THE DISTRIBUTION OF THE SOUTHERN BLUE
WHALE IN RELATION TO RECENT ESTIMATESOF ABUNDANCEJOSEPH W. HORWOOD*
ABSTRACT

Based on data from systematic sightings surveys for minke whales a
recent stock estimate was obtained of blue whales in the Antarctic. This
study reviews previous information on the distribution of blue whales andconfirms that the time and extent of the surveys was suitable to provide
information on the abundance of blue whales. It would appear that the
number of blue whales in the Antarctic is less than 2000; much less thanprevious estimates indicated.
INTRODUCTION

Blue whales (Balaenoptera musculus L.) have been caught in the Antarctic sinceat least 1905 when C A Larsen caught three from the land station at Grytviken
in January, (T0nnessen and Johnsen, 1982). Hjort, Lie and Ruud (1932)
described how whaling was initially restricted to the shore stations, mainly
from islands in the South Atlantic, but in the season of 1923/24 shore based
whaling extended into the area of the Ross Sea. True pelagic operations started
in the season of 1925/26. After forty years of exploitation all member governments of the International Whaling Commission (IWC) agreed to cease
whaling for blue whales in the Southern Hemisphere from the beginning of
the 1965/66 season and catches after this date are very small. T0nnessen andJohnsen noted that the recorded catch of blue whales from the Antarctic wasover 331,000. Part of this catch is of the pygmy blue whale (B. musculus brevicaudata) which is to be found from 0°-80°E and between 40°-55°S (Ichihara,1966).
The Committee of Three of the IWC undertook a stock assessment of blue
whales which was reported by Chapman (1964). The various techniques theyused gave different estimates of stock: size and were related to different years
depending upon the type of data used. Using catch per unit of effort (CPUE)
and Schaeffer's method gave an initial, exploited population of about 220,000.
Mackintosh and Brown (1956) reported sightings of all large whales south of* Ministry of Agriculture, Fisheries and Food, Fisheries Laboratory, Lowestoft, Suffolk NR33 OHT, EnglandSci. Rep. Whales Res. Inst.,
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HORWOOD

the Antarctic Convergence (AC) over the period 1933/34 to 1938/39 and thesedata led to an estimate of the blue whale population in this region of 33,000,
at that time and a standard De Lury analysis, using CPUE, gave an estimateof exploited stock in 1953/54of13,000, but a range of 6-10,000 was acceptedfrom subsets of the data. Masaki and Yamamura ( 1978) reported on sightingsof blue whales by Japanese, whaling, research ships over the period 1965 to
1977. They considered that the population of blue whales, south of 30°S, wasabout 12,000 over that period. Gulland (1981) reviewed these estimates. He
deducted the whales sighted in the region where pygmy blue whales were
found to give a modified sightings estimate of 5400 and concluded that thisestimate combined with others based on analyses of CPUE indicated that the
exploited population of blue whales in 1963 was around 4000. His postulated
rate of increase of 4 to 5 percent would give a current population of about
10,000.
The techniques used to provide the above estimates utilize rather crudedata. The analyses based on CPUE have to accommodate changes in catchingefficiency, species selection, time and location of whaling and operations otherthan searching. The sightings do not come from systematic surveys. Consequently, the estimates should only be regarded as very approximate. In
contrast, Butterworth and Dudley ( 1984) presented an estimate of blue whale
numbers based on systematic sightings surveys. The background to thesesurveys, designed primarily to estimate numbers of minke whales, is given by
Best and Ohsumi (1980) and Horwood, Best and Ohsumi (1981). The analysis
of Butterworth and Dudley yielded the result that south of approximately 60°S
there were about 1000 or 1600 blue whales. The lower figure is based upon
'primary' sightings which are those seen when actively searching rather thanthose seen whilst engaging in other operations such as confirming the identityof a previously observed school; these others are called 'secondary' sightings.
The second figure is based upon both combined and Butterworth and Dudleyrefer to the value 1000 as negatively biased, in respect of the selection of thenumber of sightings utilised, and the 1600 as positively biased. Other sourcesof bias are discussed later. This analysis must be regarded as the most reliable
to date and gives a figure much lower than any previous estimate. The numbers
are also small in absolute terms especially as the blue whale has not been
hunted for twenty years. Numbers in 1965 may have been half as many.
As the estimates are so low it is especially necessary to enquire if thesurveys were carried out at a time when and in the locations where blue
whales were formerly abundant. Such a concern is expressed by Butterworthand Dudley. In order to clarify this point this study reviews the previous
literature on blue whale distributions and reports on the distribution of catchesfrom the data originally held by the Bureau of International Whaling Statistics(BIWS).

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157

PREVIOUS DESCRIPTIONS

Many of the early Antarctic expeditions noted the occurrence of large
numbers of whales particularly in the regions of the Antarctic Peninsula andthe Ross Sea but most did not refer reliably to the species concerned. Froma variety of observations Racovitza (1903) and Hjort (1920) showed that blue
whales were common in these two localities. More detailed information arosewith the introduction of pelagic whaling. In a series of articles Hjort, Lie andRuud (1932, 1933a, 1933b, 1934) described the Norwegian pelagic whaling
for the seasons 1929/30 to 1933/34. They show that very little whaling took
place between 70°-150°W but throughout the rest of the Antarctic whaling
concentrated on particular grounds. The highest densities of blue whales
were found from 0° eastwards to l 70°W. A review of early exploratory cruises
seemed to confirm that the region between 70°-150°W held few whales.
Their studies show that during October and November catches were mainly
north of 60°S but, as the ice receded, catching was mainly south of 60° or 65°S.
In the area of the Ross Sea catches were taken primarily between 65° and 70°S.
Before January blue whales represented a higher percentage of the catchthan fin whales but after January fin whale catches dominated. The positionof the catches each month in December to February show that blue whales
were taken over several degrees of latitude.
Omura (1973) published the catches of blue whales from the period
1931/32 to 1971172 by ten-degree squares and lchihara (1966) has plotted out
the catch, by the Japanese pelagic fleets, of blue and pygmy blue whales byrectangles of five degrees of longitude and one degree of latitude from theseasons of 1946/47 to 1962/63. The Japanese catches show that blue whales
were caught between 55° and 65°S from 35°E to 105°E, south of 60°S from
105°E to 165°E and south of 65°S from 165°E to 90°W. Substantial catches
were taken from 90°W to 180°W, an area where the early Norwegian whaling
did not operate.
The early pelagic catch statistics (IWS, 1937) show that from October toDecember blue whales were usually over 90 per cent of the catch with theproportion of fin whales increasing in January and February. Kellog (1928),
Mackintosh (1965) and Omura (1973) interpreted this as a difference in thetiming of the migrations of the two species and this is supported by sightings
data (Gambell, 1976).
Mackintosh (1966) considered that blue and fin whales were foundthroughout the Antarctic and were distributed in a very similar way, the only
difference was that blue whales tended to frequent slightly colder waters. This
opinion is at some variance with the opinions of Ohsumi, Masaki and
Kawamura (1970) who considered the distribution of the blue whale to be
more like that of the minke whale, a species that is found in greatest numbers
around the ice edge. Racovitza (1903) considered that blue whales seek the ice.
Nishiwaki and Hayashi (1950) presented the catch of blue and fin whales
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HORWOOD

from the Ross Sea in 194 7/48 by one degree squares and showed that the two
species occurred together in those waters but Nishiwaki and Oye (1951) alsocommented that few blue whales were being caught in one expedition as they
were whaling on an aggregation of fin whales over 50 miles from the edge ofthe pack ice. Nemoto (1959) plotted the relative distribution of blue and fin
whales between 170°E-162°W for catches from 1949 to 1950 (his Figure 31).
Along the ice edge blue whales comprised between 20-100% but over 60
nautical miles away from the ice edge the percentage of blue whales was 8-78%.
He reported that "Whalers usually say they hunt blue whales in the pack ice
and chase fin whales in the off-waters." Slijper ( 1962) says that it is well known
that blue whales keep to the drifting ice whereas fin whales are found outside it.
B.I.W.S CATCH DATA

The BIWS, Norway, kept a record of all whales caught in the Antarctic fromthe 1931132 season. These data are now held on computer by the Secretariatof the IWC. Tillman and Ohsumi (1981) compared the computerized recordsof Japanese pelagic catches with Japanese government statistics and theyfound that over the seasons 1939/40 and 1940/41 records published by the
BIWS were correct but the computerized records did not include many fin
and blue whales taken in these seasons. However, these omissions represent
only a small proportion of the total number of records and the following
describes the spatial and temporal distribution of pelagic catches of blue
whales from the Antarctic based on the computerized records. T0nnessen andJohnsen (1982) recorded that 331,042 blue whales were caught between 1904
and 1978 in the Antarctic. The catch records show that from 1931/32 180,676
were taken by the pelagic fleets. Of these 123,346 were taken south of 60°S
and 57,330 were taken to the north of 60°S.
Fig. 1 shows the catch of blue whales by day by the pelagic fleets southof 60°S. Data from 29 February are omitted. Catches reach peak numbers at
the beginning of February but are high from mid-December to the beginningof March. However, various commercial, national and international agreements
have regulated the beginning and ending of whaling seasons and, consequently,
catches will not necessarily reflect abundance on the grounds. The whaling
period has varied with species in order to give protection to humpback andblue whales. Table 1 gives the whaling periods for blue whales for the pelagic
fleets extracted mainly from various volumes of the International Whaling
Statistics. Before the International Agreement for the Regulation of Whaling,
1937, the allowed whaling periods and areas were regulated by whaling
agreements amongst companies primarily of Norway and the UK. Not all
whaling countries were bound by the periods shown in any one season.
Table 1 shows that before 1948 whaling was allowed from before thesecond week in December, but after 1950 catches were to be taken fromJanuary onwards. Catches of blue whales after 1950 were relatively small,
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DISTRIBUTION OF THE SOUTHERN BLUE WHALE

1500

....
Q)

.c 1000
E
:l

z

500

Oct

Nov

Dec

Jan

Feb

Mar

Apr

Month
Fig. 1. The catch of blue whales by pelagic fleets in the Southern Hemisphere southof 60°S from the 1931/32 whaling season. The value for 29 February has not been
included.

(Horwood, 198la). Consequently, the pattern from mid-December should not
be much affected by the regulations although other operational considerations
may have played a part. The rapid decline in catches at the beginning ofMarch does look to have occurred at about the times that the closing dates forthe period have operated.
The six research cruises that provided the data for the assessment by
Butterworth and Dudley (1984) took place over the seasons 1978179 to 1983/84
and covered the extreme times of 24 December to 18 February; consequently
there is interest in the spatial distribution of pelagic catches over these dates.
South of 60°S 75,878 blue whales were recorded as caught between these dates
from 1931/32. (Another 114 whales were recorded as having been caught
between these dates but their positions of capture are uncertain). The
distribution of these catches, plotted as numbers caught in a rectangle of ten
degrees of longitude and one of latitude, is shown in Fig. 2. The cross-hatchedregion is where 500 or more whales were caught. The blank regions represent
areas in which less than 50 whales were caught; on average, over a period of
30 years (within the above dates) and in rectangles of about 15,000 square
miles, this represents less than two whales caught per year. Unfortunately
whaling regulations have also affected the spatial movements of the fleets andthese have also been summarised in Table 1.
Pelagic whaling was restricted to south of 40°S from the 1935/36 season
but, for two years from the beginning of the 1938/39 season, an area from
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HORWOODTABLE I. REGULATIONS AFFECTING THE PELAGIC CATCHING OF BLUE
WHALES IN THE ANTARCTIC

SeasonWhaling period(closing date if
different) 1

I932/33

20.10.32-30.4.33(14.4.33)

33/34
34/35

25.10.331.12.34-31.3.35

35/36
36/37
37/38

1.12.35-15.3.36
8.12.36- 7.3.37
8.12.37- 7.3.38

38/39

8.12.38- 7.3.39

39/40
40/41
41/42
42/43
43/44
44/45

8.12.39- 7.3.40
8.12.40- 7.3.41
no pelagic whaling
8.12.42- 7.3.43
8.12.43- 7.3.44
24.11.44-24.3.45

45/46

24.11.45-24.3.46

46/47
47/48

8.12.46- 7.4.47
8.12.47- 7.4.48(3 l.3.48)
15.12.48- 7.4.49(26.3.49)

48/49

49/50
50/51
51/52
52/53
53/54
54/55
55/56
56/57
57/58
58/59

22.12.49- 7.4.50(15.3.50)
22.12.50- 7.4.50(9.3.50)
2.1.51- 7.4.51( 5.3.51)
2.1.53- 7.4.53(16.3.53)
16.1.54- 7.4.54(18.3.54)
21.1.55- 7.4.55(19.3.55)
l.2.56- 7.4.56( 4.3.56)
l.2.57- 7 .4.57(16.3.57)
l.2.58- 7.4.58(16.3.58)
l.2.59- 7.4.59(16.3.59)

Area

Notes

No restrictions

Geneva Convention 1931
production agreement
June 19322
Production agreement2

S of 50°S(Norway only)s of 40°S

Norwegian law2
production agreement3
Production agreement 3
Production agreement
Int. Agreement forRegulation of Whaling
June 1937
Protocol of 1938

S of 40°S 70°W-160°W
closed for 2 yrs.
S of 40°

S of 40°S
70°W-160°W closed4

Protocol of 1944
(not ratified)Protocol of 1944
and 1945
Protocol of 1945
Protocol of 1946
Convention of 1946and subsequent schedulechanges apply to 1966

Longer period for sei
and fin

S of 40°S
70°Wopened

IWC lOth Meeting(Continued)
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DISTRIBUTION OF THE SOUTHERN BLUE WHALE

161

TABLE 1. (Continued)
59/60

60/61
61/62
62/63
63/64
64/65

1.2.60- 7.4.60(Netherlands continued
until 15.4.60)
1.2.61- 7.4.61
1.2.62- 7.4.62
1.2.63- 7.4.63
1.2.64- 7.4.64
1.2.65- 7.4.65

65/66

40° -55°S;0° -80°E
All areas closed

Pygmy blue area
Pelagic whaling nations
objected and thus
restricted to 63/64 area

All areas closedI) Actual whaling period is less if quota was reached before the end of the season. (IWS XLVI, LIV)
2) Timnessen and Johnsen, 1982, pp402-408.
3) T11mnessen and Johnsen, 1982, pp433-439.
4) This appears to be the earliest date that the closed area could be binding following the IWC meetingof December 1946. The agreement was implemented by the 1949/50 season but it also appears that thearea was not operated in, in practice, until the season of 1955/56 (Omura, 1973).

70°W to l 60°W was closed for two years. This has become known as theSanctuary. It was also legally closed from about 1947/48 to the beginning of
the 1955/56 season. There does appear to have been the intention not towhale in the area of 70°-l60°W but legislation did not keep pace with eventsand Omura (1973) showed that no catches or effort were expended in the
Sanctuary over the periods 1931/32 to 1938/39 and from 1945/46 to 1954/55.
This then leaves the region with only 10 years of whaling and means that
accumulated catches will not portray relative abundance inside and outside
this area.
The distributions of Fig. 2 show that catches were mainly taken in whaling
Areas II to V, 60°W eastwards to l 70°W. Highest numbers were taken from
Area II (0°-60°W) between 60° and 63°S, in Areas III (0°-70°E) between 60°S
and the ice edge and in a few localities in Area V (130°E-l 70°W). Very few
blue whales were caught from 60°-l20°W. From 120°W to 130°E the AC isfurther south than in the rest of the Antarctic and few blue whales werecaught north of 64°S in this region. As noted by Omura (1973) the distributions
show substantial heterogeneity with longitude but also the distribution withlatitude varies with Area.
DISCUSSION

The estimate arrived at by Butterworth and Dudley (1984), of between 1000
and 1600 blue whales in the Antarctic, was based upon data acquired during
surveys for minke whales over the period 1978179 to 1983/84. In the survey
period 30 schools of blue whales were found with an average school size of
1.7, and the positions where these schools were seen are given in Fig. 2. The
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HORWOOD

608
Fig. 2. The distribution of pelagic catches of blue whales south of 60°S from 193 l/32
over the period 24 December to 18 February by groupings of 10 degrees oflongitude and one degree of latitude. Cross hatched regions are rectangles in which
over 500 were caught over the period, hatched regions are where 50 were caught
and blank regions where less than 50 where caught. The numbers represent blue
whale schools found on the surveys: deleted as the two figures have been replacedby one.

estimate of 1OOO was obtained using the 17 primary sightings seen in over
41,000 miles travelled in a searching mode. The higher figure of 1600 was
arrived at by ignoring the distinction between primary and secondary sightings
but at the same time not increasing the miles travelled.
The review of previous studies has shown that blue whales dominatedthe early Antarctic catches until the new year, after which catches of fin
whales were more important. Sightings data confirm that blue whales doarrive in the Antarctic earlier than do fin whales, and the lower percentageof blue whales in the catch from January onwards reflects more the arrival of
fin whales than the departure of blue whales. Fig. 1 shows the distribution of
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DISTRIBUTION OF THE SOUTHERN BLUE WHALE

163

catches of blue whales south of 60°S and it can be seen that over the periods
in which the research cruises operated, late December to mid-February,catches of blue whales reached their peak. It has been shown that whaling
regulations would not have distorted this feature and that consequently, it canbe concluded that south of 60°S the research took place over the time of
maximum catches and probably maximum abundance.
The area over which the sightings estimate apply is mainly south of 60°S
but in fact only about 75 per cent of the area is used to obtain the estimate.
Large areas are not included particularly between 60° and 65°S from 0°-35°W
and between 60° and 64°S from 0°-35°E. Whaling Area IV (70°-l30°E) was
surveyed from 60°S and between 35° to 60°W the region was surveyed southof 58°S but the rest of the Antarctic was surveyed from latitudes below 60°S.
However, the previous literature does not precisely define the distribution
and behaviour of blue whales. Mackintosh (1966) referred to blue and fin
whales as being similarly distributed and all authors agree that blue whales
tend to be relatively more abundant near the ice than are fin whales. Surveys
for minke whales have shown significant differences in density between
sighting transects along the ice and those over 50 miles away from the ice(Horwood, 198lb), and Ohsumi et al. (1970) considered minke whales to be
distributed in a similar way to blue whales. Other authors have referred to blue
whales as being ice loving and the rather imprecise descriptions of blue whale
distribution is summarised by Ruud (1956) who reported that "blue whales
are supported to penetrate further into the icy regions than any other species".
The distribution of catches gives a more quantitative representation ofblue whale distribution than do the previous comments. Fig. 2 shows the
distribution of pelagic catches over the period of the sighting surveys. Over
most of the Antarctic the whales were caught over several degrees of latitude
and in general an extensive latitudinal distribution can be seen. Over the time
period of the surveys 20.2% were caught north of 60°S, some of these would
have been pygmy blue whales but, from the position of the catches, theywould have been few. It can be seen that over late December to mid-February
blue whales are distributed from the ice edge to north of 60°S. Longitudinally
there is great variation. The evidence suggests that blue whales did not occur
in large numbers from the Antarctic Peninsula westwards to the Ross Sea.
From 120°W westwards to about 110°E the AC is further south than throughout
the rest of the Antarctic and catches have largely been south of about 64°S.
Catches are concentrated to the north in the sectors from 60°W to 70°E withlarge catches to the north of 60°S from 40°W to 60°E. It would therefore
appear that the sightings cruises largely covered an area appropriate for
estimating the number of blue whales. In some areas large numbers of blue
whales were caught much further north than the ice edge and so the surveys,
designed to count numbers of minke whales, expended too much effort along
the ice edge. This would have resulted in a higher variance than if the survey
had been designed for blue whales. In some locations large catches were taken
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HORWOOD

north of 60°S and the estimates from the surveys neglected this componentof the population.
Butterworth and Dudley (1984) discuss possible biasses in their analyse~
and two aspects are particularly important. Firstly not all whales near to thevessel (strictly on the track line) will be seen. For blue whales long dive times
will mean that some will be missed but this is mitigated by large, visible blows.
Butterworth and Dudley comment that a correction factor discussed forminke whales of 1.35 still leaves the sightings estimates much lower thanothers. Secondly the area used is 75% of that south of 60°S. The catch data
show that significant numbers of whales are likely to have been neglected
mainly from 35°W to 35°E due to the survey area being south of about 64°S
in this region. At these longitudes substantial catches were also taken northof 60°S.
It is not clear that a substantial correction factor for whales missed near
the ship is warranted and cumulative catches over the survey period, but
outside the area used for estimation, are about 35% of the total. The review
indicates that the surveys for minke whales took place at a time and in locations
appropriate to extending the analysis to blue whales and that any correction
factors are unlikely to increase the estimate to that given by Masaki andYamamura (1978). Consequently, it can be concluded that the estimatesprovided by Butterworth and Dudley (1984), of between 1000 and 1600 blue
whales in 75% of the area south of 60°S, are the most reliable to date and that
previous estimates were too high. If cumulative catches are used to raise theestimates to blue whales in the area south of 40°S then this still yields a
population estimate of less than 2000 animals.
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