THE USE OF PORTABLE THERMAL IMAGING FOR ESTIMATING DEER POPULATION-DENSITY IN FOREST HABITATS

1. The reliability of deer population management could be improved wit h good density estimates, but current methods are either labour-intens ive or suffer from uncertainties regarding accuracy. 2. Visibility var ies substantially in forests depending on stand type, age and understo rey vegetation. In such conditions distance sampling would be an effic ient estimation method, but observer disturbance often results in bias when the method is applied to deer. 3. The performance of thermal ima ging for estimating deer density by distance sampling was assessed in seven forest deer populations. Thermal imaging equipment can detect th e long-wave energy. radiated by natural objects, clearly revealing war m-bodied animals even if partly obscured by vegetation. 4. Many more d eer were detected at night using a thermal imager than along the same transect routes in daytime. Detection distances were correlated with v isibility but were substantially longer than the average distances at which most animals were disturbed. Most deer were detected without cau sing prior disturbance. 5. Densities were estimated with a coefficient of variation ranging from 10.2-28.4%. Precision depended on sampling effort and sample sizes obtained. 6. A Monte Carlo simulation revealed a quadratic relationship between accuracy and visibility, with accura cy increasing with average visibility and a tendency for deer to selec t more open habitats within a forest. Under conditions that are likely to be typical of temperate forests (<40% thicket and neutral selectio n, or <70% thicket if thicket is avoided), accuracy was generally good and changed relatively little in relation to visibility and habitat s election. 7. Likely sources of bias as well as alternatives to thermal imaging are discussed. It is concluded that the method would be suita ble for estimating ungulate densities in forests with an adequate netw ork of tracks.