Our research also indicates that Puget Sound's and many other populations of killer whales are resident to areas with abundant prey resources. Most interestingly, their social characteristics are reminiscent of those of humans, in that members of orca families act according to family and group culture as much as to instinct: some pods, which can include between 5 and 20 members, lead nomadic lives, while others remain in one locality; between pods, "speech" dialects and even diets usually vary. In many ways they are our marine counterparts.
The resident population of these whales had increased from 68 mostly adult individuals in 1976 to 99 in 1995. Since then the population has dropped to 89 in 1998, which indicates that the population has apparently reached the current carrying capacity of the ecosystem, or at least it has reached a point where deaths now repeatedly outnumber births. From biopsy studies we do know that the whales have high levels of PCB contamination which may be affecting both births and mortalities. The age structure is still slightly skewed as a result of captures, but with present trends, should recover to normal age distribution by the end of this century.
We now consider the orca to be the best and most charismatic indicator of a healthy marine ecosystem. Our study's orca population is predominantly piscivorous, preying upon salmon, rockfish, and bottomfish of the region. It is not surprising, therefore, to find in their tissues the contaminants that have accumulated in the food chain: PCBs, DDTs, heavy metals, and anthropogenic hydrocarbon residues. Thus far, the population appears viable, but we have noted a high newborn-mortality rate throughout the study and have found recent evidence of nutritional problems and disease in some individuals. We are providing our evidence to the National Marine Fisheries Service for urgent management consideration as well as to media and educators.
As a result of our accumulated fieldwork, we now know that orcas are very long-lived animals; females, on average, live to approximately 50 years with a maximum longevity of 80 to 90 years, while males usually live to about 30, with a maximum life span of 50 to 60 years. Parelleling human development, orcas do not become sexually mature until their teens, at which point females begin to give birth to a single calf every three to five years until about age 40.
Two forms of killer whale, residents and transients, occur sympatrically in coastal waters of British Columbia, Washington State and southeastern Alaska. The two forms do not mix, and differ in appearance, seasonal distribution, social structure, and behaviour. Many of these differences have been attributed to apparent differences in diet, although comprehensive comparative data have been unavailable. To better understand the feeding ecology of the two forms in this region, we collected field observations of predation and stomach contents of stranded animals from both resident and transient populations during 1975-95. Resident whales were observed only to take fish, 96% of which were salmonids. Six species of salmonids were identified, with chinook salmon (Oncorhynchus tshawytscha) being the preferred prey species. Stomach contents of stranded residents confirmed this preference for chinook salmon, but also revealed other non-salmonid fish species. On rare occasions, resident whales harassed marine mammals, but no kills were confirmed. Transient killer whales were observed to prey only on pinnipeds, cetaceans, and seabirds. Six mammal species were taken, with over half being harbour seals (Phoca vitulina). Seabirds do not appear to represent a significant prey resource. Resident and transient killer whale populations thus have clearly divergent prey preferences and foraging strategies. Highly specialized, socially and reproductively isolated populations of killer whales are likely typical of the species throughout its cosmopolitan range.
(J.K.B. Ford, G.M. Ellis, L.G. Barret-Lennard, A.B. Morton, R.S. Palm and K.C. Balcomb. Diet Specialization in Two Sympatric Populations of Killer Whales (Orcinus orca) in Coastal British Columbia and Adjacent Waters. (in prep.))
One of the most fascinating results of our study is that individuals never leave the group into which they are born. The bonds among females and their offspring are extremely strong and persist throughout the whale's life. All ages and both sexes of whales tend to spend the greatest proportions of their time travelling with their mothers. It is this long-term relationship between mother and young that is the most significant feature of resident societies, and it accounts for the kinds of social structures that we see in the population. Resident killer whales live in groups that are organized along lines of maternal relatedness. Each whale belongs to a matrilineal genealogy, that is, a family tree showing an individual's ancestry through his mother and her relatives.
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