Aerial Surveys

Absolute Abundance

• Total counts over whole area

• Total counts on sample plots (e.g., quadrats, strips, irregular-shaped areas)

critical assumption is that every animal is observed and counted.

1. Delineate area

2. Apply survey-sampling methods

3. Test assumptions (e.g., all animals detected) and remove bias if necessary.

Common Sampling Designs

• Simple random sampling
• Stratified random sampling
• Cluster sampling
• Systematic sampling

Removing Bias from Sample Counts of Large Mammals

1. Correction Factor (ratio)

• Use depends upon assumption of a constant factor under highly variable conditions (e.g., cover type in survey units, use of cover by the animals, and size of groups often varies).

2. Mark-Recapture

• Well established statistical basis
• Questionable assumptions in most cases
• Costly in time and resources

3. Sightability Models

• Adaptable to a variety of conditions
• Cost efficient
• Not applicable if visibility is low

Sightability Model

1. Fly aerial survey.

2. Mark elk (deer, sheep, etc.) groups with radio-collars.

3. Determine which groups were seen and which groups missed.

• depends on group size, tree and shrub cover, snow cover, observers, type of helicopter, etc.

4. Keep some factors constant (e.g, type of aircraft used, timing of survey, etc.)

5. Develop a sightability model for other factors.

• Use logistic regression
• p = e^u / 1 + e^u, where u = a + b₁X₁ + b₂X₂ + ...b_iX_i

6. Use sightability model to correct for factors we cannot control.

Application to a Sample Unit

1. Correct each group detected based on its probability of detection (visibility).

2. Sum all corrected groups in a sample unit for an unbiased estimate of actual number of animals present.

Application to a Herd Unit

1. Calculate means, ratios, proportions, etc. according to survey design.

2. Calculate variances and confidence intervals.

• Sources of variation: sampling variance, sightability variance, model variance.

Example: Northern Yellowstone Elk

• Surveyed in 1 day with 3 Super Cubs
• Surveyed whole range (all units)
• Developed sightability model in 1980's
• u = 0.969 + 0.0369 (group size) - 0.540 (vegetative cover) + 1.701 (activity)

Aerial Survey Program (see lab notes/exercise)

• All calculations easily performed.
• Variety of sightability models.
• see Unsworth, J. W., F. A. Leban, D. J. Leptich, E. O. Garton, and P. Zager. 1994. Aerial survey: user's manual, 2nd edition. Idaho Dep. Fish Game, Boise, Id. 84pp.

Revised: 12 October 2004