – ICES Journal of Marine Science, 60:700–706.

There are many situations when it is important to know accurately the behavior of fish asa function of time and space in a fixed, three-dimensional volume. One example is theoptimal design of techniques that minimize the mortality of fish approaching hydroelectricdams or the cooling intakes of a power plant. The behavior of fish in other fixed volumes,such as estuaries and open rivers, is also of interest in the case of many migrating fishstocks. Both active (echosounding) and passive systems based on acoustic-emitting tagsimplanted in fish have been used to collect behavioral data. Active acoustic systems,including those with electronically and mechanically steered beams, only insonify a smallpart of the total volume of interest at any given time. Tag systems, on the other hand, can beused to monitor the behavior of tagged fish over the entire volume. A number of advancesin the implementation, deployment, and analysis of acoustic-tag systems have been madeover the past few years. These improvements include techniques for positioning optimallythe receiving hydrophones to minimize the location measurement errors, the developmentof acoustic-signal waveforms that provide both unique target identification and accuratelocation estimates, and the development of tracking algorithms that associate and track themultiple returns from an individual fish. These various techniques are described. Guidelinesare presented for selecting the various parameters for the tag system, including the positionsof the hydrophones. Specific examples that compare the predicted and actual performanceof the tag systems are described.

� 2003 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights

reserved.

Keywords: acoustic tags, fish behavior, fish movement, fish tracking, position accuracy.

J. E. Ehrenberg, and T. W. Steig: Hydroacoustic Technology Inc., 715 NE Northlake Way,Seattle, WA 98105 6429, USA. Correspondence to J. E. Ehrenberg: tel.: þ1 206 633 3383;fax: þ1 206 633 5912; e-mail: Consulting@HTIsonar.com.

Introduction

Many studies have been conducted on monitoring the

movement and behavior of fish using acoustic tags. Early

studies were conducted with adult Pacific salmon (Onco-

rhynchus spp.), observing the upstream migrations (Tre-

fethen, 1956; Johnson, 1960). Other studies were performed

with juvenile Chinook salmon smolts (Oncorhynchus

tshawytscha) in the Columbia River (Ledgerwood et al.,

2000) and with juvenile Atlantic salmon smolts (Salmo

salar), as they migrated through lochs, rivers, and estuaries

in the United Kingdom and Canada (Thorpe et al., 1981;

Solomon and Potter, 1988; Potter et al., 1992; Moore, 1995;

Lacroix and McCurdy, 1996; O’Dor et al., 1998; Russell

et al., 1998; Smith et al., 1998; Voegeli et al., 1998; Carr,

2000). The first reported fixed-tracking system was in-

stalled in a loch in Scotland (Hawkins et al., 1974). The

location of fish was recorded every 15min in order to

determine its movement during night and day. The authors

concluded that if the system could be automated, the po-

sition of the fish could be determined every second, and the

swimming path of each fish could then be displayed.

One automated-tracking system has been deployed in the

channels between Passamaquoddy Bay and the Bay of

Fundy in the Atlantic Ocean (Lacroix and Voegeli, 2000).

Another system has been developed for tracking fish on the

seafloor at ocean depths of up to 6000m (Bagley et al.,

2000).

Many improvements have been made in recent years

with the use of acoustic tags and automated tracking to

monitor the swimming path of a fish every second. A

tracking system has been developed and used for the past

four years at Rocky Reach and Rock Island dams on the

Columbia River (Steig et al., 1999, 2001, 2002; Steig,

2000; Steig and Timko, 2000). Other similar studies using

this tracking system have been conducted at Lower Granite

ICES Journal of Marine Science, 60: 700–706. 2003doi:10.1016/S1054–3139(03)00087-0

1054–3139/03/000700þ07 $30.00 � 2003 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.

Dam on the Snake River (Steig and Timko, 2001),

Bonneville Dam on the Columbia River, North Fork Dam

on the Clackamas River (Timko et al., 2001), Cowlitz Falls

Dam on the Cowlitz River, and Chittendon Locks on the

Washington Ship Canal (Timko et al., 2002). These studies

utilized up to 32 omni-directional hydrophones placed in

known locations. The acoustic-tag system determines the

location of the tag, using the relative arrival time of the

acoustic signal at a minimum of four hydrophones. Studies

have been conducted in a Scottish loch to compare the

theoretical position with the measured one (Smith et al.,

1998). Using simulations, Kell et al. (1994) showed that the

accuracy of the position measurements obtained with an

acoustic-tag system depended on a number of factors. A

direct method has also been developed for calculating the

positional accuracy of the location of acoustic tags without

the need for field measurements or Monte Carlo simulations

(Ehrenberg and Steig, 2002). This article expands on the

improved techniques for monitoring the movement of

acoustically-tagged fish through space and time.

Theory of operation for tag-location,measurement systems

The operating principle of acoustic-tag systems is the same

as that which accurately determines position via the Global

Position Satellites (GPS) network. The acoustic tag trans-

mits a signal that is received by at least four hydrophones.

By knowing the positions of the four hydrophones, and

measuring the relative arrival times of each signal at each

hydrophone, it is possible to estimate the locations of the

tagged fish. In particular, if we let hix, hiy, hiz specify the x,

y, z location of the i(th) hydrophone, and let Fx, Fy, Fzspecify the unknown x, y, z locations of the tagged fish,

then the travel time from the tagged fish to the i(th)

hydrophone, ti is given by

ti ¼1

c

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðhix � FxÞ2 þ ðhiy � FyÞ2 þ ðhiz � FzÞ2

q

where c is the speed of sound. Unfortunately, we cannot

measure the absolute travel time directly. We can, however,

measure the differences between the arrival times of the

signal at the various hydrophones (ti�tj) as given by

ti � tj ¼1

c

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðhix � FxÞ2 þ ðhiy � FyÞ2 þ ðhiz � FzÞ2

q�

�ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðhjx � FxÞ2 þ ðhjy � FyÞ2 þ ðhjz � FzÞ2

q �:

For four hydrophones, there are three such distinct

‘‘signal-arrival-time’’ difference equations. The system of

non-linear equations is determined by solving for the

tagged-fish coordinates, Fx, Fy, Fz, such that the mean-

squared difference between the measured (left side of the

above equation) and calculated time differences (right side

of the above equation) are minimized. There are various

algorithms for accomplishing the minimization.

If the arrival-time differences could be measured

perfectly and if the locations of the receiving hydrophones

and the speed of sound were known perfectly, the approach

described above would provide an exact measurement of

the location of the tagged fish. Unfortunately, the arrival-

time measurements will be inaccurate due to noise and the

values assumed for the hydrophone locations, and the speed

of sound will not be exactly right. Approaches for

minimizing the errors are described in the next section.

The acoustic signal on the direct path always arrives at

a given hydrophone prior to any multi-path signal. Signal

discrimination that determines the first signal arriving at

each hydrophone is used to eliminate most multi-path

signals prior to position calculations. If the direct signal is

not detected at a given hydrophone, then detection of

a delayed multi-path signal may result in systematic bias of

the position estimates.

Minimization of the errors in thedetermination of tagged-fish location

There are two strategies that can be used to minimize the

error in the measurement of the tagged-fish location. First,

the type of acoustic signal transmitted by the tag and the

associated signal processing in the acoustic receiver can be

selected to minimize the error in the measured signal arrival

time. Second, the hydrophone geometry can be chosen such

that the sensitivity of the fish-position estimate to

measurement errors in the signal arrival times is minimized.

The effects of added noise on the arrival-time estimates

at a receiver have been analysed for a variety of commu-

nications, sonar and radar applications. It has been shown

(Ehrenberg and Steig, 2002) that the standard deviation in

the arrival time due to added noise is given by

rrms ¼1ffiffiffiffiffiffiffiffiffiffiffiffi

SNR0

pBW

where SNR0 is the ratio of the signal power to noise power

out of the receiver-matched filter, and BW is the baseband-

signal bandwidth. In the case of the standard-pulsed,

continuous-wave (CW) signal, BW is the reciprocal of

the pulse duration T. For example, a 1-ms long CW signal

has a baseband bandwidth of 1/0.001¼ 1000Hz. If the

received-signal level is 10 times greater than the noise

standard deviation out of the matched filter (SNR0¼ 100

(20 dB)), then using this equation the standard deviation of

the arrival-time measurement is 0.1ms. With a CW pulse

and a given transmitted acoustic-power level compared

with the background-noise level, the standard deviation of

the arrival time is determined. If one attempts to increase

the signal-to-noise ratio by transmitting a longer pulse, the

required bandwidth is decreased as the inverse of the pulse

duration, and the denominator in the equation for the

standard deviation remains constant. Similarly, if a shorter

701Temporal and spatial behavior of fish in a fixed location

pulse is used with a wider bandwidth, the increase in the

bandwidth is cancelled by a decrease in the signal-to-noise

ratio. The way to minimize this limitation is to use

a wideband signal waveform rather than a CW pulse. The

two types of wideband waveforms used in acoustic systems

are the FM slide or ‘‘chirp’’ and the binary-coded signal. In

both cases, a special type of receiver processing called

matched filtering must be implemented. The use of FM

slides for active acoustic systems was discussed in

Ehrenberg and Torkelson (2000). An example of the type

of modulation used for binary-coded signals is given in

Figure 1.

When processed using a matched filter, this particular

coded signal has a pulsewidth of about 7.5% of that of an

equivalent CW pulse. The matched-filter outputs for the

CW pulse and the coded signal are shown in Figure 2. The

much narrower output of the matched filter for the coded

signal results in a much smaller variance in the arrival-time

estimate and a corresponding reduction in the measurement

error of the fish location.

In addition to using signal waveforms that minimize the

effect of noise, techniques have been developed for selecting

hydrophone locations that are relatively insensitive to errors.

Previous approaches have used simulations to determine the

effect of hydrophone locations on the accuracy of the fish-

location estimate. However, this process is extremely time

consuming. The alternative approach is to use a differential

analysis to derive an expression relating the arrival-time

errors and the estimated fish-position error. The details of

this derivation are contained in an earlier article (Ehrenberg

and Steig, 2002). Using the results of this analysis, it is rela-

tively easy to locate the hydrophones optimally to produce

minimal errors. Some examples are presented in the subse-

quent discussion.

Tag system implementation

Acoustic tags selected for these studies were 307 kHz

encapsulated, omni-directional pingers. The tags were 18-

mm long and 6.5-mm in diameter (0.71� 0.26 inches) and

were encapsulated with a non-reactive, inert, low-toxicity

resin compound commonly used for potting electronic

circuitry. The weight of each tag was 1.5 g (0.053 oz) in air

and <1.0 g (0.033 oz) in water. Transmit-power level was

approximately 155 dB re 1lPa@1m. Pulse rates and pulse

widths were programmable. The tags utilized encoded

pulses, which increased the detection range, the signal-to-

noise ratio, and the pulse-arrival resolution, resulting in

decreased position variability. Nominal pulse intervals

range from 1 to 6 s per pulse, with transmit pulsewidths

of 0.5–3.0ms. Once activated, the useful life of the tag has

been found to extend up to 30 days, depending on operating

parameters.

Pulse-rate encoding is used for detecting individual tags

when multiple tags are simultaneously present within the

coverage area. Up to 50 tagged fish can easily be tracked

simultaneously by the tag receiver without confusion

between the individual tags. Figure 3A, B is an example

of echograms of the same detected signals from 24 tags

operating simultaneously. The procedure for isolating the

signals from a given tag follows from the method used for

displaying the signals themselves. Each vertical scan in the

plot shows the detected arrivals in a time window equal

to the programmed pulse-repetition period of a particular

tag. The horizontal axis corresponds to the scan number.

For these plots, the number of scans along the horizontal

axis corresponds to 1 h of data collection. The upper plot

(Figure 3A) uses a time window of 4090ms. Only signals

from the tag programmed with this period will fall along a

straight line. The lower plot (Figure 3B) is for the same

dataset, but displayed using a time window of 5290ms.Figure 1. Example of a modulation used for binary-coded, wide-

band signal.

Figure 2. Matched-filter outputs showing the CW pulse and the

coded signal.

702 J. E. Ehrenberg and T. W. Steig

By using a simple tracking procedure for the data shown

on these plots, all the arrival times for a particular tag

can be isolated.

Predicted position error andhydrophone position

The error analysis described by Ehrenberg and Steig (2002)

can be used to optimally select the locations of the

hydrophones. The error contours for two hydrophone

geometries shown in Figure 4 are an example of this.

Figure 4A shows the predicted position error of a four-

hydrophone array spaced approximately 15m apart in the X,

Y, and Z directions with two near-surface hydrophones and

two bottom hydrophones (square-box geometry). Figure 4B

shows the predicted position error of a four-hydrophone

array spaced approximately 15m apart in the X and Y

directions, but only 3–4m apart in the depth (flat-box

geometry). The position error in the center of the array in the

square box (Figure 4A) is approximately 0.3m. However,

the position error in the center of the array of the flat box

(Figure 4B) is much greater, approximately 0.5m. In

addition, the position error increases dramatically towards

the corners of the box. Obviously, the position error is much

smaller for a hydrophone array that is nearly square as

compared with the one configured in the shape of a flat box,

and the errors are much smaller in the interior of the box.

Predicted position error and field results

To verify consistent three-dimensional tag positioning over

time (precision), a test was performed using a stationary tag

Figure 3. An echogram of detected signal arrivals over a 1-h time period plotted using time windows of (A) 4090ms and (B) 5290ms. The

returns from a tag with a period equal to the time window used fall along a straight line.

703Temporal and spatial behavior of fish in a fixed location

within a hydrophone array at Mayfield Dam on the Cowlitz

River (Zapel et al., 2001). The tag was fixed to a rigid

aluminum pole, which was positioned near the center of the

hydrophone array, and its transmissions were monitored for

19min. Of the 1140 individual pings produced by the tag

(60 pings per minute for 19min), three-dimensional

solutions were derived for 1084 of the pings (95%).

Physical measurements determined that the tag was located

at a position of 8.641m in the X-dimension, 13.914m in the

Y-, and .876m in the Z-dimension (depth). The calculated

measurements derived from the hydrophone array were in

agreement with the physical measurements, which in-

dicated that the array was accurately surveyed in place and

was functioning correctly.

The standard deviations of the three-dimensional posi-

tions were compared with the calculated standard devia-

tions (see Table 1). The measured standard deviation of the

arrival time of the tag signal at the four hydrophones was

used for calculating the predicted-position standard de-

viation. The actual and predicted-position standard devia-

tions were similar in the X (0.155 versus 0.165m) and Y

(0.139 versus 0.144m) dimensions. The actual and

predicted-position standard deviations for the Z dimension

were greater than those in the X and Y dimensions, but the

difference in the position error was still within 8.5 cm.

Merging fish swimming-path resultswith river-flow data

Figure 5 shows how the three-dimensional tag data can be

used to study the swimming path of a fish relative to the

river flow. The water velocity in this example was estimated

from a kinematic-flow model based on actual river-flow

data. This kinematic model was calibrated and verified with

actual river-velocity measurements. The water velocity is

represented by color, with the blue shades corresponding to

0.0–0.3ms�1 water velocity, the green shades 0.3–0.8ms�1,

and the yellow/red shades 0.8–0.9ms�1.

Figure 4. The predicted three-dimensional positional error near the center of each array with a Z-dimension hydrophone spacing of

(A) 15m (approximating a square-box configuration) and (B) 4m (a flat-box configuration).

Table 1. The predicted- and actual-position standard deviation ofa stationary tag at Mayfield Dam on the Cowlitz River in 2001. Thehydrophones were located at positions (0.88, 18.96, 0.98 m), (13.64,18.67, 9.67 m), (7.32, 1.27, 9.67 m), and (7.39, 1.27, 0.98 m).

Position’s standard deviation (m)

X (m) Y (m) Z (m)

Actual (n¼ 1084) 0.155 0.139 0.259Predicted 0.165 0.144 0.174

704 J. E. Ehrenberg and T. W. Steig

Figure 5 shows the path of a steelhead smolt released on

the morning of 14 May 1999. This fish followed the water-

velocity contour of 0.8ms�1 downstream to near the

powerhouse wall. It then traveled perpendicular to the

water flow until it reached the slow back eddy (<0.3ms�1),

where it turned again and travelled downstream toward the

end of the powerhouse. It ultimately passed into Unit 1.

Conclusions

Technological advances in the implementation and deploy-

ment of fish-tracking systems using acoustic tags have

enhanced the utility of these techniques greatly. The use of

coded pulses and matched-filter processing, for example,

has been shown to significantly improved the accuracy of

the tag-position measurements. Then again, data-processing

methods for isolating the signals from individual tags allow

many fish to be tracked simultaneously. Finally, analytical

techniques that predict the tag-position error as a function

of hydrophone placement are being used to select the

optimal hydrophone placement prior to system deployment.

In combination, these technological advances are providing

an accurate technique for monitoring fish behavior in fixed

spatial regions.

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