Hydrobiologia 371/372: 47–52, 1998. J.-P. Lagard`ere, M.-L. B´egout Anras & G. Claireaux (eds), Advances in Invertebrates and Fish Telemetry. © 1998 Kluwer Academic Publishers. Printed in Belgium.
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A dynamic combined acoustic and radio transmitting tag for diadromous fish G. H. Niezgoda1 , R. S. McKinley2, D. White2 , G. Anderson2 & D. Cote2 1
Lotek Marine Technologies Inc, 114 Cabot Street, St. John’s, NF A1C 1Z8, Canada (
[email protected]) Waterloo Biotelemetry Institute, Department of Biology, University of Waterloo, 200 University Av. W., Waterloo, ON N2L 3G1, Canada 2
Key words: diadromous, conductivity, bio-telemetry, estuary, CART
Abstract A combined acoustic and radio transmitting (CART) tag employing a dynamic conductivity switch suitable for investigating the migratory behavior of diadromous fish is described. The unique feature of the transmitter is its ability to sense the electrical conductivity of the ambient water and therefore operate in the appropriate signal mode. Under fresh water conditions the transmitter operates in radio mode; in sea water it operates in acoustic mode. To optimize range performance the conductivity threshold setting at which the tag switches mode can be changed to suit the application. This paper describes a proposed criterion for threshold selection, the physical probe characteristics and the recommended tag attachment procedure.
Introduction Several species of fish, notably diadromous varieties including salmonids, sturgeon and the American eel, migrate at different periods of their life cycle between the ocean and inland bodies of fresh water. To monitor the behavior of these animals, biologists are faced with the task of making telemetry measurements under fresh, estuarine and sea water conditions. The large variations in water depth and electrical conductivity across these zones present significant challenges for radio and acoustic telemetry. In most situations radio telemetry is restricted to fresh water applications since the conductivity levels associated with both sea water and brackish (estuary/coastal) waters, leads to severe radio signal attenuation through the water column. Also, significant attenuation may occur in fresh water depending on conductivity, transmitter frequency and depth. In the case of acoustic telemetry, severe signal loss under shallow water conditions commonly arises producing destructive interference at the receiving hydrophone. Consequently, acoustic telemetry is best applied to high conductivity, estuarine and sea water environ-
ments, with radio telemetry being most applicable in low conductivity, fresh water conditions. Therefore, for the most accurate assessment of the behavior of diadromous fish species during critical migratory phases as the animals move between fresh and saline waters, both acoustic and radio telemetry are required. Unfortunately, bio-compatibility issues such as weight and size restrictions prevent placing separate radio and acoustic tags on most animals. To address these concerns, combined acoustic and radio transmitting (CART) tags have been developed (Solomon & Potter, 1988; Armstrong et al., 1988). Current CART configurations, (Solomon & Potter, 1988; Armstrong et al., 1988), rely upon non-dynamic switching between acoustic and radio modes based on a timer. Upon release of a tagged fish, the timer setting attempts to predict the time instant when the fish will transit between salt and fresh water. This approach has two drawbacks. First, since the timer setting is based on either historical data of fish movement or intuition, some level of measurement outage must be expected. Second, to ensure the accurate monitoring
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48 of fish movement, a large number of animals must be tagged to remove the effects of outage. As a means of mitigating measurement outage and reducing the number of transmitters to an economical level without compromising the quality of measurements, a dynamic switching CART device has been developed. The present paper describes the development of such a device and suggests a suitable attachment procedure of the transmitter. Furthermore, the effects of the this attachment procedure on swimming performance of adult rainbow trout, Onchorhynchus mykiss is described.
transmission loss at the water/air interface, Lint , can be found in Velle et al (1979) and Lotek (1994). The loss in air as a function of range, r (m), is expressed as Lair (r) = 17.7 + 20 · log10 (r/λ) (dBd), where λ = 2 m equals the wavelength at 150 MHz. A prediction of acoustic range performance follows by equating the sonar equation, (Clay & Medwin, 1989), to the receiver’s minimum detection threshold level, DT (dB), DT = SL − TLspread (r) − TLabsorp (S) · r (2) +Ghydro + NL(dB)
A criterion for conductivity threshold selection In this section guidelines for selecting a conductivity threshold criterion for switching between radio and acoustic modes is proposed. The approach is based on range prediction formulae for radio (Velle et al., 1979; Lotek, 1994) and acoustic (Clay & Medwin, 1989; Coates, 1989) telemetry systems. It has been our experience that these range prediction formula provide range estimates comparable to data collected from controlled ‘in-situ’ trials. A prediction of radio range performance is derived by relating the receiver’s minimum detectable signal sensitivity, to transmitter and propagation characteristics, Rsen = ERP − Lwater (d, C) − Lint (1) −Lair (r) + Gant − Lcable (dBm) where ERP equals the effective radiated power of the transmitter in units of decibels referenced to a milliwatt (dBm). Receiver parameters include antenna gain in units of decibels referenced to a dipole antenna, Gant (dBd), and antenna cable loss, Lcable (dB). Transmission losses expressed in decibels (dB), include loss in water, loss at the water/air interface, and loss in air. Based on (Velle et al., 1979; Lotek, 1994) loss associated with radio propagation in water at 150 MHz is approximated by Lwater (d, C) ≈ (0.0171 · C + 1.5) ·d (dB) @ T = 10 ◦ C where transmitter depth is d (m) and C is electrical conductivity measured in µS cm−1 at a low frequency (i.e.: 100 kHz to 1 MHz). A detailed discussion of
where SL denotes transmitter source level in dB re 1µ P a @ 1 m. Transmission loss due to spherical spreading is TLspread (r) = 20 · log10 (r) (dB),
(3)
h
and loss due to absorption is, where S is salinity expressed in . Hydrophone directional gain is given by Ghydro (dB). The noise level, NL, will depend upon the receiver’s effective bandwidth, BW(Hz); NLambient (dB re 1µ Pa2 / Hz), the ambient noise level in a 1 Hz bandwidth; and NLwind (dB re 1 µ Pa2 / Hz), the nominal noise level due to wind action in a 1 Hz bandwidth. It is assumed that the electronic noise level of the receiver is significantly smaller than the contributions from the ambient background and wind. To compute NL, the normalized noise levels NLambient and NLwind are scaled by the receiver’s bandwidth using the expression NL = NLambient + NLwind − 10 · log10 (BW) (dB) Using study site information, system characteristics of a radio telemetry receiver, and radio/acoustic specifications of the CART tag, a prediction of range is computed from Equations (1) and (2). Note that range values generated from Equation (2) should not be considered indicative of system performance under shallow water conditions (ie: a few meters). However, for the purpose of establishing guidelines, it is assumed that the water column depth is sufficient as to make range predictions from Equation (2) relevant. As an example, the following application scenario demonstrates an approach for establishing a conductivity threshold criterion for the CART device’s dynamic conductivity switch.
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49
h
h
Figure 1. Acoustic/Radio Telemetry Ranges vs. Water Conductivity. Point A and B shows acoustic range predictions for water salinity values of 0.5 and 35 , respectively. Points C and D separate three CART tag conductivity threshold regions. In between C and D transmission mode alternates between radio and acoustic; to the left of C the transmission mode is radio; and to the right of D the transmission mode is acoustic.
Consider an application scenario that monitors the movement of fish across three zones: a shallow fresh water river (C < 200 µS cm−1 ), an estuary (200 µS cm−1 < C < 40000 µS cm−1 ) and sea water (C ≈ 40000 µS cm−1 ). The fish are assumed to transit the estuary within the top 2 meters of the water column. Across the estuary the nominal acoustic noise levels due to the ambient background and wind conditions are assumed to be known. A 150 MHz radio telemetry system to be used for the study is described by the following operating parameters:
by NLambient =40 dB re 1µ Pa2 / Hz and NLwind =15 dB re 1µP a 2 / Hz. In the open ocean, a typical ambient noise level is 20 dB re 1 µP a 2 / Hz at 76 kHz (Clay & Medwin, 1989; Coates, 1989). The acoustic system’s operating parameters are: SL = 158 dB re 1 µ Pa @ 1 m, Ghydro = 0 dB, BW = 7.5 kHz, and DT = 6 dB. Transmission loss due to absorption, (Clay & Medwin, 1989; Coates, 1989), in the estuary will range from
h
TLabsorp (S = .5 ) ≈ 0.0024 dB m−1 ,
ERP = −20 dBm, Lint ≈ 40 dB, Gant = 6 dBd, Lcable ≈ 0 dB, Rsen = −136 dBm. The study will also use a 76 kHz acoustic telemetry system. We model the estuary’s noise conditions
C ≈ 200 µS cm−1 to
@T = 12 ◦ C
h
TLabsorp(S = 35 ) ≈ 0.0027 dB m−1 , C ≈ 40000 µS cm−1
@T = 12 ◦ C
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50 Based on the radio and acoustic system parameters we compute a family of operating curves from (1), with each curve corresponding to a fixed depth. Figure 1 shows these curves for depths from 0.5 to 5 m. The dashed horizontal lines on the figure are the nominal range estimates of the acoustic telemetry system computed from Equation (2). From Figure 1 we define a threshold window from 350 µS cm−1 to 2000 µS cm−1 corresponding to the intercepts between the d = 2 m and d = 0.5 m radio operating curves with the upper and lower limits of acoustic range. In this window the CART device is programmed to operate in a combined acoustic/radio mode. Below the window the tag transmits in radio mode, and above the window the mode is acoustic.
Attributes of the CART device The CART device consists of the following subsystems: the radio section, acoustic projector section, battery pack, the microprocessor and conductivity probe assembly. A radio sub-system, including an external whip antenna, is derived from CFRT-3 type radio transmitters (Lotek Engineering Inc.). The acoustic projector assembly is taken from the CAFT family of acoustic transmitters (Lotek Marine Technologies Inc.). Battery packs for the CART device vary according to size and longevity requirements. A sketch of the CART tag is shown on Figure 2. The unique feature of this transmitter is its ability to continually sense the electrical conductivity of ambient water and through a microcomputer algorithm, select the appropriate signal mode to maximize detection range. Conductivity measurements taken between two metal conductors exposed to the water is monitored by a programmable microprocessor that controls the transmission mode of the tag. The head of the conductivity probe as shown on Figure 2, is molded around a pair of metal conductors using a high durometer encapsulant. For bio-compatibility and flexibility the probe cable entering the head is coated with a flexible bio-compatible jacket. The strong bond between the head encapsulant and cabling jacket prevents any capillary action or water creep down the wire. In addition to maximizing detection range, the continual sensing of conductivity is also motivated by the need to maximize tag life. Currently, the power consumption ratio between acoustic and radio modes of the CART tag is approximately four-to-one. Hence, a strategy of simply alternating between radio and
Table 1. Mean Fork Length, weight and Ucrit of control and tagged groups. Results, expressed as a mean ±1 standard error of the mean value. Mean values were not found to be statistically different (ANOVA, systat) Group
Fork Length (cm)
Weight (g)
Ucrit (m/s)
Control Tagged
41.73 ± 1.14 46.2 ± 1.18
912.13 ± 92 1103.5 ± 89
0.804 ± 0.094 0.694 ± 0.087
acoustic modes would compromise the longevity of the device. Other attributes of this tag include; a programmable threshold setting; and an expandable threshold setting to cover a range of conductivity over which the device is set to operate in a combined transmission mode alternating between acoustic and radio. This latter feature allows for a ‘gray’ operating area where there is uncertainty as to whether acoustic or radio telemetry is optimal. The device can also be programmed to stop transmitting in a specified conductivity window and resume transmission outside this window thus providing a means of extending tag life.
Attachment procedure Rainbow trout were obtained from the Ontario Ministry of Aquaculture and Fisheries Research station, Alma, Ontario. Tests were conducted with two groups of rainbow trout, control and tagged fish. Both groups were acclimated to laboratory conditions for four weeks prior to experimentation in a 1400 l holding tank supplied with aerated well water (T = 11 ◦ C ± 0.5 ◦ C) and lighting conditions of 16 h light: 8 h dark. Fish were fed ad libitum with commercial trout pellets (Martin Mills Inc., Elmira Ont.). However, food was withheld for a four day period prior to and during the experimental period to ensure a post-absorption state before the assessment of critical swimming velocities (Beamish et al., 1979). All fish were anaesthetized to a 200 mg l−1 solution of MS-222 neutralized with sodium hydrogen carbonate at a ratio of 1:1. Fork length and weight of each individual was recorded and the fish then placed into a 120 l Blaska type swim respirometer supplied with aerated well water (T = 11 ◦ C ± 0.5 ◦ C). All fish responded to the conditions within the respirometer and swam under a shaded area at the anterior end of the chamber. Once placed in the respirometer, fish were given an acclimation period of 30 min at a water velocity of 1.5 body lengths prior to the assessment of
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51
Figure 2. CART tag and Conductivity Probe.
the critical swimming speed. Following acclimation, fish were subjected to a forced swim by increasing the water velocity in a constant step-wise progression with a constant time increment of 5 m. Fish were deemed fatigued when they could no longer continue to swim against the current. Critical swimming speed (Ucrit ) was calculated for each individual using the formula described by Brett (1964), Ucrit = V + (t · 1t −1 )1v (m s−1 ), where 1t is the time increment in minutes, 1v (m s−1 ) is the velocity increment, t (min) is the time elapsed at the final velocity, and V (m s−1 ) is the highest velocity maintained for the prescribed time period. Dummy CART transmitters (14 mm × 58 mm) were inserted into the abdominal cavity of the fish with anaethesia induced in the tagged individuals as described above (see Mellas & Haynes (1985, Figure 1)). These fish were weighted and measured then placed on a specially designed surgical table which allowed for the irrigation of a cold (T = 6 ◦ C ± 1.0 ◦ C) solution of 100 mg l−1 MS-222 neutralized with sodium hydrogen carbonate at a radio of 1:1. Attachment of the dummy transmitters involved insertion of the tag within the abdominal cavity of the fish, the antenna wire was led through the body wall at the posterior end of the cavity through a small puncture wound made by a 16 gauge needle. The conductivity probe head was brought through the body wall in a similar fashion at
the anterior end of the cavity and protruded approximately 3.5–4.5 mm. Due to the dimension of the probe head (3 mm × 5 mm) a larger puncture wound was required to fit the probe through. This wound was closed with two small sutures to ensure that the probe head did not retract into the body of the fish during swimming. The main incision, through which the body of the tag was placed, was closed with 3–4 sutures. Total surgical time was a maximum of 4 minutes and fish were allowed to recover for a minimum of 16 hours prior to the assessment of critical swimming speeds. Tagged individuals were further anaesthetized in similar fashion as described above prior to being placed in the respirometer. All tagged fish were subject to the same protocol.
Experimental results Results are presented as a mean ± standard error. Statistical comparisons were made, using a t-test, between the control and tagged groups (n = 6) (Table 1). No significant differences were found between each group demonstrating that the attachment procedure did not significantly disrupt the swimming performance of the tagged individuals 16 hours after insertion.
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52 Conclusion
References
The CART tag described in this paper is capable of continually sensing the electrical conductivity of ambient water. This feature permits the tag to operate in an appropriate signal mode that will maximize detection range performance. The strategy for switching modes can be modified through a microcomputer program to suit the application. An attachment procedure for the CART tag and conductivity probe is presented. Swimming performance results indicate that the attachment procedure does no significantly disrupt swimming performance.
Armstrong, J. D., M. Lucas, J. French, L. Vera & I. G. Priede, 1988. A combined radio and acoustic transmitter for fixing direction and range of freshwater fish (RAFIX), J. Fish Biol. 33: 879–884. Beamish F. W. H., 1978. Swimming Capacity, Fish Physiology. Academic Press, Hoar and Randall (eds), New York, 7: 101–172. Brett J. R., 1964, The respiratory metabolism and swimming performance of young sockeyed salmon. J. Fish. Res. Bd Can. 21: 1183–1226. Clay C. & H. Medwin, 1977. Acoustical Oceanography: Principles and Applications, John Wiley. Coates, R., 1989. Underwater Acoustic Systems. John Wiley, New York. Lotek Engineering Inc., 1994, Telemetry Workshop Notes. Tondheim, Norway. Mellas, E. J. & J. M. Haynes, 1985. Swimming Performance and Behavior of Rainbow Trout (Salmo gairdneri) and White Perch (Morone americana): Effects of Attaching Telemetry Transmitters. Can. J. Fish. aquat. Sci. 42: 488–493. Solomon, D. J. & E. C. E. Potter, 1988. First results with a new estuarine fish tracking system. Fish Biol. 33 (supplement A): 127–132. Velle, J., J. Lindsay, R. Weeks & F. Long, 1979. An Investigation of the Loss Mechanisms Encountered In Propagation From a Submerged Fish Telemetry Transmitter, 2nd International Conference of Wildlife, Biotelemetry: 228–237.
Acknowledgements We gratefully acknowledge the assistance of Mark Ploughman, Ed Kennedy and Debbie Norman in the development, design and construction of the CART conductivity probe.
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