Age Composition
It is vital to the research program that samples are representative of the
total minke population. Great efforts are thus made to ensure that samples are
taken at random. The clearest indication of the effectiveness of the sampling
method is the age composition of catches compared with commercial catches.
Shown in Fig.4 are the age composition samples taken in 1989/90 and the
composition of more than a decade of Japanese commercial catches. It can
clearly be seen that younger whales are well represented in the random sample,
while the overwhelming majority were mature.
This contrast can also be seen between the lengths of samples taken
randomly and commercial catches. Dwarf forms excluded, samples taken in
1989/90 ranged from 4.8-10.1 m, with a mean of 8.1 m. In 1986/87, the last
season of commercial whaling in the Antarctic, lengths ranged from 7.2-10.3 m,
with a mean of 8.8 m.
The reason for this contrast is simple. Commercial operations naturally
focused their attentions on areas of high density, that is, in the prime
feeding grounds near the pack ice. Many young minke whales, however, remain in
lower latitudes. Furthermore, as whalers preferred larger whales, even when
young whales came within range they were seldom taken.
Some critics of Japan's research program point out that data remain
unanalysed from the days of commercial whaling, and hence it is unnecessary to
amass new data. It is true that certain nations have been reluctant to release
some old data. However, it can be seen from the age composition of commercial
catches that such data are poor representations of the total population and
would thus be of limited value anyway.
Age Determination
In the following we will take a closer look at the means by which a whale's
age can be determined. While biologists in the field perform more than 40
tasks related to sampling and measurement for each whale taken, from the
viewpoint of the program's principal objective - the age-specific natural
mortality coefficient - age determination is of paramount importance.
The age of toothed whales can be easily determined by studying the teeth.
For baleen whales such as the minke, however, the task is not so simple. In
early biological studies, age was estimated very approximately from body
length, the degree of healing of external scars, the color of crystalline
lenses, and the thickness of baleen plate.
In the 1930s a method was adopted which counted the corpora albicantia and
lutea in the ovaries. Provided that the age at sexual maturity and the
gestation cycle were known, this could provide a relatively accurate
determination of age, but of course only for females.
In the late 1950s, a method was developed which counts the number of growth
layers in a plug within the ear canal. When cut lengthwise, layers can be seen
which are formed of two laminae, one of shed skin cells which is light in
color, and one of ear wax which is darker. (Photo 1). Initially, it was believed two layers (or four laminae)
were deposited per year. However, examination of the plugs in whales which had
been marked and recaptured after many years indicated that layers were
probably deposited at one per year. Subsequent research has supported this
hypothesis by demonstrating that the light lamina, which contains a lot of
fat, forms in the feeding season, while the dark lamina, with less fat, forms
in the breeding season.
Counting the growth rings in the ear plug is now the principal method of
age determination for baleen whales. However, distinguishing laminae is
sometimes difficult or impossible, particularly in very young whales. The
Japanese research therefore uses two other methods to corroborate findings:
analysis of tympanic bullae and baleen plates. The tympanic bulla is a bone in
the ear which, like the ear plug, has growth layers which are believed to form
annually. They are seen as ridges and furrows running parallel to the outer
surface of the bulla.
On the back of baleen plates of minke whales aged up to 1.5 years are marks
known as neo-natal marks (Photo 2). These marks are not found in older whales as baleen worn
away by friction is gradually replaced by new baleen formed within the gums.
The presence of these marks is a clear indication of age, and thus for these
whales it is not necessary to observe the ear plug.
___________________________
From Japanese Research on Antarctic
Whale
Resources, published by the Institute of
Cetacean Research, 1991.
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