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.
Longevity
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.
Sympatric Speciation
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.))
Non dispersal of offspring in the resident community
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. |