where: NC=maximum number of animals which can be searched for
MD=minimum detection distance parallel to the aircraft's direction
of movement
SR= receiver scan rate
GS= maximum ground speed of aircraft
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Determining whether a ground survey, aerial survey or a combination of these monitoring techniques is most appropriate will depend on the objectives of the study, the specific species biology, and terrain as well as budget constraints. Aerial monitoring is most efficient for sampling animals that live in inaccessible mountainous terrain or disperse long distances and require searching over large geographic areas. Ground relocations allow more detailed observations of an animal to be made, and are less expensive than relocation flights.
Local climate and weather patterns can be also be an important factor in determining the suitability of ground versus aerial monitoring. Locating animals by aircraft may be largely restricted if seasonal weather is unsuitable for telemetry flights, as is the case in certain coastal areas. In contrast, other regions of the province may experience winter snowfall which will limit mobility to such an extent that aerial monitoring is the sole option.
Table 3 outlines some of the specific advantages and disadvantages of either method.
Table 3. General uses of and advantages/disadvantages of ground and aerial monitoring.
|
Ground Monitoring |
Aerial Monitoring | |
|
General Uses |
|
|
|
Pros/Cons |
|
|
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The accuracy of a radio-location varies with habitat type and may result in biased estimates of observed habitat use. A common source of error is signal bounce. Signal bounce occurs most frequently in mountainous terrain where a signal is deflected by a mountain, resulting in potential errors of many kilometres. The most effective way to overcome signal bounce during ground tracking is to take many bearings from several different places. When all signals appear to be coming from the same point then there is a good chance that the animal has been located correctly. However, if the signals are coming from a number of different points then signal bounce is likely still occurring (White and Garrott 1990).
Visual observations of radio-located animals provide the best confirmation of the accuracy of the relocation data. For large animals, a reasonable proportion of locations should be confirmed by direct visual observations (some biologists use >30% as a general rule; however, this may not be practical in all cases). In new study areas or with species which cannot be observed on a regular basis, it is strongly recommended that triangulation be used with an assessment of aerial fixes made using collars placed in known locations. Such trials can test the consistency and accuracy of triangulation using various personnel and methods under various environmental conditions. Results of the trials can be used to identify problems (e.g., signal bounce) and ensure that methods are adjusted to reliably obtain accurate radio locations.
When relocating wildlife in the field, most users judge the angle over which the signal sounds loudest, determine a bearing by mentally bisecting that angle, and follow the bearing to move closer to the signal. The process is repeated until the animal can be seen or its location can be inferred. The latter may be accomplished by circling the signal to determine a bounded area in which the focal animal must occur, tracking the animal to an obvious habitat or landscape feature, or by sandwiching the animal between the receiver and an apparent obstacle.
Alternatively, if the researcher wishes to avoid disturbing the animal, or if locations must be determined at night, the process of triangulation may be used. This requires finding the intersection of two or more bearings to determine one location. An error polygon can be calculated around the point estimate, resulting in a measure of precision equivalent to the area of the polygon. The size and shape of the error polygon is determined by:
1. the accuracy of the directional antennae;
2. the distance between the two receiving points;
3. the distance of the transmitter from the receiving points; and
4. the angle of the transmitter from the receiving points.
The most accurate estimate of an animal's location is obtained by receiving fixes that are closest to the animal and at 90o from each other. To reduce the size of the error polygon, three bearings can be taken and the animal's location estimated from the centre of the intersections. The error polygon formed by three radio bearing lines should be small enough to accurately place the animal in a single habitat polygon. If the location is near an edge, additional bearings should be obtained to accurately locate the animal on the map.
Where possible, standard telemetry base points should be established, marked and numbered by personnel experienced in use of radio-telemetry equipment. New observers should be familiarized with the base points and standard triangulation procedures by an experienced person. Triangulation of animals which are moving will produce large polygons (less accurate locations). For this reason, it is difficult to accurately determine locations of fast-moving nocturnal wildlife such as owls. If triangulation is used to determine wildlife positions, error measures should be calculated and reported along with the study results (Springer 1979; Saltz and Alkon 1985; Schmutz and White 1990; Saltz 1994). White and Garrott (1990) provide a useful compilation of error calculations for telemetry.
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The following guidelines should be adhered to when relocating animals (adapted from Page 1982):
1. Ensure that you use an antenna which is matched to the frequency transmitted.
2. As a general rule, the antenna elements should be oriented in the same direction as the transmitter antenna (i.e., when relocating a caribou wearing a radio-collar with a vertically-oriented whip antenna, the receiving elements should also be held vertically).
3. Hold antennas as high as possible or mount them on poles. Keep antennas at least 2 m away from all other objects, especially those which are large and metal objects, as these will cause detuning of the antenna.
4. Make use of null signals as well as peak signals to determine the direction to a transmitter. Using a 2-element antenna, the signal should be weakest when the tips of the elements point directly at the transmitter.
5. Make use of hills and other places of high elevation from which to receive signals.
6. Know your study area. Whenever possible take bearings through the flattest terrain with the least vegetation.
7. Take repeat bearings over a short time period, especially if the animal is active.
8. Get as close to the animal as possible. Attempt to confirm locations with direct observations.
9. Avoid sources of interference.
10. Take as many bearings as practical.
The following guidelines for aerial surveys are adapted from Gilmer et al. 1981.
1. Obtain a good set of maps and air photos of the project area. It can be useful to have both large and small scales.
2. Define the area which is to be searched for animals before the flight. If the Project Area is large, it may be useful to break it down into smaller Study Areas which can be effectively searched within an allotted time.
3. Primary power sources for receivers should be fresh and fully charged at the start of the survey.
4. An up-to-date list of transmitter frequencies should be carried, including the location of each animal from the previous search (as this may be a useful starting point).
5. Set up receiving equipment (this should be done with the pilot who has the ultimate responsibility for its safety):
1. Begin the search with the switch box set to "both" allowing the crew to listen for animals on either side of the aircraft.
2. At the outset of the flight, it may be beneficial to test equipment by making use of a test transmitter which is left at a known location on the ground.
3. Depending on the nature of the focal species and the objectives of the study, it may be useful to begin searching at the last recorded location for each animal. If this is unsuccessful, a more systematic, transect-based search design should be utilized (see item 8).
4. When a signal is detected, the control switch should be moved to "left" and then to "right" to determine from which side of the aircraft the signal is coming.
5. Once the direction has been determined, the pilot should turn the aircraft in the direction of the transmission. This will result in a temporary "null" signal until the aircraft flies close enough to the transmitter that the signal becomes audible again.
6. At this point, it should be possible to "home-in" on the transmitter position. Again, the operator changes the switch box from "left" to "right" to determine which side of the aircraft the transmitter is on. The operator will then identify an area on the appropriate side over which the pilot should begin a wide circle.
7. By moving the switch "right" and "left", the operator should be able to determine if the transmitter is within the area being circled. The circle may then be tightened, and focused based on the strength of the signal and the knowledge of the species habitat preferences.
8. Whenever possible, flight crews should attempt to verify an animals presence through direct observation.
9. For searches of a large number of highly mobile animals over a large area, it may be more appropriate to use a systematic method, using a scanner and parallel transects. For such searches, biologists should be aware of the limitations of receiving equipment to effectively scan for animals in a fast-moving aircraft. To this end, the formula below will calculate the maximum number of animals that may be effectively scanned for on a survey flight (for more information, see Gilmer et al. 1981).
where: NC=maximum number of animals which can be searched for
MD=minimum detection distance parallel to the aircraft's direction
of movement
SR= receiver scan rate
GS= maximum ground speed of aircraft
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