POPULATION TRENDS OF SOUTHERN RESIDENT KILLER WHALES (ORCINUS ORCA) FROM BACKGROUND INFORMATION

Transcription

1 POPULATION TRENDS OF SOUTHERN RESIDENT KILLER WHALES (ORCINUS ORCA) FROM David E. Bain 1 and Kenneth C. Balcomb III 2 1 Six Flags Marine World Vallejo, Vallejo, CA Center For Whale Research, Friday Harbor, WA BACKGROUND INFORMATION In the mid 1960 s, prior to live-capture collections for oceanariums, it is believed that about 100 individuals comprised the Southern Resident community of killer whales. A decline in population levels occurred as a result of collections for public display through the early 1970 s (Hoyt 1990), decreasing the number of whales to about 70 individuals. Beginning in 1974, the Southern Resident community of killer whales was censused annually using photo-identification methodology. Based on these studies, the Southern Resident killer whale population was shown to recover to nearly 100 individuals by the mid-1990 s. However, during the last few years ( ), a significant decline in population levels has been observed. In May, 1995, the population count was 98 individuals. By October, 1999, this number had dropped to 83 whales, a decline of over 15%. This recent decline led Baird (1999) to recommend to the Committee on the Status of Endangered Wildlife in Canada that this population be listed as threatened (since this population inhabits both U.S. and Canadian waters its status needs to be considered by both governments). Southern Residents are genetically isolated from other killer whales with which they share their range (Hoelzel et al. 1998, Hoelzel and Dover 1989). Olesiuk et al. (1990) summarized the life history parameters of resident killer whales from (their study included Northern Residents in addition to Southern Residents). They characterized the per capita birth rate as 5.1%, the per capita death rate as 2.2%, so population growth was 2.9% per year. Females continued to reproduce until about 40 years of age, and over 75% of viable calves reached adulthood. Olesiuk et al. (1990) realized the population would not continue to increase forever, and suggested mortality might increase, birth rates might decline, and approximately half of juveniles would die before reaching adulthood when the population stopped growing. Ford et al. (1994) published an update of the membership of the population through 1993, but did not recalculate population parameters. METHODS Recent data on births and deaths were collected by the Center for Whale Research. Expected population trends were calculated based on population parameters in Olesiuk et al. (1990), which were based on a period of population growth from Births of new population members and deaths of known individuals were determined as described by Olesiuk et al. (1990), Bigg et al. (1990), and Bain (1990). The numbers presented here are based on population estimates for July 1 of each year. This was done to allow comparison of data from recent years, when data are available from most of the year, with data from earlier years, when data are available only from a short period during the summer (and thus counts reported here may differ slightly from other reports).

2 Mortality rates were calculated for each year. To smooth out random fluctuations, a five year moving average was also calculated. Removals for public display were treated as mortalities. In addition, natural mortality of 2.2% per year was assumed for the years prior to data collection ( ). Fecundity was indexed as calves recruited per reproductive age female. A calf was counted as recruited if it was believed to be alive on July 1, typically corresponding to an age that would range from 3-9 months. Females were counted as reproductive if their age was estimated to be from 15 to 39. Expected calving was estimated at 20% of the number of reproductive females (Bain 1990). To allow testing for trends through time, data were pooled over periods corresponding to publication dates of prior research: (Olesiuk et al. 1990); and (Ford et al. 1994); and the time periods since ( and ). Expected rates in comparisons were based on the earlier period. Confidence intervals were based on a binomial distribution (Small and DeMaster 1995). Population estimates are shown in Figure 1. The population probably peaked at around 97 individuals prior to the start of extensive collections for public display in The population dropped to about 70 toward the end of collections in The population increased steadily through the 1980 census. The population then declined by 11% over the next four years. The population then resumed its increase through early 1995, when it reached its pre-capture levels. In the subsequent 4 years, the population declined by about 15%. Mortality remained near 2.2% for most of the census period. However, there was an increase in 3 of the 4 seasons from , and over the 6 seasons from (Figure 2). Pooled mortality rates are shown in Table 1. Mortality rates from 1974 through 1993 did not differ from those reported by Olesiuk et al. (1990, p.4 for adult males and females, p.25 for juveniles) Adult male and female mortality was significantly higher in than in (p<.01 and p<.001, respectively). Juvenile mortality in was significantly higher than reported by Olesiuk et al. (1990), although it was not significantly different than for Southern Residents alone in (p<.05 and p.05, respectively). Annual reproduction has been variable. Insignificantly fewer calves than expected were born in Following a period of increased calving, calf production dropped again in 1988, and has averaged lower than expected since, although there have been a few years at 1or 2 calves above expected levels. Calf production since 1993 is not significantly lower than in previous years for Southern Residents (p.15), although it remains lower than reported for "uncropped pods" (p<.001 relative to expected rates from Olesiuk et al. 1990). 2

3 Table 1. Pooled Mortality Rates for Various Time Periods. Age-Sex Class Time Period Mortality Rate Adult Male Oleisuk Adult Female Olesiuk Juvenile Olesiuk

4 DISCUSSION The population parameters observed by Olesiuk et al. (1990) lead to an annual increase in population. Thus the population can sustain itself when there is some increase in mortality, or decline in fecundity, relative to the data. Over the last 40 years, this population has experienced 3 major declines, of 25-30%, 11%, and 15%, with intervening periods of increases. These declines will be addressed below. The initial decline corresponds to the period of collection for public display. This resulted in a change in the age structure of the population, as a large proportion of calves produced in the 1960 s were removed (of calves born from , approximately 35 were taken for public display, while 11 survived to be censused in 1974). For purposes of this paper, females were assumed to be equally fecund throughout their reproductive life, although this is unlikely to be the case (Olesiuk et al. 1990). In the early 1980 s, females born prior to the capture era were late in their reproductive lives, so their fecundity would have been lower than average. Likewise, females born subsequent to the capture era were just becoming reproductive, so had probably not yet reached their peak. This could account for most (but not all) of the short-fall in calving in Similarly, a disproportionate part of the population would have been old (i.e., in above average mortality age classes), and this was reflected by the deaths of a few post-reproductive females during the 1980 s period of decline. In addition, it should be pointed out that random fluctuation in calving could account for these differences in calf production. Some of these factors would also apply to the 1990 s decline. The number of reproductive age females has been declining. Females born at the end of the 1980 s decline are just becoming reproductive. The 1960 s females are near the end of their reproductive lives. Several postreproductive females have died. However, other factors also seem to be involved. Of special concern is the failure of females born in the 1960 s to reproduce successfully. None of the offspring produced by this group of five since 1986 have survived, and their age of last successful reproduction is shown in Table 2. In addition, the recent deaths of two reproductive age females were unexpected. (It should be pointed out the ages of these females are not absolutely certain. Mortality has averaged 5.3% per year since the 1993 census. However, per capita births of viable calves have been only 3.1%, leading to a decline in the population. The bulk of the decline in calving has occurred in L Pod, the largest of the three Southern Resident pods, while calving has remained near 5% in J and K pods. (It is interesting to note that about 5% mortality and 5% recruitment would fit well with a stationary population of 100 individuals at the start of collection for public display, rather than the increasing population shown in Figure 1). Four candidates are often mentioned as potential causes for the decline, and all may play some role. These are: disruption of the age structure by collections for public display; a reduction in carrying capacity due to reduced food supply; increased mortality and reduced fecundity due to exposure to toxic chemicals; and disturbance by extensive whale watching traffic (Baird 1999). In addition, there may be non-anthropogenic factors such as competition for food with increasing numbers of Northern Resident killer whales and other marine mammals, the population may have overshot its carrying capacity and is in the process of returning to normal levels, or it may be exhibiting random fluctuations. The relative contributions of these factors will be addressed in future work on Southern Residents and neighboring populations. 4

5 Table 2. Age of last successful reproduction of females born in the 1960 s ID Birth Year Year and Age at Last Successful Reproduction L J K no surviving calves produced L L REFERENCES Bain, D Examining the validity of inferences drawn from photo-identification data, with special reference to studies of the killer whale (Orcinus orca) in British Columbia. Rep. Int. Whal. Commn Special Issue 12: Baird, R. W Status of Killer Whales in Canada. Contract report to the Committee on the Status of Endangered Wildlife in Canada. Ottawa. Bigg, M. A., P. F. Olesiuk, G. M. Ellis, J.K.B. Ford and K. C. Balcomb III Social organization and genealogy of resident killer whales (Orcinus orca) in the coastal waters of British Columbia and Washington State. Rep. Int. Whal. Commn Special Issue 12: Hoelzel, A.R.., M. Dahlheim, S. J. Stern Low genetic variation among killer whales (Orcinus orca) in the eastern North Pacific and genetic differentiation between foraging specialists. Heredity. 89: Hoelzel, A.R.; Dover, G.A Molecular techniques for examining genetic variation and stock identity in cetacean species (SC/39/O 7). Rep. Int. Whal. Commn. Special Issue 11: Hoyt, E Orca: the whale called killer. Olesiuk, P. F., M. A. Bigg, and G. M. Ellis Life history and population dynamics of resident killer whales (Orcinus orca) in the coastal waters of British Columbia and Washington State. Rep. Int. Whal. Commn Special Issue 12: Small, R. J. and D. P. DeMaster Acclimation to captivity: a quantitative estimate based on survival of bottlenose dolphins and California sea lions. Mar. Mamm. Sci. 11:

6 Figure 1. Population trends in Southern Resident killer whales. Figure 2. Annual Mortality rates. 6

7 Figure 3. Annual reproduction of Southern Resident killer whales. The following is an additional table to supplement the original paper. Comparison of Number of Surviving Offspring Per Female of Northern and Southern Residents Matched for Age Birth Year of Southern Resident Number of Offspring Difference N Southern Residents Northern Residents (Ford et al. 1994) 1950 s s