Optimal timing to estimate moose Alces alces demographic parameters using remote cameras

Abstract

Obtaining estimates of demographic parameters are fundamental for managing species. However, survey timing and duration influences the precision and accuracy of estimates. We used motion-activated camera images to investigate the effect of survey duration, timing, camera density and on- or off-trail placement on detection rates, sex and age ratios, and relative abundance estimates of moose (Alces alces) in Isle Royale National Park (IRNP), Michigan, USA. Variations in detection rates reflected moose life history patterns and suggested the optimal times to estimate demographic ratios and population relative abundance. We recommend camera surveys of 25-days during mid-June-mid-July and early December-early January to produce consistent and precise calf: cow and bull: cow ratios. On-trail cameras returned greater detection rates and relative abundance estimates, but decreased precision for summer bull: cow and calf: cow ratios than off-trail cameras. Subsampling camera densities to 3 cameras/km² decreased precision and consistency for density and ratio estimates. We recommend estimating moose relative abundance during early December-early January, using > 3 cameras/km² placed on and off-trail. Pairing life history events with high detection rates can be used to identify optimal survey periods and could be applied to other species.

Keywords: Alces alces; Age ratios; Detection rate; Moose; Relative abundance; Ungulate.

Fig. 1

Moose mean daily detection rates across 10-days using on- and off-trail cameras (n = 156), Isle Royale National Park, Michigan, USA, 2020 (lighter colors) and 2021 (darker colors). Moose detections included unknown age and sex. Moose neonates are born, breeding season, and shedding of antlers occur approximately mid-May-early June, mid-September–late October, and mid-December–early January, respectively.

Fig. 2

Moose relative abundance/km² estimations (95% confidence intervals) across all pooled (blue; n = 156) and on- (pink; n = 98) or off-trail (orange; n = 58) cameras, Isle Royale National Park (544 km²), Michigan, USA, 2020 and 2021. Estimations calculated from moose detections within 60-day periods.

Fig. 3

Moose relative abundance/km² estimates (95% confidence intervals) using camera density subsets of 5 (100%), 4 (80%), 3 (60%), and 1 (20%)/km², Isle Royale National Park (544 km²), Michigan, USA, 2020–2021. Estimations calculated from moose detections within 60-day periods. Camera numbers have equal proportions of on- and off-trail cameras and are scaled to 1 km².

Fig. 4

Daily mean calf: cow and bull: cow ratios across 25-day moving windows (95% confidence intervals) in summer (June–August) and early winter (November–January), Isle Royale National Park, Michigan, USA, 2020 (red) and 2021 (blue). For each set of ratios, differences in 95% confidence intervals (CI diff – [upper CI – lower CI]) and coefficient of variation (CV) are plotted for 2020–2021. Periods with low CI diff values and CV indicate higher precision.

Fig. 5

Camera locations (n = 156), Isle Royale National Park, Michigan, USA, 1 April–31 March 2020–2022.