Northern Prairie Wildlife Research Center
We used Jolly-Seber models with time-specific survival and capture probabilities to estimate numbers of females (Model A of Pollock et al. 1990) because they fit better than models with constant survival probabilities (Model B: all females χ²5 = 12.3, P = 0.03; adults only χ²3 = 10.9, P = 0.01; truncated capture histories χ²3 = 8.6, P = 0.03) and models with constant survival and recapture probabilities (Model D: all females χ²13 = 27.3, P = 0.01; adults only χ²13 = 25.84, P = 0.02; truncated capture histories χ²8 = 14.4, P = 0.07). Further, goodness-of-fit was satisfactory only for Model A (all females χ²9 = 4.07, P = 0.91; adults only χ²5 = 3.52, P = 0.62; truncated capture histories χ²4 = 3.28, P = 0.51). Estimated numbers of females (Table 1) were relatively stable from 1968 to 1970, increased abruptly during 1971-72, remained high through 1974, then declined abruptly (Fig. 2).
|Table 1. Estimated numbers of black bears using a 218-km² area near Cold Lake Alberta, 1968-77. Estimates of total numbers (all bears) are from Young and Ruff (1982) unless otherwise noted. Estimates for females are from this study and are based on Jolly-Seber models with time-specific survival and recapture probabilities.|
|Year||All bears||All females||Adult females|
|1968||84|| a|| a|| a|| a|
|1976||123 b||34.2||5.26||25.0|| c|
|1977||99 b|| c|| c|| c|| c|
| a Not estimable by Jolly-Seber
b R.L. Ruff, unpublished data
c Limited data precluded estimation
|Fig. 2. Trends in estimated numbers of bears using the CLSA during 1968-77, standardized for comparison. Estimates from Young and Ruff (1982) are based on male and female bears aged 2 years (). Others are Jolly-Seber estimates for females (age 4 years ; age 2 years ; captures prior to 1975, age 2 years ).|
The abrupt nature of the increase in estimated numbers of female bears suggests a change in spatial distribution, rather than a change in reproduction or survival. Survival records of radiomarked bears suggest that mortality may have been partially responsible for the abrupt decline in 1975 (Table 2). K-M estimates of mortality caused by humans were similar for radiomarked male and female bears (M:F risk ratio = 1.05, χ²1 = 0.014, P = 0.91) and were not significantly lower for subadults than for adults (ad:sa risk ratio = 0.80, χ²1 = 0.20, P = 0.66).
|Table 2. Kaplan-Meier estimates of mortality rates from deaths due to all agents and natural agents only for radiocollared black bears at Cold Lake, Alberta, 1974-77.|
|Year||Natural mortality||All mortality||Bears||Deaths|
Human activity was the proximate cause of at least 25, and probably 28, of 30 deaths that befell radiomarked bears during 1974-77. Principal causes of mortality were depredation control (11 deaths) and a combination of legal hunting by native people and poaching (11 deaths), which could not be distinguished. Legal sport hunting, a drug overdose, a collision with a train, another bear, and other natural causes each caused 1 death. Fates of 3 bears could not be confirmed by recovery of a carcass or radiocollar, but each was rumored to have been shot.
The number of subadult males captured on the CLSA was larger during each year of 1972-75 (range = 21-25) than during any year of 1968-71 or 1976-77 (range = 2-17; Table 3). We believe this result indicates a difference in the number of subadults that were at least temporarily present on the study area because estimated capture probabilities of subadults were not significantly higher (χ²1 = 1.13, P = 0.29) during the post-removal period (post, sa = 0.55, SE = 0.11) than during the pre-removal period (pre, sa = 0.72, SE = 0.13).
|Table 3. Numbers of individual black bears captured annually at Cold Lake, Alberta, May-Sep, 1968-77.|
Moreover, estimated settling rates did not differ enough between periods (pre, sa = 0.49, SE = 0.10; post, sa = 0.49, SE = 0.07) to offset the increase in captures. Thus, numbers of subadult males using the study area were probably larger during 1972-75 than during other years.