Northwest Minnesota 

Moose Mystery Research 

Summary Report

 

Acknowledgements: Results are finally in on the moose research project initiated in 1995. 

The death of the graduate student Eric Cox and pilot Grant Coyour, June 11, 1999, on the last 

calf production flight of the study was a great set back to getting the data analyzed and put into 

publication. Dr. Warren Ballard of Texas Tech University, who has vast experience working 

with moose and predators in Alaska, was hired to complete the job. Eric was an ambitious 

researcher and left no stone unturned. The volume of data that was collected made Dr. Ballard’s 

undertaking monumental. 

Our sincere thanks and appreciation goes out to all of the citizens and land owners who 

assisted with the study by allowing access to your property, reporting sick or dead moose and 

monetary donations. Many of you became involved with captures, transportation and pulling our 

vehicles out of mud or snow. Without your interest and assistance the project would not have 

been as complete. Thank you to the many private citizens, local businesses, sportsmen and 

conservations groups who provided funding for the study and the very successful Adopt-AMoose program. We are also indebted to the numerous student volunteers that endured the cold 

and the mosquitoes just for the experience. We hope it has benefited your careers. 

~~~~~ 

The moose research project was initiated in 1995 to investigate the reasons for the sharp decline 

in moose numbers during the early 1990s. 

Northwest Minnesota Moose Population 1960 to 2001 

Year

Number of moose

0

1000

2000

3000

4000

5000

6000

1960-611962-631964-651966-671968-691970-711972-731974-751976-771978-791980-811982-831984-851986-871988-891990-911992-93

1994-95 

1996-971998-992000-01
Three study areas were chosen to examine what was perceived, at that time, to be three 

different densities and rates of decline. They were: 

 Red Lake Wildlife Management Area (WMA) in the Beltrami Island State Forest 

 Agassiz National Wildlife Refuge

 Viking Area agricultural lands up to and including Thief Lake WMA 

152 moose (52 calves, 100 cows) were radio collared between 1995 and 1998. The radio 

collars had motion sensors that sent out a different signal rate when the collar (or moose) had not 

moved for 4 to 6 hours. This signal is often called a “mortality signal” indicating that an animal 

had died and needed to be found to determine the cause of death. Between May 1995 and July 

2000, 76 of the radio collared moose died. Thanks to the reporting efforts of the public an 

additional 84 dead moose that were not radio collared were examined by the research crew. 

The mortality results are summarized in the following Table. 

Percent of moose mortalities by apparent cause of death, 1995-2000. 

Radio Marked 

Moose

Non-Radio 

Marked Moose 

Cause Percent Percent 

Liver Fluke 32 7 

Disease/starvation 25 33 

Unknown 25 5 

Predation 7 0 

BrainWorm 5 20 

Vehicle Collision 4 26 

Poaching 2 3 

Starvation 0 1 

Natural Accident 0 5 

Total 100 100 

Deaths attributed to brain worm and vehicle collision are much higher for the non-radioed 

animals. This reflects a bias toward more easily observed causes of death, since most of the 

non-radioed moose were found by the public, rather than by radio tracking. The deaths due to 

accidents, vehicle collisions and poaching were combined as a sample of the “healthy, normal 

population” to compare health parameters to the other causes of death. Many aspects were 

looked at for their possible relationships to these apparent causes of death. 

Hunting was unlikely to be the cause of the population decline because hunting had been at a 

very low rate (3-25% per year and only every other year). The population usually grew in years 

after hunting seasons. Hunting seasons were closed in 1996 and yet the population has not 

recovered. 

Cows with calves: (from DNR’s annual census data) 

1984 to 1997 - varied from 54 to 94%. 

After 1997 - this percentage dropped below 50%. 
Ratio of Bulls to Cows: (from DNR’s annual census data) 

Prior to 1996 - percent of bulls to the number of cows varied between 57 to 94%. 

After 1996 - percent of bulls increased to 129 to 165% after hunting seasons closed. 

Bull to cow ratios were never below 50% which means there were an adequate number of bulls 

in the herd, and the population has not shown an increase with the increased percentage of bulls. 

This indicates that other factors besides availability of bulls is decreasing production. 

Age: Age was determined by counting cementum annuli of teeth removed from dead animals. 

From these data we inferred the age structure of the herd and found that it was skewed toward 

younger age classes as shown in the next figure. There were relatively few old cows and nearly 

all males were less than 5 years old. The lack of a high percentage of prime breeding ages (4-7 

year olds) suggests a declining population.

Age Structure of Moose Herd in Northwestern Minnesota 

0 5 10 15

Age

0

10

20

30

40

P

erc

e

nt

Male

Female

Fecal samples were collected at time of capture and in the snow from radio-tagged moose 

during subsequent winters. Blood samples were also taken at the time of capture. Pregnancy 

rates were checked by a combination of assays for fecal progestagens, blood serum progesterone 

and aerial observations of calves in late May. Pregnancy rate averaged 47.9%. There were no 

statistically significant differences among study areas, age class, or years but there was a trend 

for the lowest rate to be among yearlings and the highest rate at age 6 or 7. The lowest year was 

38.5% in 1996 and the highest year was 58.6% in 1999. The highest pregnancy rate by area 

was 51% for the Agricultural area. Twinning rate was only 17%. These pregnancy and 

twinning rates are low compared to other populations. For example, pregnancy rates in other 

studies in Canada and Alaska vary between 60 to 100% and averaged 84%. Twinning rates 

averaged 38%. In our population the pregnancy rate for yearlings was less than 20% compared 

to a population in Montana where it was 32%. The low pregnancy and twinning rates in 

Northwest Minnesota suggest a nutritional limitation. If quantity of forage had been limiting, 

it should be replenished after several years of a low population, however, since the population 

has not recovered even after a series of very mild winters it would not appear that the amount of 
forage is limiting. We also examined the quality of browse by collecting twigs from the plants 

that moose were found eating in the winter and found the amounts of nutrients and protein (6.7 

to 9.3% protein) to be adequate. 

Low pregnancy rates were directly correlated to low bone marrow fat and a low blood 

condition index. The blood condition index was based on the blood samples that were collected 

when the animals were captured. Bone marrow fat was determined by taking a sample of bone 

marrow from the leg bone of dead moose. Bone marrow fat was lower for moose dying of 

natural causes than for those dying from accidents (vehicle collision, poaching and other natural 

accidents) implying that malnutrition contributed to moose mortality. Moose that eventually 

died within 18 months after capture had body condition index ratings of Poor at the time of 

capture, indicating that the malnutrition was a long term chronic condition. This type of 

malnutrition is consistent with a parasite-mediated condition. 

Liver samples were taken from dead moose to determine concentrations of 19 minerals and 

trace elements. Analysis of 106 liver samples revealed deficient levels of copper and selenium

(based on threshold levels for domestic cattle and compared to several other moose studies). 

Copper deficiencies in livestock have been shown to contribute to low reproductive rates and 

may have an influence on low reproduction in Northwest Minnesota moose. The analysis, 

however, failed to find any correlations with apparent causes of death or other factors. In 

Sweden, the affects of acid rain on copper and molybdenum relationships resulting in copper 

deficiency has been proposed as one of the plausible causes for a moose population decline 

which resembles Northwest Minnesota’s decline. One of the other plausible causes of the moose 

decline in Sweden is a viral disease. We sent tissue samples to Sweden but they did not find the 

same virus present in our samples. 

Compared to other populations in Alaska and Canada, cows and yearlings in Northwest 

Minnesota had lower annual survival rates but calves had a higher annual survival rate (the 

chance of living for one more year). The adult cows in Northwest Minnesota had an annual 

survival rate of 79% compared to 75 to 94% in other studies. Calves had an annual survival rate 

of 66% compared to 2 to 27% in other studies. In most other places predators play a significant 

role in calf mortality but it was only 4% in Northwest Minnesota. The difference in the annual 

survival rate for calves is very significant. A moose calf in Northwest Minnesota has a 66% 

chance of surviving to be a yearling, where a moose calf in Alaska or Canada only has a 20+% 

chance of becoming a yearling. Whatever is limiting the population in Northwest Minnesota is 

not affecting calf survival. 

Blood serum was checked for leptospirosis and brucellosis. There were no positives for 

brucellosis. There were differences between the study areas for leptospirosis, but no correlations 

to pregnancy status or pathology. Brains that were submitted to the Veterinary Diagnostic Lab, 

North Dakota State University, were examined for chronic wasting disease. No CWD was found. 

There were no correlations between deer population and the decline in moose population, 

however, liver flukes, brain worm and winter ticks are deer parasites that have adverse affects 

on moose. During the time period of this study winter ticks were counted on skin samples but 

did not appear to have caused any mortality. Liver flukes and brain worm were a major 

apparent cause of death and may have been an even larger part of mortality as they may have 

contributed to the unidentified disease and starvation and the unknown deaths. 
SO...The BIG question is ... what could have changed since 1984 

that would make the above mortality factors more prevalent? 

Climatic factors based on data from a weather station near Norris Camp were examined for 

changes in the past 41 years that may have affected these mortality factors. The results are 

graphed in the figures below. Correlations were not found between population change and 

precipitation or snow depths, however, population decreases often occurred the year after 

summers with higher mean summer temperatures. Winter and summer temperatures in the 

past 41 years have increased by about 12 ̊F and 4 ̊F, respectively and the growing season has 

lengthened by about 39 days. Other moose research has shown two temperature thresholds for 

moose. March temperatures above 23 ̊F and September temperatures above 57 ̊F require 

moose to expend energy to keep cool. Since the peak moose population year in 1984, there 

have been more years when March and September mean temperature exceed these thresholds 

than between 1960 and 1984. Warmer temperatures may have contributed to increased energy 

demands for moose to keep cool causing a disruption of energy balance. While the exact 

mechanism involved in this response is not understood, the additional heat stress may accentuate 

poor body condition due to parasite induced chronic malnutrition, resulting in lower reproductive 

rates that may also be impaired by copper deficiency. 

Climate Data for Norris Camp, 1960 to 2001 

-25

-20

-15

-10

-5

Jan.-Feb. Temperature (oC)

15

17

19

21

23

25

Jul.-Aug. Tempera ut re (oC)

130

150

170

190

Da et of last spring freeze

260

280

300

320

te of first fall freeze

160

140

120

Da

80

100

180

Length of growing season (days)

1960 1970 1980 1990 2000

-61 -71 -81 -91 -01

1960 1970 1980 1990 2000

-61 -71 -81 -91 -01

1960 1970 1980 1990 2000

-61 -71 -81 -91 -01

1960 1970 1980 1990 2000

-61 -71 -81 -91 -01

1960 1970 1980 1990 2000

-61 -71 -81 -91 -01

*

*

* NOTE: date of last spring freeze & first fall freeze are based on the number of days after January 1 (Julian Date). 
0 1000 2000 3000 4000 5000

Population size (N)

-1.0

-0.5

0.0

0.5

Ln P( opulation rate of change)

0 1000 2000 3000 4000 5000

-1.0

-0.5

0.0

0.5

NOTE: Summer temperature (t-1) is the 

mean summer temperature one year prior 

to the population change on the vertical 

axis. 

16 17 18 19 20 21 22 23

Summer temperature (t-1)

-1.0

-0.5

0.0

0.5

Ln P( opulation rate of change)

16

0.5

0.0

-0.5

-1.0

17 18 19 20 21 22 23

 

The study concluded that climatic changes combined with increases in 

deer numbers and parasite transmission rates may have rendered 

Northwest Minnesota inhospitable to moose. The decline of the moose 

population in Northwest Minnesota does not appear to be from causes that management actions 

can change. Until the climatic factors that are making the moose range shrink to the North are 

reversed we will probably see fewer moose in Northwest Minnesota.

 

Local Contacts 

Agassiz NWR Red Lake WMA Thief River Falls Thief Lake WMA

22996 290th Street NE P.O. Box 100 246 125th Ave. NE 42280 240th Ave NE 

Middle River, MN 56737 Roosevelt, MN 56673 ThiefRiverFalls,MN 56701Middle River, MN 56737 

218-449-4115 218-783-6861 218-681-0946 218-222-3747 

This report is based on the manuscript submitted to The Wildlife Society’s publication Wildlife 

Monographs. Submitted December 23, 2004. Parasite-Mediated Decline in a Moose Population at the 

Southern Range Periphery. Eric W. Cox, Dennis L. Murray, Warren B. Ballard, Heather A. Whitlaw, 

Mark S. Lenarz, Thomas W. Custer, Terri Barnett, Todd K. Fuller.