Evaluation of Maternal Penning to Improve Calf Survival in the Chisana Caribou Herd

Abstract

Predation is a major limiting factor for most small sedentary caribou (Rangifer tarandus) populations, particularly those that are threatened or endangered across the southern extent of the species' range. Thus, reducing predation impacts is often a management goal for improving the status of small caribou populations, and lethal predator removal is the primary approach that has been applied. Given that predator control programs are often contentious, other management options that can garner broader public acceptance need to be considered.Substantial calf losses to predation in the few weeks following birth are common for these small caribou populations. Therefore, we employed a novel experimental approach of maternal penning with the goal of reducing early calf mortality in the Chisana Caribou Herd, a declining population in southwest Yukon and adjacent Alaska thought to number around 300 individuals. Maternal penning entailed temporarily holding pregnant females on their native range in a large pen secure from predators from late March through the initial weeks of calf rearing to mid-June. During 2003-2006, we conducted 4 annual penning trials with 17-50 pregnant females each year (n = 146 total), assessed survival of calves born in the pens, and evaluated survival and nutritional effects of penning for females that were held. We also investigated the herd's population dynamics during 2003-2008 to determine effects of maternal penning on calf recruitment and population growth. In addition to information gained during maternal penning, we determined natality and survival patterns via radiotelemetry, conducted autumn age-sex composition surveys each year, and censused the population in mid- October 2003October , 2005October , and 2007. Based on our penning trials and demographic investigations, we used simulation models to evaluate the effects of maternal penning relative to a population's inherent growth rate (finite rate of increase [λ] without maternal penning) and penning effort (proportion of calves born in penning) to provide perspective on utility of this approach for improving the status of small imperiled caribou populations.Pregnant females held in maternal penning tolerated captivity well in that they exhibited positive nutritional responses to ad libitum feed we provided and higher survival than free-ranging females (0.993 and 0.951 for penned and free-ranging females, respectively). Survival of pen calves from birth to mid-June was substantially higher than that of free-ranging calves (x = 0.950 and 0.376, respectively). This initial period accounted for 76% of the annual calf mortality in the free-ranging population. Pen-born calves maintained their survival advantage over wild-born calves to the end of their first year (x = 0.575 and 0.192, respectively) during years penning occurred.Females in the Chisana Herd were highly productive with 57% producing their first offspring at 2 years of age, and annual natality rates averaging 0.842 calves/female ≥2 years old. Age-specific natality rates exceed...

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“…Benefits from demographic augmentation are calculated using empirically informed assumptions about vital rates (Supplementary Tables S1, S2). The assumption made under all demographic augmentation actions is that these recovery actions increase both calf and adult survival of penned/captive individuals (Adams et al, 2019). Once captive animals are released to the wild, their vital rates are reduced to baseline levels observed in the wild, under current landscape conditions.…”

“…Maternal penning involves temporarily protecting adult female caribou and their calves from predation, with 10-50 females cohabiting in each 0.06-0.12-km 2 pen (0.002-0.003 km 2 /adult female; Adams et al, 2019;. Adult females are typically captured about a month before parturition and they are released back to the wild when calves reach about a month old (Adams et al, 2019).…”

“…Maternal penning involves temporarily protecting adult female caribou and their calves from predation, with 10-50 females cohabiting in each 0.06-0.12-km 2 pen (0.002-0.003 km 2 /adult female; Adams et al, 2019;. Adult females are typically captured about a month before parturition and they are released back to the wild when calves reach about a month old (Adams et al, 2019). Calves are most vulnerable to predation during the first few weeks of life, and protection from birth through one month of age can increase annual survival rates threefold (Adams et al, 2019; see Supplementary Material for further discussion).…”

“…Adult females are typically captured about a month before parturition and they are released back to the wild when calves reach about a month old (Adams et al, 2019). Calves are most vulnerable to predation during the first few weeks of life, and protection from birth through one month of age can increase annual survival rates threefold (Adams et al, 2019; see Supplementary Material for further discussion).…”

“…Predator-prey management involves wolf and moose reductions (WR and MR, respectively), which have succeeded in stabilizing or even increasing some caribou populations (Hervieux et al, 2014;Serrouya et al, 2017bSerrouya et al, , 2019. A demographic augmentation method particularly relevant for caribou recovery is maternal penning (MP), which are small fenced areas (< 10 ha) where pregnant and post-parturient individuals and their calves are housed for the spring to reduce predation risk (Serrouya et al, 2015b;McNay, 2018;Adams et al, 2019). Conservation breeding (CB) programs and predator exclosures (PEs), which are methods widely used for species recovery (Magin et al, 1994;Hayward and Kerley, 2009), have also been considered for caribou (Antoniuk et al, 2012;Serrouya et al, 2015a;Hayek et al, 2016).…”