Copyright © 1997 Province of British Columbia
Published by the Resources Inventory Committee
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Recommended method(s): Fixed-width (Strip) transects
Determining relative abundance requires holding bias constant in the survey method. Indices of abundance are usually expressed as animals per unit area, distance, or time. Techniques for obtaining indices have been developed for several species and habitats in British Columbia and are being used increasingly for management and research purposes. Consistent methodology is critical to maintain constant bias. As noted in Section 3.4.1, encounter transects can sometimes be used for relative abundance. Fixed-width transect sampling, however, is the recommended protocol.
3.5.1 Fixed-width transects
When using fixed-width transects (FT), only animals within a defined survey area (strip) are counted. Fixed-widths can be defined by marks on the airplane struts, or by placing a board across the helicopter skids and calibrating that with a mark on the bubble window. The strip width is usually < 200 m, but may vary with vegetative cover. Transects may be laid out in either a systematic or random pattern. Ideally, transects should be located in the direction of greatest density variation, usually perpendicular to contours (Caughley 1977). While the number of animals seen can be converted to a density estimate with an associated variance (Caughley 1977:31), it is recommended that fixed-width transect estimates be used for relative abundance (trend line monitoring), and only where visibility bias can be held constant.
Fixed-width transects provide the best results when animals are randomly distributed over large areas of homogeneous habitat. Variation in habitat types can cause large variation in the number of animals seen per transect. Animals which are aggregated can also cause large variations in the density estimate per transect and reduce the precision of the survey. The Boreal and Taiga Plains ecoprovinces in northeastern B.C., and possibly Nechako Lowland (Sub-Boreal Interior ecoprovince) are areas in British Columbia which appear to be best suited for fixed-width transect surveys.
The most common bias in fixed-width transects is associated with the difficulty in maintaining precise transect width. Small changes in the height of the aircraft or in observer position can result in large variations in the width of the area surveyed. Maintaining consistent transect width is often difficult or impossible in rugged terrain. When available, LORAN-C or GPS equipment is useful to track position in terrain with few landmarks. Flying by compass or using a distant object on the horizon helps on flat homogenous habitats.
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