By The Institute of Cetacean Research, 1991

Objectives of the Research

In 1982, the International Whaling Commission decided to impose a moratorium on all commercial whaling. Many species of whale, heavily exploited, had already been given full protection prior to that date, in line with the advice of the IWC's Scientific Committee. However, the Scientific Committee had made no recommendation to extend that moratorium to the plentiful Southern Hemisphere minke whale. The blanket moratorium was thus imposed without scientific justification.

For pelagic whaling, the moratorium entered effect with the end of the 1986/87 season. Since then, the Institute of Cetacean Research has been engaged in a program of scientific research to resolve the "uncertainty" which the IWC cited as its justification for imposing the moratorium.

The principal objectives of the research program are as follows:

Estimation of the biological parameters required for stock management of Southern Hemisphere minke whales

The primary objective of the research program is to estimate the age-specific natural mortality rate of the Southern minke whale through stochastic sampling carried out in combination with systematic sighting surveys. The program is also designed to ascertain stock size and the means of forecasting changes in that stock by studying other parameters including the recruitment rate and changes in reproductivity.

Elucidation of the roles of whales in the Antarctic marine ecosystem

A broader objective of the program is to elucidate the roles of whales in the Antarctic marine ecosystem. This involves primarily the study of the food web, and in particular the prey-predator relationship between whales and their food sources, krill, fish and squid.

The Need for Scientific Research

For the proper stock management of any undomesticated animal, it is essential to have knowledge of three parameters relating to population dynamics: the current size of the stock, the natural mortality rate, and reproductivity. It is also preferable to know how the dynamics change in response to external influences such as changes in the food web or competition from other species. With this knowledge it becomes possible to predict changes in population, and hence identify a level of harvesting which can be sustained while the population remains stable or even increases.

The first requirement for assessing the state of any stock is knowledge of its present size. Fortunately, our knowledge of the population size of the Southern minke is clearer than for any other whale.

In 1978, a program of annual sighting surveys was commenced under the International Decade of Cetacean Research (IDCR) program, sponsored by the IWC and financed largely by Japan. The findings of these surveys have enabled statisticians and biologists to arrive at ever better estimates of the population size of the minke whale in high latitudes, where it congregates during the summer feeding season. The latest IWC estimate for minkes south of 60° S in the austral summer is 760,396. Not included are whales which remain in lower latitudes and those which enter the pack ice, so the total population is presumably somewhat larger.

While sighting surveys conducted over many years can tell us whether a population is increasing or decreasing, they do not tell us why changes in size occur or, by themselves, enable prediction of future changes. Expertise in these areas is essential for successful stock management, but requires knowledge of two other parameters, the natural mortality rate and reproductivity, which can only be acquired through biological sampling.

Why is knowledge of these parameters so important? In the case of human populations, planning for the future requires knowledge of the present age structure, and how it will change with time. If a "baby boom", for example, appears at the base of the population pyramid, these year classes will require extra facilities such as schools, and in 20 year's time will themselves produce children resulting in another baby boom. Knowledge of the age structure thus allows us to predict demographical changes, and to plan ahead to meet demand for facilities. Demographical changes in whale populations can be predicted in the same way.

When an animal population's supply of nutrition improves, the animals typically respond by producing more offspring, resulting in a population increase. There are indications that this effect is being seen in the minke whale population. As larger species of baleen whale were depleted in the Antarctic during the course of this century, there was an explosion in the krill population, their staple diet. The minke, which went almost untouched by commercial whalers until the 1970s, was able to exploit this. As a consequence, the age at maturity fell and the reproductive rate is believed to have risen to a new high.

To identify such changes and enable prediction of future population size, it is thus vital to accumulate knowledge of age structure, age at sexual maturity, pregnancy rate, etc.

By the time commercial harvesting of the minke whale began, it had already become clear that serious mistakes had been made in the management of larger species. In 1975, the IWC introduced the so-called New Management Procedure to rationalize the fundamentals of whale stock management. Using the biological information and data already accumulated on many of the whale species, analyses were begun using the latest in computer modelling techniques to ensure that management of such stocks as the minke was conducted in a more rational manner. The results, however, were flawed. Though the data themselves were of great value, they had one major drawback: they came from commercial operations. As these operations had taken almost exclusively large animals in areas of high density, the data were not representative of the population as a whole.

What is now required, therefore, is uncompromised science based on biological samples taken at random. By this research, we will arrive at a better understanding of whale population dynamics, which in turn will enable us to manage stocks more productively and with a greater degree of certainty.

The results of Japan's research program may also have positive repercussions for other species of whale. It has been hypothesized that the reason why certain depleted species of baleen whale have shown little or no apparent recovery may be due to the explosion in the minke population. If this is the case, aiming for the fastest possible recovery of depleted stocks and managing minke whale stocks would not be independent and separate concerns, but directly related ones. We have no clear answer to this problem at the moment, because our knowledge of interactions among whale stocks is insufficient. We can greatly improve our understanding of the dynamics of the various stocks by studying the dynamics of the ecosystem as a whole, including other whale species and prey species. Studying the population dynamics of the minke whale will thus provide valuable information on the role of whales in the Antarctic ecosystem, and this knowledge, combined with future studies of the food web, will lay the foundation for multi-species management, a significant advance on the single-species approach adopted hitherto.

Research Areas

To facilitate management of whale stocks, the IWC had divided the Antarctic Ocean into Areas I - VI. From 1978/79 to 1985/86, the Japanese commercial catch of minke whales was concentrated heavily in Area IV (70 - 130° E) and Area V (130° E - 170° W). As biological data were collected for all whales caught during this period, the best information on stocks available prior to the moratorium thus came from these two areas. Given that navigators were also most familiar with sea and ice conditions there, it was decided that the Japanese research program could be conducted most efficiently if sampling and sighting methods were first tested in these areas.

In the 1987/88 season, a two-year feasibility study was launched. For the first cruise (Jan. 17 to Mar. 26, 1988), a part of Area IV was chosen where the coastal topography and ice formations are relatively simple. For the second cruise (Jan. 12 to Mar. 31, 1989), a more extensive part of Area V was covered in which topography is complex and weather conditions are comparatively inclement. The time frames chosen for these cruises were considered to coincide with the period following completion of the minke whale's migration, when the majority of the population would be engaged in feeding.

Having tested the methodology and found it to be satisfactory, the Institute of Cetacean Research launched the program proper in the following season. In the first stage of the program, it has been decided to alternate research between Area IV and Area V, covering these areas in their entirety. The research periods are also divided into two halves, so that each area is surveyed twice. In this way, changes in distribution of whales over time can be observed.

In 1989/90, Area IV was surveyed from Dec. 6 to Mar. 12. At the time of writing, the 1990/91 cruise is currently under way in Area V and a part of Area VI ⁽¹⁾.

In addition to sighting and sampling south of 60° S, the research program includes the less intensive study of lower latitudes. South of 55° S, sighting and sampling are conducted, while north of 55° S research is confined to sighting (see Fig. 1 ). The purpose of these surveys is to observe the distribution and abundance of minke whales which do not migrate all the way to the Antarctic in summer.

* (1): A part of the Ross Sea technically belongs to Area VI. However, for practical purposes both Japan and the IDCR include it in Area V.

Method for Sighting and Sampling.

After breeding each winter in warm waters, Southern Hemisphere minke whales migrate to the Antarctic in summer to feed. Their staple diet, krill, is most abundant near the pack-ice edge, and it is here that the whales congregate. However, the entire population does not migrate en bloc. The research program is thus designed to determine which classes of whale (based on age, sexual maturity, etc.) complete the migration, which whales remain in lower latitudes, and at what times the various classes arrive at their feeding grounds.

To ensure that sighting and sampling data are representative of the total population in the area under study, the research area is divided into three: "northern", "off-shore", and "ice-edge". To observe temporal changes in distribution, the latter two divisions are further divided into east and west, each of these divisions being covered twice during the research period, once in the first half and once in the second.

These divisions are covered following carefully devised tracklines. In the "northern" division, a comparatively simple trackline is used, reflecting the low density of whales, the absence of land masses, and only occasional pack ice. In higher latitudes, however, where whale density is high but irregular, tracklines are calculated with great care to ensure that there is an equal probability of coverage at any point. The complexity of calculating these tracklines is compounded by irregularities in coastal topography and unpredictable seasonal fluctuations in the shape of the pack-ice edge, and each year modifications are made in the quest for perfection. Fundamentally, however, the method does not change, being based on the IDCR method in which tracklines reflect from boundary edges at predetermined angles.

Tracklines are followed by three sighting/sampling vessels, one following the main trackline while the others cruise in parallel at a perpendicular distance of 9 nautical miles ⁽²⁾.

Sightings are classified as either primary or secondary. The former refers to whales sighted while a vessel is on the trackline. To take a sample, the vessel must usually leave the trackline, so other schools sighted during the chase are thus classified as secondary sightings. Only primary sightings are used for estimation of the population as the sighting effort applied to primary sightings is comparatively easy to calculate.

Great pains are taken to minimize variations in sighting effort, as such variations could introduce biasses into population estimates. Factors influencing the effort required in sighting include weather and sea conditions, school size, and the skill of the spotters. To eliminate biasses as far as possible, a number of controls are used. The distance covered per day is fixed, and should a vessel be forced to leave the trackline to avoid bulging pack ice, sighting temporarily halts. Similarly, if visibility falls below 1.5 nautical miles, or the wind exceeds Force 6, research stops. Three skilled spotters are constantly on lookout in each sighting vessel, each covering a different sector, and their performances are monitored. However, the factors of weather and school size cannot be discounted, and statistical techniques are thus adopted when estimating the population to correct any biasses.

Samples are taken only from primary sightings. In the case of a solitary whale or a pair, all animals are sampled if possible. From larger schools, two animals are sampled. Which whale to sample is determined by using a random-number table. Once the sample has been selected, there can be no change in the selection, even though other whales in the school might be more easily taken. The bias seen in commercial catches towards larger animals is thus removed.

* (2): During the 1987/88 and 1988/89 surveys, only two sighting/sampling vessels were used 12 NM apart.

Research Items

During the course of each cruise, large amounts of data and biological samples are amassed. The principal biological samples and measurements, and studies based thereon, are shown in Table 1. The range of studies in which these materials are used is too broad to be covered in detail here, but the materials themselves can be categorized as follows:

1) Data from sighting surveys. Although the program is designed with emphasis on the minke whale, sightings of other species are also recorded.

2) Biological samples from minke whales only. These samples provide important information on biological parameters, migration patterns, stock delineation, the environment, etc.

3) Data on meteorological, oceanographic and environmental conditions. Data are collected on meteorological conditions, distribution of pack ice, and sea water temperature, and samples are taken to assess possible changes in the environment such as fluctuations in the levels of pollutants such as organochlorines and heavy metals present in sea water and the atmosphere.

4) Biopsy/photo-identification. Two means are being studied on a preliminary basis by which it might become possible to identify individual free-swimming minke whales. Biopsy darts are being tested as a means of sampling skin and muscle tissues for analysis of the DNA sequence. Photographs are taken of sampled whales to determine the feasibility of identifying free-swimming minke whales by photography.

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 colour 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 a rate of one a 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 plus, 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, with one ridge and one furrow constituting a layer.

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.

Population Estimates

As it is logistically impossible to cover the entire research area in the time available for each cruise, the total population must be estimated by extrapolation. The procedure used by the Institute of Cetacean Research is basically the same as that used by the IWC/IDCR, with certain modifications. Only data on schools classified as primary sightings are used.

The effective search width is derived from the perpendicular distances of sightings from the trackline. This method, developed by our institute, is intended to compensate for the fact that efficiency of sighting can be affected by the size of schools in the area. The search width is then multiplied by the length of the trackline to obtain the effective search area. The total population of the whole research area can then be extrapolated using the effective search area, the number of schools sighted, and the mean school size.

As the research program requires simultaneous sighting and sampling, some concern has been voiced that the act of sampling might influence the behavior of other whales in the vicinity and hence introduce bias into the sighting data. Consequently, this bias might also appear in the estimation of population size. Tests are now being conducted to observe the behavior of other schools while sampling is in progress, but preliminary findings indicate no apparent change in the perpendicular distribution of primary sightings.

The techniques employed for sighting whales and extrapolating the total population are constantly being improved. In this regard, it is interesting to note that the population estimate for the Southern Hemisphere minke whale, derived by the IWC/IDCR from sighting surveys conducted continuously since 1978, has increased remarkably over the years. Scientists credit this not to a sudden increase in the population but to improved techniques for estimating that population. Fig. 3 shows the latest IDCR estimates of population by area. Combined, these give a total abundance of minke whales in the Antarctic in the austral summer of 760,396 ⁽³⁾.

* (3): This is not an estimate of the total population. Although the majority of minke whales are believed to be in these areas at this time, substantial numbers are thought to remain north of 60° S, while smaller numbers feed within the pack ice. These areas are not covered by IDCR surveys.

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 is the age composition of 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 in commercial catches the overwhelming majority were mature.

This contrast can also be seen between the lengths of samples taken randomly and commercial catches. Dwarf forms ⁽⁴⁾ 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 unanalyzed 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.

* (4): Little is known about dwarf minkes as they were ignored by commercial whalers. During the first three seasons of the research program, only nine were taken. Our Institute's biologists believe it to be a separate subspecies and thus exclude it from analyses of the ordinal minke population.

Characteristics of Migration and Segregation

Like other rorquals, the Southern minke shows marked seasonal changes in distribution, breeding and calving in winter in low latitudes and migrating south in the summer for feeding. The southward migration is directly related to the seasonal abundance in polar waters of the minke's staple diet, krill.

The migration of the minke population is not, however, a simple north-south movements. Evidence suggests, for example, that different classes of the population arrive at the feeding grounds at slightly different times. It is also believed that some classes do not migrate as far south as others. Furthermore, there is lateral movement within the feeding grounds as the summer draws on.

Understanding such parameters of migration and segregation is important for a number of reasons. It can help us to understand population dynamics and to delineate the various stocks which are believed to merge in the feeding grounds. There are consequently important implications for stock management.

The findings of the 1989/90 survey of Area IV suggested some definite trends. The first half of the survey began in the western part, where the pack-ice edge was found to be bulging prominently up to about 62° S. Samples taken here in mid- and late December showed a marked imbalance in the sex ratio, 75.2% being males ( Fig. 5 ).

Research then moved to the eastern part, where the pack-ice edge was found to be similarly unstable. Here the ratio of the sexes was found to be more or less balanced, with 44.2% of samples being males.

Upon returning to the western part in the second half of the research period, the pack ice had retreated to about 65° S. It was found that a balance of the sexes had been achieved here also, with 45.4% being males.

No samples were taken in the second half in the eastern part as the predetermined number of samples for the entire cruise of 330 had already been collected. Controls have been introduced for the 1990/91 season to ensure that this situation does not recur. Nonetheless, comparison of available data from the two halves clearly suggests that males precede females in the southward migration.

Other observations of interest concern differences in school size by time and area. In the western part in the first half, 82.4% of all schools observed were small (1 - 2 animals). In the second half, however, this figure was 70.2%. In other words, large schools (> 3 animals) had become relatively more frequent. This observation warrants further investigation to ascertain whether larger schools arrive later in the feeding season, or whether small schools merge over time to form large schools.

Also observed was a marked difference in the sizes of schools between the western and eastern halves. As we have seen, small schools predominated in the western half throughout the research period. In the eastern part, however, small schools accounted for no more than 37.3% of all schools.

Segregation of Minke Whales - Part 2

In the foregoing section, it was explained that variations occur in the whale population both in terms of sex ratio and of school size. Data on samples taken from small schools during the feasibility studies in 1987/88 and 1988/89 have been used for an analysis of geographical variations in the sex ratio, and also the ratio of mature to immature females.

For the purpose of this analysis, the part of Area IV surveyed in 1987/88 was divided into "offshore" and "ice-edge", while the part of Area V surveyed in the following season was divided into "offshore", "ice-edge" and the Ross Sea. In both survey periods, the pack-ice lines were fairly stable. It should be noted that small schools, on which this analysis is based, were predominant.

The results are shown in Fig. 6 . Immediately apparent is the predominance of mature females in the Ross Sea. At the ice-edge, in both Areas IV and V, the sexes are more or less balanced, and about half of the females are mature. Offshore, a slight difference appears in the sex ratio between the two areas. More pronounced is the difference in the ratio of mature females to immature females: in Area IV the overwhelming majority are immature, while in Area V they are almost balanced.

These findings suggest that both the sex ratio and the maturity of females change with latitudes, and that mature females tend to migrate to high latitudes (The 1989/90 survey of Area IV supported this interpretation: mature males were found throughout the area, immature whales mainly offshore, and pregnant females at the ice edge.)

As these patterns of segregation are found in both Area IV and Area V, it is possible that there is movement of whales between the two. There is, for example, a distinct possibility that, in view of the high concentration of mature females in the Ross Sea in Area V, mature females migrate there also from Area IV. Such studies are an important tool in attempting to delineate the different stocks which are thought to be present simultaneously in the Antarctic, and hypotheses will need to be tested using data from future studies.

Birthrate and Rate of Population Increase

Here, birthrate is defined as the value obtained by dividing the number of foetuses by the number of animals in the population. Since all foetuses found in samples of minke whales would have been born in the subsequent breeding season, excepting still-births and miscarriages, the birthrate is an important tool for monitoring population dynamics.

However, knowledge of the birthrate alone is not enough. To assess its impact on the trend of the population, we must also know the natural mortality coefficient. There is some uncertainty about this with regard to the minke whale, which is why Japan has adopted this as the principal objective of its research program.

For our present purpose, therefore, let us take the mortality coefficient for juveniles (Mj) in their first year of life as 0.5. This figure is hypothetical, but not without tentative justification. Reliable estimates of Mj for marine mammals are available only for Californian gray whales and North Pacific fur seals. In the case of the former, Mj = 0.5, while for the latter it is actually lower.

For whales aged older than one year, let us assume a mortality coefficient (M) of 0.08. Combined with Mj = 0.5, we have an effective mortality coefficient for the whole population which conforms with the 0.086 adopted by the Science Committee of the IWC until 1983 for setting quotas.

For the parts of Area IV and Area V surveyed in the two-year feasibility study, birthrates of 15% and 35% were obtained, respectively. The coefficient of variation of these estimates is about 10%. The higher birthrate obtained in Area V may reflect the migration of mature females from Area IV, as described in the foregoing section. In the 1989/90 survey of all of Area IV, of 329 whales sampled, 82 were found to be carrying foetuses, giving a birthrate of 25%. This figure not only falls between the values obtained from the feasibility study but was also obtained from a larger area, and can thus be considered the most representative.

Using the two coefficients of mortality rate (M = 0.08 and Mj = 0.5) and a birthrate of 25%, we can calculate the rate at which the population will increase or decrease. The result of this calculation is a growth rate of 7.5%, higher than previously considered to be the case.

Some modifications of this result are required if it is to be applied to the minke whale population as a whole. Downward revision is necessary to reflect the whales which do not migrate as far south as the Antarctic, as these are believed to be mainly immature whales and resting females, i.e. non-pregnant whales. On the other hand, upward revision might be necessary to reflect the possible movement of pregnant females out of Area IV and into Area V.

In order for the rate of population increase to be estimated with a high degree of confidence, further data are required, including data on whales in low-latitudinal waters. However, it can be concluded from the above that the rate of population increase of minke whales in Area IV is certainly not as low as, say, 1%.

References

Fujise, Y., Kato, H. and Kishino, H. 1991.
Some progress in examination on age distribution and segregation of the southern minke whale population using data from the Japanese research take.
Document submitted to the 43rd meeting of IWC/SC. SC/43/Mi18.

Fujise, Y., Yamamura, K., Zenitani, R., Ishikaea, H., Yamamoto, Y., Kimura, K. and Komaba, M. 1990.
Cruise report of the research on southern minke whales in 1989/90 under the Japanese proposal to the scientific permit.
Document submitted to the 42nd meeting of IWC/SC. SC/42/SHMi25. 56pp.

Kato, H., Hiroyama, H., Fujise, Y. and Ono, K. 1989.
Preliminary report of the feasibility study on southern minke whale under the Japanese proposal to the special permit.
Rep. int. Whal. Commn 39:235-48.

Kato, H., Fujise, Y., Yoshida, H., Nakagawa, S., Ishida, M. and Tanifuji, S. 1990.
Cruise report and preliminary analyses of the feasibility study on southern minke whales in 1988/89 under the Japanese proposal to the scientific permit.
Rep. int. Whal. Commn. 40:289-300.

International Whaling Commission, 1991.
Report of the Scientific Committee. Rep. int. Whal. Commn. 41 (IWC/42/4)

Kato, H. and Zenitani, R. 1990.
Neonatal mark on minke whale baleen plate.
Abstract. Spring meeting of Jap. Soc. Sci. Fish., April 1990. (In Japanese).

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