TROUBLED WATERS - Whale and Dolphin Conservation Society

chance of fog decreases from 15 to 5 per cent. In the following months to November there is no clear
trend. In December, the probability of precipitation is 24 per cent. These results indicate that March
has the highest chance of precipitation of all the whaling months. The probability of fog is highly
variable in the winter, but from September onwards, the probability of fog increases from 3 per cent to
around 15 per cent in December and January. Thus, in the period when the probability of precipitation
is low, the probability of fog is relatively high. So if the issue of reduced visibility is considered, fogrelated
problems add to those caused by precipitation in these months. How the whalers respond to fog
or precipitation is unclear, and it would be of great value if more information was made available by the
whalers to help understand how these weather states affect the efficiency of whale killing.
Wind speed and wave height
The average wind speed, regardless of its direction, increases from 7.4 ms-1 (a moderate breeze) in
January to 9.3 ms-1 (fresh breeze) in March. However, the wind speed is highly variable from April
until October, with the highest variation in the months with the highest averages. These are September
and October (10.5 ms-1; fresh breeze). Thereafter the wind speed reduces to 7.7 ms-1 in December.
Given the variation, probabilities of wind speeds higher than 11.2 ms-1 (strong breeze and higher) in
March or November, or 14.3 ms-1 (moderate gale and higher) in October are significant (10, 13 and
20 per cent respectively).
The average height of the waves (calculated from the upper third of all wave heights, known as
‘significant wave height’, see also www.oceanweather.com ) and the time between reoccurrences of the
most violent and energetic waves (known as ‘Tpeak’) is estimated at one location: 67.5° S and 180° W.
This has been done using the GROW model of Oceanweather Inc. and is based on the years 1970 until
2001. The average significant wave height tends to increase from 2.2 to 3.4 metres between January
and March and tends to remain relatively high until June with an average wave height of 3.2 metres.
The height is not known for the following months, but averages 1.9 metres in December. The missing
values are for months in and after the winter, when the ice covers the sea. The model excludes data in
these cases. The Tpeak seems to follow the same trend as the wave height, starting with around 10.5
seconds in December and January and increasing thereafter to about 11.5 seconds in the period April
to June.
The wind increases from January until March, but becomes more variable thereafter. This together with
the growing percentage of ice in the area, blocking wave formation after March, may explain why the
highest waves are found in March before the significant formation of sea ice. It can, therefore, be
postulated that March may be one of the most severe months (in terms of adverse weather conditions)
in which to perform whaling operations in the research area.
Ship motions in March and December at an Antarctic whaling ground
Ship motions have been calculated using the SHIPMO computer programme of the Maritime Research
Institute in the Netherlands (Anon 2002). The ship used for these calculations was similar to the
Japanese whale catching vessel Toshi Maru No.25. A sailing speed of 6 knots, head seas coming in at 30
degrees and local sea depth of 1,000 metres were used in the model. The model was run for estimated
sea conditions during March and December, as described above. The motions considered were, the
sway (from left to right), heave (up and down) and surge (forward and backward), as would be
experienced at the level of the harpoon on top of the bow. Table 1 provides the results for these
different motions in December and March.
WEATHER, SEA CONDITION AND SHIP MOTIONS AFFECTING ACCURACY IN WHALING
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