Transactions of the American Fisheries Society 136:423–427, 2007 Ó Copyright by the American Fisheries Society 2007 DOI: 10.1577/T06-055.1
[Note]
An Improved Technique for Small-Scale Radio-Tracking of Crayfish and Benthic Fishes in Upland Streams BEN BROADHURST*
AND
BRENDAN EBNER
Environment ACT, Wildlife Research and Monitoring, Post Office Box 144, Lyneham, ACT, 2602, Australia Abstract.—This study measured the accuracy of two radiotracking techniques: (1) triangulation using a Yagi antenna and (2) a form of direct location, the extended-reach technique, which uses a small loop antenna attached to a lightweight 9-m telescopic pole. The study was conducted in the Cotter River, an upland stream in the Australian Capital Territory. Radio tags were positioned instream to determine whether depth, substrate, or local bank profile affected radiotracking accuracy. The extended-reach technique was more accurate (mean error 6 SE ¼ 1.00 6 0.21 m) than triangulation (2.57 6 0.21 m). Decreased accuracy resulted from the triangulation of radio tags positioned close to boulders or in water more than 1 m deep. These variables had no effect on the accuracy of the extended-reach technique. The presence or absence of a steep rocky bank did not affect the accuracy of either radio-tracking method. The increased accuracy of the extended-reach technique provides an improvement over traditional methods for studying the small-scale movement and microhabitat use of crayfishes (e.g., Eustacus spp.) and benthic fishes in upland streams.
Radio-tracking has been used to study the movement of biota in streams on a scale ranging from meters (e.g., Jellyman and Sykes 2003; Khan et al. 2004) to hundreds of kilometers (Eiler 2000; Karppinen et al. 2004; Zurstadt and Stephan 2004). Radio-tracking has been especially useful in revealing large-scale movement including migration (Clarke and Gee 1992). At the low end of the scale (submeters to meters) it can be difficult to resolve movements and therefore habitat use accurately by means of radio-tracking (Niemela et al. 1993), though visual verification can increase the accuracy of location estimates. However, success of this method is dependent on visibility. The ability to accurately track small-scale (;1-m) movements is not trivial. These movements may reflect changes in microhabitat use that can be critical for individuals and ultimately the success of populations (Railsback et al. 1999; Doerr and Doerr 2005). Understanding the accuracy of radio-tracking is important in small-scale movement studies, as it allows determination of the degree of certainty in a location estimate (Simpkins and Hubert 1998; Brenden et al. * Corresponding author:
[email protected] Received February 28, 2006; accepted August 13, 2006 Published online March 12, 2007
2004), which affects the measurement of movement and habitat scale variables. There are at least two causes of inaccuracy in these types of studies. First, signal infidelity arises when radio waves are deflected by the rocky, irregular-shaped stream bed or the bordering steep-sided rock faces typical of narrow upland stream valleys (Kenward 1987; White and Garrott 1990). Second, radio waves emitted from a submerged radio tag at an angle of 68 or more from the vertical are reflected back under the water’s surface, whereas those within 68 of vertical refract towards the water’s surface (Priede 1980). Because of this refraction, radio signals are strongest directly above the source and weaker towards the horizontal (Priede 1980). In addition to the spatial error in locating an animal’s position, there may be distortions because the observation process itself affects the behavior of an animal. There is increased potential for this observer effect (Martin and Bateson 1994) when radio-tracking alongside or within upland streams owing to the difficulty of moving quietly and inconspicuously in such terrain (White and Garrott 1990). Researchers that enter the stream or skip across instream boulders are especially likely to disturb fish, which would compromise a true record of animal movement. This effect would be further exacerbated if radio-tracking were repeated over short time periods (White and Garrott 1990). Radio-tracking is used to obtain the location of animals based on either triangulation or nontriangulation (White and Garrott 1990). In lotic systems triangulation facilitates shore-based research, can be rapid (Kenward 1987), and has the potential to minimize observer effect since the researcher is not required to enter the aquatic habitat nor to encroach on the radio-tagged individual (Matthews 1996; Serena et al. 1998; Crook et al. 2001). However, location error increases with distance from the radio tag as a function of antenna accuracy (about 58 for the commonly used three-element Yagi antenna; Macdonald and Amlaner 1980; Kenward 1987; White and Garrott 1990). Another source of error associated with triangulation is that caused by the movement of the individual between triangulation fixes (Schmutz and White 1990).
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Direct location involves holding an antenna over a radio-tagged individual. This typically involves the use of a boat (e.g., Niemela et al. 1993; Thorstad et al. 2001) or requires the researcher to enter the stream on foot (e.g., Khan et al. 2004), increasing the likelihood of observer effects. Direct location of the radio tag is accurate; however, it is impractical where there are high flows, turbulent water, or slippery and irregular substrates, all of which would incur observer effects. In these circumstances triangulation is the only recognized technique of manual radio-tracking despite its appreciable spatial error. Our primary interest was in developing a radiotracking method that did not require a researcher to enter the water, thereby minimizing the observer effects, while positioning an aerial directly over the fish or transmitter to increase the accuracy of the location estimate. Ideally, this would enable researchers to detect and record small-scale movements and microhabitat associations with an accuracy of less than 1 m, without visual verification of the target. While designing the new technique consideration was given to reducing observer effect (assuming levels comparable with triangulation); however, this factor was not tested in this study. Our aims were to compare the accuracy of the new technique with the commonly used technique of triangulating from shore and to quantify the effect of common environmental features in upland streams (deep water, boulders, and steep rocky banks) on the accuracy of each technique. Methods Study site.—The study was conducted in the Cotter River, an upland river of Namadgi National Park, located in the southwestern part of the Australian Capital Territory (ACT), Australia. The river valley is typically well vegetated, steep, and rugged (National Capital Development Commission 1986) and the maximum river width is 18 m. The Cotter River is a typical high-gradient river with a streambed that is rocky and heterogenous, being dominated by boulders, cobble, and gravel and having steep rocky banks (Maddock et al. 2004). The study site was located approximately 60 km from the river’s source, immediately downstream from the Spur Hole, which has a moderate reach gradient (0.00480) and is typified by runs and glides (Maddock et al. 2004). Field trial.—A hierarchical design was used to test the effect of three variables—the presence of steep rocky banks, the presence of boulders, and water depth—on the accuracy of locating a radio tag. Pool 1 had steep rocky banks and pool 2 had open banks. Eight radio tags (2 stage, 150–152 MHz; Advanced Telemetry Systems) were placed in each pool in
shallow (0–0.99-m) or deep (1.0–1.48-m) water and near (,1 m or underneath) or away from (.1 m) boulders. Tags were divided equally among treatments in each pool. Along one bank of each pool a straight transect pegged at 2-m intervals was aligned approximately parallel to and 1 m from the water’s edge. Two operators who had no previous knowledge of the experimental design or radio tag locations tracked the radio tags using two techniques: (1) triangulation using a Yagi antenna and (2) a form of direct location, the extended-reach technique (ERT), that uses a small loop antenna (Titley Electronics, Ballina, Australia) attached to a lightweight 9-m telescopic pole, which collapses down to 1.1 m and weighs 1.12 kg. Both methods used the Titley Australis 26K scanning receiver (Titley Electronics). Volume on the receiver was fixed at 6 (the midpoint available), the gain level being manipulated by the operator. The technique used and the order in which the radio tags were located were based on a predetermined random sequence. Operators commenced locating each radio tag from the middle of the transect. An observer prevented the operators from looking into the water for the hidden radio tags and recorded the time taken to locate each radio tag. Operators were instructed to focus on accuracy, not speed, when locating each radio tag. Three readings were taken in the triangulation process. The first was an estimate of where the radio tag was positioned longitudinally (i.e., along the stream), which was achieved by the operator, who scanned the pool along the transect until the radio tag was determined to be directly perpendicular to the transect. The second and third readings were taken upstream and downstream of the perpendicular at positions where the signal was coming from angles of 458 along the transect. A portable brace was used to hold the Yagi antenna at a 458 angle. All positions on the transect were measured to the nearest 0.1 m. Trial of the ERT involved each operator moving a small loop antenna (diameter of loop ¼ 10 cm) fitted to the end of the telescopic pole immediately above the water surface to detect each radio tag. A record was made of the distance along the transect where the radio tag was directly perpendicular to the transect. Distance from the transect to the radio tag was measured by reading from 0.1-m increments marked along the pole. Data analysis.—Four-way analysis of variance (ANOVA) was conducted using Statistica for Windows (version 5.5, Statsoft). Correlations and two-sample ttests were conducted using Statistix for Windows (version 2.0). Data were transformed, when necessary, to achieve the assumption of a normal distribution for the analyses (based on guidelines for transformations in Tabachnick and Fidell 1989). An alpha level of 0.05
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FIGURE 1.—Scatterplots showing the deviations of fixes from the location of the radio tags based on (A) the extended-reach technique (n ¼ 32) and (B) triangulation (n ¼ 32).
was set for discrimination of significant results for all analyses.
min), but the difference was not significant (t ¼1.92, df ¼ 62, P ¼ 0.06).
Results
Discussion
The ERT (mean error 6 SE ¼ 1.00 6 0.21 m) was more accurate than triangulation (2.57 6 0.21 m) (F ¼ 29.2, df ¼ 1, 48, P , 0.01). Fixes recorded with the ERT ranged in accuracy from 0.22 to 4.28 m and showed little bias laterally or longitudinally (Figure 1A). The accuracy of triangulation ranged from 0.08 to 8.42 m and fixes generally underestimated the distance from radio tag to transect (Figure 1B). The frequency of occurrence of sub-meter accuracy from the ERT and triangulation was 68.8% and 18.8%, respectively.
This study demonstrates that the ERT is significantly more accurate than triangulation. The accuracy of triangulation in this study was comparable with reports from previous studies (Matthews 1996; Crook et al. 2001; Simpson and Mapleston 2002), whereas that of the ERT was often within 1 m. This has important implications for studying the movement and habitat use of biota in upland streams, especially biota that make relatively small movements. The positioning of radio tags in close proximity to boulders significantly decreased the accuracy of triangulation but did not affect the accuracy of the ERT. The ERT allows direct positioning over the transmitter, which increases the likelihood of detecting the true signal (Priede 1980) and not a signal reflected off boulders. Owing to the indirect nature of triangulation, discrimination between reflected signals and true signals becomes difficult and in this case led to inaccuracy. Consequently, the ERT is superior to triangulation for locating species that frequently use boulders as shelter (e.g., Khan et al. 2004; Nykanen et al. 2004; Katano et al. 2005). Water depth did not affect the accuracy of the ERT, whereas the accuracy of triangulation was significantly compromised by deep water. This finding supports the claims of Priede (1980) that radio waves of highest fidelity emanate directly above the radio tag, and radio waves of lesser fidelity are refracted along the horizontal surface of the water. Owing to the direct overhead position of the loop antennae in the ERT,
Treatment Effects The mean accuracy of the ERT was consistently close to 1 m across all treatments (Figure 2); there were no significant differences across treatments (Table 1; Figure 2). The mean accuracy of triangulation was significantly lower for radio tags placed near boulders (3.2 m; SE, 0.43) than for those not placed near boulders (1.93 m; SE, 0.3; Figure 2; Table 1). Depth had a similar effect on triangulation accuracy, which was significantly lower for radio tags in deep water (3.29 6 0.43 m) than for those in shallow water (1.85 6 0.28 m) (Figure 2; Table 1). The presence of a steep rocky bank led to a higher error (2.84 6 0.46 m) than in the pool with open banks (2.3 6 0.33 m) (Figure 2) for triangulation, although this difference was not significant (Table 1). The ERT took an average of 8.46 min (SE, 0.58) to locate a radio tag. Triangulation took slightly longer to locate a radio tag (10.04 6 0.58
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FIGURE 2.—Mean errors of the extended-reach technique and triangulation across habitat treatments. Whiskers represent SEs; n ¼ 16 for each treatment and method.
radio waves of highest fidelity can be located, thus reducing the error associated with using refracted radio waves to locate the radio tag. Triangulation from shore relies on locating a radio tag by using both the true radio waves and the refracted and reflected radio waves, which leads to a high degree of signal infidelity that culminates in decreased location accuracy. The presence of a steep rocky bank on the side of a pool did not have a significant effect on the accuracy of either technique in this study. It is possible that either technique may be used in many upland streams. However, this conclusion may be premature because the moderately sized steep rocky bank selected as a treatment in this study may have been insufficient to cause major deflection of radio waves. It would be informative to conduct further experiments that include trials in streams bordered on both banks by larger rock faces.
The time taken to locate each individual by manual radio-tracking determines the maximum sample size used in studies of diel movement. Both techniques had comparable application times in our study (8–10 min), and it is likely that familiarization with the technique will lead to an increase in the speed of application without compromising accuracy. The time taken to locate an individual with the ERT will invariably increase when the individual is moving, though the level of accuracy will not be affected. This provides an advantage over triangulation, the accuracy of which would be affected as described in Schmutz and White (1990). Therefore, the ERT is a highly accurate radiotracking technique suitable for describing small-scale movements of instream fauna, particularly crayfishes (e.g., Eustacus spp.) and benthic fishes, in small upland streams. However, an evaluation of the extent to which the ERT and other manual radio-tracking techniques
TABLE 1.—Results of the four-factor analysis of variance of the treatment effects on the radio-tracking accuracy of the extended-reach technique and triangulation. (Values in bold italics are significantly different at the 0.05 level). Treatment or interaction Method Boulder Depth Bank Method 3 boulder Method 3 depth Boulder 3 depth Method 3 bank Boulder 3 bank Depth 3 bank Method 3 boulder 3 depth Method 3 boulder 3 bank Method 3 depth 3 bank Boulder 3 depth 3 bank Method 3 boulder 3 depth 3 bank
Sum of squares
Mean square
F
P
39.53 3.37 7.08 2.03 10.53 9.45 0.677 0.53 0.17 0.60 0.21 0.88 0.31 0.96 0.57
39.53 3.37 7.08 2.03 10.53 9.45 0.677 0.53 0.17 0.60 0.21 0.88 0.31 0.96 0.57
29.16 2.49 5.21 1.49 7.77 6.97 0.50 0.39 0.12 0.44 0.16 0.65 0.24 0.71 0.42
2.04 3 106 0.12 0.03 0.23 0.01 0.01 0.48 0.54 0.73 0.51 0.69 0.43 0.63 0.41 0.52
NOTE
disturb instream biota is required before a best-practice method can be advocated. Acknowledgments Luke Johnston, Jason Thiem and Katie Ryan assisted with fieldwork. Murray Evans and David Pederson provided statistical advice. Don Fletcher, David Shorthouse and Mark Lintermans commented on drafts of the manuscript. Funding for this research was provided by Environment ACT and the Natural Heritage Trust. References Brenden, T. O., B. R. Murphy, and E. M. Hallerman. 2004. Estimating total error in locating radiotelemetry transmitters by homing in a riverine environment. Journal of Freshwater Ecology 19(2):295–304. Clarke, D., and A. S. Gee. 1992. Applications of telemetric tracking in salmonid fisheries management. Pages 444– 455 in I. G. Priede and S. M. Swift, editors. Wildlife telemetry: remote monitoring and tracking of animals. Ellis Horwood, New York. Crook, D. A., A. I. Robertson, A. J. King, and P. Humphries. 2001. The influence of spatial scale and habitat arrangement on diel patterns of habitat use by two lowland river fishes. Oecologia 129:525–533. Doerr, E. D., and V. A. J. Doerr. 2005. Dispersal range analysis: quantifying individual variation in dispersal behaviour. Oecologia 142:1–10. Eiler, J. H. 2000. Fish movements: tactics for survival. Pages 85–92 in D. A. Hancock, D. C. Smith, and J. D. Koehn, editors. Fish movement and migration. Australian Society for Fish Biology, Sydney. Jellyman, D. J., and J. R. E. Sykes. 2003. Diel and seasonal movements of radio-tagged freshwater eels, Anguilla spp., in two New Zealand streams. Environmental Biology of Fishes 66:143–154. Karppinen, P., J. Erkinaro, E. Niemela, K. Moen, and F. Okland. 2004. Return migration of one-sea-winter Atlantic salmon in the River Tana. Journal of Fish Biology 64:1179–1192. Katano, O., T. Nakamura, S. Yamamoto, and S. Abe. 2005. Summer daytime habitat and population density of the torrent catfish, Liobagrus reini, in the Urano River. Ichthyological Research 52:50–56. Kenward, R. 1987. Wildlife radio-tagging: equipment, field techniques, and data analysis. Academic Press, London. Khan, M. T., T. A. Khan, and M. E. Wilson. 2004. Habitat use and movement of river blackfish (Gadopsis marmoratus R.) in a highly modified Victorian stream, Australia. Ecology of Freshwater Fish 13:285–293. Macdonald, D. W., and C. J. Amlaner. 1980. A practical guide to radio-tracking. Pages 143–160 in C. J. Amlaner and D. W. Macdonald, editors. A handbook on biotelemetry and radio-tracking. Pergamon, Oxford, UK. Maddock, I., M. Thoms, K. Jonson, F. Dyer, and M.
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