Dall’s sheep productivity, and survival rates in
interior Alaska in mitigated and non-mitigated
military overflight areas
CHAPTER 2
Dall’s sheep productivity, and survival rates in
interior Alaska in mitigated and non-mitigated
military overflight areas
The Yukon-Tanana Uplands of interior Alaska are overlain by airspace that is designated as a military operations area (MOA). Military aircraft from the United States Air Force (USAF) and its allies utilize this airspace for air combat training, fighter intercept training, basic fighter maneuvers, and low altitude fighter operations (Department of the Air Force 1995). Dall’s sheep (Ovis dalli) also occupy the Yukon-Tanana Uplands. Aircraft have been shown to have varying impacts on wild sheep (Krausman and Hervert 1983, Lenarz 1974, MacArthur et al. 1979, Stockwell et al. 1991, Krausman et al. 1993, Bleich et al. 1994, Weisenberger et al. 1996, Krausman et al. 1998) leading to concerns of potentially negative impacts on Dall’s sheep from low level military aircraft. In an effort to minimize potentially negative impacts, the USAF has implemented mitigation measures at a known lambing area, Cirque Lake (Fig. 2.1). Mitigation measures applied to this area occur from 10 May – 15 June. During this time, military aircraft are restricted to flying 5,000 feet (1,500 m) above ground level in the Cirque Lake mitigation area. At other times of the year, and in the surrounding area, military aircraft may fly as low as 100 feet (30 m) above ground level (Department of the Air Force 1995).
The fundamental concern regarding aircraft impacts on wildlife species is the potential for long-term negative effects on populations due to human disturbance. Population decline can be brought about by a decline in survivorship, a decline in fecundity or changes in immigration and emigration. Studies investigating aircraft impacts on wild sheep populations have focused on short-term effects (Krausman and Hervert 1983, Lenarz 1974, MacArthur et al. 1979, Stockwell et al. 1991, Krausman et al. 1993, Bleich et al. 1994, Weisenberger et al. 1996, Krausman et al. 1998). While insightful, these investigations do not provide a direct measure of the demographic processes that control population levels. Therefore, inferences of long-term effects based on short-term responses are strengthened in those instances in which long-term effects can also be assessed.
In this study, we investigated long-term effects of military jet overflights on 6 parameters. We investigated body weights, age, pregnancy rates, the number of lambs, the number of yearlings, and survival rates of ewes in 2 Dall’s sheep populations exposed to military jet overflights. One population is mitigated for potential negative impacts (Cirque Lake) and the second (West Point/Puzzle Gulch) is not. Because there were no sheep populations within the general vicinity that were not exposed to military overflight activity, our study is restricted to comparisons of the mitigated area versus a non-mitigated area. We predicted Dall’s sheep ewes at the non-mitigated location would be lighter, would have lower reproductive success (pregnancy rates, lamb to ewe ratio and yearling to ewe ratio), and lower survival than would sheep in the mitigated area.
Dall’s sheep were captured, and indices of productivity and survivorship, were gathered at 2 study locations in eastern interior Alaska in the Yukon-Tanana Uplands (Fig. 2.1). One site is mitigated for potential impacts of low level military jets (Cirque Lake) and the second site is not (West Point/Puzzle Gulch). The 2 study sites occur approximately 35 km apart and can be typified as rugged alpine landscapes separated by wooded areas. The climate and vegetation typical of the study site has previously been reported (Chapter 1). Dall’s sheep density were 0.10 sheep per km2 and 0.19 sheep per km2 at Cirque Lake and West Point/Puzzle Gulch, respectively during the final year of this study (2002) and the sheep population at the 2 study sites was relatively stable throughout the course of this study (chapter 1). Potential predators known to exist in the area include black bears (Ursus americanus), grizzly bears (Ursus arctos), wolves (Canis lupus), wolverines (Gulo gulo), lynx (Lynx canadensis) and golden eagles (Aquila chrysaetos)(Appendix C).
Sheep captures were accomplished with a hand-held netgun using a helicopter (Robinson R-44) as a platform. Ten sheep were radiocollared at each study site in March of 1999, 2000 and 2002 (20 radiocollared Dall’s sheep per year). During March 2001, 10 sheep were radiocollared at West Point/Puzzle Gulch but due to problems locating sheep, only 7 were captured at Cirque Lake. Out of 84 total sheep that were netgunned during this project, one died during capture. This animal stopped breathing and could not be revived. A necropsy performed on this animal did not reveal the cause of death (C. Rosa, DVM, University of Alaska Fairbanks, personal communication). A second sheep died 6 days after capture from an unknown predator. All remaining radiocollared sheep survived at least a minimum of 27 days following capture. Capture methodology was within the guidelines established by the American Society of Mammalogists (Animal Care and Use Committee 1998).Physical measurements recorded for captured sheep were metatarsal length, chest girth, body length, neck circumference, horn length, horn circumference, and body weight (Appendix B). Blood samples (30 cc) were collected from captured animals and assayed for progesterone concentrations by the Institute of Arctic Biology, University of Alaska Fairbanks to determine pregnancy rates of captured sheep. Sheep with blood levels of >2ng of progesterone/ml of serum were classified as pregnant (Ramsay and Sadleir 1979; Brundige et al. 1988). A subjective determination of relative body condition was assigned to each animal (Gerhart et al. 1996). Subjective scores were assigned based on a scale from 1 to 5 (1 = emaciated, 5 = obese). Sheep age was estimated by counting horn annuli (Geist 1971) and by assessing incisor wear. Age estimated by counting horn annuli has a tendency to underestimate sheep ages in comparison to canine tooth cementum analysis in older ewes (Kleckner et al. 2002) and for this reason, we consider ages of sheep listed in this report to be conservative.
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| Figure 2.1. Location of study sites in interior Alaska. One study site is mitigated (Cirque Lake) for potential impacts of low-level military jet overflights, and the other (West Point/Puzzle Gulch) is not. Ten Dall’s sheep were captured in March of each year from 1999 – 2002 at each study site with the exception of Cirque Lake in 2001 when 7 sheep were captured. |
Each year during this study (1999-2002), aerial surveys were conducted to count sheep within study units in early to mid-July (Appendix A; Chapter 1). Sheep were classified during this survey as rams (all animals with >1/4 curl horn size), ewe-like (rams < 1/4 curl and all adult ewes), lambs, and yearlings (the yearling category contained primarily ewes as yearling rams would likely be classified as “ewe-like”). In addition to the total number of sheep observed in each of the study units (Chapter 1), other measures of survival and productivity can be derived from this data. The number of lambs:100 ewe like sheep provides an indication of productivity. As this value is estimated in mid-summer, it is not a measure of lambing rates but is a combination of lambing rates and 1-2 month lamb survival. The yearling:100 females value provides an upper index for the number of ewes from a specific cohort that can be recruited into the breeding population.
Dall’s sheep mortalities were initially investigated from the fixed-wing aircraft. At the earliest opportunity, sites were visited on the ground but this often occurred long after the animal had died. Indication of cause of death was recorded when possible. We did not directly observed any predation but assumed sheep were depredated if there were signs of a struggle or if there was an abundance of bright red blood in the area. We assumed blood from an animal that had died while still intact would be dark and coagulated and, in the wintertime, frozen, thereby limiting dispersal.
Sheep survival was investigated using the Kaplan-Meier procedure extended to allow staggered entry of radiocollared sheep during each year of this study, and allowing failed radiocollars to be right censored from the data set (i.e., failed collars were excluded from analysis after disappearance; Pollock et al. 1989).
A multivariate analysis of variance (MANOVA) was used to compare body weights and age (dependent variables) of sheep at the 2 study sites and between years (independent variables). We used two-tailed t-tests to examine differences in pregnancy rates, ewe survival rates (from mid-March – mid-November), lamb:100 ewes ratios, and yearling:100 ewe ratios between the 2 study sites. Because rates and ratios vary between 0 and 1 and, therefore, typically have a binomial rather than a normal distribution (Zar 1996), we arcsine transformed all rate and ratio data prior to analysis to avoid problems with data distribution. Body weight and age of pregnant versus non-pregnant ewes, and body weights and age of sheep that lived and died in the months following capture were also compared using two-tailed t-tests. The relationship between body weight and pregnancy status, and body weight and survival was described with univariate logistic regression. We tested for differences in Kaplan-Meier survival functions using a Chi-square test (Pollack et al. 1989). Statistical tests were considered significant if P < 0.05.
Mean (+SD) age of captured sheep was similar between the 2 study sites (6.7 [+1.59] and 6.9 [+1.84] years for West Point and Cirque Lake respectively; Fig. 2.2). Age of captured sheep varied between 3 years of age (Cirque Lake and West Point) and 11 years of age (Cirque Lake)(Fig. 2.2). There was a slight tendency for age of capture sheep to increase as the study progressed. Annual mean body weights of captured Dall’s sheep have been consistently greater at West Point in comparison to Cirque Lake (Fig 2.3). Differences in weights between the 2 study areas were greatest when the study was initiated and approached parity during the final year of the study.
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| Figure 2.2. Age distribution of Dall’s sheep captured in March of 1999 (n = 20), 2000 (n = 19), 2001 (n = 17), and 2002 (n = 20) in interior Alaska at a study site mitigated for military aircraft overflights (Cirque Lake) and at a non-mitigated site (West Point). |
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| Figure 2.3. Body weights (mean + SE) of Dall’s sheep captured in March of 1999 (n = 20), 2000 (n = 18), 2001 (n = 12), and 2002 (n = 17) in the Yukon-Tanana Uplands of interior Alaska at a study site mitigated for military aircraft overflights (Cirque Lake) and at a non-mitigated site (West Point). |
Examination of multivariate results indicates the dependent variables (sheep age and body weight) were effected by study site selection (F2, 62 = 6.699, Pillai’s trace P = 0.002) and not by sampling year (F6, 126 = 1.929, Pillai’s trace P = 0.081). The interaction between sampling year and study site selection was also significant (F6, 126 = 6.699, Pillai’s trace P = 0.045). Univariate results as a follow up to the mutivariate approach indicate body weight differed significantly between the 2 study sites (F1, 63 = 9.254, P = 0.003) and age did not (F1, 63 = 0.289, P = 0.593). Univariate results also indicated that body weight was significant in the area by year interaction (F3, 63 = 3.608, P = 0.018) and age was not (F3, 63 = 0.653, P = 0.584).
Annual pregnancy rates tended to be higher at West Point in comparison to Cirque Lake. For both areas, pregnancy rates were lower in 2000 and 2001 in comparison to 1999 and 2002 (Table 2.1). Differences in pregnancy rates at the 2 study sites were not found to be significant (t = 1.030, df = 4.1, P = 0.360).
| Table 2.1. Dall’s sheep pregnancy in interior Alaska at a study site mitigated for the impacts of military aircraft overflights (Cirque Lake) and at a non-mitigated site (West Point) during March of 1999, 2000, 2001 and 2002. | ||||||||||||||||||||||||||||||||||||||||||||||||
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A t-test indicated mean age (+SE) did not differ between ewes that were pregnant and those that were not (6.8 [+0.23] years and 6.4[+0.35] years, respectively; t = 1.071, df = 70, P = 0.288). Mean (+SE) body weights of pregnant Dall’s sheep (58 kg [+0.8 ] kg) were greater than non-pregnant sheep (51 kg [+0.23kg] kg) and this difference was significant (t = 4.607, df = 66, P <0.001). A logistic regression model indicated that sheep needed to weigh at least 50 kg before they had a >0.50 probability of being pregnant (Fig. 2.4). The model correctly predicted the pregnancy status (preganant or not) of 77% of the sampled sheep.
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| Figure 2.4. The logistic relationship between the probability of pregnancy and body weight (kg) for adult Dall’s sheep ewes in the Yukon-Tanana uplands, Alaska. Coefficients presented are for the logistic equation of the form: Pr(X) = eg(x)/(1 + eg(x)), where g(x) = β0 + β1X1, and Pr(X) is the probability of pregnancy at a particular X1. |
The number of lambs:100 ewes observed during July surveys varied between 57 (Cirque Lake, 1999) and 25 (Cirque Lake, 2001). No trend was apparent in lamb:100 ewes when comparing study locations or when comparing years (Fig. 2.5). In contrast, the yearling:100 ewes ratio tended to decline from 1999 to 2002 and Cirque Lake had higher yearling:100 ewes ratios than did West Point/Puzzle Gulch (Fig. 2.5). The number of yearlings:100 ewes varied between 45 (Cirque Lake, 1999) and 13 (West Point/Puzzle Gulch, 2001). Statistically, no difference between study sites was found for lambs:100 ewes (t = 0.043, df = 4, P = 0.967) or for the number of yearlings:100 ewes(t = 2.258, df = 4, P = 0.087). We were only able to evaluate the change in the number of lambs: 100 ewes to yearling: 100 ewes in one year (2002). At Cirque Lake during 2001, 25 lambs: 100 ewes resulted in 25 yearlings:100 ewes in 2002, a 0% decline. At West Point, 33 lambs: 100 ewes in 2001 resulted in 15 yearlings:100 ewes in 2002, a decline of more than 50%.
Out of 77 sheep that were collared over 4 years in the 2 study units, 14 died (18%) while radiocollared (Table 2.2). The number of sheep mortalities was comparable between the 2 study sites (8 at Cirque Lake and 6 at West Point). Yearly mortalities were the same during 1999 and 2001, slightly higher during 2000 and nonexistent in 2002 (Fig 2.6). It should be noted however, that 3 of the mortalities at the West Point study site may have been the result of 1 predation event (by wolves) as the mortalities were detected at the same time and appeared to be of similar age. Upon investigation, the remains of 6 sheep (3 with radiocollars) were detected within a 1 km radius.
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| Figure 2.5. Lambs:100 ewes and yearlings:100 ewes observed in July in interior Alaska during an aerial survey. Sheep were surveyed at a study site mitigated for military aircraft overflights (Cirque Lake) and at a non-mitigated site (West Point/Puzzle Gulch). |
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Table 2.2. Cause of death for Dall’s sheep captured in March of 1999 (n = 20), 2000 (n = 20), 2001 (n = 17), and 2002 (n = 20) in the Yukon-Tanana Uplands of interior Alaska at a study site mitigated for military aircraft overflights (Cirque Lake) and at a non-mitigated site (West Point). |
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| Figure 2.6. The Kaplan-Meier survival probability function for Dall’s sheep radiocollared in the Yukon-Tanana Uplands, interior Alaska. Data presented is combined from 2 study sites and is monthly survival by year. |
Survival from mid March – mid January was comparable at Cirque Lake and West Point (0.80 and 0.82, respectively) during the 4 years of this study (Chi-square = 0.463, P = 0.50). The pattern of survival we observed indicate most mortalities occur from late winter through lambing (mid March – mid June; Fig. 2.7). The sharp decline in survival at West Point between mid-November and December was due to the probable wolf predation event previously described.
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| Figure 2.7. The Kaplan-Meier survival probability function for Dall’s sheep radiocollared in the Yukon-Tanana uplands, interior Alaska at 2 study sites. One site (Cirque Lake) is mitigated for the potential negative impacts of military jet overflights and the second (West Point) is not. Data presented is for monthly survivorship and was collected over 4 years (1999-2002). |
Mean (+SE) age of radiocollared sheep that died (7.2[+0.55]) during this study did not differ significantly (t=0.844, df = 74, P= 0.402) from those that survived (6.7[+0.21]) Dall’s sheep survival was significantly effected by body weight (t=2.015, df = 70, P= 0.048). Mean (+SE) weights of radiocollared Dall’s sheep that died during this study were 53 kg (+2.1) and those that survived were 57 kg (+0.8). A logistic regression model supported the conclusion that heavier sheep tended to have a higher probability of survival (Fig. 2.8). The model correctly predicted the survival (dead or alive) of 82% of the sampled sheep.
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| Figure 2.8. The logistic relationship between the probability of survival and body weight (kg) for adult Dall’s sheep ewes radiocollared in the Yukon-Tanana uplands, Alaska. Coefficients presented are for the logistic equation of the form: Pr(X) = eg(x)/(1 + eg(x)), where g(x) = β0 + β1X1, and Pr(X) is the probability of survival at a particular X1. |
DISCUSSION
Our predictions that Dall’s sheep ewes at the non-mitigated location would be lighter, would have lower reproductive success (pregnancy rates, lamb to ewe ratio and yearling to ewe ratio), and lower survival than would sheep in the mitigated area was generally not supported (Table 2.3). Indeed, the only parameter that was found to be statistically different between the 2 study sites was body weight, and ewes in the non-mitigated (West Point/Puzzle Gulch) site weighed more than ewes at the mitigated site (Cirque Lake). The trend in pregnancy rates also leads to the conclusion that conditions were better for Dall’s sheep at West Point/Puzzle Gulch than at Cirque Lake. The only result that contradicts this and supports our initial predictions was the higher yearling: 100 ewe ratio at Cirque Lake in comparison to West Point and this result was not significant. It appears likely, therefore, that factors other than the presence or absence of mitigation measures are responsible for observed Dall’s sheep productivity and survivorship at these 2 study sites during this study.
Not surprisingly, many of these productivity parameters appear to be related (Table 2.3). In those instances in which ewes were heavier, pregnancy rates were higher and lambing rates and survival tended to be better. The reverse was true when ewes were lighter. The effect of body weight on pregnancy rates is not surprising as autumn body weight has been shown to correlate to pregnancy rates in other northern ungulates (Albon et al. 1986, Cameron et al. 1993, White et al. 1997). It is also logical that lambs: 100 ewes would be related to pregnancy rates.
| Table 2.3. Summary of measures of productivity over a 4 year study of Dall’s sheep in the Yukon-Tanana Uplands of interior Alaska. Productivity parameters were collected at at 2 study sites. One site was mitigated for the potential impacts of low level military overlfights (Cirque Lake) and second site was not. Data is summarized by year and by study site. The highest measure of a particular parameter is highlighted in blue. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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* Body Weight in kg. |
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Of the results presented in this paper, likely the most difficult to interpret is the mean age of captured ewe. Because of small sample sizes in each age category, a few individuals could make substantial changes in the distribution of ages of captured sheep. Two observations are possible however. The mean age of captured sheep at the 2 study sites (6.7 and 6.9 at West Point and Cirque Lake, respectively) is near the median for all sheep captured (7) indicating a relatively even distribution between the oldest and youngest sheep we captured (3 and 11 years, respectively). Survival, therefore, appears to have been relatively constant for this age bracket of ewes. This pattern is consistent with life-expectancy tables that have been published for this species (Murphy and Whitten 1976, Hoefs and Cowan 1979). The second observation is the lack of younger sheep in the sample. Because we specifically selected mature adult ewes in our capture efforts and this selection was based on comparative body size, we may have selected against any ewe younger than 4 years with 2 exceptions. Therefore, sheep in this population may be reaching adult size when they are 4 years old. Because smaller sheep are less likely to be pregnant, the successful breeding of 2 and 3 year old ewes seems doubtful.
Mean (+SE) body weights we recorded during this study (55.9[+0.77]) were high in comparison to that reported in the literature. Mean body weights for adult ewes in the spring during a study conducted in the Alaska Range was 47.2 kg (Heimer 1972). For Dall’s sheep in the Yukon Territory in the late winter, mean body weights were 48.8 kg (Bunnell and Olsen 1976). In a study in the Central Brooks Range, mean body weights were 50.0 (+2.09) kg (Lawler 2004). Therefore, in comparison to populations of Dall’s sheep that were not overlain by MOA airspace , body weights in both study sites the Yukon-Tanana Uplands (mitigated and non-mitigated) do not appear to be suffering from exposure to low-level military jet aircraft.
Our results for pregnancy rates, lamb:100 ewe ratios, and yearlings:100 ewe ratio all showed considerable variation. This sort of variability appears to be the rule in Dall’s sheep populations (Nichols and Bunnel 1999). Pregnancy rates and lambing rates would both depend on a ewes nutritional status which in turn is effected by the current forage available to the ewe as well as the forage available during the previous winter and summer. Similarly, yearling survival is dependent on the current forage available to them, their nutritional plane during the previous winter and summer and their vulnerability to predation pressures. Forage quantity and quality has been proposed as the ultimate factor limiting growth and populations of Dall’s sheep (Nichols and Bunnel 1999). Forage quantity and quality appear to be simlar at the 2 study sites in this study (Appendix D). Winter weather, and particularly snow cover, is extremely important as it can control habitat availability. Both Murphy (1974) in Denali National Park Alaska, and Nichols (1978) on the Kenai Peninsula of Alaska found lambing rates in the spring to be inversely related to snow depth the previous winter. In soft-snow, 30 cm has been proposed as the limit under which Dall’s sheep can effectively forage (Hoefs and Cowan 1979, Nichols 1988). Therefore, some of the variation we observe could be related to factors such as weather.
Relative to other Dall’s sheep populations, lamb:100 ewe ratios, and yearling:100 ewe ratios we observed in the Yukon-Tanana Uplands were comparable. Nichols (1978) presented statistics from numerous surveys of Alaska Dall’s sheep populations in the Brooks Range, Central Alaska, and the Kenai Mountains and found a mean (+SE) lamb: 100 ewe ratio of 37 (+2.1) (n =57). The mean (+SE) lamb:100 ewe ratio we observed during this 4 year study was 40 (+5.2) (n = 6). Statistics for mean (+SE) yearling:100 ewes ratios presented by Nichols (1978) were 21 (+0.02) (n = 59) ewes. We observed mean yearling:100 ewe ratios of 25.7 (+4.75) (n = 6). The over winter survival of lambs during their first winter in this study is difficult to assess as we only have 1 year in which to make this comparison. In this 1 year we saw 100% survival at one of the study sites (Cirque Lake) and a 45% survival rate at the second site (West Point). Therefore, in comparison to populations of Dall’s sheep in areas ouside of the MOA structure, lambing rates, yearlings rates and first year survival of lambs are all good and do not appear to have been compromised by military flying currently occurring in the Interior Alaska MOAs.
Survival of adult ewes in this study was comparable to that observed in other populations. Hoefs and Cowan (1979) reported an average annual mortality rate of 20% for ewes in Kluane National Park, Canada. Simmons et al. (1984) estimated an average annual mortality of 15% for adult ewes in both the Mackenzie Mountains of Canada and in Denali National Park. We found mortality rates at the 2 study sites to be 20% and 18% (Cirque Lake and West Point, respectively). Based on the observation that these populations appear to have been stable over the last 4 years (Chapter 1), these mortality rates appear sustainable and are not extraordinary in comparison to other populations.
Weisenberger et al. (1996) and Krausman et al. (1998) investigated heart rate and behavior of bighorn sheep (Ovis canadensis nelsoni) and found short-term effects on these variables from simulated noise of low-level jet overflights or from overflights by F-16 jets. Within a short time however, heart rates returned to pre-disturbance levels (generally <3 minutes) as did behavior (<5 minutes) and these authors concluded that mountain sheep reactions to F-16 overflights were minor. Responses to military jet aircraft or from simulated military jet aircraft appeared to be short-lived in their study and response times declined with repeated exposure (Weisenberger et al. 1996) suggesting sheep habituate to aircraft overflights. In regard to long-term effects, Weisenberger et al. (1996) did not find long-term effects on heart rate in bighorn sheep exposed to simulated noise of low-level jets. Krausman et al. (1998) present antidotal information that low-level military jet aircraft did not adversely effect reproduction in a group of bighorn sheep in Nevada. These animals were kept in a large enclosure over a 2 year period and were subjected to military overflights during the second year. In the 1st year of their study, 8 ewes were observed with 6 lambs. In the following year, when the sheep were exposed to overflights, 8 ewes were observed with 7 lambs. In both years of their study, sheep within the enclosure had higher pregnancy rates than did free ranging sheep in nearby populations who were rarely exposed to overflight events (1/month).
In conclusion, documenting long-term effects of aircraft overflights on wildlife species, such as effects on reproduction or survival, is difficult (National Park Service 1994). Previous workers examining long-term effects of military overflights on other northern ungulates have reached varying conclusions regarding the effects of military overflights on wildlife species from no effect on population dynamics in caribou (Rangifer tarandus; Davis et al. 1985), changes in activity budgets in caribou (with the potential to have long term effects; Murphy et al. 1993) to increased calf mortality in woodland caribou (Rangifer tarandus caribou; Harrington and Veitch 1992). In this study, we looked at long-term effects of military overflight on Dall’s sheep body weights, age, pregnancy rates, the number of lambs, the number of yearlings, and survival rates of Dall’s sheep ewes in an area mitigated for the potentially negative impacts of military overflights, and an area that isn’t mitigated and is heavily overflown by military aircraft. The preponderance of evidence suggests that the current mitigation measure is not providing any measurable long term advantage to sheep at Cirque Lake (mitigated) in comparison to West Point (not mitigated). However, we could find no indication that current levels of military overflights are causing long-term harm to sheep in the Yukon-Tanana Uplands as the population parameters we measured are comparable to sheep populations that are not exposed to low-level military aircraft.
We investigated body weights, age, pregnancy rates, the number of lambs, the number of yearlings, and survival rates of ewes in 2 Dall’s sheep populations exposed to military jet overflights. One population is mitigated for potential negative impacts (Cirque Lake) and the second (West Point/Puzzle Gulch) is not. Body weight was the only parameter out of the six examined that differed significantly between West Point and Cirque Lake. Sheep at West Point were heavier than those at Cirque Lake. Based on these six measures of individual and population health, the current mitigation measure is not providing any measurable long term advantage to sheep at the mitigated site in comparison to a non-mitigated site.
2004 Sheep Report
http://www.nps.gov/yuch/Expanded/key_resources/sheep/sheep_2004.htm
Doug Beckstead
December 8, 2004