H
0
3: The kill rate by wolves on moose calves depends on
the proportion of calves alive in winter;
H
0
4: The kill rate by wolves on moose decreases when the
availability of caribou exceeds that of moose.
Methods
We studied wolf kill rates in winter in the 23 000-km
2
Finlayson
Lake Study Area (FSA) in the east-central Yukon (62°N, 128°W)
from February 1990 through March 1994. Hayes and Harestad
(2000a) describe the study area. Wolves were reduced in the area
during the 1980s (R. Farnell, Yukon Fish and Wildlife Branch, Box
2703, Whitehorse, YT Y1A 2C6, Canada, unpublished data), and
the wolf population increased rapidly during our study (Hayes and
Harestad 2000a).
Other biologists estimated moose abundance in two regions in
the FSA before (Jingfors 1988) and during our study (Larsen and
Ward 1995). In the North Canol area, moose annually increased at
a finite rate of 1.16 from November 1987 to 1991, for a density of
339 ± 61 (mean ± 90% confidence interval (CI)) moose/1000 km
2
(Larsen and Ward 1995). Similarly, in the Frances Lake area,
moose increased at a finite rate of 1.18, for a density of 381 ± 80
(mean ± 90% CI) moose/1000 km
2
in winter 1992 (Larsen and
Ward 1995). We calculated the mean moose density for the two
areas in winter 1990 and 1991 by interpolating between these sur-
veys, assuming a constant rate of increase (Appendix, Table A1).
We extrapolated the rate of increase between 1992 and 1993 from
Larsen and Ward (1995). From 1993 to 1994, we projected popula-
tion change on the basis of adult mortality and calf recruitment
rates, after the formula of Hatter and Bergerud (1991):
λ
= (1 – M)/
(1 –
R), where
M is adult mortality rate and
R is the proportion of
moose calves observed in March 1994 (Appendix, Table A1). We
estimated that overall moose density in the FSA increased from
263/1000 km
2
in 1990 to 443/1000 km
2
in 1994 (Appendix, Ta-
ble A1). After 1994, moose-calf survival rates and moose density
apparently declined in the area. In 1996, R. Ward (Yukon Fish and
Wildlife Branch, Box 2703, Whitehorse,YT Y1A 2C6, Canada, un-
published data) estimated moose densities of 278 ± 53/1000 km
2
(mean ± 90% CI) in the North Canol area and 337 ± 71/1000 km
2
(mean ± 90% CI) in the Frances Lake area. Densities were not sig-
nificantly different from 1991 estimates, but apparently declined
from our projected estimate in 1994 (Appendix, Table A1).
Caribou were counted using stratified random block surveys in
1987 (Farnell and MacDonald 1988), 1991 and 1996 (R. Farnell,
unpublished data). The herd increased at a mean finite rate of 1.18,
growing to 5950 ± 18% (mean ± 90% CI) animals in winter 1991
(R. Farnell, unpublished data). After 1991, herd growth slowed and
possibly declined by 1996 as recruitment dropped. Herd size in
1996 was 4536 ± 12% (mean ± 90% CI) animals, but was adjusted
to about 5000 because of bulls missing from surveys.
Hayes and Harestad (2000a) describe methods for estimating
wolf density and radiotelemetry techniques. We defined the kill
rate as the number of moose killed per day by each wolf (to study
moose population dynamics) or the total biomass (kg) of ungulate
prey killed per day by each wolf (to study wolves’ consumption
rates). The daily area traveled by each pack was estimated from
100% area convex polygons (Ackerman et al. 1990).
We estimated kill rates by locating packs of radio-collared
wolves at regular intervals during February and March of 1990 and
1992 and during March of 1991 and 1994. We defined each series
of consecutive daily or bi-daily relocations as a predation period.
We defined wolf-pack size as the mean number of wolves seen dur-
ing each predation period (Messier 1994; Dale et al. 1995). Air-
craft crews observed wolf behavior using methods of Mech (1974).
When observers located a radio signal, they counted wolves and
searched the area for ungulate carcasses. If most pack members
were not seen, aircraft crews followed wolf trails to find missed in-
dividuals and locate any kills. From the air we classified all dead
moose as calf or non-calf (yearling and adult combined) according
to differences in size and body shape (Peterson 1977).
The interval between locations varied according to the composi-
tion of ungulate species in pack territories. Wolves usually spend
more than 48 h handling a moose carcass (Peterson et al. 1984;
Messier and Crête 1985; Ballard et al. 1987; Hayes et al. 1991).
Therefore, we located a pack every 24–48 h if only moose prey
were available and twice each day, usually between 9:00 and 11:00
and between 16:00 and 19:00, if caribou were also available. We
compared kill rates with location intervals to test for any temporal
bias. If a pack was not seen for more than 3 consecutive days, we
ended the observation because a moose could be killed and con-
sumed within that period (Peterson et al. 1984; Hayes et al. 1991).
We divided causes of ungulate mortality into wolf predation,
and other natural and human causes. We assumed that wolves
killed an animal when there was fresh blood spoor, or when snow
trails showed that the animal had been recently attacked by wolves.
We assumed that wolves were scavenging if a carcass was found
lying on its sternum (Stephenson and Sexton 1974; Ballard et al.
1987; Hayes et al. 1991) or there were signs that other animals had
fed on the carcass before wolves did. Human causes included kill-
ing by hunters or trappers or being hit by a vehicle.
We visited a sample of in situ prey carcasses each winter to de-
termine their sex, age, and physical condition. Moose sex was de-
termined from antler pedicels and ileum morphology and caribou
sex from the size and shape of antlers. We collected incisor bars
from killed moose to determine age (Sergent and Pimlott 1959).
We also collected long bones from killed moose and caribou to
assess nutritional condition (Neiland 1970). We kept bones frozen
to minimize dehydration loss (Peterson et al. 1982). Even when
moose carcasses were mostly consumed, many could still be classi-
fied as either calf or adult from the size and shape of moose fecal
pellets on site.
We estimated the live mass of adult female moose in late winter
at 375 kg (Franzmann et al. 1978) and adult bulls at 413 kg
(Schwartz et al. 1987). We assigned a mass of 400 kg to animals of
unknown sex, 250 kg to yearlings, 150 kg to calves (Ballard et al.
1987), 152 kg to adult caribou (R. Florkiewicz, Yukon Fish and
Wildlife Branch, Box 2703, Whitehorse, YT Y1A 2C6, Canada,
unpublished data), 55 kg to calf caribou (Skoog 1968), and 75 kg
to Dall sheep (Ovis dalli dalli) (Sumanik 1987; Hayes et al. 1991).
Consumable biomass of caribou was 75% of live mass (Ballard
et al. 1987). We estimated that consumable biomass of moose was
65% after weighing 7 moose carcasses on the day that wolf packs
abandoned them. Ravens (Corvus corax) were important scaven-
gers in our study area during winter (Promberger 1992). We used
data from Promberger (1992) to adjust wolf consumption to ac-
count for raven scavenging, depending on wolf-pack size.
We defined the predation rate as the proportion of prey that were
killed daily (Messier 1994). We estimated the winter predation rate
by multiplying daily kill rates by 182 days, then dividing by the
mean moose density.
Annual snow data were collected in early March at 7 stations in
our study area (G. Ford, Government of Canada Water Resources,
Whitehorse, Yukon, unpublished data). We compared kill rates
with March snow depth obtained from the station nearest each
pack’s territory. We used linear regression analysis to examine rela-
tions between kill rate and several independent variables.
Results
Types of ungulates killed by wolves
During all winters we found 326 ungulate carcasses, in-
cluding 291 moose (89%), 30 caribou, 1 Dall sheep, and
4 unidentifiable kills. We determined that 286 moose were
killed by wolves (Table 1). We visited 51 kills in situ. During
© 2000 NRC Canada
50
Can. J. Zool. Vol. 78, 2000
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