© 1995 Elsevier Science B.V. All rights reserved
Whales, seals, fish and man
A.S. Blix, L. Wall0e and 0. UUtang, editors
425
On the life history and autecology of North Atlantic rorquals
Jóhann Sigurjónsson
Marine Research Institute, Reykjavík, Iceland
Abstract. Background and methods: this paper briefly reviews in qualitative terms available information on exploitation, stock development, life history and autecology of North Atlantic rorquals (mainly
blue, fin, sei, minke and humpback whales) with special reference to interspecific variations in abundance. Historical catches and information on growth, reproductive rate and food preferences are examined in relation to present stock sizes. Results and conclusions: recent sighting surveys show that the
North Atlantic rorquals cannot be regarded as threatened or endangered by exploitation, although some
species are depleted locally in some areas. The minke and fin whales are both euryphagic seasonal
feeders that prior to exploitation and still today number in the 50,000-100,000 and 100,000-200,000
range, respectively. Their opportunistic feeding habits and choice for vast open ocean breeding areas
seem to give risé to relatively large stocks and to their apparent sustenance for high catches. The near
stenophagic crustacean feeding blue and sei whales seem on the other hand to have had smaller historical stock levels of well within 20,000 animals. While the sei whale is probably at present in the 10,00015,000 range, the blue whale is still at a low level, although increasing in some areas, such as off Iceland. The humpback whale seems to be the species that historically occurs in smallest numbers, although the northwestern stock seems to be in a healthy state (increasing by -10% per year). The strong
recovery of the species in recent decades may be linked with its euryphagous lifestyle. However, the
relatively small population size raay also be correlated with its dependence on the rather limited coastal
zones for breeding.
Key words: balaenopterids, stock size, exploitation, status, feeding, competition
Introduction
The past century has been a dramatic epoch with respect to the development and
exploitation of North Atlantic balaenopterid whales or rorquals, since with the
introduction of modern whaling in the late 1860s they became the main target of the
newly developing industry [1,2]. The rorquals are part of the baleen whale group and
are all, except the minke whale (Balaenoptera acutorostrata), true large whales. The
other rorquals occurring in the North Atlantic comprise the largest animal on earth,
the blue whale (B. musculus), in addition to fin (B. physalus), sei (B. borealis),
Bryde's (B. edeni) and humpback whales (Megaptera novaeangliae). The minke
whale was, however, not subject to large scale exploitation until during the first
decades of this century with the introduction of small-type whaling activities in
several areas of the North Atlantic Ocean [3-5].
The intense whaling pressure caused significant reduction in many of the rorqual
stocks even before the turn of the century. This paper briefly reviews the history of
Addressfor correspondence: Marine Research Institute, P.O. Box 1390, 121 Reykjavík, Iceland.
426
exploitation and discusses a few qualitative aspects of development of stock sizes
and the present situation in light of several life history and ecological characteristics
of the different species and stocks.
Main Factors Affecting the Survival of the Stocks
There are a number of factors tl\at need to be considered in this context (see e.g. Ref.
[6]), some of which one may explore, while others are very difficult to approach. We
have mentioned the exploitation that evidently is important here. Biological limits
are important, the rate of reproduction, the growth rate, etc., i.e. the life history
characteristics of each species and stock; also, the selection of habitat, food
preference and feeding strategy that the species have adjusted to, and the availability
of preferred habitats and food in time and space. Finally, behavioural aspects such as
territorialism that we know so little about, may play a central role in this game of
survival and competition, One can say that the optimization of all these factors forms
each of the species' competitive strength.
Fig 1. Traditional IWC stock divisions of N Atlantic fin whales.
427
Migration, Distribution and Stock Units
The typical migratory cycle of a large baleen whale is a summer feeding migration to
high latitudes and a return migration to wintering/breeding grounds in autumn and
winter at low latitudes [2,19]. Although this seems to be the general pattern with
respect to the rorquals at high latitudes, there are clear exceptions, e.g. the humpback
whales that annually occur during winter at the capelin (Mallotus villosus) grounds
north off Iceland and at the turn of the century they occurred during January-March
at the northwest coast of Finnmarken, N Norway [20,21]. Detailed accounts on the
distribution and movements of the rorquals in the N Atlantic are reviewed, e.g. by
Kellogg [22] and Jonsgárd [23] and more specifically for blue and fin whales in
Jonsgárd's comprehensive papers [24,25], for sei and minke whales in Horwood's
two monographs [3,26] and for humpback whales in Winn and Reichley [27].
The question of stock discreteness of each of the species of rorquals has been
dealt with in detail by many authors in conjunction with management of whaling [714]. However, scientists in the mid-1970s made postulations regarding division of
the N Atlantic baleen whales into stocks that gave ríse to the traditional IWC management units [13]. These were mainly based on the distribution of catch grounds
and recent sightings, catch history and whale markings. Accumulating evidence favours the idea of several genetically discrete or near isolated entities for minke, fin
and humpback whales [10-12]. Fidelity to site, demonstrated well by traditional
whale marking/recoveries in fin, sei and minke whales [9,14] and by repeated sightings of photoidentified blue, fin, humpback and minke whales [15-18], is also an
important element in this discussion on reasonable units to manage whales, that accommodates some restricted intermingling, even between distant feeding populations.
Figure 1 shows the seven traditional stock areas or management units for fin
whales in the N Atlantic. Similarly, some four stock areas have been suggested for
minke whales, i.e. two westera units, one central and one northeastern stock unit.
Three sei whale stock units (Nova Scotia, Iceland-Denmark Strait, and Eastern
stock) have been suggested [13].
All these rorquals, except the sei whale, have summer feeding grounds ranging
north to the ice edge and as far south as the Gulf of Mexico, the Mediterranean Sea
and the coast of NW Africa, and more oceanic offshore winter distribution at lower
latitudes. The sei whale on the other hand (Fig. 2a), does not normally reach as far
north in summer, is more confined to temperate/boreal waters, and stays also offshore but has more southern distribution in winter than the other species, possibly as
far south as the equator [26].
Figure 2b shows the feeding and breeding areas of humpback whales at high and
low latitudes, respectively. Due to the humpback's coastal affinity and its often
clearly identifiable marks on the ventral fluke that can be photographed, this species
has been the subject of intensive investigations since the 1970s. The feeding aggregations seem to form quite discrete matrilineal units, apparently with a panmictic
genetic pool [12,17]. The whales from the western Atlantic including the grounds off
428
Sei whale
a
70°
50°
30"
10°
10*
30'
Humpback whale
b
70"
50°
30«
10"
10'
30'
Fig. 2. Summer feeding grounds and winter breeding grounds of N Atlantic sei (a) and humpback (b)
whales.
429
3000
2500
2000
<D
.Q
£
3
1500
z
1000
500
0
1868
1878
1888 u 1898
1908
1919
1929
1939
1949
1959
1969
1979
Year
Fig. 3. Catches of blue, fin, sei and humpback whales in the N Atlantic 1868-1985 (cf. text).
Iceland, migrate in winter to the breeding grounds in the Caribbean Sea. There are
large feeding aggregations off Iceland and Newfoundland, and smaller ones at W
Greenland, in the Gulf of St Lawrence and Gulf of Maine. The humpbacks off N
Norway may have a relationship with the breeding grounds in the southwest,
although we have traditionally assumed that this feeding aggregation migrates to the
eastern breeding area off Cape Verde Islands, which at present is evidently sparsely
populated [27,59].
Exploitation
The catches of the four large rorqual species from the start of modern whaling last
century (based on Refs. [28,29]) are shown in Fig. 3. We can see a period of learning
and expansion of whaling activities into new areas that gave improved catch results
up to the mid-1920s when over 3,000 animals (where unspecified catches have been
incorporated) were taken annually. The largest catches were taken off Great Britain
and Spain, while whaling first commenced off the coast of N Norway in the 1860s
and in the 1880s at Iceland, but both these areas played a major role in the history of
modern whaling in the N Atlantic. Note that in the early years, strike and loss rate
was quite high [1], so here we are only talking about landed animals that may have
been well over 100,000 in the last century or so.
Examination of the species composition indicates that some 79,000 fin, 12,000
blue whales, less than 10,000 humpbacks and 16,000 sei whales were landed during
commercial whaling since the late 1860s. The blue and humpback whales were
mainly taken during the first period of modern whaling, while catches of fin whales
430
increased until the mid-1920s and then decreased with the closures of the fisheries in
the British Isles and later the commercial operations in the Faroes in the 1960s, in
Canada and N Norway after 1972, and Spain and Iceland in the mid-1980s.
In contrast to the large whaling industry, minke whaling with cold-harpoons and
motor vessels did not commence until well into this century. It was confined to small
fishing boats in nearshore waters of Norway [4] and Iceland [5,31] at the beginning
of this century. After the World War II, Norwegian minke whaling expanded to the
west [30] and local whaling commenced on the Canadian [32,33] and Greenland
coasts [34,35].
The bulk of the minke whale catches were taken by the Norwegian fishery, which
gradually expanded towards the open sea on board well equipped and larger vessels
that were able to catch and process the whales at sea. At most the annual take by the
Norwegian fishery was over 4,000 whales [28,29], but the minke whaling activities
were less widely distributed than whaling for other rorquals.
Present Status
Intensive hunting in the last century, involving mainly the five species of rorquals,
has had major impact on the status of the stocks. This applies particularly to blue and
humpback whales, which probably were reduced to very low levels throughout the
ocean just after the turn of the century [17,24,38]. Now recent sighting surveys and
other investigations have given us information on the current stock levels (see Table
1).
Although apparently the humpback whale never was in great numbers in the N
Atlantic [59], after decades of depleted status the stock has more or less fully recovTable 1. Present status of stocks of North Atlantic rorquals
Species
Blue
Stock size
1,000-2,000
Fin
50,000+
Sei
13,500+
Minke
Humpback
100,000+
5,500+
Status
Sources
Low level, mainly off Iceland and Gulf of
St Lawrence, far less in other past whaling
grounds, increasing by 5% per year off Iceland
Still in good numbers, although depleted off W
Norway and UK; lack of recent sightings
estimate in NW Atlantic, so tagging estimate from
1970s and CETAP results from the US coast used
Recent survey estimate in the central N Atlantic
available, NW Atlantic marking estimate (1,800)
used, depleted in some earlier whaling grounds,
such as off N Norway
Stock size reduced but still abundant in NE
Atlantic, the smaller NW stock may number
several thousands
NW Atlantic near pre-exploitation level, increasing
by -10% per year, eastern stock depleted
15,24,33,37-39,59
8,33,39,40,46
26,33,38,42,46
7,46
17,36,37,59
431
ered in the western and central distribution area and may still be increasing
[17,36,37,59]. However, the whaling grounds at the eastern side of the N Atlantic are
almost vacant for this species. It seems that, prior to exploitation, both sei and blue
whales were in somewhat greater abundance or in the 10,000-15,000 range [38].
The present level of sei whales may be of similar order of size in some areas as prior
to exploitation, while its absence from the earlier grounds off N Norway has been
noted [26]. On the other hand, the blue whale is evidently still at a low level, while
showing some significant signs of.recovery (5% per year) off Iceland [37].
Moving to the far more abundant fin whale stock, which evidently was also subject to heavy taxation at the end of the last century and the first half of this century
[25,28], the situation is different. Nevertheless, some major past whaling grounds,
such as off W Norway and the British Isles, seem still to be sparsely populated [8].
Finally, we have the stock of minke whales, which may originally have numbered in
excess of 150,000 animals and still is probably well above 100,000 in the entire N
Atlantic Oceap [7].
Life History Parameters
One may like to speculate why these rather closely related species exhibit such
different historical stock levels and what are the likely factors affecting recent
developments of the stocks. Such speculations may be of help in understanding stock
development and can be useful in generating models to study the stocks in the future.
Let us first take a look at the life history characteristics.
As we see from Table 2 the ranges in life history parameters are not very different
in the large species, the age at sexual maturity ranging from 6 to 12 years of age in
blue, fin and sei whales, while it seems that both humpback and minke have a
slightly faster growth potential, with age at maturity being somewhat lower. The
pregnancy rates are of the same order of magnitude, i.e. normally a 2-year reproductive cycle in the larger species with possibilities of up to 1.5 calf per 2-year period.
Table 2. Life history parameters for balaenopterid whales
Bluea
Length of newborn (m)
Length (m) at sex. maturity
Males
Females
Age (years) at sexual maturity
Males
Females
Pregnancy rates
Length of gestation (months)
~7
20-21
21-23
-10
-10
-
10-11
Finb
Seic
6.4
4.5
Minked
2.4-2.8
4-5
11.6
12.1
17.7
18.3
12.0-12.8
13.1-13.4
6.8-7.0
7.3-7.4
8-12
6-10
0.5-0.73
11.2
7-11.7
5.6-11.7
0.36-0.47
10.7
3—6
5-7
0.86-0.99
10
Sources: a[44]; b[44,45,55j; c[26]; d[3,7]; e[27,57].
Humpbacke
2-5
2-5
0.3-0.43
11-11.5
432
S
10
15
19^0
65
1970
75
1980
85
1990
Year at transition
Fig. 4. Changes in age atmaturatíon in fin whales off Iceland,
However, the typical minke whale cyclus is annual, which presumably gives it a plus
on the competition record.
The ranges of ages at maturity shown in Table 2 are to a large degree a reflection
of growth rate in the populations where we have available information of this kind.
Evidence for dramatic changes in the growth rate of fin whales off Iceland demonstrate this well, where after years of decline in age at sexual maturity [41], a reversed
trend in the most recent year classes has followed [43,45] as shown in Fig. 4. This
has been related to the available food resources [6,19,46], where great fluctuations in
fecundity have also been linked with changing availability of food.
In conclusion, changes in growth rate and the corresponding changes in age at
maturity are important elements in the survival of these stocks and here all the
balaenopterid species may have a similar chance. Likewise, it seems as if minke
whales may have greater potential in utilizing favourable conditions, when these are
available, than the larger species, if only reproduction would decide who is to win.
Table 3. Prey group preference in North Atlantic fin and sei whales
Species
Area
lst prey
2nd prey
Fin a
N Norway
W Norway
Faroes
Icelandc
Nova Scotia
Newfoundland
N Norway
W Norway
Icelandc
Nova Scotia
Krill
Krill
krill
krill
krill
Capelin
Copepods
Copepods
Krill
Copepods
Capelin/herring
Sei b
Sources: a[25,47]; b[26,47]; cunpublished.
Herring
Blue whiting, herring, capelin
Sandlance/mackerel
Sandlance/lantern
Krill
Krill
Copepods
Krill
433
Table 4. Main fish species found in N Atlantic minke whales
Area
Herring3
Capelina
Barents Sea
Norwegian coast
Great Britain
Iceland
E Greenland
W Greenland
Newfoundland
Eastern US
X
X
X
X
X
X
X
X
X
X
X
Ammod. sp.a
Gadoidsb
X
X
X
X
X
X
X
X
Mackerelb
X
X
X
X
Sources: [3,47-50,53,54].
Greatestin quantity; bless in quantity.
a
Food and Feeding
Table 3 lists the favourite/common food (lst prey/2nd prey) of fin and sei whales.
Usually fin whales go for krill as the first choice, but they are also taking fish in
considerable amounts in certain areas, seasons or time periods [25,47]. The fish
species are then most often capelin (Mallotus villosus), herring (Clupea harengus),
sandlance or Ammodyte spp. The humpbacks feed even more on fish, while also
taking krill.
The sei whales eat nearly exclusively crustaceans [26], most often the smallest of
these, the copepods. The blue whales are also solely crustacean feeders [59], and are
Fig. 5. Sightings of blue, fin, sei and humpback whales on board vessels west and southwest of Iceland,
June-September 1979-1985.
434
even more specialised, nearly always preying on krill. The minke is perhaps the most
extreme opportunistic feeder of the N Atlantic balaenopterid whales, taking mainly
fish, but aiso krill, this all varying greatly between seasons and areas. The main species of fish found in the stomach of minke whales in different areas of the N Atlantic
are listed in Table 4.
In conclusion, it can be stated that there are generalists/opportunists as Mitchell
categorized it [47], the minke, humpback and fin whales forming that group; there
are the specialists, the biue and, sei whales. No doubt the position in the food web
and the degree of specialization counts very much in the whale's ability to cope with
variations in the availabiiity of different prey.
It is important for us to monitor the status of the principal species of fish stocks as
well as crustacean production in order to capture a picture of the situation the whales
are in at any given time. Many of the pelagic fish species undergo major natural
fiuctuations that can greatiy influence the livelihood of the whale popuiation in the
area, such as the capelin off Newfoundland [51] and off Iceland [46], or the herring
off Norway and Iceland. Off W Iceland herring comprised up to 30% of the food of
fin whales at the turn of the century (unpublished data), while in recent years there is
and 2000m depth contours shown).
435
a
Blue whale
Minke whale
Fig. 7. Sightings of bíue (a) and minke (b) whales west and southwest of Iceland June-September
1979-1985.
436
a
b
1951-1955
1971-1975
Fig. 8. Distribution of fin whales caught west an southwest of Iceland during the years 1951-1955 (a)
and 1971-1975 (b) (see legend for depth in Fig. 7).
437
no herring found in the stomachs of fin whales at Iceland, and in fact only some 3%
of stomachs during the summer contained remains of fish [52].
Temporal/Spatial Distribution
The migration time and spatial utilization of the different species also needs to be
considered, although general conclusions on this point are difficult to make. Figure 5
demonstrates the variations in rnigration behaviour of the different species off
Iceland which relate to temporal utilization of the space. Noteworthy, fin and sei
whales tend not to occur concurrently in great numbers and have a different peak
migration period in the area. Figure 6 shows the distribution of the two species in the
Icelandic NASS-89 survey that confirms this apparent tendency.
The baleen whales off Iceland also show considerable areal or depth preferences,
the blues, humpbacks aíid particularly minke whales staying close to the coast (Fig.
7), while fin and sei whales stay offshore, mainly outside the banks beyond 400500 m depth and as deep as 2,000 m. Although this may partly be explained by local
depletion of fin whales due to the whaling and possibly by invasion of blue and
Fig. 9. Distribution of catches of fin whales during the years 1904-1908 in the waters around Iceland.
438
humpbacks as time has passed since they were protected in the 1950s, fin whales
seem not to have inhabited the shallow waters in the postwar years (Fig. 8). Examination of catch data in eastera Iceland at the turn of the century shows whales in both
deep and shallow waters (Fig. 9). Off the US and Canadian coasts, however, the bulk
of the baleen whales are found within 100 or 200 m depth [56,58].
It seems thus that although species seem to have a preference for certain depth
intervals in each area, that may be shared with other species, the topography and
more generai ecological chara^teristics of the area are not less important, Baleen
whales seem to be rather eurytherm [59] and when for example examining satellite
images of temperature distribution and rorqual occurrence during the NASS-89 survey (unpublished data), the distribution of whales appears not to be critical with respect to temperature. More Hkely, ocean fronts, currents and upwellings, which give
support to great prey stocks, are influencing the distribution of the whales.
*
Conclusions
This brief examination of the status of the N Atlantic rorquals shows that they need
not be regarded as threatened or endangered by exploitation, although some species
are depleted locally in some areas. The minke and fin whales are both euryphagic
seasonal feeders that prior to exploitation and still today number in the 50,000100,000 and 100,000-200,000 range, respectively. Their opportunistic feeding habits
and choice for vast open ocean breeding areas seem to give rise to relatively large
stocks and to their apparent sustenance for high catches,
The near stenophagic crustacean feeding blue and sei whales seem on the other
hand to have had smaller historical stock levels of well within 20,000 animals. While
the sei whale is probably at present in the 10,000-15,000 range, the blue whale is
still at a low level, although increasing in some areas, such as off Iceland.
The humpback whale seems to be the species that historically occured in smallest
numbers, although the northwestern stock seems to be in a healthy state (increasing
by -10% per year). The strong recovery of the species in recent decades may be
linked with its euryphagous lifestyle. However, the relatively small population size
may also be correlated with its dependence on the rather limited coastal zones for
breeding.
It is likely that the stronger competitors, particularly fin, minke and humpbacks,
which all are to some extent fish feeders, will in the near future come increasingly
into conflict with fisheries. Therefore, it is important to intensify investigations that
may help us to evaluate the potential conflicts that may arise. In that context we need
to monitor the whale stocks and associated resources in order to model the interactions. This also requires some basic information on stock discreteness in order to
understand the recovery of depleted stocks or stock units. Here further genetic work
may be of help, while new technology, such as satellite tracking of the large whales
[60], may in the near future help us better to understand the movements, distribution
and behaviour of rorquals.
439
Acknowledgements
The author wishes to thank his colleagues at the Marine Research Institute,
Reykjavik, for help in preparing this paper, particularly Sverrir D. Halldórsson for
compiling catch data and Jóhanna Erlingsdóttir for help with analysis of catch data
and graphical support.
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