Population dynamics of moose in Alaska: Effects of nutrition, predation, and harvest

Project 1.57, Federal Aid in Wildlife Restoration Grant. Project Duration: July 1, 2001–June 30, 2006. Principal Investigator: Rodney D. Boertje, Fairbanks.

This 5-year study of age-specific productivity and survivorship focused on the effects of nutrition and predation on moose at a high density in Unit 20A. The research was prompted by the need to know when and why population fluctuations are occurring in order to propose and evaluate management options for maintaining moose above predation-limited, low levels. Several prior moose studies in penned and wild populations provided data indicating that low twinning rates, delayed reproduction, and low short-yearling weights indicated poor nutritional status. More recent studies of browse removal rates have indicated that browse removal rates are highest where twinning rates are low, reproduction is delayed, and short-yearling weights are low.

The PI concluded that density-dependent nutritional limitation was apparent in Unit 20A, an expected result of maintaining moose at high density. There was a maximum 10% decline in the expected proportion of calves in the immediate postcalving population. Although the population increased during this study, predation by wolves and bears limited moose population growth more than reduced productivity.

Cow parturition rate was directly related to density. Strong age-specific indicators of nutritional stress existed, including: 1) no 24-month-old moose (n = 38) were pregnant, 2) only 32% of 36-month-old moose gave birth, and 3) no marked moose less than 60 months old produced viable twins. Since 1978, production declined at a minimum 20% with a 3.2-fold increase in density, but the increased moose population allowed greater sustainable yields than would have been possible at the lower density. Newborn singleton and twin birth weights were not significantly different with regard to location or capture year in GMU 20A. As expected, newborn weights on the Tanana Flats were relatively low compared with those from the Yukon Flats, where moose density is 85% lower and the observed twinning rate (63%) indicated a high nutritional status during ovulation.

Mean maximum depth of rump fat was significantly greater among pregnant versus nonpregnant adult cow moose. Mean maximum depth of rump fat was also significantly greater for moose observed parturient versus those never observed with a calf and for dams giving birth to twins versus those with singletons. The fattest dams produced the heaviest calves and calved earlier than dams with low rump fat. Also, weighing short yearling moose appears to be a particularly useful and relatively inexpensive tool for evaluating moose population condition. The adjacent Denali and Unit 20A populations had substantial different weights. We concluded that adult rump fat depths are less sensitive indices of nutrient regime compared to short yearling weights and twinning rates.

Annual calf survival was 53% from May 1996 to May 2006 (n= 79 newborn calves and 292 short-yearlings). Predation was the major proximate cause of death. Wolves killed more calves than either bear species, but combined predation by both bear species exceeded calf predation by wolves. In addition to mortality detected using radiocollared calves, mortality prior to birth or neonatal mortality during the first 24 hours after birth apparently occurred in 7 (17%) of 42 pregnancies in 1996 and 3 (13%) of 23 pregnancies in 1997.

The annual composite yearling survival rate for females from mid-May 1997 through early-May 2006 was 83%. The 2-year-old through 5-year-old annual composite rates ranged from 97% to 100%. These rates averaged 92% for ages 6 through 10 years, and 80% for ages 11 to 16. No moose were known to live to 18 years. Female moose appeared to be most vigorous and capable of avoiding predation from 2 through 5 years of age.

Wolf predation was the major cause of death among adult and yearling moose. In 47 cases where we were able to investigate natural causes of death among radiocollared moose older than 24 months, wolves killed 25 (53%), grizzly bears killed 8 (17%), and 14 (30%) died from factors other than predation or harvest. Of 47 yearlings (12 to 24 months old) that died, wolves killed 33 (70%), bears killed 9 (19%), and 5 (11%) died from factors other than predation or harvest.

These data were incorporated into discussions with area biologists, the Board of Game, local students, and wildlife professionals. Nine different area biologists contributed data on moose productivity to help rank nutritional status in 15 different moose populations in Alaska (see appendix). Management activities related to achieving nutrition-based population objectives have increased because of this work.