Provide Assistance to Isle Royale National Park in the Monitoring of Wolf and Moose Populations During the Winter of 1987

Peterson, Rolf

Wolves declined while moose and beaver increased during 1986-87, the 29th consecutive year of research into wolf-moose dynamics at Isle Royale. We have hypothesized that this predator-prey system is slowly cycling on a very long time scale, with population peaks approximately every three decades (Fig. 1). According to this hypothesis, wolves are expected to remain stable at a relatively low level through the 1980's, while moose steadily increase. Wolf numbers stood at only 16 in 1987, the second lowest total documented £or this population. Mortality rate for the wolves present last year was 40-45%, very high £or a protected wolf population. The nature of this mortality is unknown, but low kill rate, extensive trespassing and continued interpack conflict all point to a chronically low food supply. We expect that the current food shortage will continue until the end of the 1980's, when abundant cohorts of moose born since 1981 will attain middle age and hence be more vulnerable to wolves. Only two wolf packs persisted through the 1987 study. The East Pack II (EPII) enlarged its territory considerably to about half of the land area of Isle Royale. Researchers witnessed an attack by the EPII on a group of three wolves, apparently a remnant o£ the Harvey Lake Pack (HLP). The alpha male in the latter group was killed and the two surviving wolves were dispersed. The moose population was estimated at 1380 +/- 204 (95% confidence interval), suggesting a substantial increase over last year. Calf recruitment in 1986 was strong, especially in the eastern half of the island where wolf density has plummeted within the last two years. Overall, moose mortality remains low, and malnutrition remains a significant source of mortality for very old moose. Radio-collared moose were used in 1987 to continue an assessment of moose "sightability" on census plots, or the proportion of moose seen and counted on plots. After moose have moved into conifer cover and midwinter distribution has stabilized, 75% of the moose on plots can be detected from circling fixed-wing aircraft.