Copyright © 1997 Province of British Columbia
Published by the Resources Inventory Committee
3.4 Presence/not detected (possible)
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Recommended method: Encounter transects (Linear Transects)
Presence/not detected (possible) inventories (also called reconnaissance or distribution inventories) usually involve relatively low intensity surveys to determine animal distribution, composition and broad habitat associations. Habitat protection investigations and surveys associated with habitat suitability/capability mapping may fall into this category. Such surveys usually produce limited data on population size, and composition ratios obtained from these surveys may be biased. Thus, they are most useful in areas where there is minimal or no information. A specialized and important use of these surveys is as pre-surveys for intensive sample-based absolute abundance inventories.
3.4.1 Encounter Transects
The recommended protocol for conducting presence/not detected (possible) surveys are encounter transects. These should not be confused with line transects or transects of indefinite width where the right-angle distance of each animal from the transect line is recorded (Caughley 1977:39). Encounter transects (ET) are flown by either fixed-wing aircraft or helicopter, and all visible animals are counted and classified. Encounter transects may follow predetermined straight lines, contours, or drainages. When classification is conducted, it will normally be necessary to deviate from the transect line to ensure accurate classification of the animals. Following the classification, the pilot should be instructed to resume the transect line. While the method provides information on population composition and general distribution, it lacks a measure of the area surveyed, and consequently neither densities nor population size can be estimated. The number seen on each transect flown will vary greatly depending on vegetative cover and prevailing weather conditions, regardless of the number of animals present. The technique is most useful for species in homogeneous habitats, and where visibility is good (e.g., moose in deciduous forests, sheep on alpine ridges). Systematically spaced lines throughout the survey area may allow for calculation of indices such as animals per hour or animals per kilometre (relative abundance). It probably also provides the least biased composition data.
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