THE LENGTHS AND DISTRIBUTION OF
ANTARCTIC SEI WHALESJOSEPH W. HORWOOD*
ABSTRACT

Data from the Bureau of International Whaling Statistics are used to
examine the distribution and lengths of sei whales caught by the pelagic
fleets in the Antarctic over the period 1931 to 1978. Over this period peak
catches occurred in January but from 1931 to 1950 the peak was in Marchand from 1950 to 1960 the peak was in February. This difference withtime is probably due to an earlier preference for blue and fin whales.
Maximum lengths found were 61 feet for males and 64 feet for females.
Having stratified the data by latitude, longitude and month it was foundthat for males there was no difference in the mean lengths between themonths January to March but for females mean lengths were longer inJanuary than those in February and March. This may reflect the differencein timing of migration of the different age and sexual classes. In all
months and longitudinal bands mean lengths of whales of both sexes weregreater south of the Antarctic Convergence. These mean differences
ranged from 0.5-3.8 feet for males and 0.9-3.9 feet for females. The
distribution of larger whales is similar north and south of the Convergenceand the difference is caused by most of the smaller whales staying northof the Convergence.
INTRODUCTION

Following the development of the attached, explosive harpoon large rorquals
could be caught and numerous sei whales (Balaenoptera borealis Lesson) werecaught off Finmark, the Faroe Islands and Iceland before the turn of the century(Collett, 1886; Hjort and Rudd, 1929; T0nnessen and Johnsen, 1982). In theSouthern Hemisphere sei whales were first taken from the land stations on
the Falkland Islands and South Georgia before larger catches were taken by
the pelagic fleets. The first record which I can trace of sei whales caught fromthe Antarctic land stations, is during February or April of 1906 at New Islandin the Falkland Islands (T0nnessen and Johnsen, 1982). Catches by the
Antarctic Pelagic fleets remained at less than 1,000 per year until 1957/58
although the land stations at South Georgia usually caught more than the
pelagic fleets (Ohsumi and Yamamura, 1978). With restrictions on the capture* Ministry of Agriculture, Fisheries and Food, Fisheries Laboratory, Lowestoft, Suffolk NR33 OHT, EnglandSci. Rep. Whales Res. Inst.,
No. 37, 1986, 47-60

48

HORWOOD

of fin whales the fleets took progressively more sei whales and extensive
exploitation occurred over the seasons 1963/64 to 1967 /68 (Horwood, 1978).
Ohsumi and Yamamura record that over the period 1909 to 1978 over
171,000 sei whales were caught from the Southern Hemisphere. Under the
regulations of the International Whaling Commission (IWC) sei whales werecompletely protected from all whaling in the Southern Hemisphere from thestart of the 1978179 whaling season (Anon., 1979).
The distribution of sei whales is seasonal, as is that of the other rorquals,
with breeding occurring in the sub-tropical waters and migrations to the polar
feeding grounds. The distribution of catches and catch per unit of effort showthat in the Antarctic sei whale numbers are highest from January to Marchand off South Africa are highest in August to October (Bannister and Gambell,
1965; Best, 1967; Budylenko, 1977; Gambell, 1968; Matthews, 1938). Sightings
data also confirm this movement (Bannister and Gambell, 1965; Best, 1967;
Gambell, 1968; unpublished information provided by Far Seas Fisheries
Research Laboratory, Japan; Masaki, 1979). Paiva and Grangeiro (1965, 1970)
show a similar winter abundance off Brazil. Marking has shown that sei whales
seen in the subtropics do migrate to the Antarctic (Brown, 1977). During the
migration, segregation of sexual classes has been observed. Pregnant females
leave the warmer waters first, arrive first at the feeding grounds and departearlier, and older animals generally precede younger ones (Gambell, 1968;
Lockyer, 1977).
In the Antarctic, catches of sei whales were predominantly between 40°S
and 50°S from longitudes 60°E eastwards to 130°E but at higher latitudes in
the rest of the Antarctic (Horwood, 1980a). As shown by Ohsumi (1977) fromthe North Pacific, Nasu and Masaki (1970) and Kawamura (1974) .found thewhaling grounds for sei whales to be in temperatures of between 8°C and 18°C,
but numerous whales also have been caught south of the Antarctic Convergence (AC). Japanese sighting data show that in some locations sei whales
may be found north of 40°S in the whaling season (Masaki, 1980). Within the
Antarctic sei whales are not distributed homogeneously and more localized
whaling grounds exist. As the whaling season progresses there is a tendency forwhaling, and presumably the whales, to be found more to the south (Nemoto,
1959; Kawamura, 1974).
Doi et al. (1967) reported on the size distribution of sei whales in theJapanese catch from 60°W eastwards to 170°W (Whaling Areas 11-V) over theseasons 1962/63 to 1964/65. They showed that in the catch females were about
4 feet longer than males and that south of 45°S the average length of males in
the catch was about 48 feet and the length of females was about 52 feet. Northof 45°S the average length of both sexes was much shorter. Similar features
were noted by Woolner and Horwood (1980).
The latitudinal segregation by size observed by Doi et al. ( 1967) is notfound in the other large rorquals, the catches of which are in higher latitudes.
The segregation can be assumed to be of some consequence to the biology ofSci. Rep. Whales Res. Inst.,

No. 37, 1986

SEI WHALE DISTRIBUTION

49

the Southern Hemisphere sei whale but, in addition, the sampling andinterpretation of demographic parameters will be affected by the non-random
distribution of the whales. The material reported upon by Doi et al. (1967)
was collected over a limited number of seasons and longitudinal sectors. This·
study makes use of the material recorded by the Bureau of International
Whaling Statistics (BIWS), from pelagic operations since the 1931/32 season,
to describe in greater detail the distribution of sei whales and in particular the
distribution of lengths. The spatial and within season distribution by sex andage described above have necessitated analyses by month, latitudinal andlongitudinal divisions. Of particular interest is the relative sizes north andsouth of the AC. The rapid increase in exploitation over the middle 1960s
resulted in a substantial change in the mean length of whales in the catchwithin ten degree latitudinal zones (Woolner and Horwood, 1980), andconsequently investigations have been carried out before and after this period.
MATERIALS AND METHODS

The data used are the individual catch records originally collated by the
BIWS, Norway. They consist of information about each whale caught bypelagic expeditions in the Southern Hemisphere from the 1931/32 whaling
season onwards. Each record is from one whale caught and gives details ofexpedition, date, position, sex, length and foetal data. The lengths are measured
in feet from the tip of the rostrum to the notch between the tail flukes andare recorded to the nearest foot. These records are now held by the Secretariatof the IWC. Pelagic whaling for baleen whales in the latitudes 0° -40°S has notbeen allowed under the International Convention for the Regulation of
Whaling, 1946, and so catch data for sei whales are restricted to the higher
latitudes. The statistics give 67,853 male sei whales caught, 63,683 females
and one unsexed by the pelagic fleets over the period 1931 to 1978 inclusive.
Woolner and Horwood (1980) described changes in the mean length ofsei whales from the 1961/62 season with time. Here we are more interested
in the spatial distribution of lengths and the effects of the changes with timeneed to be removed. This has been done by considering two separate time
periods, that before and including the 1961/62 season and that after andincluding the 1968/69 season. Before 1961 /62 exploitation was relatively lightand mainly restricted to the South Atlantic region. After 1968/69 exploitation
diminished and lengths in the catch were stable in most regions (Woolner andHorwood, 1980).
The most important oceanographic feature at about 45° to 50°S in the
sector 60°W eastwards to 130°E is the AC and it is likely to be this feature,with its associated hydrographic and biological characteristics that is responsible
for the smaller whales staying to the north. South of the AC the waters arecolder and contain different food organisms (Kawamura, 1974). Consequently,
we are interested in looking at mean lengths to the north and south of theSci. Rep. Whales Res. Inst.,

No.37,1986

50

HORWOOD

AC. The position of the AC has been approximated by the position of the 4 °C
surface isotherm (Machida, 1974) and this can be seen in the Southern Ocean
Atlas as the surface averages from January, February and March (Gordon andMolinelli, 1982). Its position is similar to that described by Mackintosh ( 1946).
Marking of sei whales and the distribution of sei and other whales has indicatedthat different stocks of sei whales can be found in the Antarctic, but the positionis ambiguous with substantial mixing of marked whales. Consequently, theregion has been divided into twelve sectors of 30° of longitude. This fine divisionalso makes the recognition of the latitudes north and south of the AC easier
since the AC varies from below 40°S to above 60°S. The higher position of
the AC from 60°W westwards to 180° corresponds to the region where sei
whales were caught in the higher latitudes. The position of these sectors andof the AC is given in Fig. 1.
Further arrangement of the data consists in their presentation by months,
since previous studies have indicated a differential migration by age and sexual
status. Initial inspection of the data is by groupings of two degrees of latitude
within each longitudinal sector.
The data thus have been disaggregated into two time periods, all months,
12 longitudinal and 20 latitudinal zones. ANOVA techniques cannot beapplied due to the large number of empty cells and unequal variances and sodirect comparison of means has been done using Students-t tests and experimentwise significance levels.
RESULTS(a) Maximum sizes
From the 131,537 total sei whales recorded in the pelagic data twelve
were 60 feet or over. Two were males of 60 and 61 feet. The largest female
was measured as 64 feet and was caught when pregnant by boats of theSovietskaya Ukraina at 68°S 99°W in 1962.
(b) Catch by month
The percentage catch by month over all seasons since 1931 by the pelagic
fleets is given by sex in Table 1. It can be seen that peak numbers occur inJanuary and February with large catches also in December and March. No
catches occurred until November and after May. Catches in December before
1961 /62 were very few and after 1968/69 tended to be restricted to the more
northern latitudes. Consequently, subsequent analyses have looked at sizes
only in January, February and March.
(c) Size by month within latitude and longitudinal bands
Mean lengths were extracted by the two time periods, the twelve
longitudinal zones, three months and two-degrees of latitudes. Within eachSci. Rep. Whales Res. Inst.,

No. 37, 1986

51

SEI WHALE DISTRIBUTION
0.... /,,.------- -....

''

...

--- ' '

"' \I

\

'I
I

,

\ - ......(J

'\
I
I

I
I
I

I
I

\

,...,,.., /
\

I
I
I

/

I

\\

\

__
'\. __ _-- - -""...._.

........

_...

,.. ,..//

Fig. l The Antarctic south of 40°. The dashed line shows the position of the Antarctic
Convergence and the radiating lines show the 12 sectors used in the analyses. Ticks
are at 50, 60 and 70°S.
TABLE I. PERCENTAGE OF THE
CATCH BY MONTH FROM
THE PELAGIC FLEETS, 1931-78

Sci. Rep. Whales Res. Inst.,
No. 37, 1986

Month

Male

FemaleNovember
DecemberJanuary
February
March
April
May

0.0
20.0
32.6
28.7
17.7
1.0
0.0

0.0
17.9
28.0
30.9
21.5
1.6
0.1

52

HORWOOD

time period and longitudinal zone the mean lengths in each two-degree
latitudinal block were compared by month to see if mean lengths in January
were greater than in February, February greater than March and January
greater than March. Cells in which less than six whales were caught wereignored. A plus was given if the earlier month was larger and for each sex theplusses and minuses were summed over the entire Antarctic. The results are
summarized in Table 2. A signs test can be applied with a chi-square distribution
with one degree of freedom and the pair-wise significance level for eachcomparison is given in Table 2.
TABLE 2. COMPARISON OF MEAN LENGTHS BY TWO-DEGREES OF LATITUDE
WITHIN THE 12 ZONES SUMMED OVER THE ANTARCTIC.
J = JANUARY, F = FEBRUARY, M = MARCH.
A PLUS(+) IS GIVEN IF THE MEAN LENGTH WAS LARGER IN THE EARLIER MONTH
Pair-wise
probability

Time

Sex

Months+""61162""61162""61162

M
M
M

]IF
FIM
JIM

20
12

13
17
14

> 0.5
>0.5
>0.5

""61162""61162""61162

F
F
F

]IF
FIM
JIM

23
22
18

12
25
10

0.08
> 0.5
0.09

~ 68169

M
M
M

]IF
FIM
JIM

18
13
18

15
17
8

>0.5
>0.5
0.05

F
F
F

]IF
FIM
JIM

21
15
22

11
20
9

0.08
> 0.5
0.02

~ 68169~ 68169~ 68169~ 68/69~ 68169II

The signs tests is not powerful but has the advantage of making few assumptions
and here the test overcomes any concerns about different sizes between
longitudinal zones due to any possible population differences. The results
show that in general no differences exist in mean lengths by month of males,
but that for females those in January may be larger than those found in Februaryor March. The subsequent analyses retain the segregation by months.
(d) Sizes north and south of the ConvergenceFig. I shows the position of the AC as defined by the 4 °C isotherm andeven after dividing the Antarctic into 30° sectors the AC covers several degrees
of latitude in many sectors. For each of the 12 sectors the latitudes taken as
Sci. Rep. Whales Res. Inst.,
No. 37, 1986

SEI WHALE DISTRIBUTION

53

above and below the AC are given below (Table 3); this division thus excludes
whales caught within the Convergence.
The larger gaps in some of the sectors, e.g. 5, are because there is somevariance between the position of the 4 °C isotherm from the Atlas and thatgiven by Mackintosh (1946).
Inspection of the data from the early period showed that the catches
were very largely south of the AC and do not provide enough material for
TABLE 3. DEFINITION OF SECTORS
SectorLongitude

Above AC:north oflatitudes

Below AC:south oflatitudesI

0- 30E30- 60E
60- 90E
90-120E
120-150E
150-180E
0- 30W30- 60W
60- 90W
90-120W
120-150W
150-180W

48
48
50
50
50
52
48
50
58
58
60
60

54
52
56
56
60
62
54
60
64
64
62
64

2
3
4
5
6
7
8
9
10
11
12

further investigation. The rest of the analysis is based on data obtained during
and after the 1968/69 season.
Only a few sectors have sufficient numbers in two-degree latitudinal sectors
in one month to enable an inspection of trends from north to south. They are
shown in Fig. 2.
From the data on males .no trend is obvious in sector 8 in February (Fig.
2a), but sector 9 in January (Fig. 2b) does show a difference between the lengthsof whales caught north of 50°S and those caught south of 58°S. Sector 6 inMarch has the AC covering ten degrees of latitude and most of the data arefrom within the Convergence but Fig. 2c shows the lengths south of theConvergence to be larger than the others. Sector 3 in February (Fig. 2d) has
no values totally south of the AC but the data from the lowest latitudes indicate
smaller sizes. This is not seen in the same sector in the previous month (Fig.
2e) and the southernmost point removes the impression of a consistent trendwith latitude.
The data from females in sector 8 in January and February (Fig. 2f and g)also show no trend. However, data from sectors 6, 3 and 2 (Fig. 2h, i and j)
all show a discontinuity at about the position of the AC, with larger lengths
to the south.
Sci. Rep. Whales Res. Inst.,
No. 37, 1986

54

HORWOOD
55

a

:

52

52

49

49

46

46

43

43

40

40

52

52

49

49

46

46

43

43

40

40

a; 52
.!!

52

- 49
.5

.s 49

46

- c 46

~ 43

_, 43

°'.

=
"'

"

c

40

40

52

52

49

49

46

46

43

43

40

40

52

52

49

49

46

46

43

43

40

40
40

50

60
Latitude

40

50

60
LatitudeFig. 2 Mean lengths in feet and ± 2 standard errors of lengths by latitude. 40°
corresponds to a grouping of whales caught in 40-42°5. Males: a-e. Females: i-j.
(a) February, 30-60°W(b) January, 60-90°W(c) March, 150-180°E(d) February, 60-90°E(e) January, 60-90°E(t) February, 30-60°W(g) January, 30-60°W(h) March, 150-180°E(i) January, 60-90°E(j) February, 30-60°E.

It is possible to aggregate the data over latitudes north and south of theConvergence (as defined above), but retaining the segregation by month andsector. The mean lengths so obtained and the differences of the means are
given in Table 4.
Data have only been used if six or more lengths were obtained both
north and south of the AC and consequently many gaps are found in thetable. In all cases (35) the mean value was greater south of the Convergence.
In the light of this further tests are hardly required but for completenessStudent's t-tests were done.
In all cases the variance of the lengths was greater in the north, a feature
Sci. Rep. Whales Res. Inst.,

No. 37, 1986

55

SEI WHALE DISTRIBUTIONTABLE 4. MEAN LENGTH OF WHALES NORTH OF THE AC AND IN BRACKETS DIFFERENCES
IN THE MEAN LENGTH IN FEET OF SEI WHALES CAUGHT SOUTH AND NORTH OF THE AC
BY MONTH AND LONGITUDINAL SECTOR IN THE SEASONS OF 1968/69 AND AFTER.
ALL VALUES ARE GREATER IN THE SOUTH. AN ASTERISK INDICATES THAT LESS THAN
6 WERE CAUGHT EITHER NORTH OR SOUTH OF THE CONVERGENCE
Month

SectorJanuary(a) FEMALES
1
2
3
4
5
6
7
8
9
10
11
12
(b) MALES
1
2
3
4
5
6
7
8
9
10
11
12

March

February

*

*

*

*

*

47.3 (2.50)
48.0 (2.67)

47.5 (2.94)
46.7 (2.75)
47.4 (2.08)

47.1 (2.80)

*
*
*

*
*
*

47.0 (3.87)

49.2 (l.43)
49.5 (l.44)

49.1 (l.23)
50.0 (0.87)
47.7 (2.96)
49.4 (1.53)
49.0 (1.64)*
49.6 (0.91)***
46.3 (1.71)
46.8 (1.36)
47.3 (0.68)**
47.2 (0.55)
47.7 (3.33)*
47.9 (l.23)

*

*

*
*
*
*
*
*
48.6 (2.32)
47.2 (3.85)

*

*
*
*
*
*

*

45.6 (3.79)

46.l (2.23)
46. l (1.30)
45.5 (1.88)**

47.l (0.84)
47.4 (0.74)
46.5 (l.01)
47.8 (0.50)
47.l (0.95)

44.5 (2.50)**
47.7 (0.99)
46.6 (2.24)

seen in the data of Doi et al. (1967), and consequently tests involving unequalvariances and unequal sample sizes have been used (Snedecor and Cochran,1980). An example of the distribution of lengths from the 60°-90°E sector in
January is given in Table 5. The t-values are given in Table 6. Very few values
are not significant at a pair-wise significance level of P = 0.01. If the males
and females are treated as two independent groups then a better test is tolook at the experiment-wise significance rates and these are shown in Table
6. A large number are significant even on this basis.
Sci. Rep. Whales Res. Inst.,

No. 37, 1986

56

HORWOODTABLE 5. LENGTH FREQUENCY DISTRIBUTION OF FEMALES CAUGHT FROM THE 1968/69
SEASON AND AFTER IN JANUARY AND IN THE 60-90°E SECTOR
Length38 39 40 41
North of AC
South of AC

0

1
0

29 41
0
0

42 43 44 45 46 47 48 49 50 51
47 43 65 52 80 85 84 85 91
2
2 3
4
2
7
5
11

52 53 54 55 56 57 58

84 76 25 12 6
13 14 7 0
3

0
0

0

0

TABLE 6. STUDENT'S T-VALUES OF COMPARISON OF MEAN LENGTHS NORTH AND SOUTH
OF THE AC. THE DEGREES OF FREEDOM BASED ON UNEQUAL VARIANCES ARE GIVEN IN
BRACKETS. SIGNIFICANCE LAVELS OF P.:; 0.02 ASSESSED ON AN EXPERIMENT-WISE
BASIS FOR MALES AND FEMALES SEPARATELY ARE GIVEN BY+(EQUIVALENT TOP.:; 0.001 FOR PAIR-WISE TESTS)
SectorJanuary

February

7.0 ( 93)+
6.7 ( 23)+

7.6 ( 48)+
4.5 ( 42)+
3.4 ( 83)

March(a) FEMALES

1
2

3
4

3.3 ( 10)

5

6
7
8
9
10
11
12

10.7 (247)+
5.7 (329)+
2.9 ( 23)
1.7 ( 49)

2.2 ( 65)
1.9 ( 43)
3.9( 21)+5.5 (262)+
5.5 (367)+

4.9 (119)+
5.1 ( 35)+(b) MALES

2
3
4
5
6

7
8
9
10
11
12

5.6 ( 86)+
3.0 ( 16)
1.3 ( 6)5.7 ( 22)+2.9 ( 30)
3.7 ( 79)+
13.5 (286)+

2.5 (152)
8.8 ( 22)
3.7 (34)+

1.9 ( 64)
2.2 ( 52)
4.1 ( 48)
2.8 (292)
4.5 (254)+

2.8 ( 11)

2.6 ( 48)
3.4 ( 27)DISCUSSION AND CONCLUSION

The lengths of sei whales in catches are affected by the location of the catches.
Sci. Rep. Whales Res. Inst.,

No. 37, 1986

SEI WHALE DISTRIBUTION

57

For instance, this study has shown that there are differences north and southof the Antarctic Convergence and Gambell ( 1968) found that more immature
whales were caught off South Africa than off South Georgia. This presents a
problem in describing lengths in the population. However, large numbers of
sei whales have been caught from many localities and the aggregation of thedata from the Southern Hemisphere is likely to yield a better description of
the length at age in the population than presentation of the data from single
regions. Even so there has been selection for the larger of the young animals
and that has forced the construction of 'ideal age-length keys' (Ohsumi and
Masaki, 1978). The age length keys have given maximum mean lengths (L 00 )of 48 to 49 feet for males and about 51 feet for Southern Hemisphere sei whales.
The maximum length of each of the whale species is useful to know, atleast for identification purposes. Zemski (1980) reported that the maximum
sizes of sei whales were 19 m (62 feet) in the Southern Hemisphere and 18 m
(59 feet) in the Northern Hemisphere. The BIWS pelagic catch data show 12
whales caught of 60 feet or over. The largest male was 61 feet and the largest
female 64 feet. The International Whaling Statistics (IWS, 1967, 1968) showthat off Kamchatka a 59 foot male and 62 foot female were caught. Thompson( 1928) reported males of 62 and 64 feet in catches from Scottish whaling stations
and mentioned that Haldane's notes recorded a male of 65 feet. Thompson( 1928) observed, in his section on right whales (p. 7), that he suspected that atleast some of the lengths of right whales were measured along the arc of the
body. No such comment is made about sei whales but it is probable that he
was referring to length measurements of all whales. Martin (1983) remarked
that the Scottish length measurements were much bigger than any known
reliably documented measurement from the North Atlantic. The 12 whales
from the BIWS data were recorded over the period 1956-64 and although
Clark ( 1983) has indicated that there might have been differences inmeasurement by different countries of up to 5 per cent, the techniques ofstandard measurement were established by this time and the lengths can be
accepted. We can conclude that maximum lengths of sei whales are 61 feet for
ma1es and 64 feet for females with a possibility that larger ones have existed.
Harmer ( 1931) and Matthews (1938) showed that over the years 1913 to1931 peak catches of sei whales at South Georgia occurred in March withreasonable numbers caught in February and April. Gambell (1968) reported
peak catches at South Georgia from 1963 to 1965 to be in January and peak
catch per effort in the same month. Catches were high from January to
March. Gambell (1968) argued that although the wave of migrations of blue
and then fin whales past South Georgia affected the desirability of sei whales
to the industry the late influx of sei whales was real and this is supported by
the low percentage of sei whales with diatom infestation at South Georgia
from January to March. The pelagic data over the years 1931 to 1978 show
peak catches in January and February with less in December and March andvery few in other months. Japanese sightings data from 1965 also confirm
Sci. Rep. Whales Res. Inst.,
No. 37, 1986

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HORWOOD

that peak abundances occur about January in the latitudes 40°-50°5 but witha year-to-year variation that shows the general abundance of sei whales tooccur between December and March. At higher latitudes peak sightings occur
later. The impression is of sei whales starting to arrive in large numbers in thewaters south of 40°S about December and generally remaining until Marchwith a few penetrating further south later in the season. The more southern
penetration of sei whales as the season progresses has been noted by Kawamura(1974). Nemoto (1959) considered that relatively large catches of sei whales in1957 and 1958 in the Antarctic were associated with higher water temperaturesin those years and that higher surface water temperature later in the summer
allows sei whales further south. The sensitivity of sei whales to cold water andcold weather is well documented (Ingebrightsen, 1929; Nemoto, 1959; Mitchelland Kozicki, 1974).
Laws (1977) suggested that the changes in peak abundance from the
1927-31 period to that in the late 1950s could be due to a relaxation ofinterspecific competition or some environmental change but Matthews (1938)
pointed out that absence of catches in January may well be because of themuch greater demand for fin whales. Subsequent catches described by Gambell( 1968) and the large majority of the pelagic catches described here were taken
earlier in the season but were taken after fin and blue whales had been
substantially depleted. Consequently, we cannot tell if the timing of migration
has altered. Further inspection of the catch data by ten-year blocks revealed
that before 1950 pelagic catches peaked in March, over the period 1950 to1960 they peaked in February and only later were the large catches taken in
January. One factor suggesting that the change is not real is that the catches
described by Harmer ( 1931) and Matthews ( 1938) and early pelagic catches
were over only three months whereas we now see sei whales in substantial
numbers over four months. A three-month feeding season is very short.
However, temperatures in the Antarctic Peninsula during the late 1920s werethe coldest of the century (Horwood, l 980b) and subsequent warming or
relaxation of competition may have allowed sei whales to arrive earlier in theseason, but even so the late pelagic catches peaking late in the season continued
until 1960 and a change in selection by the industry is the likely explanation.
The results given in Table 2 show that for males there is no evidence that
migration to or residence in the waters south of 40°S is related to length. For
females, however, the average lengths are greater in January than in Februaryor March. This would indicate that larger females arrive earlier. The earlymigration of older females is documented by Gambell ( 1968) and Lockyer( 1977) but the sexual status of the whales is also important with immature andpregnant females preceding lactating females. Lockyer ( 1977) shows (herFigure 3) that there is a high incidence of pregnant and mature sei whales in
mid-January. The larger whales found in January thus may be the older mature
whales which are joined later by the smaller, younger whales. Reasons for thismay be that the larger whales can tolerate the slightly colder temperatures
Sci. Rep. Whales Res. Inst.,

No. 37, 1986

SEI WHALE DISTRIBUTION

59

found in January or that their sexual condition needs a longer feeding season.
The figures showing the distribution of lengths of whales with latitude
do not generally show sharp discontinuities associated with the AC. In somelocalities the changes are more pronounced than others, such as 30°-90°E, but
usually only slight trends can be seen. In contrast, when the lengths are grouped
north and south of the Convergence large differences in the mean lengths arefound. In January to March in both sexes and in all longitudinal zones, whereenough data existed, whales were longer south of the Convergence. In males
the means difference ranged between 0.55 to 3.79 feet and in females from
0.87 to 3.85 feet. An example of the distribution of lengths is given in Table
5 and its character is similar to that observed by Doi et al. (1967). The differenceis generated by the distribution of larger sizes being about the same both
north and south of the Convergence but with many smaller whales to thenorth. This was reflected in the variance of the lengths being always greaterin the north. The reason for the segregation is not known. As previously
described, sei whale distribution is known to be sensitive to surface temperature.
This may be a direct thermal effect in that they do not like cold water or aneffect moderated through their food supply. In the Antarctic it is possible that
only the larger whales can tolerate the colder waters to the south of theConvergence or that the distribution of the prey species, known to be different
south of the Convergence (Kawamura, 1974), can only be harvested by thelarger animals.
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