_Life Expectancy_
Life expectancy is another very useful statistic for population studies.
The whaling industry has no easy way of finding this out because whaling
never gives an individual whale the chance to attain old age. Perhaps we
can eventually determine life expectancy for the gray whale if it can
continue to be protected from whaling. By counting the number of babies
produced each year, and knowing the life expectancy, We might indirectly
obtain the number of whales which die before they have realized their
life expectancy. This number should represent the surplus on which
whaling could be based. A further study would be necessary to determine
what percentage of the surplus could be diverted to the whaling
industry.
_Age Determination_
The problem of aging whales is an important one, and one to which much
thought has been given. Actually, it is desirable to know at what age
sexual maturity occurs; at what age physical maturity; at what age
reproduction is no longer possible, and lastly, the age at death. In
addition, we need to know the gestation period and the average number of
pregnancies of which the female is capable. Many of these statistics can
be obtained from the examination of a freshly killed whale. Such age
determinations are made on every specimen which comes aboard the factory
ships and also at the shore stations. There is some doubt as to the
accuracy of some of these. Physical maturity is delayed in whales for
many years, and it is judged complete when the cartilaginous end plates
fuse to each vertebrum (backbone). After fusion, further lineal growth
is impossible and after this time whales have been noted to shrink
slightly each year.
The other structures which a biologist collects for aging are the
ovaries of the female whale. These are sliced across into half-inch
slabs, and the number of exposed grayish masses are counted. There is
one of these structures produced for each pregnancy, and they persist as
scar tissue throughout the life of the whale. These structures are
functional during pregnancy and nursing and are responsible for a
successful pregnancy. A count of these gives the investigator the total
number of offspring which have been produced. If one assumes that the
whale has not missed an opportunity to produce a baby, the count of
these gives the age, assuming one knows the age of sexual maturity and
the maximum length of bearing.
In recent years other structures have been tried for aging, so that
males might also be aged, as well as females past the reproductive
period. The best structure is the cylinder of wax which fills the outer
canal of the ear leading to the eardrum. This enlarges as the whale
grows, and the growth is marked by banding very similar in appearance to
tree rings. Slicing the plug, the rings can be exposed and counted.
There are produced but two rings a year. Toothed whales can be aged by
sawing the teeth in half and noting the banding. Baleen also is
periodically enlarged, but this is more difficult to check.
_Distribution and Population Rise_
The whaling industry spends a few weeks before the whaling season and at
the end, hunting whales with a much lighter gun which implants a
numbered cylinder deep in the skin of the whale. Each whale so marked is
logged as to the species and the latitude and the longitude, and lastly,
the date. Should the marked whale be processed in any succeeding season,
a second entry is completed giving the date and place of capture. These
records are used to determine the migration routes and the degree of
intermingling of adjacent populations. The ratio of marked whales to
unmarked whales allows the estimation of the probable size of the whale
population. The length of time elapsing until a marked whale is
recovered is an indication of the intensity of whaling.
Unlike the situation in the fishing industry where the reproductive
potential of the fish is more than adequate to repopulate quickly, in
the whaling industry the reproductive potential is small. Only
one-fourth of the population can contribute replacements each year, and
then only a single baby. Of course, each offspring has a reasonable
chance of reaching maturity, whereas with fish only a very small number
achieve adulthood.
One might expect that whales would be less affected by the fluctuations
in environmental conditions, but this is only a surmise. We do not know
as yet how to ascertain whether the food of whales is ever inadequate.
Certainly the fact that baleen whales do best around the polar seas
suggests that climatic fluctuations of the magnitude and duration of
those producing the ice ages, may have profound effects on them.
We can only hope that man will learn to exploit whales in such a way as
to take only the surplus and leave the main stock, creating thereby a
perpetual resource. Man can best profit esthetically and scientifically
by leaving the gray whale free of whaling as a control population.
[Illustration: Gray whale making a deep dive. _Photograph by T. J.
Walker._]
[Illustration: Gray whale spouting. _Courtesy Scripps Institute of
Oceanography._]