Northern Prairie Wildlife Research Center
For the period 1975-86, 437 wolves were captured in foot traps and 262 destroyed or otherwise removed during control activities (Fig. 7). The number of wolves captured did not increase during this period (r = 0.021, P = 0.475), but the number destroyed has increased with time (r = 0.756, P= 0.002). The killing of depredating wolves was prohibited by regulations governing the program from 1975 through 1977 (Fritts 1982). No significant increase occurred in the number of wolves captured from 1979 through 1986 (r = 0.494, P = 0.107), but the number killed increased (r = 0.702, P = 0.026). The highest percentage of the Minnesota population killed (assuming 1,200 wolves) was 3.4% in 1983, well below the 30% harvest that wolf populations are believed capable of sustaining in fall (Keith 1983).
Fig. 7. Wolves captured and killed in depredation control activities in the Minnesota wolf range. Data from 1987 to 1989 are discussed in the Epilogue.
From 1979 to 1986, 232 attempts were made to trap problem wolves at 207 farms (sum of number of farms trapped annually). Some farms required more than one trapping attempt per year, and some received trapping in several years. The mean number of trap-nights per complaint was 145 (min-max = 2-1,224). There were 271 individual wolves (average, 34 per year) among the 279 captures, and 216 were destroyed. Successful trapping was defined as the capture of one or more wolves during a period of continuous trapping at a farm. One hundred and twenty-four (53%) trapping efforts resulted in capture of one or more wolves and thus were considered successful.
When trapping efforts were successful, the number of wolves captured was one in 44% of the efforts, two in 27%, three in 14%, and four to eight in 15%. The mean number of trap-nights per capture was 126. The mean number of wolves destroyed per verified complaint was 1.1 during 1983-86, compared with 0.6 during 1979-82, primarily because of longer trapping periods, but also because of pan-tension devices on traps, increased skill of personnel, and earlier killing of pups.
Periods of trapping lasted 1 to 38 days; about 70% lasted 1 to 15 days. When wolves were captured, the average time to the first capture at a complaint site was 4.8 days (min-max = 1-17). Seventy-five percent of wolves were captured in the first 10 days of trapping and 88% in the first 15 days. An analysis of trapping efficiency was made for 5-day periods after 10 days and as long as 35 days, although most trapping lasted for 10 to 14 days. When calculated per period, capture efficiency remained stable up to the maximum period trapped. We left traps out beyond 10-15 days only under special circumstances (e.g., at farms where there was continued wolf activity); otherwise, efficiency probably would have declined. A trapping period of 10-15 days seemed to be the best approach for balancing costs and trapping efficacy at most farms.
Wolves were most likely to be captured when a pack or pair lived nearby. Ninety percent of successful trapping attempts involved a pair or pack of wolves (determined from sign present or the age and breeding status of wolves captured); 10% involved single animals. When no wolves were captured during a trapping attempt, one of the following factors was responsible: (1) a transient lone wolf was involved but left the area shortly after the depredation, (2) members of a pack or pair were involved but returned only sporadically to the site, or (3) members of a pack or pair were involved and simply eluded capture. We concluded that about 90% of depredations were caused by members of packs. In British Columbia, about 82% of the problems were believed to be caused by packs (Tompa 1983a, 1983b). Lone wolves associated with cattle more often than packs did on grazing leases in Alberta; however, their relative importance as predators of cattle was not clear (Bjorge and Gunson 1983).
The age ratio of adult or yearling wolves to pups captured during 1979-86 was 187:84; 48% were males. Pups began showing up in the catch as early as 22 June and outnumbered adults by September (Fig. 8). Breeding adults were difficult to identify with certainty; we identified 21 breeding females in the May, June, and July catches because they were lactating or had recently lactated. In total, some 80 wolves were thought to be breeding adults. In many respects, the May-June period was an ideal time to trap; the probability of a captured wolf being a depredator was higher during that period because packs were not yet traveling with pups of the year.
Fig. 8. Number of adult (including yearling) and pup wolves captured by month, 1979-86.
We found it extremely difficult to determine the age and breeding status of the wolves that committed depredations. Depredations were rarely observed, and the only evidence available for assessment was tracks, droppings, and other signs at the kill site. We expected that adult wolves, especially breeders (dominant pack members), were most likely to be responsible because they are known to play a lead role in killing wild prey (L. D. Mech, unpublished data). However, our evidence to support that assumption was scarce. Accordingly, when a pack was involved we usually found it difficult to determine whether the wolves captured were actually the ones responsible for depredations. Proximity of capture site to kill site usually provided the best inference possible.
Captured wolves were generally in excellent condition. Few debilitating conditions were found that might have predisposed wolves to seek domestic prey. Eleven wolves sustained foot injuries from being trapped previously (primarily in coyote or fox traps set by private coyote and fox trappers) that were judged severe enough to hinder their killing of wild prey.
Forty-eight pups were released from 1979 through 1986 (Federal Judge P. McNulty, court order, 14 July 1978, unpublished data). Their fates are largely unknown. Four pups were recaptured at one farm and two at a neighboring farm. Two additional wolves were recaptured as yearlings at other farms where they were implicated in depredations. Pups were not believed to be depredating animals until late fall, when they were capable of killing sheep and turkeys. However, the food demand of rapidly growing pups, especially in July and August, probably contributed to depredation when rendezvous sites were established near farms. Wolves that were exposed to livestock as pups may have been more inclined to cause problems later in life; however, because we recaptured so few of the 48 pups (all had been ear-tagged) in later trapping, that hypothesis lacks support. Survival of the released pups could have been low because adults were removed from their packs.
Evaluating the effectiveness of wolf removal programs for reducing livestock depredations has been difficult (Fritts 1982; Tompa 1983a, 1983b; Bjorge and Gunson 1983, 1985). It might be possible to determine the effectiveness of reactive trapping of wolves to control livestock depredation by trapping and removing wolves at some farms having verified wolf depredations, conducting mock trapping at other comparable farms (controls), and comparing subsequent losses. However, such an experiment was not the purpose of our program, nor a viable option. A large sample of affected farmers-willing to forego what they believed to be corrective action and risk financial losses-would have been required.
The only method we had for evaluating trapping effectiveness was to compare posttrapping losses at farms where trapping was successful with posttrapping losses where trapping was unsuccessful. Therefore, we analyzed the 124 successful trapping efforts (108 farms) and 108 unsuccessful efforts (99 farms) for followup losses in the same and succeeding years. Of the 108 farms where trapping was successful, 37 (34%) had another loss in the same year, and 34 (31%) had another loss in the succeeding year. Of the 99 farms where trapping was not successful, 23 (23%) had another loss in the same year, and 23 (23%) had another loss in the succeeding year. As in the past, depredations at many farms stopped even though few or no wolves were removed, but continued at other farms despite regular removal of wolves. Thus our comparison method proved inconclusive.
The preceding results were unrelated to type of livestock except that unsuccessful trapping was followed by additional losses in the same year more often when the prey was sheep-13 of 29 instances-than when the prey was cattle--8 of 64 instances (X2 = 11.93, df = 1, P = 0.001). Thus, depredation problems with cattle were more likely to be sporadic than those involving sheep.
We examined the relation between number and age-class of wolves captured and subsequent verified depredations in the same and following year. In this assessment, a trapping effort in which wolves were captured was considered effective if no further depredations were verified later that year or in the following year, or ineffective if any were verified during that period. Trapping efforts in which one wolf was captured (regardless of age) were effective 54% of the time, two wolves 53% of the time, and three or more 43% of the time. This result is misleading, however, because pups are included. Trapping that resulted in the capture of only pups was effective in 4 of 18 instances (22%) compared with 57 of 105 (54%) instances in which only adult or yearling wolves or both were captured (X2 = 6.319, df = 1, P = 0.012). With only adult and yearling wolves considered, the capture of one wolf was effective in 27 of 47 (58%) instances, and 2 wolves in 14 of 25 (58%) instances The capture of three or more adult or yearling wolves was effective in 11 of 17 (67%) instances (but not significantly higher than for captures of one or two wolves (X2 = 0.355, df = 2, P = 0.837).
Contrary to our expectations, no higher effectiveness resulted from breeding wolves being caught. The effectiveness rate for efforts in which either one or both breeding adults alone were captured versus the rate when nonbreeding adults or yearlings were captured were identical (55%). Even when we captured both breeding adults (18 instances), the effectiveness rate was still only 50%. Therefore, capture of nonbreeding adult or yearling wolves evidently had as positive an effect as capture of breeding adults, suggesting that nonbreeding adults or yearlings may be as likely to initiate depredations as breeding adults.
The major difference in types of farms must be clearly understood before attempting to interpret the preceding results. Wolves were most readily captured at chronic problem farms (Type I farms; Fritts 1982) where one or more packs were present year round, and where depredations usually resumed within a year despite successful trapping (34% of all captures occurred at 6% of the farms). The reasons for resumption of depredations at those farms were not understood, but probably included high density of wolves locally, differences in wolf traditions, farm management practices, and the degree to which the pastures were forested (Fritts 1982). Also, the incidence of subsequent losses at farms that were unsuccessfully trapped was probably naturally low because many of those farms were of the sporadic problem type (Type II farms; Fritts 1982).
No significant relation was found between the number of verified complaints in a year and the total number of wolves captured the previous year (r = 0.062, P = 0.883). Removal of depredating wolves may have had a positive local effect on losses at some farms during the current and subsequent year, but usually did not prevent annually recurring losses at chronic problem farms. Therefore, the finding of no overall year-to-year relation may have resulted from the blending of new and repeat farms each year. If we had trapped mostly the same farms from year to year, a reduction in losses due to a positive effect of trapping at some of the farms might have been more apparent. However, because the total number of farms with verified complaints each year consisted of farms trapped the previous year (30-40% of total) and farms trapped for the first time (60-70%), the positive effect of trapping at farms across the wolf range the previous year was negated by losses occurring at new farms each year. Also, some repeat farms that have persistent losses regardless of the trapping efforts would be chronic problem farms. Depredations ceased at some farms after trapping. In 1983, for example, six wolves were captured after the loss of at least nine calves at a large farm having exclusively open pasture near the edge of the wolf range; no further losses occurred there. At some farms, removal of wolves was followed by long-term absences of depredation; generally those farms did not meet the same criteria as chronic problem farms and in many instances they were not situated adjacent to high density wolf areas.
Roseau and Kittson Counties showed notable differences in the depredation problem compared with the rest of the wolf range (Fig. 1). Many farms in these two extreme northwest counties were disjunct from the primary range but adjacent to small parcels of wilderness capable of supporting only a pack or pair of wolves. Several of the depredations in Roseau and Kittson Counties involved turkey flocks, whose husbandry favors extremely selective wolf control. We therefore expected trapping in that area to be more specific for offending wolves and abate complaints for a longer time. However, no relation was found between the number of wolves captured and verified complaints during the subsequent year (r 0.030, P = 0.944).
We suggest that the Roseau and Kittson County findings resulted from the ease with which sheep and turkeys are captured, enabling wolves to have a higher probability of killing livestock. The number of wolves captured never demonstrated a positive effect of wolf removal, especially with new wolves developing livestock-killing behavior on a regular basis.
The critical question in evaluating the effect of trapping is whether losses would have been higher without removal of problem wolves. Although we believe that to be true, no proof can be offered for the Minnesota wolf range as a whole. Any positive effect of wolf removal on losses throughout the entire wolf range probably was obscured by the blending of recurrent and new farms experiencing wolf depredations each year. Control trapping and wolf removal had its most obvious value in seasonally curtailing the most serious problems where wolves returned regularly to commit depredations (e.g., nightly surplus killing of turkeys and sheep). One such example involved a farm in Roseau County where 61 sheep were killed in repeated incidents in 1984 before two wolves were captured and killed. Removal of wolves conditioned to livestock undoubtedly reduced the accumulation of such individuals in the population and may have impeded long-term growth of the depredation problem. Moreover, we suspect that the level of annual losses at Type I farms would have been higher without wolf removal. Another probable, but unmeasurable, benefit of wolf removal was the reduction of illegal local control efforts.
Conceivably, regular removal of wolves could have been counter-productive in some instances by making remaining pack members more dependent on livestock. A wolf in Alberta became more dependent on cattle after the removal of the majority of his pack and his dispersal (Bjorge and Gunson 1985). A yearling wolf in Montana preyed on calves during the spring after the removal of other pack members (E. E. Bangs, J. Fontaine, and S. H. Fritts, unpublished data). Clearly, much remains to be understood about the effects of wolf removal, especially at Type I farms, but it is obvious that disruption of social organization within the packs occurs. The effectiveness of trapping improved during the program. Personnel gained expertise at trapping, refined trapping techniques, and became familiar with the physiography of farms and travel routes of wolves at farms that were trapped more than once. Also, a major advance in trapping efficiency came with the use of tension devices or underpan spring devices in 1984, which drastically reduced captures of smaller, nontarget species (Turkowski et al. 1984). This refinement reduced the amount of time spent resetting traps and left more traps available for catching wolves. The use of all-terrain vehicles and improvements in lures and baits also improved efficiency.