Research Document - 2005/045
Life History and Population Dynamics of Northern Resident Killer Whales (Orcinus orca) in British Columbia
By Olesiuk, P.F., G.M. Ellis and J.K.B Ford
Annual photo-identification surveys conducted between 1973-75 and 2004 were used to estimate life history parameters and develop a population model for the northern resident population of killer whales that inhabits coastal waters of British Columbia. During the 1970’s, 80’:s and early 90’s, the population grew exponentially (r2=0.986; F1,22=1,568.5; P<0.001) at an annual rate of 2.6% (95% CI 2.48-2.76%). Although the population almost doubled in size from about 125 to 217 animals, there was no evidence of a slowing of the growth rate (F1,21=0.25; P=0.622), suggesting the population was unrestrained and increasing at its maximum intrinsic rate. The population peaked abruptly in the mid-1990s, declined by 7-9%, and then exhibited a small increase, resulting in no discernible trend over the last decade (F1,10=1.36; P=0.271), indicating that something was restraining its growth. Life history and population parameters were thus estimated separately for 1973-96, a period of unrestrained growth; and 1996-2004, a period of no net change. During the period of unrestrained growth, females had a mean life expectancy of 46 years and maximum longevity was on the order of 80 years. Females typically gave birth to their first viable calf at 14.1 years of age (SE=0.050; range 10-21 years) and those that survived produced a total of 4.7 calves at mean intervals of 4.9 years (SE=0.18; range 2-11 years) over a reproductive lifespan typically lasting about 24 years. Older females exhibited reproductive senescence, with about 50% being post-reproductive by 38 years of age, and none reproducing after 46 years of age. Based on development of the dorsal fin – a secondary sexual characteristic – males typically attained sexual maturity at 13.0 years of age (SE=0.046; range 9-18 years) and the fin continued to develop for an average of 5.5 years (SE=0.113; range 3-7 years), such that males had typically attained physical maturity by 18.5 years of age. Males had a mean life expectancy of 31 years and maximum longevity was probably on the order of 60-70 years. Mortality curves were U-shaped for both sexes, indicating most mortality occurred early and late in life, but the right limb was steeper for males, resulting in a sex ratio that was progressively skewed toward females with increasing age (1:1 at age 15, 2:1 by age 34, and 3:1 by age 41 years). A sex- and age-structured model incorporating these parameters predicted that a population would increase at a rate of 2.4% per annum and be comprised of 46% juveniles, 22% reproductive females, 10% post-reproductive females, and 22% adult males. During 1973-96, the study population actually increased at 2.6% and was comprised, on average, of 46% juveniles, 21% reproductive females, 11% post-reproductive females and 22% adult males, indicating a good fit with the model predictions. Surprisingly, there were no major changes in reproductive parameters as the population stabilized during 1996-2004. Mean age at first birth increased slightly but significantly from 14.1 to 15.4 years (t49=3.23; P=0.002), mean age of onset of post-reproductive senescence increased from 38.4 to 40.6 years (t61=2.84; P=0.006), and calving intervals were marginally longer (5.5 versus 4.9 years; t97=2.92; P=0.091). The overall effect was a slight drop in the estimated reproductive potential of females from 4.7 to 4.5 calves. The recent decline in productivity was due almost entirely to increases in mortality, which were evident and statistically significant (7.63<X2<8.14; P<0.01) across all sex- and age-categories. Survival of viable calves to age 15 (about the age they are recruited to the adult population) dropped from 80% to 61%, and mean life expectancy declined from 46 to 30 years for females and from 31 to 19 years for males. Because the increase in mortality was broadly distributed across all sex- and age-classes, the predicted sex and age structure of the stable population remained almost unchanged at 47% juveniles, 24% reproductive females, 11% post-reproductive females, and 18% adult males. The life history parameters for neighbouring resident killer whale populations in Alaska and Washington appear to fall within the range of our unrestrained and stable models for northern BC residents, suggesting the models represent the general population biology of the resident ecotype of killer whale. We believe such models provide a useful construct for exploring and developing a better understanding of the factors that may regulate or impact killer whale populations.
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