Southern Minke Whales Balaenoptera Acutorostrata

MARINE ECOLOGY PROGRESS SERIES Published March 23 Mar. Ecol. Prog. Ser. I

Breeding areas and southbound migrations of southern minke whales Balaenoptera acutorostrata

Fujio Kasamatsu*, Shigetoshi Nishiwaki, Hajime Ishikawa

Institute of Cetacean Research, 4-18 Toyomi. Chuo, Tokyo 104. Japan

ABSTRACT: Southern minke whales Balaenoptera acutorostrata are genetically separated from Northern Hemisphere minke whales. Seasonal migrations take them from tropical latitudes in winter to high latitudes of the Antarctic in summer. Breeding takes place in warmer waters but llttle is known of breeding areas. Breedmg areas and southbound migrations were studied using sighhng data derived from the Japanese sighting surveys dunng 1976 to 1987. Relatively higher concentrations were observed In the waters mainly around 10" to 20" S in October, the end of the main conception period of this species in the Southern Hemisphere Spatial distribution in tropical and subtropical waters during the latter half of the conception period suggests that there are 2 breeding areas In the eastern and west- ern South Paclflc and 2 others in the eastern and western Indian Ocean. It appears that breeding populations of the southern minke whale are relatively dispersed in open waters while coastal species such as right whales Eubalaena glacialis, humpback whales Megaptera novaeangliae and gray whales Eschrichtius robustus migrate along the shore and congregate in near-shore breeding areas. Lati- tudinal occurrences by month suggest that southern minke whales moved southward from the breed- ing areas by October-November, and that most of them had migrated into Antarctic waters by January. They tend to move almost directly south from the breeding areas to feeding areas, and subsequently disperse after arriving at the feeding areas. Southbound migration speeds were estimated to average 20 nautical miles (n miles) d-' in waters north of the subtropical convergence and 40 to 50 n miles d-' in waters south of the convergence.

KEY WORDS. Southern minke whale . Balaenoptera acutorostrata . Breeding area . Migration Population cycles

INTRODUCTION Since 1978, as a consequence of the International Whaling Commission International Decade of Ceta- All larger southern balaenopterids except Bryde's cean Research (IWC/IDCR) Southern Hemisphere whale Balaenoptera edeni are thought to undertake Minke Whale Assessment Cruises, enormous seasonal migrations between winter breeding areas amounts of data have accumulated concerning the in tropical or subtropical waters and summer feeding distribution and abundance of southern minke areas in the Southern Ocean (Mackintosh 1942, 1966). whales in their Antarctic feeding areas (Best & In the case of the southern minke whale B. acutoros- Butterworth 1980, Kasamatsu et al. 1988). At the trata, direct and indirect evidence of migration has same time, large amounts of data and materials have come from 2 mark recoveries between the Antarctic accumulated from whaling on this species, which and Brazil and from the presence of diatoms on the began in 1971 (Ohsumi & Masaki 1975, Ohsumi skin of southern minke whales (Nemoto et al. 1980, 1979, Kasamatsu & Ohsumi 1981, Kato 1982). Com- Best 1982, Buckland & Duff 1989). pared with this accumulation of information from the Antarctic, and despite useful studies in coastal waters off Durban, South Africa (Best 1982), off Cos- 'Present address: Marine Ecology Research Institute. tinha, Brazil (Williamson 1975), and in the Indian Teikoku-shoin Bldg, 3-29 Jinbo-cho, Kanda, Chiyoda, Tokyo Ocean (Gambell et al. 1975), information concerning 101, Japan distribution, abundance and breeding areas of the

O Inter-Research 1995 Resale of full article not perrnltted 2 Mar EcoI. Prog. Ser. 119: 1-10, 1995

southern minke whales in low latitudes has been (Williamson 1975) and a predominance of the northerly surprisingly limited. moving animals in the mating season off Durban (Best The purpose of the present study was to identify 1982), we guessed that breeding areas might be breeding areas and describe southbound migrations located between these latitudes, possibly between 5" of southern minke whales. It was based mainly on and 30" S. Thus, sightings anywhere north of 30" S sighting data from surveys in the Southern Hemi- were considered useful for identifying the breeding sphere since 1976. areas. While 2 forms of minke whales, the 'dark-shoulder' The conception season of southern minke whales is form and the 'diminutive' or 'dwarf' form, were re- considered to be from July to December with the peak cently identified in the Southern Hemisphere (Best in August to October (Best 1982, Kato 1990) and nearly 1985, Arnold et al. 1987), only the more abundant half of conceptions occur from October to December 'dark-shoulder' form was considered for this study. (Kato 1990). The first half of the survey period (October to December) is thought to have coincided with the latter half of the conception season of this species. MATERIALS AND METHODS However, the second half of the survey period (January to March) did not cover the conception Sighting surveys and data. Since 1976 Japan has season. Therefore, only the sightings data from been conducting sighting and marking surveys in the October to December were used to identify the breed- Southern Hemisphere from October to March using ing areas. Southbound migrations were analyzed on 2 or 3 sighting vessels, independent of whaling opera- the basis of spatial and temporal density variations in tions (Ohsurni & Yamamura 1982). The vessels used all latitudes and periods. are all converted whale catchers (750 to 900 gross tons) Encounter rate. To analyze temporal and spatial and thus are ideally suited for whale sighting surveys. occurrences of southern minke whales, encounter While searching, the vessels normally kept a constant rate (number of whales seen per 1 n mile distance speed of 12 knots. A constant watch for whales was searched) was used as an index of density. The maintained from sunrise to sunset each day when coefficient of variation (CV) of the encounter rate weather permitted [normally wind speed was less than was calculated, based on variation of distance 22 knots h-' (or Beaufort Scale 5 or less) and cue searched and number of whales seen by day. The visibility more than 1.5 nautical miles (n miles)]. There encounter rate (n/L) and its CV were calculated as were always 2 principal observers on watch in the bar- follows: rel of the foremast. These observers had the primary responsibility to find whales. In addition, there were always 2 or 3 crew members present on the upper bridge. All these observers, and particularly those in where'n is the total number of whales seen, L is the the barrel, used 7 X 50 binoculars for scanning the sea total distance searched, k is the number of research ahead of and up to 90" on either side of the vessel's days, nj is the number of whales seen in i days, and bow. The vessels traveled along pre-decided track- L, is the distance searched in i days (Kasamatsu et al. lines. When a whale or group of whales was seen, the 1990, Kishino et al. 1991). vessel closed with the animals at a speed of 15 knots. After closing with the animals, the species and number was ascertained. In addition to this, a chronological RESULTS account of the vessel's activities for the searching effort and weather conditions was recorded in the ship's log- Breeding areas book by the officer on watch. At the end of each day, the officer extracted total search distances for that day Total search distance by 10" squares of latitude and and total number of whales seen by species. This infor- longitude, between the equator and 30" S, from Octo- mation was recorded on the sighting format with other ber to December are presented in Fig. 1 and those with information such as noon position, weather and sea number of whales seen in Table 1. Since search efforts surface temperature at noon. were not distributed homogeneously in the study area, With the exception of the whaling grounds off relationships between search distance and relative Costinha in Brazil (7" S) and possibly Durban in density (encounter rate) were investigated. There was South Africa (30°S), locations of breeding grounds of no significant relationship (the regression slopes are southern minke whales have not been well identified. not significantly different from zero at the 5 % level) Since there were few observations of northward move- between those values in 10" squares in any cases. It is, ments of southern minke whales in waters off Costinha therefore, assumed for the present calculations that Kasamatsu et a1 . Breeding areas of southern minke whales 3 P P P -

20E 40 60 80 100 120 140160E 180 16OW 140 120 100 80 60W Long~tude ( degree )

0 1 1000 2000 3000 4000 5000 6WO7W0 8000 9WO- n . m I l e s Search d~stance

Fig. 1. Distances searched by 10" squares of latitude and long~tudein waters 0" to 30" S from October to December

there is no relationship between search efforts and Encounter rates in 10" squares are illustrated in Fig. 2. southern minke whale density and that an unstratified They ranged from 0 to 0.0154 whale per n mde searched estimate of density should be unbiased in this respect. with a mean of 0.0023 (CV = 0.09, or 9%). A total of 26 333 n miles was surveyed in October in In October, higher encounter rates (>0.003 whale waters north of 30" S, 65719 n miles in November and per n mile) were in waters 0"-10" S, 90"-100°E and 20 140 n miles in Decerriber. A total of 57 minke whales 10"-20" S, 100"-110" E in the eastern Indian Ocean, were seen in the waters in October, 166 in November and in waters 10"-20" S, 150"-180" W in the western and 30 in December. South Pacific. Effort in the area 0"-10" S, 90"-100" E

30s 20E 40 60 80 100 120 140 160E 180 16OW 140 120 100 80 6OW Long I rude ( degree)

Encounter rate

Fig. 2. Balaenoptera acutorostrata. Encounter rates in 10" squares of latitude and longitude in waters 0" to 30' S r-r-rWOaUmLnaW0 umm-OOOOOOO~n %%%%%I~I~I~,',°I~I5 I I I I I -wmumoer -+r++OOOOOOTIDm~m~roo o o o a %%%%%gmmmmmrn7mmmm U 2

ONO-

CC~CN~N~Werrr rrrr NW r -rr-N4A~~rrNmmWrr- m~m~r N-.IN m-mc.am~r~~w-~~uwmN mrmmwm~ma- ~W~N-~W~OUN~U~CLM~~~WN memwmm eewm rn~wwmmwem~rnm~mm~N meermr~um~ mem~wc~wmmemeeeo~mmw~~ w-m-r~ *ONO, ~uoww~a~~amm-euu~~ee UNUWNWW~~W ~rmw~o~r~rr~mmomowecn&

+ + rr C CNNN~ err -mu NNWP~P mwrr r m - ~N~ONW&NWWW~W r NUWO~N-um me-rw P r erer m N N~~O~OU~W~~WW~WU~W U o ~cnwmmm mmmq-mo a wwrn~w~mo~oaoum m m mm-& m ~wrnwam~wmwowmm~ucno m m -wm~mm OOUONOO a m~mmoewwomamww N r mm-- m- N m~mo~~wmw~memmweoo

OOOOOON~ 0-0-00 OOOOOOOO~WOO C's --C's -8 -W --%S --m0 WW --mu

--mggg ------ggs5'gg m----ggggg Urn* -wweoo uwo.Ju --W -----

C C 0 0 ONwNPmrNrOOCnWAN +NO --7------A--- --m m-- PPPP PPP PPPPPPPP PPPPP PPP mmwm g S'5,"gZ &g8zzXg ZzJgZ wasl wmmwwm~cn mwmcnm mmm ammm m ummwcn -0NNrUm PENSS au- -omowoum ---U --- 2022~.WW~ Kasamatsu et al.: Breeding areas of southern minke whales 5

was very limited (261 n miles searched in 2 d). In assume the presence of 2 concentration areas in the November, the higher encounter rates in the eastern South Pacific with a possible boundary at 120"-130" W. Indian Ocean were in waters 10"-20°S, 100"-110" E On the basis of the observations, we suggest the and 20"-30" S, 90"-120" E. The areas of higher en- possibility that the breeding areas of the southern counter rates in the western South Pacific were rela- minke whale populations may not be as concentrated tively dispersed between 10"-30" S, 130"- 170" W in as those of coastal species such as humpback whales November. In the eastern South Pacific, higher en- Megaptera novaeangliae, right whales Balaena glaci- counter rates were in waters 10"-20°S, 100"-120" W. alis and gray whales Eschrichtius robustus. The main In December, higher encounter rates were in waters breeding areas of southern minke whales may be 20"-30" S, 80"-90" E in the eastern Indian Ocean, and located mainly between 10" and 20" S. We suggest that in waters 20"-30" S, 30"-40" E in the western Indian there are at least 4 separate breeding areas, 1 in the Ocean, while the effort in the waters 20"-30" S, eastern South Pacific, 1 in the western South Pacific 30"-40" E of the western lndian Ocean was small (main area 150"-170" W) with a possible boundary (300 n miles in 2 d). In the South Pacific, higher at ca 120"-130" W, and the other 2 in the eastern encounter rates were observed in the eastern South Indian Ocean (main area 100"-110" E) and the western Pacific from 10"-20" S and 100"-110" W. There were Indian Ocean with a possible boundary at ca 60"- no very clear concentrations in other areas. 70" E. Since the temporal and spatial variation of search It is noteworthy that the vast breeding areas in effort makes it difficult to draw clear distribution pat- the western South Pacific (mainly between 170" and terns in waters north of 30" S from October to Decem- 130" W) are in zones where many islands are located. ber, the search effort and sightings for the 3 months Also, this area is directly north of Antarctic Area V were pooled (Fig. 2 bottom, Fig. 3). This revealed (International Whaling Commission management area relatively high encounter rates in the eastern South 130" E - 170" W) and west of Area V1 (170"-145" W) Pacific (100"-120" W, the highest in 10"-20" S, 110"- where the largest population estimates of southern 120" W), in the western South Pacific (130"-130" W, minke whales have been made (International Whaling the highest in 20"-30" S, 130"-140" W), in the western Commission 1991). Indian Ocean (30"-40" E) and in the eastern Indian Ocean (80"-120" E, the highest in 20"-30" S, 110"- 120" E). Southbound migration A distribution gap was observed between the eastern and western Indian Ocean at 60"-7O0E To investigate the southbound migration of southern (Fig. 3). In the South Pacific sector, a relatively lower minke whales, we first examined latitudinal occur- encounter rate was observed in waters between 120" rence by month. Because the amount of open water and 130" W compared with other areas. Since this low south of 60" S varied greatly due to recession of the ice density area has been continuously observed in waters edge as the season progressed, the encounter rates do 30"-50" S during the southbound migrations and in not always reflect the actual abundance there. Thus waters south of 50" S through the early stage of the we also calculated an index of abundance of minke feeding season (see 'Migration routes' below), we can whales on the basis of area size and the encounter rate.

Long1 tude ( degree

Fig. 3. Balaenoptera acutorostrata. Encounter rates by intervals of 10" longitude in waters 0" to 30" S from October to December. Vertical Lines show the standard errors of the encounter rates 6 Mar. Ecol. Prog. Ser. 119: 1-10, 1995

x10-3 3358 12528 lad11 19 o o o x10.3 o o o 39% 32032 19225 59076 observed a low density of this species in 8- October a> January + waters between 20" and 40" S in Janu- ary and February. In addition, we 6 3- - observed a very low encounter rate in

4 - B- waters north of 30" S in February and ,, the highest rate in January in the 2- 10 - , + Antarctic waters. From these results, we a I S l-l I I I I 1-1 I I can assume that more than 90% of x,0.3 l4531 299~21215 10512 10323 1AW 22579 .&-3 11232 13619 southern minke whales will have February Q) 75; November - 341 migrated into the Antarctic by January 4- - (see Fig. 6). In February, encounter rate 9- 9- 9 L J in the Antarctic decreased slightly and CQ) S 6- 6- no clear peak was seen in waters north of 60" S. In March, a small peak was 3 3 - 1.5 .-l%r; also observed in waters 30"-40" S. -H' -111~11 11~1~On the basis of these observations, x10.3 L211 1528 11401 19321 L68YI 18807 19681 x10.3 96Y 12677 12W 1595 62lE 1352 2U we can assume that a part of the south- 178: 178: Decder - ltRil-- March - / ern minke whale population migrates 9- IS- southward from the waters around 10"-20" S to the waters around 20"- 6- .'i 10- I 30" S during November, arriving in

S- Antarctic waters by January. On the basis of monthly catches and biology in l m I p-'! le,lj the Antarctic (Ohsumi & Masaki 1975, 10' 2 0" 3 0" 4 0" 5 0' 6 0' S 1100w2100 300 400 6100S Latitude Lat i rude Ohsumi 1979, Kasamatsu & Ohsumi 1981), we suggest that this group is Fig. 4. Balaenoptera acutorostrata. Encounter rates by latitude and by month. compose^ of mature animals, mainly Vertical lines show the standard errors of encounter rates. Numbers above panels are total distances searched pregnant females, and that the group in the Antarctic by November consists mainly of young animals. We note that The index of abundance (I) is thus calculated as the first groups of the northbound migration may leave I = A(n/L), where A is area size. the Antarctic by February and reach waters between Fig. 4 shows the encounter rates by month and by 30" and 40" S in March. Such a northbound migration intervals of 10" latitude. The highest rate during October was in waters between 10" and 20" S, and this (%) peak shifted to waters between 20" and 30"s during 100 November. Another mode was also observed in waters south of 60" S in November. /. -2.- 8 0 Fig. 5 shows the distribution of the index of abun- mL dance by month and by intervals of 20" latitude. a, According to this figure, approximately 40% of south- : 60 ern minke whales have already migrated into the TJ3 Antarctic waters by November and about 35% remain L3 in waters north of 40" S in November. We can see a -0 40 X clear shift of abundance of minke whales from the U01 lower latitudinal waters to higher latitudinal waters c - 20 with time. In December, the latitudinal distribution was also bimodal (Fig. 4). In waters 30"-40" S the mode was not o Nov. Dec. Jan. Feb. clear, compared to those in October and November. Month The second mode was in waters south of 60" S. The encounter rate in waters south of 60" S increased 2.4 0 0 times from that in November. Although no encounter 0-70"s 20-40"s 40-60's 60"s - rate data for January in waters north of 30" S were Fig. 5. Balaenoptera acutorostrata. Index of abundance [ratio available from this study, Gambell et al. (1975) = 1(2O0-40"S)/Total I] by latitude and by month Kasamatsu et al.. Breeding areas of southern rnlnke whales 7

Long I t ude ( degree )

Fig. 6. Balaenoptera acutorostrata. Encounter rates by intervals of 10" longitude and by month in waters 0" and 30" S from October to December (upper panel), in waters 30" and 50" S from November and December (middle panel), in waters south of 50" S during December and January (lower panel). Vertical lines show the standard errors of the encounter rates pattern would be consistent with the fact that the first From these results, we can probably assume that minke whales appeared off Durban (30" S) in March southern minke whales move generally straight south (Best 1982). from the breeding areas to the feeding areas.

Migration routes Migration speed

On the basis of longitudinal occurrences by month, The speed of the southbound migration of southern we tried to describe the possible migration routes minke whales was crudely estimated based on the of southern minke whales. Fig. 6 shows the encounter peak of seasonal occurrence by latitude. From the rates by intervals of 10" longitude in the 3 latitudinal examination of encounter rate by intervals of 5" zones. Southern rninke whales in the eastern South latitude by half-month (Table 2), the peak of occur- Pacific breeding area (0"-30" S, 120"-100" W) moved rence in late October was between 15" and 20" S and southward or south-southeastward and reached the peak shifted to between 25" and 30" S in early and Antarctic waters between 120" and 70" W. Encounter late November. In early December, the peak was rates were consistently low in the area around 120" W between 30" and 3.5" S. This progression shows that during the southbound migration and feeding periods. a component of the population, mainly mature ani- Southern rninke whales in the western South Pacific mals, traversed a total of 15" of latitude (900 n miles) in moved generally southward but trending towards the roughly 1.5 mo, or an average of about 20 n miles d-'. west. Those in the eastern Indian Ocean moved south- After mid-December, there was no clear peak in ward almost directly, apparently diverging in eastern waters north of 60" S. In contrast, the density of minke and western directions around 100" E. Those in the whales in Antarctic waters substantially increased western Indian Ocean possibly moved southward from December and the maximum density occurred in or south-southeastward. In the Antarctic, it seems that late January. This suggests that a component of the the whales tend to disperse after arriving at the feed- minke whale population, mainly mature animals, ing areas. There was no clear peak of the distribution moved from around 35" S in mid-December to south of in the Antarctic feeding areas during the main feeding 60" S in January. Such movement indicates an average season (January and February) with 1 possible excep- migration speed of 40 to 50 n miles d-'. Southern tion of minke whales in the eastern South Pacific. This rninke whales thus may migrate relatively slowly in the feature was also recently suggested by mtDNA diver- waters north of the Subtropical Convergence and sity of this species (Pastene et al. 1993). increase their speed once having crossed the Conver- 8 Mar. Ecol. Prog. Ser. 119: 1-10, 1995

Table 2. Balaenoptera acutorostrata.Encounter rates (ER,~10'~) with coefficient of variations (CV) by intervals of 5" latitude and half-month. L: late; E: early

Latitude L.Oct E. Nov L. Nov E. Dec L. Dec E. Jan L. Jan E. Feb L.Feb E. Mar interval ER CV ER CV ER CV ER CV ER CV ER CV ER CV ER CV ER CV ER CV

gence. Although there was no previous information on immature. Therefore, another problem is judging the migration speed of this species, Dawbin (1966) esti- whether or not immature whales and resting females mated that humpback whales averaged 30 n miles d-' migrate into the breeding areas. While Best (1981) sug- en route to the Antarctic. gested that juvenile right whales may be distributed farther off shore than adults during the breeding sea- son off South Africa, little is known of the segregation DISCUSSION between mature and immature whales on the breed- ing grounds for pelagic species such as blue, fin, The use of sighting information to identify breeding sei and minke whales. Bannister & Gambell (1965) areas was thought to involve 2 problems: first, it is observed that immature whales, resting females, preg- difficult to know whether or not the surveys effectively nant females and lactating females of fin whales covered the breeding season of the target species; Balaenoptera physalus and sei whales B. borealis second, it is necessary to evaluate the significance of appeared together at the same whaling grounds off sightings during the breeding season in tropical or Durban while the timing of migration differed accord- subtropical waters. ing to reproductive class. Williamson (1975) and Best The breeding areas can be generally denoted as (1982) reported that both mature and immature areas where mating and/or calving take place. This whales were present on the Brazilian whaling grounds study mainly examined data from the period of con- and the Durban whaling grounds, respectively, but ception (mating season) to identify the breeding areas. that there were clear differences in the timing of The breeding season of southern baleen whales, gen- migration. erally, is considered to be austral winter, generally Considering the lack of direct information on May to August (Lockyer 1984) but our surveys covered segregation by reproductive class, further surveys are only the period of austral spring-summer (October to needed especially during the main breeding season March). Therefore, generally it is difficult to identify within the suggested breeding areas. the breeding areas on the basis of sightings obtained This study provides information on the breeding during the non-breeding season. However, as men- areas and the southbound migrations mainly during tioned before, most conceptions are considered to October to March, but our surveys did not cover the occur from July to December with the peak in August main northbound migration period of this species. The to October (Best 1982, Kato 1990). The first half of the lack of coverage from April to September makes it survey period (October to December), therefore, was difficult to draw clear conclusions about the entire thought to overlap sufficiently with the latter half of the migration pattern, or cycle, of southern minke whales. mating season of southern minke whales such that Best (1982) and Holt et al. (1982) described seasonal sighting distribution, especially October, would pro- changes in the abundance of southern minke whales, vide useful insights about breeding areas of southern off Durban (30" S) and Costinha (7" S), respectively. minke whales. Data from the Brazilian land station from June to The observers on survey vessels could not properly December indicated increasing abundance from June determine whether the animals seen were mature or to October and a rapid decline thereafter (Fig. 7). In the Kasamatsu et al.: Breeding areas of southern minke whales 9

Feb Yar Apr May Jun Jul Rug Sep kt So\, Dec Jan Feb War pr and DNA analyses (Hoelzel & Dover 1~111'1'111'1''111989, Wada et al. 1991). However, stock divisions for southern minke whales in the Antarctic have not been well defined. Substantial mixing of individuals of different stocks has been considered in the feeding grounds, where nearly all sampling has taken place. Ours is the first study to provide evidence about the possible separation of stocks and the presence of breeding areas in tropical waters, based on sightings data obtained toward the end of the mating season. We may o!slllllllllllllInow be able to answer the 2 questions Feb Mar Apr May Jun Jul Aug Sep Ocr Nov k Jan Feb Yar dpr Mont h posed by Mackintosh (1966; see above). Southern minke whales Fig. 7. Balaenoptera acutorostrata. Seasonal abundance of southern rninke migrate far north, but their main whales off Brazil (sightings per research hour, relative to October), Durban breeding areas are probably between (whales seen per 12 X 103 miles flown, relative to September), and in waters and 200 S. The breeding popula- 20" to 40" S (encounter rate, relative to November) and south of 60" S (encounter rate, relative to January) tions of southern minke whales may be relatively dispersed in open waters. This distinguishes them from right, latitudes 20" to 30°S, Japanese sighting data show a humpback and gray whales, which migrate between declining trend from November to March, and data near-shore breeding concentration areas and oceanic from Durban indicate increasing density there from feeding areas. This difference may suggest differences February to September. These trends in the 3 sets of of meeting probability of mature males and females in data, taken as a whole, are consistent with the hypoth- the breeding areas between oceanic species such as esis that southern minke whales undertake regular minke whales and coastal species such as humpback annual migrations between tropical and polar regions, whales. They can be interpreted as suggesting that younger Finally, sighting data do not provide any information whales leave the breeding areas earlier than mature on genetic differentiation between the breeding areas. ones, while the mature whales and pregnant females Such information can only be obtained through bio- depart the breeding areas by approximately October logical and genetic surveys in the breeding areas dur- to November; the segment of the population consisting ing the main breeding season. of mainly young whales begins returning to the breed- ing grounds after February. Most other whales leave Acknowledgements. All vessels were provided by the Fish- the feeding grounds by March, and the northbound eries Agency of the Japanese Government and the sightings migration continues into the early austral winter. data were provided by the National Research Institute of Far Mackintosh (1966) called attention to the general Seas Fisheries. We are grateful to Dr P. B. Best, Mammal problems of identifying breeding grounds and sug- Research Institute, University of Pretoria, Dr S. Ohsumi, Insti- tute of Cetacean Research, and Mr M. Cawthorn, Plimerton. gested the need for direct observations in low lati- Wellington, New Zealand for their critical and valuable com- tudes. He raised 2 specific questions: (1) How far ments on this study. Dr Randall R. Reeves, Deputy Chairman, do whales travel toward the equator? and (2) Are Cetacean Specialist Group, IUCN and Dr P. G. H. Evans, Dept breeding populations concentrated or dispersed? Little of Zoology, University of Oxford, read the final manuscript progress has been made since the 1960s toward and made valuable suggestions for improvements. The study could not have been undertaken without the assistance and identifying and characterizing the breeding grounds cooperation of the crew and researchers on board the sighting of pelagic whale species. vessels. We also thank 3 anonymous reviewers for their useful Effort has been made to identify stocks using various suggestions. methods, such as analysis of catch distributions in the Antarctic (van Beek & de la Mare 1982, van Beek 1983), mark-recapture (Buckland & Duff 1989), com- LITERATURE CITED parisons Of 1979' Wads Arnold, P,, Marsh, H., Heinsohn, G. (1987).The occurrence of Numachi 1979, Bushuev & Ivashin 1986, Bushuev two forms of minke whales in east Australian waters with 1990), isozyme analyses (Wada & Numachi 1979), a description of external characters and skeleton of the 10 Mar. Ecol. Prog. Ser. 119: 1-10, 1995

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This article was subnutted to the editor A4anuscript first received: June 28, 1994 Revised version accepted: December 9, 1994