The Mark-Recapture Research and the Sighting Survey

(from "The Research on the Whale Stock in the Antarctic - The Result of the Preliminary Study in 1987/88", ICR, 1989)


Data on animal abundance can be collected either by examining the same individuals after a certain interval of time, or by not using the same individuals repeatedly for each research. A typical example of the former is the mark-recapture technique which can be quite effective if practised strictly according to the theory.

Let us suppose that an "m" number of whales were marked and released to the sea and an "n" number of whales were captured at a later time. If a "k" number of whales among those captured were found to have the marks, the total population can be obtained from the following formula: 

n x m/k
In the case where a long period of time has lapsed since the marking of the whale to the recapture, the mortality of the whale can also be estimated by analysis of the time lapsed to the recapture.

This procedure is theoretically sound. However, the drawback is that it is difficult to know for sure whether the whales were successfully marked. Because the technique also requires capturing of whales, it is subject to strong objections from anti-whaling groups. Maybe for these reasons, the materials obtained from the Southern Hemisphere minke whale research expeditions have been unused by the IWC Scientific Committee.

Thanks to the studies in recent years, it has become possible to overcome problems arising from uncertainties in the number of individuals successfully marked and to obtain more reasonably precise estimates of mortality rate and stock size.

In addition to the artificial marks, studies have been embarked on natural marks such as patterns and scars on the body of the whales. Natural marking studies are feasible on coastal whales. For the Southern Hemisphere minke whales, however, it is not only virtually impractical but also financially impossible to make observations on whales with an estimated stock size exceeding 600,000 in the poor presence of visible marks that distinguish one individual from another.

Census surveys that avoid the repeated use of the same individuals can be conducted with or without the captures of whales. A well-known example of the survey that does not require any capture is the sighting survey.

Gray whales that inhabit the Pacific coasts of the U.S. are known to migrate between California and the Arctic Ocean. When they pass the coasts of California, a majority of these whales move either southward or northward quite close to the coasts. Sighting survey on gray whales are conducted from lookouts located on hills by the coasts or at the capes. The estimated stock size of the gray whales according to those surveys is around 15,000 although some whales are missed during sighting. This stock of gray whales continues to be harvested today by the U.S.S.R. at a rate of around 170 animals per year in the Arctic Ocean.

Offshore type Southern Hemisphere minke whales cannot be observed by land-based surveys. Details on sighting survey of minke whales are given under the corresponding title.

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