(from "ISANA" No. 10, 1994)
Dr. Seiji Ohsumi
Executive Managing Director, The Institute of Cetacean Research
Thirty years earlier, the IWC's Scientific Committee banned the hunting of the Southern Hemisphere blue whale under complete protection in compliance with the recommendation by the Committee of Three. According to a population assessment at that time, there were an estimated 150,000 ordinary-type blue whales before the exploitation started (Chapman, 1974) and the number of the whales in 1963 was estimated at 650 - 1,950 animals. The Committee of Three therefore estimated that ordinary blue whale resources would be on the way to extinction and need 65 years to recover to a healthy level on the assumption that the population would increase by 10% annually (IWC, 1964). It has been commonly accepted that the stock size in the early 1970s numbered 5,000 to 10,000 animals (Chapman, 1974).
Those results indicate that any of three phenomena may have taken place: (1) although the population has increased at a normal pace, the increase could not be detected; (2) the population has failed to recover steadily; or (3) the population has kept decreasing. The method of analyzing the resources in the 1960s was different from the present methods. Needless to say, there are errors in estimation. If the population was overestimated in the past and is underestimated at present, the first and second cases above appear practical. Otherwise, the third case must be taking place. The question of which one of the three cases is correct is difficult to answer in the absence of long-term monitoring data on the population as a whole. But if the population size estimated by the Committee of Three at that time and the population size recently calculated by the IWC are both correct, the correct answer to the question is that the blue whale has not recovered as expected despite the whaling ban. If the rate of recovery in the baleen whale population from their low level is put at a reasonable 7% on the average, the population is calculated to increase 7.6-fold 30 years later. At 4%, the increase should be 3.2 times.
Increase or decrease in biological resources are brought about by differences between the number of newly recruited whales, natural mortality and catches. Accordingly, the reason why Antarctic blue whale resources have failed to recover is one of or a combination of three factors: (1) catch continued even after the whaling ban was imposed; (2) the number of newly recruited whales has been extremely small; or (3) natural mortality of whales was high. A recent report revealed that a whaling fleet of the former Soviet Union had caught more whales than statistics showed in the 1960s (Yablokov, 1994). But their main target after the ban on taking of Southern Hemisphere blue whale was sei whales, the whaling grounds for which were far north of the ordinary blue's feeding grounds. Blue whales caught by the Soviet fleet, if any, are presumed to have been pygmy blue whales (B.m. brevicauda). It is fair to say that the number of ordinary blue whales caught in 1960s was small and that no blue whales have been caught since the 1970s because the international observer scheme has been in force since 1972 when the main target of catch shifted to the minke whale. It is practical to conclude that the ordinary blue whale's failure to recover is mainly attributable to the extremely slow pace of recovery caused by either a low rate of recruitment to the population or a high rate of natural mortality.
It is assumed that the cause of the drop in the rate of recruitment or an increase in natural mortality has been deteriorating environment surrounding the ordinary blue whale. The environment can be classified into biological and non-biological categories. Marine pollution can be considered to be the factor of deteriorating non-biological environment. But if it is the cause, similar trend must be found for the minke whale that share the common habitat. The view attributing the failure of recovery to deteriorated non-biological environment could be refuted as there is a number of evidence that the minke whale showed an increase while the ordinary blue decreased. Accordingly, there is no other choice but to conclude that deteriorated biological environment has been the main cause of the slow recovery.
Biological environment consists of three components: predators, prey, and other organism competing for the same food and habitat. Major food for ordinary-type blue whales in the Antarctic is Antarctic krill. Exploitation of krill, which started in the Antarctic in the 1960s, stayed only hundreds or thousands of tons in the peak year and is by no means considered to have decreased the abundant feed resources. Rather, a surplus of krill should be considered to have resulted from hunting of major baleen whales. And the surplus does not square with what has actually happened, because it contributes to an increase in recruitment rate. It is assumed that retarded recovery of the ordinary blue whales has been caused by killer whales, one of few natural enemies of the blue whale. The chances of being attacked by killer whales probably increase as the number of blue whales decreases. The possibility cannot be ruled out that such an attack is one of the factors causing an increase in the rate of natural mortality. Nevertheless, the biggest cause may be competition with minke whales for a niche.
While the ordinary blue whales dropped to 0.5% of their initial stock size, the minke whale increased 9.5 times. When whaling started in the Antarctic Ocean, the stock size of minke whales was only 4% of ordinary blue whales - 590,000 tons for minke whales vis-a-vis 15 million tons for ordinary blue whales. At present, the ratio is estimated to have reversed to 59 times in favor of the former - 5.6 million tons for minke whales against 70,000 tons for blue whales. The wide difference of population size between the minke and blue means that their ecological status have been drastically inverted. The situation can be illustrated by saying that a large number of minke whales have occupied houses left vacant by ordinary blue whales. Minkes share feeding and breeding grounds with the blues. Besides, they compete most strongly with each other as they feed on the same food species. The competition possibly generates stress as a result of friction in their habitat including feeding and breeding places. Ordinary blue whales have not recovered probably because they, as a minority vis-a-vis minkes, have built up stress, become undernourished and reduced the rate of their net recruitment rate through declining recruitment rate and increasing natural mortality. Anti-whaling groups are fiercely denying this view because it means that recovery of ordinary blue whales cannot be guaranteed unless minke whales are culled to some extent.
At its annual meeting in 1993, the IWC accepted the Japanese government's proposal to start international joint research to find causes hindering the recovery of the blue whale population and to prop up the pace of the recovery. We sincerely hope that the research will promptly get under way and find the causes so that proper measures can be taken to help the resources recovery. We are looking forward to the day when this valuable resource of the Antarctic ordinary blue whale recover to an optimum level and their rational use resumes.