(from "ISANA" No. 10, 1994)

Dr. Peter B. Best
Mammal Research Institute, University of Pretoria

No surveys of this nature were carried out prior to the onset of the exploitation of the baleen whale stocks of the Southern Ocean at the beginning of this century, so authentic figures for the original numbers of blue, fin, humpback, sei and minke whales do not exist. We do know how many whales were removed from these populations, however, and, given a mathematical model of how they might have responded to depletion, these catches can be used to provide population trajectories over time. The trajectories in turn can be tested either by seeing how accurately they hit an estimate of current population size, or how closely they fit some measure of the rate of depletion such as an index of relative abundance. Such "assessments" were carried out some years ago for the four main baleen whale species in the Antarctic that were depleted by whaling (blue, fin, sei and humpback), and the results suggested original populations ranging from about 100,000 (humpback) to 400,000 (fin whales).

The estimates obtained on the IWC cruises are of a different nature, being based on the numbers of whales seen on specially designed cruises independent of whaling operations, and, most importantly, only refer to the area of ocean roughly south of 60 degrees South, whereas whaling operations for the four species ranged over time over virtually the whole Southern Ocean. To make the two sets of estimates more comparable, therefore, it is necessary to adjust the IWC figures so that they refer to the whole Southern Ocean. This can be done using another set of information, the sightings obtained over a number of earlier years from scouting boats attached to Japanese whaling expeditions.

If the IWC estimates so adjusted are compared with the "assessments" of original population size (see Fig .), it is clear that the populations of these four species are still substantially depleted, so justifying their continued protection. Of all four, the status of the blue whale gives by far the greatest concern: the best estimate of current population size of 700 animals is only 0.35% of the original population estimated at some 200,000 whales. Since Antarctic blue whales were given protection by the IWC more than 25 years ago, does this mean that the population is not recovering, and if so, what could be the reason for the apparent failure to increase?

Firstly, estimates of the size of the population of Antarctic blues around the time of IWC protection vary widely, but one, produced after extensive analysis by the Committee of Three specially appointed by the IWC, was that the stock in 1963 probably numbered between 600 and 1,950 whales. The current estimate of 700 whales (with wide confidence limits that do not exclude the possibility of the population being twice as big), is essentially of the same order as that produced in 1963. On this evidence one might conclude that no increase has taken place.

There are two important qualifications to this conclusion. The first is that the methods used to assess the size of the blue whale stock at protection relied heavily on catch per unit of effort as an index of abundance. The basic measure of effort used in these calculations (catcher-day) was crude, and most recent interpretations of series of such data are that they probably underestimate the rate of decline of a stock. Hence there may have been even fewer blue whales at protection than the Committee of Three thought. Secondly, the catch series used in their assessments was probably incorrect; it has recently been revealed that Soviet catches of blue whales for one of their factory ships operating in the Antarctic in the 1960s were about 10 times those reported to the IWC, and (more significantly) that such catches continued for many years after protection was supposedly afforded by the IWC. If this pattern was common to all four of the Soviet factory ships operating at that time, then one might not have to look very far for the reason for the apparent failure of blue whale stocks to recover.

For the same reasons it is impossible to determine whether or not Antarctic fin or sei whales have been recovering (or have been prevented from recovering) since protection: thorough reassessments of the historical data, including the use of a correct catch series, are needed before this question can be answered. For humpback whales, however, we are fortunate enough to have independent information from outside the Antarctic.

Off the west and east coasts of Australia, humpback whales on migration to their winter breeding grounds have been counted in shore-based or aerial surveys for the last 12-15 years. In three independent series of such counts, two on the east and one on the west coast, the populations have been shown to be increasing. The word populations is used because it is known that the whales overwintering on west and east coasts of Australia spend the summer in different longitudinal sectors of the Antarctic (Areas IV and V respectively). The rates of increase are surprisingly high, ranging from 9 to 14%, but are similar to those being seen in North Atlantic humpback whales, and are probably close to the maximum biologically possible for the species.

The humpback whale is not the only southern baleen whale which has been shown to be recovering: surveys of right whales in Argentina, South Africa and Western Australia have all demonstrated significant rates of increase that must also be close to the maximum biologically attainable. Until recently, southern right whales were considered to be the most depleted and endangered of all the southern baleen whales, having undergone 150 years of exploitation before finally being given protection in 1935, when their numbers were so low that they were rarely seen by whalers. The apparent ability of this species to stage a recovery from such a low level would seem to be a good omen for what must now be considered the most threatened of all the southern whales, the Antarctic blue whale.

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