ASSESSMENT OF HEALTH, MORTALITY, AND POPULATION DYNAMICS OF MOOSE IN NORTHERN NEW HAMPSHIRE DURING SUCCESSIVE YEARS OF WINTER TICK EPIZOOTICS
New Hampshire’s moose population has declined from ~7,500 to ~4,000 moose since the mid-1990s. Early decline is linked to harvest, but since the mid-2000s frequent winter tick epizootics have likely caused decline through high calf mortality and reduced productivity despite good habitat. This research used radio-marked moose (n=87) in northern New Hampshire to measure health at capture and cause of mortality, and better understand population dynamics.
Blood analysis from moose captured in January 2014 and 2015 indicated that moose were generally in normal health. Albumin, glucose, phosphate, and total protein values indicated that calves in better condition in January were more likely to survive. Iron levels (30%) were lower than in populations without winter ticks, were 20% lower in calves than adults, 11% lower in non-pregnant adult cows, and 17% lower in unsuccessful adult cows.
Substantial late-winter mortality of calves (69%) was associated with high infestation of winter ticks both years, whereas adult mortality was low (15% average) and attributed to multiple causes. Estimates of tick infestation on dead calves ranged from moderate (34,000 ticks) to severe (70,000 ticks) that would cause substantial protein and blood loss and presumably acute anemia when energy and protein are deficient in the diet.
Pregnancy (76%), calving (54%), and twinning rates (0%) were similar to those in declining populations and lower than 10 years earlier. The largest declines were in yearling pregnancy (20% to 0%), twinning (11 to 0%), and annual successive calving rates (75 to 29%). Lower productivity presumably reflects the frequent winter tick epizootics that collectively reduce physical and nutritional condition of adult cows and neonates. Adult cow (83%) and summer calf (76%) survival were high and similar to that measured in 2002-2005. Average late winter calf survival (31%) was 50% lower than in 2002-2005. The sequential epizootics and successive high calf mortality rates in 2014 and 2015 are previously undocumented in the northeastern United States.
The female population is projected to halve in 18 years given the current frequency of epizootics (5 in 10 yr); to achieve population stability, the frequency must decline to 3 in 10 years. If the shorter winter weather pattern persists, a cyclic relationship will likely ensue where a moderate moose density is sufficient to support conditions for continued epizootics until the moose (host) density declines to limit winter tick abundance; thereafter productivity and survival will increase. A liberal harvest that quickly reduces population density would shorten the phase of population decline, allowing for faster population recovery. Such an approach has somewhat limited risk given that recovery would be optimal in good habitat, while proactively addressing the insidious impact of winter ticks that cause high annual mortality of calves and low productivity, neither indicative of a healthy moose population.