Adjusting for seasonal harvest bias in the lactation index for white‐tailed deer management

Abstract

Recruitment estimates are critical for making wildlife harvest decisions. White-tailed deer (Odocoileus virginianus) lactation rates are often used to estimate fawn recruitment. Relatively long hunting seasons in the southern United States commonly include the period when females cease lactating, with concomitant reduction in the detection of lactation by hunters. We used 1,587 site-and year-specific lactation rates from 383 deer populations in Mississippi, Alabama, and Virginia, USA, to model the rate of decline in lactation detection across the hunting season. For each population, we calculated distance of mean female harvest date from median parturition date, and included linear, quadratic, and cubic functions of mean harvest day as potential explanatory variables. We grouped populations into 18 regions based on median conception date and tested 12 models using year and region as possible random effects. Mean female harvest occurred 92.5-210.7 days past median parturition date ( x ¼ 148.4 days, SE ¼ 0.5). Seasonal decline in lactation detection followed a linear trend with a daily decline of 0.25%. Given that our data contained properties where mean female harvest date varied by up to 81 days across years, this equates to a potential change in lactation index of 20% attributable only to differential detection. Adjusting hunter-estimated lactation indices should dampen variability of annual lactation indices simply associated with harvest date of females and clarify spatial and temporal trends in recruitment needed to advise wildlife managers. Ó 2016 The Wildlife Society.

Sensitivity analysis demonstrates limits to utility of lactation index for white‐tailed deer management

“…Fawn mortality is typically very low beyond 10 weeks of age; therefore, any bias from falsely assigning living fawns to harvested lactating females should be very low (Ballard et al , Vreeland et al , Rohm et al , Saalfeld and Ditchkoff ). Females frequently cease lactation during hunting season, potentially biasing lactation estimates downward; however, underestimates of lactation rate can be avoided using adjustments based on average harvest date or possibly by limiting data to early season harvest before substantial reduction in detectability (Scanlon and Urbston , Campbell et al ).…”