MASS STRANDINGS OF TOOTHED WHALES (ODONTOCETI)
AS A POPULATION PHENOMENON
D. E. SERGEANT
Fisheries and Oceans Canada, Arctic Biological Station, 555 Blvd. St.
Pierre, Ste. Anne de Bellevue, Quebec, H9X 3R4, Canada

CONTENTS

Abstract
Introduction
Three case histories
a. North Atlantic pilot whale Globicephala melaena in the northwest
Atlanticb. White-sided dolphin, Lagenorhynchus acutus in the northwest Atlanticc. Sperm whale Physeter catodon at New Zealandd. Conclusions
Distribution of mass strandings among the Odontoceti
a. Definitionb. Offshore and onshore mortalityc. Reporting bias
d. British strandings(i) Frequency of mass strandings(ii) Consistency of mass strandings through timee. Strandings in the southern hemispheref. Strandings in Florida
g. United States Eastern Seaboard
h. Geographical variation in mass stranding rate within a species
i. Individual species considered
Social structures compareda. Orcininae
b. Delphininaec. Phocaenidae
d. Sperm whale
Herd size
Age-specific mortality in Cetacea
a. Generalb. Mysticetes
c. Odontocetes
Reproductive cycles of Mysticetes and Odontocetes
Density-dependence
a. Time series within one populationb. Comparisons of two populations of the same species(1) by the same author(2) by different authors
Sci. Rep. Whales Res. Inst.,
No. 34, 1982, 1-47

2

SERGEANT

Proximate causes of strandings
Mass strandings as emigrations
Practical applications
Summary
Acknowledgments
References
ABSTRACT

From a study of Globicephala melaena and Lagenorhynchus acutus in thenorthwest Atlantic, and Physeter catodon at North Island, New Zealand, it isshown that mass strandings of Odontoceti occur at high population densities.
Moreover, within the range of a single species, mass strandings occur withinthe " core " of the range but not at its periphery. They occur at high frequency among species of the subfamily Orcininae. In Globicephala spp. andPseudorca crassidens the stranded herds include all sizes of animals of both
sexes, and these species are therefore highly social. Amongst most Delphininae, mass strandings occur at lower frequency. Herds consist of reproductively mature animals and calves, or of immatures, or solitary immatures
may strand. These species are therefore less social and, in captivity, an adultmale Tursiops truncatus is aggressive towards immature animals. The social
Orcininae nevertheless show greater sexual dimorphism than the Delphininae,
a more delayed sexual maturity and a lower frequency of males, features
usually associated with polygyny. The sperm whale Physeter catodon shows
frequent mass strandings of harem herds and occasional mass strandings ofadolescent males in warm seas, singleton strandings of solitary males in coolseas.
Age-specific mortality rates of Odontoceti and Mysticeti were studied
from length-frequencies of stranded as compared with randomly-captured
animals. Two Mysticetes (minke, humpback) showed a high mortality of
newly-weaned animals, and minke-a high mortality at greatest lengths,
among strandings. A weakly social Odontocete (Phocoena phocoena) showed
the first feature. Three Delphinid species showed evidence of more evenmortality through all ages. This feature is related to their sociality, including
long parental care. Nevertheless there is evidence for enhanced mortality ofimmatures of both sexes in species showing aggression by adults. There isalso evidence for density-dependent reproduction in several species of Odontoceti, though not so far in the most highly social Orcininae. Since most
mortality in these latter species is mass mortality, mass mortality must regulate their populations. Psychological mechanisms leading to mass strandings remain to be investigated, but could include stress. The animals about
to strand are regarded as being in a passive, moribund state, drifted to thecoastline by currents.
INTRODUCTION

The Cetacea are divided into two suborders which differ in many ways, not least
Sci. Rep. Whales Res. Inst.,
No. 34, 1982

MASS STRANDINGS OF TOOTHED WHALES

3

in size. The baleen whales or Mysticeti are all large (6 to 30 m adult body length)and feed by engulfing organisms much smaller than themselves which they trap
and strain with their baleen plates. The toothed whales or Odontoceti are smaller(2 to 15 m) and feed on fish or squids not so much smaller than themselves. Veryoften, active cooperation aids capture of their prey, a habit which may have resulted
in their commonly found and highly-developed social behaviour. Only among
the Odontocetes, especially in the most highly social families, do we find the very
curious tendency to come ashore and die in groups; the phenomenon of mass strandings. Mysticetes are found stranded en masse in only one circumstance, whenpressed by pack ice on to the shore, a phenomenon found also in Odontocetes thatlive in cold seas (Taylor 1957; Sergeant, Mansfield and Beck 1970). This seems
to be an accidental phenomenon, which does not necessarily imply social behaviour,
other than that individual whales may be feeding or travelling together.
Mass-stranded animals are not obviously diseased or in an abnormal bodystate, and the phenomenon has so far defied explanation (Geraci 1978). Observers
who have sought the cause at the level of the specific site and set of circumstances
have so far produced no convincing explanations. \Vhile the stranding animals
must be exhibiting abnormal behaviour, since it is almost always lethal, I havefound it useful to look first at the phenomenon at the level of the population.
Opportunity to do this came from the lucky chance that 30 years' data existed,from 1950 to date of writing (1981), on the abundant north Atlantic pilot whale
Globicephala melaena at Newfoundland. I also found some less complete data on
another delphinid, the white-sided dolphin Lagenorhynchus acutus in the northwest
Atlantic, and on the sperm whale Physeter catodon at New Zealand. I have also been
fortunate in that networks of published strandings have increasingly developed inrecent years, notably in France and the United States.
THREE CASE HISTORIESNorth Atlantic pilot whale Globicephala melaena in the northwest Atlantic.
Northern pilot whales have long been known to mass-strand in cool watersof the North Atlantic, from Cape Cod to Newfoundland, in Greenland, and from
France to Iceland (Figure 1). In several regions of their abundance, especiallyat the Faeroes and at Newfoundland, inshore fishermen have taken advantage ofthis tendency to drive the animals ashore (Mitchell 1975a).
Catching in Newfoundland was intensive, with an organised industry, from1947 to 1971 (Fig. 2). In the later years of this drive-fishery, annual catches became very low. It appeared that a population, of ill-defined extent, but probably
inhabiting the area east of Newfoundland, had been decimated; the total catchhad been approximately 54,000 animals (Mercer 1975). Figure 2 shows that highcatches at the outset, peaking in 1956, were steadily reduced, lesser fluctuations
being associated with variable abundance of the main food, the squid Illex illecebrosus, from year to year. There was, however, no overall decline of squid overthe period of decline of pilot whales.
a.

Sci. Rep. Whales Res. Inst.,
No. 34, 1982

4

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MASS STRANDINGS OF TOOTHED WHALES

5

Globicephala melaena
Newfoundland

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1960

1950
Fig. 2.

Globicephala melaena.

1970

1980

Catches and strandings at Newfoundland, 1948 to 1980.

1950's to early 1970's, they were still common off Nova Scotia (Fig. 3), and natural
mass strandings took place both on the mainland and at offshore Sable Island(Table 1). This adjacent popula,tion must have contributed to the rather rapid
recovery that took place when hunting stopped at Newfoundland, where by 1980
pilot whales were again abundant off the eastern coast (K. Hay, unpublished MS
1981 ).
In 1972 all whale hunting in Canada, with the exception of aboriginal hunting
in the arctic, was prohibited. In 1975, natural strandings of pilot whales began
to occur at Newfoundland (Mitchell 1977, Geraci and St. Aubin MS 1975; Table
1). Such had been the change in the world climate of opinion about Cetacea thatfishermen were employed to save the animals by towing them to sea. In 1978 twomass strandings occurred at Miquelon Island off Newfoundland's south coast, andone in Notre Dame Bay on the east coast. In 1979 one large stranding occurred on
the Burin Peninsula on the south coast. In 1980 strandings occurred on the south
and northeast coasts of Newfoundland, and one at Prince Edward Island (Table 1).
In 1981 there were two mass strandings in Newfoundland, one at the Magdalen
Islands, Quebec, four in Nova Scotia and Cape Breton Island, and one at Cape
Cod, Massachusetts. In 1980 K. Hay (unpublished MS 1981) estimated fromaerial survey at least 14,000 pilot whales in Newfoundland's eastern waters up to
150 km offshore, and animals were seen to enter inshore waters in large numbers
Sci. Rep. Whales Res. Inst.,

No. 34, 1982

SERGEANT

6

Globicephala melaena
o May-Oct.
• Nov.-April
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60°

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Fig. 3. Sightings and strandings (s) of G. melaena in the northwest Atlantic, 1957 to1972 (from Sergeant 1979).

in early September (M. Moore, pers. comm.). In 1981 large numbers agam
entered inshore waters in late summer.
Mass strandings had therefore begun, coming from a population which had
increased again and resumed its habit of moving inshore in late summer. A roughassessment of mortality due to mass strandings of G. melaena at Newfoundland in1975 to 1980 can be made from the mean number stranded (Table 1), which is
not less than 150 out of the population estimate of at least 14,000, or about 1%;
this assumes that all animals were vulnerable to this kind of death, and excludes
animals further than 150 km offshore. This is not a large proportion, but fromthe 1980 and 1981 data, mass strandings seem to be increasing in frequency andin area affected, if not in total number of animals dying.

b.

White sided dolphin Lagenorhynchus acutus in the northwest Atlantic.
Lagenorhynchus acutus was observed at Newfoundland (Sergeant and Fisher
Sci. Rep. Whales Res. Inst.,
No. 34, 1982

7

MASS STRANDINGS OF TOOTHED WHALESTABLE 1. MASS STRANDINGS OF PILOT WHALES IN EASTERN CANADA, 1957-1980
Sources: Sergeant, Mansfield and Beck (1970); Geraci and St. Aubin (1975, 1977),
Mitchell (1977, 1980, 1981, MS 1981).
Date

Lat.

Long.

Locality

Province/State

Number

VIII 1957
x 1959VIII 1960
XII 1967
24
IX 1975
28
XII 197626
IX 1978
19
3
x 1978VII 1979
16
x 198018
x 198025
9-18 XI 1980

49°51'
43°58'
44°
46°13'
48°35'
43°58'
47°
49°28'
46°54'
49°09'
47°15'
46°23'

57°48'
59°48'
66°10'
59°57'
53°30'
59°48'
56°15'
53°56'
55°33'
55°20'
55°30'
63°48'Cow Head
Sable I.
Yarmouth
Glace Bay, Cape Breton I.
Charleston, Bonavista Bay
Sable I.
Miquelon-LanglandeMusgrave Hbr., Notre Dame Bay
Pt. au Gaul, Burin Peninsula
Pt. Leamington, Notre Dame Bay
Grand Beach, Burin Peninsula
Bedeque Bay

Newfoundland
Nova Scotia
Nova Scotia
Nova Scotia
Newfoundland
Nova Scotia(France; S. ofNfld.)
Newfoundland
Newfoundland
Newfoundland
Newfoundland
Prince Edward Island

12 or more
18
58
15
300
130 or more
93+63
70
135
75
18
52

4
2

1957) and at New England (Schevill 1956) in the early 1950's and its noteworthiness, as well as the infrequency of previous reports, suggests that it was nota particularly abundant species in the northwest Atlantic at this time. The
same was true in the 1960's when Mercer (1973) recorded it in southern Newfoundland and far to the east of the island over deep water, and Sergeant, Mansfieldand Beck (1970) recorded it stranded on Sable Island. During the 1970's, however, it has very often been reported in the waters off New England and Nova
Scotia, and in summer in the Gulf of St. Lawrence (Katona, Testaverde and Barr
1978; Sergeant, St. Aubin and Geraci 1980; Testaverde and Mead 1980). Whilesome of this increase must be due to an increase in observers, it seems certain thata real increase of population has occurred. The aerial surveys carried out by the
University of Rhode Island's Cetacean and Turtle Assessment Program (CETAP;
see Hain, Edel, Hays, Katona and Roanowicz 1981) found Lagenorlrynchus acutus in1979 to be the most numerous cetacean along the northeastern seaboard of the
U.S.A., exceeding 10,000 individuals observed. The species occurred throughout
the Gulf of Maine, especially in the vicinity of Cape Cod and over Georges Bank,and south to waters off Chesapeake Bay.
During the 1970's there were two mass strandings of L. acutus: at Cape Cod
in May 1973 and in the northern Gulf of Maine in September 1974. There wasa third multiple stranding at almost the same site in the Gulf of Maine in September
1981 and also a small multiple stranding at Prince Edward Island in the Gulf of
St. Lawrence in November 1979 (Mitchell 1981). There have been numerous
single strandings in New England, Nova Scotia and the Gulf of St. Lawrence
throughout the 1970's. In September and October 1981, the species several
times entered the waters of Bedford Basin, Halifax Harbour, Nova Scotia. Thus
irregular mass strandings, and frequent single strandings, have been a feature of
the biology of L. acutus during the period of its population abundance in the northwest Atlantic.
Sci. Rep. Whales Res. Inst.,
No. 34, 1982

8

SERGEANT

1. Singleton strandings, Britain

North Atlantic Pilot Whale
G lobicephala melaena

N=65

10

5

10

•.

~......
200

300

400

,. •.·...

..

··-...........

500

600

700

800

Body length in cm.
Fig. 4.

Length frequencies of G. melaena, stranded naturally and driven shore.

Sci. Rep. Whales Res. Inst.,
No. 34, 1982

MASS STRANDINGS OF TOOTHED WHALES

9c.

Sperm whale Physeter catodon at New Zealand
At North Island, New Zealand, multiple strandings of harem herds of sperm
whales have repeatedly occurred up to at least 1972 (Gaskin 1968; Robson and
Van Bree 1971; Stephenson 1975). Best (1974, Figs.-1 and 11-2) shows that thesurrounding area of sea has one of the highest known densities of sperm whales,
and the highest known density of female sperm whales, in the southern hemisphere.
Conclusions
From the preceding review of three commonly-stranding species one may
conclude that mass strandings come from a dense population and not from a reduced one. In ecological parlance, they are either density- or resource-dependent(DeMaster 1981 ). As to the condition of naturally-stranded G. melaena, I am
indebted to K. A. Hay (in litt. 1979) and ]. Lien and staff (in litt. 1981) for
measurements and sexings of animals in three herds stranded at Newfoundland in1979 and 1980. The distribution of lengths (Fig. 4) differs little from length
frequencies which were obtained from large samples of driven animals in the 1950's(Sergeant 1962). The sex ratio is more variable, being low for males in the two
1980 samples, and high for a stranding at Sable Island in 1976 described by Geraci
and St. Aubin (1977), but with little difference if the samples are summed (Table2). The length-specific blubber thickness is comparable (Table 3). There is
therefore no obvious difference between the state of animals driven ashore by man
and those naturally stranded. Both sets of beachings result from the dense herds
which form up in the coastal waters when the population is abundant.
d.

TABLE 2.

SEX RATIOS OF NORTHWEST ATLANTIC PILOT WHALES,DRIVEN AND STRANDED. Date from Sergeant (1962)
Geraci and St. Aubin (1977), Hay (pers. comm.), and Lien (pers. comm.).

Sample

Date

Males

Females

Male ratio

DrivenStranded

1952-1957
1977-1980

1,825
141

2,778
173

.395
.450

TABLE 3. MID-LATERAL BLUBBER THICKNESS OF FEMALE PILOT
WHALES AT NEWFOUNDLAND, DRIVEN AND STRANDED
Data from Sergeant (1962, Table IV) and Hay (pers. comm.).
Date

SampleDrivenStrandedAugust
July

1952
1979

N

Range of bodylengths (cm)4074

185-470
206-470

Blubber thickness (cm)
Range

Mean

2.1-3.2
2.3-4.0

2.7
3.3

DISTRIBUTION OF MASS STRANDINGS AMONG THE ODONTOCETI

Definition
I define a mass- or multiple-stranding as one consisting of two or more animals,
other than the simultaneous stranding of a female and her calf.
a.

Sci. Rep. Whales Res. Inst.,
No. 34, 1982

10

SERGEANT

Offshore and onshore mortaliry
Although I have assumed up to now that all dead cetaceans arrive on the
coastline, this cannot be true. Skulls of pilot whales G. melaena are dredged bytrawlers from Georges Bank off New England (Lord and Chandler 1969). Thisbank is separated by channels from the land, showing that animals which die at
sea may sink there. Mass strandings, by definition, occur on the coast and it isdifficult to imagine mass, almost simultaneous deaths occurring at sea among aherd of Cetacea. If they do occur, no concentrations of carcasses have been reported at sea, to my knowledge, nor of a sequence of carcasses drifting ashore.
Mass strandings are of live animals, though singleton animals can arrive on thebeach dead or alive. That some dead singletons do not reach the shore could
bias upward the percent of animals recorded dead en masse. Other sources ofbias work in the same direction.
b.

Reporting bias
For most past reporting, it is likely that multiple strandings are over-reported
as compared with single animals because they are the more conspicuous phenomenon.
It is extremely hard to quantify this statement, but it seems likely that themore intensive the stranding network, the more this bias is removed. I have identified three relatively intensive stranding networks: that of Great Britain from 1913
to 1966, of France from 1971 to the present, and of the United States from 1976 to
the present. Since none of these is complete or perfect, not all bias has been removed. For France I compared reports of strandings of pilot whales in the period
1961-1960 (Duguy and Baudouin-Bodin 1965, Budker 1968 and Le Tallec 1968)
to those in 1971-1980 (Duguy and Budker 1972, Duguy 1973, 1974, 1975, 1976,
1977, 1978, 1979, 1980, 1981). In the earlier decade, there were reports of 310
individual G. mqlaena of which 303 (97. 7%) were mass-stranded. In the later
decade there were only 5 out 80 (6.25%) G. melaena mass-stranded. Clearly thechange cannot be due solely to a change in sighting intensity since the absolute
frequency of mass strandings of pilot whales decreased. This could be due, inpart, to chance: mass-strandings are rare events and more than a decade may benecessary to obtain an index of their frequency. Unfortunately, the density ofthe population involved may change during a decade (see section 4 d (ii), so thatsolution of the problem seems impossible There seems little left to do except touse the most complete data available, and to use the much more evenly distributed
singleton strandings, together with the mass strandings, as an index of natural
mortality rate, and therefore of abundance.
The classification of the Odontoceti as usually divided (Rice 1977, Fraser and
Purves 1960) is shown in Table 4.
c.

British strandings
Systematic observations of stranded Cetacea around the British coasts were
begun in 1913 and I have drawn freely on this long-continued record (Harmerd.

Sci. Rep. Whales Res. Inst.,
No. 34, 1982

II

MASS STRANDINGS OF TOOTHED WHALESTABLE 4.

SYSTEMATIC ORDER OF ODONTOCETI ACCORDING TO RICE (1977)
Genera including species known to mass-strand shown with an asterisk.

Ziphiidae

Tasmacetus* Mesoplodon* -?,iphius* Hyperoodon
Berardius
Physeteridae* Physeter
Kogia

Platanistidae
Stenidae

Phocaenidae
Delphinidae
Delphininae

Sotalia
Sousa* Steno* Phocoena
Phocaenoides* Delphinus* Grampus* Lagenorhynchus
Lagenodelphis* Stenella* Tursiops
Cephalorhynchinae
Lissodelphinae
Orcininae

Cephalorhynchus* Lissodelphis? Feresa* Globicephala
Orcaella* Orcinus* Peponocephala* Pseudorca

1929; Fraser 1934, 1946, 1953, 1974.) While it is impossible to define its completeness, the existence of a reporting network, together with a responsible central
museum over a long period of time, makes it a good starting point.
(i) Frequency of mass strandings:
As stated at the outset, there are no mass strandings of Mysticeti. The British
records give the number of individual Odontoceti in each mass stranding and also
the number of singleton strandings of each species. Table 5 summarises these data.
There are rare species, such as the arctic white whale Delphinapterus leucas andnarwhal Monodon monoceros, and the more southerly blue-white or euphrosyne
dolphin Stenella coeruleoalba, that give few records. The others can be divided intoa group of mainly mass-stranders, comprising pilot whale G. melaena, false killer
whale Pseudorca crassidens and white-sided dolphin Lagenorhynchus acutus and a groupof species stranding mainly as individuals, comprising killer whale Orcinus area,
several other dolphin species (Lagenorhynchus albirostris, Grampus griseus, Delphinusdelphis, Tursiops truncatus), bottlenose whale Hyperoodon ampullatus and common porpoise Phocoena phocoena. All sperm whales Physeter catodon stranded singly, althoughSci. Rep. Whales Res. Inst.,
No. 34, 1982

12

SERGEANTTABLE 5.

FREQUENCY OF MASS-STRANDED TO TOTAL STRANDED INDIVIDUALODONTOCETI BY SPECIES IN BRITAIN, 1913 TO 1966
Data from Harmer (1927) and Fraser (1934 to 1974).
Number of animals
Species

Delphinapterus leucas
Delphinus delphis
Globicephala melaena
Grampus griseus
Hyperoodon ampullatus
Lagenorhynchus acutus
Lagenorhynchus albirostris
Monodon monoceros
Orcinus orca
Phocoena phocoena
Physeter catodon
Pseudorca crassidens
Stenella caeruleoalba
Tursiops truncatussinglemultiple

total

121
66
63
61
24
78
2
36
596
13
5
3
175

14
429
4
11
67
9

135
495
67
72
91
87
2
42
631
13
232
3
189

Percent
multiple
(-)

6
35
227
14

10.4
86.7
5.9
15.3
78.2
10.3
(-)
14.2
5.5
0.0
97.8
(-)
7.4

the species is known to mass-strand elsewhere and has done so in western Europe(Boschma 1938). However, mainly solitary males reach northern seas, and singleton strandings are the rule also in Norway (Haug and Gulliksen 1981) and in
Newfoundland CJ. Lien MSS 1980, 1981).
(ii) Consistency of mass strandings through time:
Because of the uniquely long (53 year) period of the British strandings, it isinstructive to examine the consistency through time of mass-strandings of a species.
In Table 6 I have done this for 6 species, retaining the unequal time periods into
which Harmer and Fraser divided their reports. [To use decades, as S. G. Brown(1975) did, would not materially alter results.] The analysis shows that one species,
Pseudorca crassidens, stranded at high frequency during its short occurrence in seasaround Britain. Three species, Grampus griseus, Lagenorhynchus albirostris, andTursiops truncatus, mass-stranded at low frequency throughout the 5 episodes of time.
Two species remain. Lagenorhynchus acutus decreased markedly after 1932, whereas
Globicephala melaena increased markedly after 1937. Not unexpectedly, both species
TABLE 6. MULTIPLE STRANDINGS PER TIME PERIOD IN BRITAIN
Number of individuals stranded in multiple strandings and in total.
Years
Species

1913-1926(14)

1927-1932(6)

1933-1937(5)

1938-1947(10)

1948-1966(19)

G. melaena
G. griseus
L. acutus
L. albirostris

2/10
2/10
32/40
2/30

405/447
0/19
0/6
5/28

4/27

2/6
0/10
0/3
2/14
77/82
4/23

20/26
2/19
0/3
0/7

P. crassidens
T. truncatus

0/6
0/9
35/39
0/8
150/150
2/27

2/35

2/77

Sci. Rep. Whales Res. Inst.,
No. 34, 1982

;->·

l;<
;:i.

~

~

""""'

.._
~~

"° ~
v~~

~

~~

-

25

-

24
-

19
24
81
152
6
10
19

19
4
16
4
6
8
19

0
20
65
148
0
2
0

-

51.0

-

49

-

6

-

0
83.3
80.2
97.4
0
(20.0)
0.0

-

(0)
4.5
15.0
(100)
0

-

0.0
0
0
99.7

%M

17
16
15
859
1
2
22
20
3
6

T

0
0
0
856
0
0
1
3
3

M

-~-----

17
16
15
3
1
2
21
17

s

--- - - - - - - - - - - " -New Zealand
S. Australia

6
8

1
1
3
1
3
1
13
6

1

-

15

129
243

-

-

-

-

-

-

-

21
8

1
1
3
1
3
1
142
249

1

120
917

114
908

6
9

71.4
0.0

0
0
0
0
0
0
90.8
97.5

0

95.0
99.0

0

-

%M

T

-

M

1

s

12
1

1
3
(a few)

0

1
10

-

7

1

s

( )
12
1

-

3

1
10

250

-

0

%M

-

0.0

0
32
(high)

0

0
0.0

7
0
5 (high)

1
3

5

-

T

-

-

-

-

-

-

M

----------·-------- ------------------

Tasmania

7
1
30
1
22
6
32
10
6
26
28

-

19
16
28
12
1
3
32
17

s

-

1
3
3

17

-

20
194
391

-

-

25

-

-

-

-

0
0
114
1,769

M

%M

--~---

0.0
19
0
16
80.2
142
1,781
99.3
1 (0)
3
(0)
33
3.0
15.0
20
(100)
3
0.0
7
1
(0)
55 45.5
(0)
1
0.0
22
76.9
27
226 85.8
97.5
401
(0)
6
43
39.5
0.0
28

T

Overall

FREQUENCY OF SINGLE (S), MULTIPLE (M) AND TOTAL (T) STRANDED INDIVIDUAL ODONTOCETIIN NEW ZEALAND (Gaskin 1968, Fraser 1955), Tasmania (Guiler 1978) and South Australia (Aitken 1971)

Berardius arnouxi
Cephalorhynchus commersoni
Delphinus delphis
Globicephala melaena
Grampus griseus
Hyperoodon planifrons
Kogia breviceps
Lagenorhynchus obscurus
Lissodelphis peroni
Mesoplodon bowdoini
M. densirostris
M. grayi
M. hectori
M. layardi
Orcinus orca
Physeter catodon
Pseudorca crassidens
Tasmacetus
Tursiops truncatus
Ziphius cavirostris

Species

TABLE 7.

~

-""

rri

l:.'j

::r:
>
t""

~

t:I

::r:
l:.'j

>-1

>-1

0
0

>rj

0

rri

z>t:I
......
zCl

:;o

>-1

rri

rri

>
rri

SERGEANT

14

show high mass stranding rates with high overall stranding rates. None of thesedata support the idea that mass-stranding rates vary with time, in contrast to the
French data discussed earlier (section 4-c). The data do, however, support theconcept that populations of Odontocetes may change in abundance with time.

Strandings in the southern hemisphere
In amplification of the British stranding records, there is an excellent set of
tabulated data on strandings on New Zealand coasts, chiefly due to Gaskin (1968).
To these I have appended details given for Tasmania by Guiler (1978) and forsouth Australia by Aitken (1971), since all are cool-water coasts with much thesame fauna (Table 7).
In these seas Globicephala melaena and Pseudorca crassidens retain a very highpercentage of individuals mass-stranded (97.5-99.3%). Nearly as high are Physetercatodon with 85.8% of individuals mass-stranded, Delphinus delphis with 80.2% andOrcinus orca with 76.9%. High values are also found for lvlesoplodon grayi (45.5%),
Tursiops truncatus (39.5%) and Lagenorhynchus obscurus (15.0%). Other Ziphiidae(Berardius arnouxi, :(,iphius cavirostris and other Mesoplodon spp.) had no multiple
strandings, nor did the pigmy sperm whale Kogia breviceps.
e.

Strandings in Florida
Table 8 shows strandings from a well-documented sub-tropical region, thatof Florida with its extensive sandy coasts.
High multiple stranding frequencies are again shown by pilot whales, thistime the tropical short-finned species Globicephala macrorlryncha, and by false killer
whales Pseudorca crassidens, which are here within their normal range of distribution
at sea in the sub-tropical North Atlantic (Morzer-Bruyns 1969), Gulf of Mexico(Bullis and Moore 1956; Brown, Caldwell and Caldwell 1966) and Caribbean Sea(Caldwell, Caldwell, Rathjen and Sullivan 1971 ). Stenella longirostris shows a highmass-stranding rate (92. 7%) with two mass strandings, and Steno bredanesis with
but a single one. Both Tursiops truncatus and Kogia breviceps show singleton strandf.

TABLE 8. STRANDINGS IN FLORIDA
Sources: 1Moore, 1953; 2 Layne, 1965; 3 Caldwell, Caldwell and Walker, 1970; 4 Mead,
Odell, Wells and Scott, 1980; 1 0dell, Asper, Baucom and Cornell, 1980.
SingleGlobicephala macrorhynchal
Globicephala macrorhyncha2
Kogia brevicepsi,2Mesoplodon europaeus 1
Physeter catodon1,2
Pseudorca crassidenst,s,oStenella longirostris 4
Steno bredanensis2
Tursiops truncatus1, 2:(iphius cavirostris 1

6
12
5
3
5
13
3

Multiple

Total

%M

323
212

324
218
12

99.7
97.2
0.0

5
242
69
16
13

0.0
99.6
92.7
(100.0)
0.0
(0)

241
64
16

Sci. Rep. Whales Res. Inst.,
No. 34, 1982

MASS STRANDINGS OF TOOTHED WHALES

15

ings only; for other species data are scarce.

United States Eastern Seaboard
Six years' intensive observations, from 1976 to 1981, were available to me
for this region from the monthly reports of strandings given in SEAN (Smithsonian
Scientific Event Network).
The resulting summary of data (Table 9) refers to 32 species, such a large
fauna being the result of the wide geographical extent of the coast from a cold toa warm sea. Some cold-water species, e.g. Hyperoodon ampullatus, were absent,
others, e.g. Orcinus orca, were rare.
g.

TABLE 9.

UNITED STATES EASTERN SEABOARD. STRANDINGS, 1976-1981(Source-SEAN monthly reports).
1977-1981
Species
Total

Balaenoptera acutorostrata
B. borealis
B. edeni
B. musculus
B.physalus
Delphinapterus leucas
Delphinus delphis
Eubalaena glacialis
Feresa intermedia
Globicephala macrorhyncha
G. melaena
Grampus griseus
Hyperoodon ampullatus
Kogia breviceps
K. simusLagenorkvnchus acutus
L. albirostris
Megaptera novaeangliae
Mesoplodon bidens
M. densirostrisM. europaeus
M. mirus
Orcinus orca
Phocoena phocoena
Physeter catodon
Pseudorca crassidens
Stenella caeruleoalba
S. longirostris
S. plagiodon/attenuata
Steno rostratus
T ursiops truncatus
Ziphius cavirostrisMultiple

Multiple
Frequency

27

3
22
1

29
3

4

.138

42
21

28
18
5

.651
.429
.238

105
30
78

2

.066

19

.245

(12)
30
6
29

(.062)
.366
1.000
.133
.906

21
(7)*

(.913)(.013)

4

43

10

3
14

3
3
. 177
30
30

45
32

11

32
23

543
6

* Strandings of several animals close in space and time were reported but it could not be determined if
these were in fact multiple strandings.

Sci. Rep. Whales Res. Inst.,
No. 34, 1982

16

SERGEANTThe data allowed comparison with older data (Table 8) from parts of thesame seaboard. Thus, multiple strandings of Globicephala macrorhyncha fell from99.7-97.2% for earlier data from Florida to 65.1 % for the whole coast. At first,
I thought this was the result of more thorough examination of beaches for singletons. However, examination of SEAN records by state showed a geographicaltrend. A high multiple stranding rate was still recorded for Florida but northward this decreased to zero. I therefore analysed data for G. macrorhyncha and G.
melaena by latitude in order to quantify such geographical changes (section h).
The strandings from the east coast of the United States show a similar sequence of frequency of mass-strandings to that found in other regions. Ten species
show multiple stranding frequencies exceeding 10%, in order of descending rank:
Number ofSpecies

Pseudorca crassidens
Sterw rostratus
Stenella longirostris
Globicephala macrorhyncha
G. melaena
Physeter catodon
Lagerwrhynchus acutus
Grampus griseus
Delphinus delphis
Stene/la caeruleoalba

Frequency

I.OOO
.913
.906
.651
.429
.366
.245
.236
.138
.133

--~-----·-

animals

mass events

30
23
32
43
42
30
78
21
29
45

1
2
4

4
2
1
2

The three commonest species to strand: Tursiops truncatus, Phocoena phocoena
and Kogia breviceps, all showed extremely low rates of multiple strandings, as noted
earlier by Mead (1979). Indeed, for P. phocoena and T. truncatus, there was doubt
whether animals that stranded at the same time and place were, in fact, from thesame group. These data agree with those from Britain (Table 5), although mass
strandings of Tursiops truncatus were noted from the southern hemisphere (Table 7),
where perhaps a different species or stock with different social behavior is involved.

Geographical variation in mass stranding rate within a species.
This problem is sufficiently common to require resolution. In order to study
it I selected the east coast of North America where the excellent data from the
U.S.A. (from SEAN and from earlier fauna! studies) could be supplemented byless complete data from Canada. I selected the two pilot whale species G. melaena
and G. macrorhyncha because of their high mass stranding frequency, and listed all
strandings known to me by geographical position, and whether single or multiple,
then analysed these by half-degrees of latitude (Table 10).
·
The table shows that the cold-water species G. melaena mass-strands mainlyin the northern part of its range north of 46°N. Sable Island, located at 44°N,
accounts for most mass strandings south of this latitude. There are in modern
times few mass strandings at Cape Cod, at ea 41°30'N Lat., although the species wash.

Sci. Rep. Whales Res. Inst.,
No. 34, 1982

17

MASS STRANDINGS OF TOOTHED WHALESTABLE 10. STRANDINGS OF THE PILOT WHALES GLOBICEPHALA MELAENAAND G. MACRORH'YNCHA ALONG THE EASTERN SEABOARD OF
CANADA AND THE UNITED STATESThe records are summed by half degrees of latitude, and separated into single
and multiple strandings. Sources: Various, from varied starting
dates up to end 1981 in SEAN.

G. melaena
Latitude Zone

51°31'-52°
-5W
-51°
-50!0
-50°
-49!0
-49°
-48!0
-48°
-47! 0-47°-46! 0-46°-45! 0-45°-44! 0
_440
-43!
-430
-42!0
-42°
-4W
-41°
-40!0
-40°
-39! 0-39°-38! 0
-38°
-37!°
-37°
-36!0
35°31'-36°(I)

G. macrorhyncha

(2)

12
129
125
3
4
3

1
3
2

5
4
3

18
291
15

203

18

3
(I)*
1

Latitude Zone
39°00'-39!0(1)

35°31'-36°
-35!°
-35°
-34! 0-34°-33! 0-33°-32! 0
-32°
-31!°
-31°
-30! 0
-30°
-29!°
-29°
-28!0
-28°
-27!0
-27°
-26! 0-26°-25! 0-25°
24°00' -24!0

4
3

(2)

3

5
19
13+
26
3
30

2

150
72
57+
63
64

1
2

2

51

70+
12
50

(I) Number of animals stranded singly.
(2) Number of animals in multiple strandings.
* Species uncertain.

driven here in the past and herds are sometimes seen (e.g. by Starrett and Starrett
1955 ). Singleton strandings occur further south as far as about 36°N.
The warm-water G. macrorhyncha has its highest frequency of mass-strandingsin the southern part of its range. Mass strandings occur along the U.S. east coast
Sci. Rep. Whales Res. Inst.,
No. 34, 1982

18

SERGEANTfrom the Florida Keys at 24°30'N to about 34°'30'N. Northward only singleton
strandings occur, as far as 39°N. Thus, both species show a " core " area where
mass-strandings occur, and a peripheral area where they do not. I would deducefrom this that mass-strandings occur where the population of the species is high.
Mass strandings of G. macrorhyncha continue through the West Indies southward toat least Isla Margarita, Venezuela, at 12°N Lat. (Casinos and Bou, 1980). Thusin G. melaena there seems to be no northern limit to the mass strandings (in relation
to the population as a whole), in G. macror!ryncha no southern limit, suggesting that
it is inter-specific competition between the species that determines density of both.
I know no other pair of contiguous species where such an analysis is possible,
but note other cold-water species that show mass strandings towards the north oftheir range, e.g. Orcinus orca in Norway but not in Britain, see footnote this p.; otherwarm-water species that show mass strandings mainly in the south of their range, e.g.
Delphinus delphis in west Africa but not in Europe, see pp. 37 and 11 respectively.

Individual species considered.
The false killer whale Pseudorca crassidens is unusual in that virtually all of its
mortality on the coasts of Britain, New Zealand, Tasmania, South Australia, South
Africa (Smithers 1938) and Florida, occurs as mass strandings. It is a warm-water
species (Bullis and Moore 1956, Morzer-Bruyns 1969), that does not normally occur
round the British coasts; thus the mass strandings that occurred there in 1927, 1935
and 1936 were in the nature of immigrations followed by mass mortality. However,its mass strandings in Florida occur within the species' normal range. As its dietconsists of large fish and squids, including tuna which it frequently takes offlongline hooks (Moore 1953, Brown, Caldwell and Caldwell 1966, Mitchell 1975), we
have the impression of a top carnivore exerting pressure on its food supply.
In tropical seas, the short-headed electra dolphin Peponocephala electra has massstranded in eastern Australia (Dawbin, Noble and Fraser 1970), the New Hebrides(Rancurel 1974) and Japan (T. Kasuya, pers. comm.). Analysis of some of theserecords by Perrin (1975) gives a very high frequency (about 95%) of individuals
mass-stranded. The genus is placed within the Orcininae as is another tropicalgenus Feresa, of which we know little. We now have three genera from among
this subfamily: Globicephala, Pseudorca and Peponocephala, which show high frequencies of multiple strandings. The killer whale Orcinus orca has shown a low
incidence of mass strandings in Britain\ but a high incidence in New Zealand.
Killer whales mass-stranded in British Columbia before live captures for aquaria
started in 1964 (Ford and Ford 1981). Carl (1946) and Pike and MacAskie (1969)
together record 39 animals stranded in the period 1941-1967, of which 36 or 92.3%
represented multiple strandings.
Among the typical dolphins (Delphininae) many species have been recordedI.

1

None were recorded by Harmer or by Fraser from 1913 to 1966, but Turner (1871) described a drive-huntof 18 Orcinus orca at Bressay Sound, Shetland in February 1871, which makes it seem likely that mass
strandings might occur to the north of Britain. A pod of 14 animals indeed stranded at Lofoten, Norway
in June 1981 (Haug and Sandnes, 1982).

Sci. Rep. Whales Res. Inst.,
No. 34, 1982

MASS STRANDINGS OF TOOTHED WHALES

19

with a high frequency of strandings, yet there is considerable variability in thisfeature; e.g. Delphinus delphis and Tursiops truncatus may show a low incidence in
one region and a high incidence in another. There is also a high incidence of
strandings in the pelagic long-snouted dolphin Stenella longirostris in Florida. The
species Stenella coeruleoalba and S. attenuata are driven ashore in large groups byfishermen in Japan, which makes one believe that they would mass-strand there
naturally, if not being hunted.
Among the porpoises Phocoenidae, the harbour porpoise Phocoena p!wcoena
normally strands singly or in groups of 2-4 animals. It is known to be weakly
gregarious, with schools of 9 animals or less observed by Gaskin, Arnold and Blair(1974).
There are about 20 known mass strandings of Physeter catodon in the world in
the last 300 years and more than half of these have come from New Zealand (Anon
1979). I have also heard of two unpublished mass strandings (in the Seychelles
and Falkland Is.) since beginning this study. Most mass strandings are of harem
herds (Robson and van Bree 1971, Stephenson 1975). Considering that the main
mass of harem herds of sperm whales live in the tropical oceans, which are grossly
under-reported for Cetacea (consider the late discovery of tropical genera such as
Feresa, Peponocephala and Lagenodelphis), the scarcity of reported mass strandings ofPhyseter is not surprising. At high latitudes most strandings of sperm whales areof single adult or subadult males. The pigmy sperm whale Kogia breviceps seems
always to strand singly or in numbers not exceeding three, suggesting a group size
no larger than a family.
Mass strandings are also known among several genera of the Ziphiiidae: e.g.
a herd of 25 Mesoplodon grayi at the Chatham Is., New Zealand (Gaskin 1968);
a herd of 4 :(iphius cavirostris at the Netherlands Antilles; and records of Hyperoodon
ampullatus in Britain (van Bree and Kristensen, 1974). There are no known mass
strandings from among the freshwater Platanistidae, nor from among the small
group Cephalorhynchinae, but we have noted one of three animals from among
the Lissodelphinae (Lissodelphis peroni at New Zealand, Fraser 1955).
Considering the high incidence of mass strandings among members of theOrcininae it is worth looking at the biological characteristics of the subfamily, as
compared with those of members of the typical dolphin subfamily Delphininae.
There are now sufficient studies oflife histories of a number of genera to make sucha comparison possible. The social structure of the sperm whale Physeter is also
well known.
SOCIAL STRUCTURES COMPARED

Orcininae
As Norris and Dohl (1980) have previously observed, mass strandings occur
especially in the group Orcininae of the Delphinidae, which contains the genera
Pseudorca, Orcinus, Orcaella, Globicephala, Feresa, and Peponocephala. Of these only
Globicephala is well known bionomically, Orcinus is beginning to be so (Bigg 1981),
a.

Sci. Rep. Whales Res. Inst.,
No. 31:, 1982

SERGEANT

20

and knowledge on Pseudorca must be assembled from the few data of various authors.
I studied the social structure of G. melaena from a drive fishery in Newfoundland in the 1950's (Sergeant 1962), having a large number of animals available toexamine. Main features relevant were: (I) a marked sexual dimorphism, males
reaching 20 ft (6 m) in length compared with 16 ft (5 m) for females; (2) an extremely delayed sexual maturity for males-16 ft (5 m) length and 12 years of age
compared with 12 ft (4 m) and 6 years of age for females; (3) a higher male mortality rate throughout life. The last two features resulted in a low ratio of adultmales to adult females. Most importantly (4), the composition of 91/92 schools
was homogeneous, consisting of all sizes of both sexes of animals. The only atypical
herd was a group of adult males with a few very large females. I deduced thatthe species was exceptionally social, with no signs of aggressiveness, there being nosegregation of size groups associated with aggression.
McCann (1974) found tooth scars or scratches on the body of almost all
Odontocete genera studied which he attributed to various behaviour patterns,
including play between younger animals and aggression by adult males. In driven
G. melaena I found fresh scratches on all animals, which I attributed to the panicof the drive, since I could find no old scars. Norris and Prescott (1961) found
tooth scratches on young male G. scammoni and not on adult males, leading Norris
Pseudorca crassidens

clcl

10

N=126

5

...

20

!i!!i!

QI .

N=152

.Q

E
::s

z 15

10

5

3 0

350

400

450

500

Length in cm.
Fig. 5. Length frequencies of naturally stranded male and female Pseudorca
crassidens.

Sci. Rep. Whales Res. Inst.,
No. 34, 1982

MASS STRANDINGS OF TOOTHED WHALES

21(1967) to deduce that these were produced by schoolmates in play. Thus I supposethat tooth scratches are not necessarily an index of aggression. McCann notedthat Ziphiids have deeper scars than most Delphinidae and he believed that suchless social species show more aggression.
Information on Pseudorca crassidens comes from samples examined by Hinton(1928), Fraser (1936), Peacock, Comrie and Greenshields (1936), and Yamada(19S6) from animals stranded in Britain; by Smithers (1938) from strandings inSouth Africa; by Scott and Green (197S) from strandings in Tasmania; and by
Odell, Asper, Baucom and Cornell (1980) from strandings in Florida. Comrie
and Adam (1938) examined reproductive tracts collected by Peacock et al. (1936).
Figure S shows length frequencies which indicate clearly that the species is sexuallydimorphic. Extreme recorded lengths in all these strandings are males S89 cm
and females 489 cm, or a ratio of about 1.2: I, which compares with 1.3: 1 inextreme and peak lengths for large samples of G. melaena (Sergeant 1962). If one
omits the length frequencies of Scott and Green (197S), who examined part of thesame set of females twice, the sex ratio is about S3.S% females for a sample size of
198. Lengths at first maturity of female Pseudorca were well-studied by several
authors, as follows:
Body length (cm)Hinton 1928
Fraser 1936
Comrie and Adam 1938
Harrison, Brownell and Boice 1972
Scott and Green 1975
Odell et al. 1980
Highest/lowest

largestimmature

smallest mature

418
284
293
337
358
418

427 (pregnant)429 (several corpora albicantia)
425 (9 corpor~ albicantia)
414 (large corpus luteum plus corpora albicantia)
360 (pregnant)
440 (8 corpora albicantia)
360

This analysis gives a range of 360 to 420 cm for length at first pregnancy,
a wide spread. One wonders if all workers measured length in the same way, or
whether the populations differed in size. For males, we know only that a large
male S20 cm in length was mature, with testis in active spermatogenesis weighing
8.2 kg (Odell et al. 1980), and that a male 366 cm was immature, while males
SI I, S30 and S34 cm in length were mature with testis volumes SOX 16X8 cm orlarger (Fraser 1936). Thus, we can only say that male Pseudorca crassidens mature
at body lengths of S 11 cm or less. Since the animal measured by Fraser as S 11 cm
in length had a testis significantly smaller in volume (SOX 16x8 cm) than those of
S30 and S44 cm in length (60 X 20 X 16 cm and 62 X 22 X 11 cm) it is possible that
it was just mature; this compares with a S20 cm male P. crassidens measured by
Odell et al. (1980), the testis of which weighed 8.2 kg (see also Sergeant 1962,
Fig. 19 for a curve relating testis length to body weight in G. melaena). If P.
crassidens males mature at about SOO cm body length, as compared with about
400 cm for females, then the ratio of numbers of mature females to mature males
from the data of Fig. S is about 90/34 or 2.6S: 1, a ratio comparable with that
Sci. Rep. Whales Res. Inst.,
No. 34, 1982

SERGEANT

22
TABLE 11.
Species
Delphininae
Delphinus delphis
Stenella attenuata

Tursiops truncatus

RATIO OF ADULT LENGTHS OF ODONTOCETES (cm)Region of Study

Peak

Extreme

M

F

M

F

Ratio
M/F

170

170

205
200

200
190

1.02
1.05

46

270

250

1.08

120
16

250
254
260

250
252
245

1.00
1.01
1.06

944 822

1.15

610
590

1.15
1.21
1.20
1.44

Sample
size

4,217
3,504

Black Sea
E. Tropical
Pacific
FloridaN. W. Atlantic
Tursiops truncatus
Tursiops aduncus
S. Africa
Lagerwrhynchus acutus N. W. Atlantic
Orcininae
Orcinus orca

567

Globicephala melaena
Pseudorca crassidens
Physeter catodon

TABLE 12.

655

609

890

780

15.8

Length (cm)Age (yr)

M

F

M

F

170
195
245
230

158
181
235
210

12
13
(10)
12
12
19

579
488
487
366
12m 8.5m

TABLE 13.

Delphininae
Delphinus delphis
Stenella attenuata
Orcininae
Orcinus orca

Globicephala melaena
Pseudorca crassidens

Nishiwaki & Handa
1950
Bigg 1981
Sergeant 1962
(seep.21)
Lockyer 1976

Ratio
lengths

Authors

9
12
(9)

1.08
1.08
1.05
(1.11)

Sleptsov 1941
Perrin, Coe & Zweifel 1976
Sergeant, Caldwell & Caldwell 1973
Sergeant, St. Aubin & Geraci 1980

8
6-8
9

1.18
1.33
2.1

Bigg 1981 and in litt.
Sergeant 1962
Best 1974

SEX RATIO OF ODONTOCETESOverallSpecies

Sleptsov 1941
Perrin, Coe & Zweifel
1976
Sergeant, Caldwell &Caldwell 1973
Hohn 1980
Ross 1977
Sergeant, St. Aubin &Geraci 1920ATTAINMENT OF SEXUAL MATURITY OF ODONTOCETES

Species
Delphininae
Delphinus delphis
Stenella attenuata
Tursiops truncatus
Lagenorhynchus acutus
Orcininae
Orcinus orca
Globicephala melaena
Physeter catodon

503
490

Authors

MatureNo. of No. ofmales females

Male
ratio

No. of No. ofmales females

Male
ratio

Authors

2,079

2,138

0.493
0.449

890

1,179

0.435
0.43

Sleptsov 1941
Perrin, Coe & Zweifel 1976

247

320

0.436

60*

72*

0.455

92

106

0.396
0.465

418
90

1,364
34

0.234
0.265

Nishiwaki & Handa 1958;
Bigg 1981
Sergeant 1962
(See discussion)* excluding juveniles and calves.
Sci. Rep. Whales Res. Inst.,
No. 34, 1982

MASS STRANDINGS OF TOOTHED WHALES

23

found in G. melaena. Unfortunately the variance of estimate of this ratio is high as
between samples: using Scott and Green's (1965) mean female maturity length of348 cm it would be 128/34 or 3.8: 1, using Fraser's (1936) value of 423 cm it would
be 68/34 or 2: 1 (Fig. 5 ).
For Orcinus orca, Nishiwaki and Banda (1958) and Bigg (1981 and in litt.)
provide good data on body lengths of adult males and females, allowing calculation
.45
.40
0

~... Ul .35.... iii
Cl):s E .30

"O

<t

.25

0

~... UlCl).50=~ iiiE .45
Cl)

>

.40

0

.s

1.3

.... Cl):!:o E""'.!2 ~ 1! 1.2
ia)( c,a:: Cl)] 1.1

0

i 1.3

0

..c 1.2

I

~

~...

•)

Ul-

....Cl
c:

1.1

2

3

Delphininae

4

5

6

7

Orcininae

Fig. 6. Summary of information on sexual dimorphism within the Delphinidae.
I. Delphinus delphis, 2. Tursiops truncatus, 3. Stenella attenuata, 4. Lagenorhynchus acutus,
5. Orcinus orca, 6. Globicephala melaena, 7. Pseudorca crassidens. (The ordering ofspecies within each subfamily was random.)Sci. Rep. Whales Res. Inst.,
No. 34, 1982

24

-SERGEANTof the degree of sexual dimorphism, which seems to be somewhat less than in
Globicephala and Pseudorca. There is less good information on lengths and ages at
sexual maturity. Data are summarised in Tables 11 to 13 and Figure 6. One
gains the impression that Orcinus in some ways is an intermediate between thestrongly sexually dimorphic Orcininae and the weakly dimorphic Delphininae (seebelow).
For Peponocephala electra, data are as yet too few to assess the degree of sexual
dimorphism in length and at sexual maturity with any accuracy (see Bryden,
Harrison and Lear 1977).

Delphininae
Good information on the bionomics exists for several genera. Sleptsov (1941)
studied large numbers of Delphinus delphis in the Black Sea, this species proving toshow very little sexual dimorphism in either body size or length at sexual maturity.
Perrin, Coe and Zweifel (1976) studied equally large numbers of Stenella attenuata
in the eastern tropical Pacific with rather similar results. There is some information
for the species Tursiops truncatus (Sergeant, Caldwell and Caldwell 1973, Hohn
(1980); and T. aduncus (Ross, 1977), summarised in Tables 11 and 12.
In Lagenorlrynchus acutus (Sergeant, St. Aubin and Geraci, 1980) mass stranded
herds were found to be composed of adults, mostly females, together with young
calves. Singly-stranded animals by contrast were found to be made up chiefly ofadolescents. This kind of heterogeneity could account for much of the variability
which we have found in the frequency of mass strandings in various members of
the Delphininae, as discussed in section 4 above.
In both Stenella attenuata and S. coeruleoalba driven ashore in Japan, abundantdata show the existence of juvenile schools and adult schools, with the juveniles
rejoining the adult, mating schools at puberty (Kasuya, Miyazaki and Dawbin
1974; Miyazaki and Nishiwaki 1978).
A valuable study on living animals, which indicates the behaviour that may
be involved in such segregation, comes from the work of Tavolga and Essapian(1957) and Tavolga (1966) on the former captive colony of Tursiops truncatus at
Marineland, St. Augustine, Florida. This colony in the 1950's was not subjected
to heavy pressure to perform in public; it was large (13 animals) and composed ofvariably sized animals. Its behaviour was thus probably close to that of a wild herd.
In this colony a dominant male drove off three immature males and maintained
a harem structure of females and calves. There was also a dominance hierarchyof adult females, although other pubertal and older females took much care of
the calves. Another study, from the USSR, gave similar findings. Shurepova(1973) states that a large male in a group of 11 captive animals "had the dominating position in the school: it grabbed fish from the others during feeding, chased
young females which shied away from its mating advances, chased young males,
bit them, beat them vigorously with its tail and snout and so on."Aggressiveness and dominance hierarchies could give rise to the segregation of
immature animals seen among the Delphininae, and we have seen, at least inb.

Sci. Rep. Whales Res. Inst.,
No. 34, 1982

MASS STRANDINGS OF TOOTHED WHALES

25

Lagenorhynchus acutus, that it is often these immature animals which strand singly.
Indeed such animals may attach themselves to a group of one of the social species,
such as Globicephala melaena (Sergeant and Fisher 1957) or Orcinus area (Sergeant,St. Aubin and Geraci 1980 fide R. D. Elliot). Hence, probably, arises the lower
frequency of mass strandings among the Delphininae. But of course in all species
sick and injured animals can strand singly.
In summary the Orcininae are more sexually dimorphic than the Delphininae,
males having a greater excess of asymptotic body length over females. There isalso a greater delay in attainment of male sexual maturity, and with this delay
comes a greater male mortality, resulting in a lower overall male sex ratio than in
the Delphininae.
All these features are usually associated with polygyny and male aggression insea mammals (e.g. in elephant seals, fur seals and the sperm whale). Yet, paradoxically, the polygynous Orcininae are the more highly social of the Delphinidae,
with a more homogeneous herd composition and with a higher mass stranding rate.
Thus polygyny is not necessarily associated with male aggression.
Phocaenidae:
Phocoena phocoena shows a larger size of females than males (Mohl-Hansen
1954 ). It seems likely that this is due to the need to produce a neonate which islarge enough to be viable in cold water (Kanwisher and Sundnes 1965). In
support of this, a larger member of the family Phocoenidae, Phocoenoides dalli, hasa male asymptotic length about 1.05 times that of the female (207 and 198 cm,
Kasuya 1978), as typical for Delphininae. I have noted previously (Sergeant 1969)
that Phocoena phocoena has a high metabolic rate throughout life, as determined fromits food consumption and heart weight/body weight ratio, and that the same istrue for Phocoenoides dalli. The high activity appears to be an adaptation to life
in cold water with a thin blubber layer.
c.

Sperm whale
The sperm whale Physeter catodon is a well-researched species because of its
commercial importance. Because of the migrations of males to high latitudes wealready know that its sociality is not total. A number of mass-strandings havebeen well-documented (e.g. by Robson and van Bree (1971) and by Stephenson(1975) in New Zealand), and these are usually harem herds. It seems likely fromthe records of Gilmore (1957, 1959) and Bryant (1979) in the Gulf of California
that herds of adolescent males may also strand. This distribution of strandings
would accord with the types of sperm whale schools classified by Gaskin (1970)
and Best (1979), which include bachelor schools of young males. As the males
mature, they become less and less social and some become the harem bulls, loosely
attached to the schools of females and calves, others the northern wanderers. Sexual
maturity in male Physeter is much delayed as compared with the female (Best 1969,1974), sexual dimorphism is marked and harem-masters are intolerant of other
adult males. The social structure of the sperm whale therefore comes closer to thatd.

Sci. Rep. Whales Res. Inst.,
No. 34, 1982

26

SERGEANTof the Delphininae than of the highly social Orcininae, which fits the sperm whale's
lower frequency of mass-strandings to total strandings.
HERD SIZE
Herd size is extremely difficult to define. As Norris and Dahl (1979) and others
have shown, there is a whole range of groupings within a species (pod, herd, school)from the family up to large schools with varying cohesion and varying stability.
The basic subgroup does not vary very much and is thus not a useful measure ofsociality. Thus for Pseudorca crassidens, which we would judge from its stranding
behaviour to be a species travelling in large aggregations, Morzer-Bruyns (1969)
has this to say from observations of animals feeding at sea:
" The normal school of Pseudorca crassidens contains 20 to 30 animals divided
into "family" groups of four to six animals, covering an area of about 0.5 mile indiameter."
About the only generalisation possible is that inshore species e.g. Sousa plumbea
have only small school sizes, while in offshore species, e.g. Delphinus delphis, Stenella
coeruleoalba, large aggregations can occur (Saayman, Bower and Tayler 1972,
Saayman and Tayler, 1979); but the basic unit remains small, a family of females
and calves with loosely attached males of all ages older than calves in Stenella
longirostris (Pryor and Kang 1980 ), much as in Tursiops truncatus (Tavolga 1966 ).
In both these species in the wild groupings may have a very temporary, changing
existence (Norris and Dahl 1979, Wursig and Wursig 1979).
There seem to be two features which must be separated in considering therelation of mass strandings to school size: 1. Species such as Phocoena phocoena,
Kogia breviceps and perhaps also some of the Ziphiidae (Morzer-Bruyns 1968) havesmall herd size; that is, one scarcely or never finds large aggregations. These show
mass strandings rarely because they are rarely in groups; 2. Social species may
have aggressive adult males, which split the herd into harem (or reproducing)herds and the rest, chiefly immature (occasionally adult) males, and immature
females. Harem herds tend to mass-strand, presumably because females andcalves are closely associated and females are rarely aggressive (Tavolga 1966 for
Tursipos truncatus). The rest tend to strand singly because they are weakly associated, so mass stranding frequency is reduced.
For Globicephala melaena I noted (Sergeant 1962) that the mean number ofanimals in groups driven ashore was about 85, the same as in natural massstrandings reported around the world; however the mean number for animals
observed far to sea was about 20. This could be due to aggregation of herds as
they increase in density in inshore waters. For Orcinus orca stranded herd sizes
recorded are: in British Columbia 20, 11 and 5 (Carl 1946); 10 for an ice-stranded
herd in Newfoundland (Dearden 1958); 19 for a herd stranded in Norway (Haugand Sandnes 1982), for an overall mean of 14. This compares with means of
10.5 and 11.4 for observed pod sizes in the southern and northern components of
the killer whale population observed in the sounds of British Columbia andSci. Rep. Whales Res. Inst.,
No. 34, 1982

MASS STRANDINGS OF TOOTHED WHALES

27

Washington by Bigg, MacAskie and Ellis (MS, 1976).
AGE-SPECIFIC MORTALITY IN CETACEAa.

GeneralIt is rather generally assumed that age-specific patterns of mortality in Cetacea
are similar; that Odontocetes can be lumped together with Mysticetes, in spite of
their very different social structure. I shall here show that this is not the case,and the evidence I shall use is again from strandings.

b.

Mysticetes

British strandings again prove useful and the source is a compilation by Fraser(1953) of strandings of the minke whale Balaenoptera acutorostrata on British coasts,
reproduced here as Figure 7. Fraser showed a peak stranding length of 16 ft (5 m)and one again at the greatest lengths attained. The latter peak, one must assume,contains a good many estimated lengths. However the peak at 16 ft must be
measured or estimated accurately and I believe it is significant that this is thelength at which weaning occurs (Jonsgaard 1951). In other words, minke whales
strand (die) especially at the length of first independence, and again in old age.
These peaks do not represent the normal occurrence of these length-classes in
the North Sea, as can be seen from data (1951) on length-frequencies of catchesfrom different areas around Norway given by Jonsgaard (1951). For areas suchas Lofoten, immature whales were commonly hunted, but in Jonsgaard's area 1(Skagerrak and the eastern North Sea), catches consisted mainly of adults (Fig. 7).
Thus the evidence suggests that newly weaned animals strand selectively.
For a long time I was unable to obtain information on mortality patterns ofother Mysticetes owing to the paucity of stranding information. However, recently
there became available information on the length frequency of humpback whales
Megaptera novaeangliae caught in nets at Newfoundland (Perkins and Beamish 1979,
Lien MS 1981), and further information on some natural deaths and several accidental deaths from gill-netting and a ship collision on the U.S. east coast (Mead
12

I 10

Minke B. acutorostrata
20

"O

~

8

,.

6

!'!

4

~

g

!

·.......

...
w n

u

m m w

~

M

~

u

w

Length in feetFig. 7. Solid line: mortality by length for minke whales B. acutoroslrata stranded on
British coasts. Dotted line: catches by length in the North Sea.

Sci. Rep. Whales Res. Inst.,
No. 34, 1982

SERGEANT

28(a) Netted and stranded, 1978-1980
16

D Nfld.

12~ E'n U.S.A.

8

N=80

4
6

7

8

9

10

11

12

13

14

15

(b) Caught with minimum length limit, Nfld., 1969-1971

N=41

12
8
4

6

7

8

9

10 11 12 1.3
Length in m.

14

15

Fig. 8. Lengths of humpback whales M. novaeangliae caught in nets (a) and harpooned at sea with a lower length limit (b).

1980). Both sets of data come from the same population, which migrates between
the Caribbean in winter and the Gulf of Maine and eastern Newfoundland insummer (Kellogg 1929).
Analysis of these data (Fig. 8) shows that the dead humpbacks, many of whichwere imperfectly measured because of their entrapment in floating nets, werelargely immature animals, 6 to 12 m in length. Weaned immatures, about 9-10
m in length, greatly exceeded younger calves and older animals, though sometimes
an adult female entrapped itself with its calf. The lower frequency distribution
in the Figure essentially shows the length of adults, upwards of 12 m, from thispopulation; it is derived from a selective and specially licensed catch taken in
1969-1971 off eastern Newfoundland (Mitchell 1973). Thus, mainly immature
humpbacks die in nets, owing to their inexperience. I infer that immature humpbacks probably have a higher death rate than adults to other forms of hazard whichmight be considered more "natural" than nets. Putting minke whales and humpbacks together, I would expect that Mysticetes as a whole show similar patterns ofmortality. Allen (1980) can throw little light on the subject, doubtless becauseof the imposition of a lower size limit on whaling for all species except the minke
whale.

Odontocetes
Natural mortality rates of Odontocetes are profitably examined from strandings, owing to the large numbers of animals involved and to some precision of size
estimation. For several genera (Phcooena, Delphinus, Tursiops, Globicephala) somecomparison is possible between length frequencies of stranded animals and nonselective length-frequencies coming from fisheries made by mass-netting or driving.
c.

Sci. Rep. Whales Res. Inst.,

No. 34, 1982

MASS STRANDINGS OF TOOTHED WHALES

29

Phocoena phocoena
100

50

I
,,

40

!j.

i

]

c

~
i;j

30

0

~.,

~
~ 20

l"

"

C"

....2!

10------------100

80

120

140

160

180

length in cm

Fig. 9. Common porpoise P. phocoena. Length frequencies of animals stranded in
Britain (solid line) and taken by driving in Denmark (dotted line).

Phocoena phocoena (Fig. 9). Size frequency of strandings in the U.K. (fromFraser 1934 to 1974) is compared with a revived drive-fishery in Denmark during
the second world war (from Mohl-Hansen 1954). The size range of the animals
is the same, as might be expected from the nearness of the two samples. The maindifference is a peak of stranded animals at 4 ft (120 cm) which seems to be real,in spite of rounding of estimates of size in strandings. This size corresponds toanimals aged about 1 year, which is also the age at weaning (Fisher and Harrison1970). Thus it is possible that Phocoena phocoena endures a somewhat higher mortality rate at weaning than at younger or older ages, just as do minke whales.
The distribution of older deaths seems to be proportional to the abundance ofanimals.
Delphinus delphis (Fig. 10). The distribution of stranded animals comes from
British strandings (Fraser, op. cit.); that of unselected hunted animals from Soviet
fihseries in the Black Sea (Sleptsov 1941 ). Black Sea D. delphis constitute a dwarf
race (Kleinenberg 1956), although exceedingly abundant. Thus, the peak of adultlengths is very different. However there is no sign of a peak of juvenile deaths ineither graph.
Tursiops truncatus (Fig. 11 ). The sample of stranded animals again comesfrom Britain. The very small sample of driven animals comes from Cape Hatteras,
North Carolina, as described by True (1890). From what is known about relative
sizes of T. truncatus from Europe and North America (Hohn 1980), adults from thesecond area should be considerably smaller. There seems to be no difference inthe distribution of deaths.
Globicephala melaena. If one compares driven and naturally-stranded animals
from Newfoundland, there are no differences in the size frequencies (Fig. 4).
Owing to the high percentage of multiple strandings there are only small numbers
Sci. Rep. Whales Res. Inst.,
No. 34, 1982

SERGEANT

3030Delphinus delphis

N=128

25
20
15
10

120

100

140

180

160

200

220

240

260

N =2079 d'cf
=2138 'i?'i?

600

r:!cl'

./

f

!

E 500

:I

z

'i?'i? ..........

\

160

180

400

300

200
............•······
100............
IOO

120

140

200

220

Length in cm
Fig. 10. Length frequencies of saddleback dolphin D. de/phis stranded in Britain(above) and taken in net fisheries in the Black Sea (below).

of measured, singly-stranded animals from Britain. Thus, although there may bea tendency for neonates and old animals to die selectively in these strandings, thenumber of animals involved must be small.
In summary, the porpoise Phocoena phocoena may show raised juvenile mortality,
Sci. Rep. Whales Res. Inst.,.No. 34, 1982

MASS STRANDINGS OF TOOTHED WHALES

31

Tursiops truncatus: Strandings in Britain

20

N=175

10

.c

E

Drive-in catch, Cape Hatteras, 1886

N=25

z
5

400
Length group (cm)Fig. 11. Length frequencies of bottlenosed dolphin T. truncatus stranded in Britain(above) and taken in a drive fishery at Cape Hatteras (below).

but the three Delphinid genera examined show no sign of this phenomenon. Theirnatural mortality, as measured by their strandings, appears to occur in proportion
to the abundance of animals of different sizes in the population. If this is so,then the Delphinidae (at least) must have evolved adaptations to reduce juvenile
mortality. What these adaptations might be involve a comparison of Mysticete
and Odontocete reproductive cycles (see next section).
However, many weaned, immature dolphins remain independent of the reproductive herds: in Lagenorhynchus acutus they may travel as singletons or in loose
immature herds (Sergeant, St. Aubin and Geraci 1980); in S. coeruleoalba they may
keep together in semi-permanent herds (Miyazaki and Nishiwaki 1978), and perhaps in Stenella attenuata also when not aggregated by purse-seining (Pryor andKang 1980). Perrin and Powers (1980) found a higher incidence of skull lesions
caused by the nematode Crassicauda sp. in juvenile than in older Stenella attenuata;highest rates were at 2-8 years, which are just those animals expected to be inseparate schools. A higher mortality is implied because of the permanent scars
left by the parasite on the skulls. Therefore in this species, there is evidence froma unique source for higher immature than adult mortality; there are not sufficientSci. Rep. Whales Res. Inst.,
No. 34, 1982

32

SERGEANT

strandings of the same species to make a comparison by the method we have used
for the more inshore species.
REPRODUCTIVE CYCLES IN MYSTICETES AND ODONTOCETESAs Laws (1959) made clear, the length of gestation in most Balaenopterid whales is
one year. This feature involves an accelerated foetal growth rate in late pregnancy.
Coupled with it is a short lactation period of 6-11 months (Gambell 1973). Since
pairing occurs in winter, and a migration to high latitudes in summer, the two
features appear to ensure that birth will occur in the winter, and independence inthe following summer, so that the lactating female and young animal are loosed
upon a rich food supply. The whole process of gestation and lactation is extremely
rapid for such large animals.
The Odontocetes by contrast have gestation lengths which increase with thesize of the animals, from 10-11 months in Phocoena phocoena (Fisher and Harrison1970), 11-12 months in genera such as Stenella (Perrin, Coe and Zweifel 1976;
Kasuya 1976) and Delphinus (Sleptsov 1941), to 16 months in large Delphinidae
such as Globicephala (Sergeant 1962) and 14-15 months in the Monodontid genus
Delphinapterus (Sergeant 1973). The sperm whale Physeter catodon also has a gestation of 16 months (Best 1968).
The lactation period also increases with body size, being 8 months (as short
as in a Mysticete) in Phocoena phocoena (Fisher and Harrison 1970), ranging up to
two years in Tursiops, Globicephala, etc. From Tursiops truncatus in captivity it is
known that, though solid food may be taken at 6 months, lactation may continueintermittently to 2 years (Tavolga and Essapian 1957).
Care of juveniles continues during this period or beyond (Tavolga 1966),
and adult females ("aunties") other than the mother may share this task. Caregiving or succorant behaviour is well-known in many Delphinids, in sperm whales
and in Ziphiids (Caldwell and Caldwell 1966; Best 1979; Benjaminsen andChristensen 1979). It is little wonder, then, that juvenile mortality is reduced
among Odontocetes.
We may suppose, therefore, that one major direction of evolution in Odontocetes has been towards increasing the length of the reproductive cycle coupledwith an increase in size. The resulting lowered reproductive rate is offset bya decrease in juvenile mortality. One may ask, therefore, if density-dependent
control of reproductive rate still occurs in the Odontoceti.
Mysticeti (Balaenoptera sp.) exhibit a shorter period between pregnancies, andage at sexual maturity falls, when populations are reduced (Gambell 1973 ).
Attempts to find the same phenomenon in Odontoceti have been more recent andhave shown less clear-cut results. Let us start with the examination of a time series
within one population which has been reduced by hunting-or the reverse, apopulation which has been allowed to increase by relaxation of hunting-and pass
to comparisons of adjacent populations of the same species believed to exist at
higher and lower population densities.
Sci. Rep. Whales Res, Inst.,
No. 34, 1982

MASS STRANDINGS OF TOOTHED WHALES

33

Density-dependence:a. Time series within one population
In 1952 to 1954 and again in 1957 I measured the reproductive rate in unselected samples of Globicephala melaena in search of a change of reproductive rate
(Sergeant 1962, Table XIX). I failed to find such a change (Table 14), there
being a decrease rather than an increase of pregnancy rate and no change in lactation length. Yet we now know (Fig. 2) that 15% of the population had been removed at the mid-point of the first sample, 50% at the beginning of the second.
At the time, I put the failure to find a change in reproductive rate to the short time
period between samples and to a possible lag in the system, since female pilot whales
take 6 years to reach sexual maturity. This still seems a reasonable explanation.
However, had I returned to resample, it might not have been possible to obtainadequate further data owing to depletion of the population (Fig. 2).
Perrin, Coe and Zweifel (1976) studied the reproductive rate of a populationof the spotted dolphin Stenella attenuata in the eastern tropical Pacific from 1968 to
1973. Perrin, Miller and Sloan (1977) continued these studies up to 1975.
Reproductive rate was best calculated as the proportion of females in the population, times the proportion of females reproducing, times the annual pregnancy rate;
this was 0.146 in 1968-73, 0.14+0.008 in 1973, 0.135+0.008 in 1974, 0.140+0.007
in 1975 and 0.140+0.004 in 1973-75, thus showing no significant change in 7 years.
In 1974 this population was at or near its maximum productivity, with a kill of
about 80,000 to 100,000 occurring annually in a purse-seine fishery for tuna (Perrin,
Smith and Sakagawa 1975). Kills had been higher in 1971 to 1973 and the population was believed to have declined, though the amount was unknown.
Length oflactation period was 11.2 months in 1968-1973.
Henderson, Perrin and Miller (1980) calculated a reproductive rate of about
0.12 for northern S. attenuata from the eastern Pacific in 1974-1978. Length oflactation increased steadily from 8 to 36 months between 1973 and 1978. They
noted a "dramatically decreased (porpoise) kill over the same period" in tunapurse-seines. These data would tend to suggest that length of lactation can be very
sensitive to exploitation rate in Stenella spp., given a long enough time for study.
Here it increased, making the reproductive rate fall, as the rate of exploitation
slackened.
TABLE 14.

REPRODUCTIVE RATES OF PILOT WHALES THROUGH TIME
Data from Sergeant (1962, Table XIX).
1952-1954

1957

FemalesMaturein active reproduction in oestrus
or ovulated
pregnant
lactating
Percent animals removed from population
Sci. Rep. Whales Res. Inst.,
No. 34, 1982

No.

%

No.

%

415
395
35
140
220

100
95.2
8.4
33.7
53.0

114
105
16
29
60

100
92.l
14.0
25.4
52.6

15

50

SERGEANT

34

TABLE 15.

INCIDENCE OF FEMALE SPERM WHALES IN EARLY AND LATEPREGNANCY FOR TWO TIME PERIODS
from Best (1980).

No females examined
Percent in early pregnancy
Percent in late pregnancy
Percent with large corpus luteum but no foetus
Total

1962-1967

1973-1975

1,029
17.1
4.5
2.2
23.8

1,127
20.7
3.1
6.2
30.0

Kasuya (1976) studied Stenella coeruleoalba off eastern Japan. The population
was depleted to about half its initial size between about 1945 and 1972-73. The
mean calving interval beeame shorter, changing from 3.2 years in 1952 to 2.4 years
in 1972-73, with a decrease in lactation period of from two years to less than one
year. This is the same process in reverse as seen by Henderson et al. (1980) in
S. attenuata.
Best (1980) studied female sperm whales off Natal, South Africa (Table 15).
Sampling was done twice with an interval of 10 years, during which period thefemale population was believed to have been halved. The results are a little hardto interpret, because a category "females with large corpus luteum but no foetus "might have been either females with large foetuses lost in towing, or recently calved
females. The total number of pregnant females, if this category is included, increased from 23.8% to 30.0%, if excluded, from 21.6% to 23.5%. Even thougheach sample exceeded 1000 animals, Best regarded them as barely adequate. It
cannot be said that this example is clear-cut, but the conditions of sampling were
not ideal.
Judging from the studies reported above, about a decade of studies is neededon a population subject to a changing exploitation pattern, probably because of
the long time required to attain sexual maturity in the genera studied.
b. Comparisons of two populations of the same species(1) by the same author
Perrin and Henderson (1979) compared reproductive rates in two populationsof spinner dolphins Stenella longirostris which had been exploited at markedly different rates. There was no difference in gross reproductive rates (proportion of
females times proportion mature times pregnancy rate) although there were differences in these individual components of reproduction.
(2) by different authors
These comparisons are listed separately since methodology cannot always beclosely compared.
Perrin, Coe and Zweifel (1976) and Perrin, Miller and Sloan (1977) examined
the reproductive rate of Stenella attenuata in the eastern tropical Pacific Ocean, apopulation which had sustained a heavy fishery. Kasuya, Miyazaki and Dawbin(1974) examined the reproductive rate of the same species off Japan, from a popuSci. Rep. Whales Res. Inst.,
No. 34, 1982

MASS STRANDINGS OF TOOTHED WHALESTABLE 16.

35

WHITE WHALES: A COMPARISON OF LIFE HISTORYPARAMETERS IN TWO POPULATIONS
from Brodie 1971, Sergeant 1973.

Character
Sample size:
HuntedNettedTotal
Maximum no. tooth layers
Maximum no. corpora albicantia
Mean age at sexual maturity (tooth layers):
malefemaleGestation
Lactation
Frequency of calving

Western Hudson Bay
257
293
550
50
15
16-18
8-13 (10)14 months
20 months
1 in 3 years

Cumberland Sound
17
107
124
40
9
16
9-11 (10)14.5 months
2 years
1 in 3 years

lation which had only been exploited for about 10 years at a low rate (6000 caught
in 1963-1973), both fisheries exploiting populations of unknown size. Reproductive
rate was calculated in a different way from Perrin et al., but the data allow recalculation by Perrin et al.'s (1976) method: thus percent females (193/323)=0.60;
percent reproductive females (124/193)=0.64; and annual pregnancy rate=0.256,
including simultaneously pregnant and lactating females assigned half to eachcategory. This gives a reproductive rate close to 0.100 as compared with Perrin
et al.'s rate of 0.140 to 0.145. The difference appears to be due to a lower rate ofovuiation prolonging lactation in the population from the Japanese seas.
Brodie (1971) examined the reproductive rate of a population of white whales
Delphinapterus leucas as Cumberland Sound, Baffin I. This population has undoubtedly been under heavy exploitation for a long time (Brodie, Parsons and Sergeant1981) and should show the maximum reproductive rate of which the species iscapable. Sergeant (1973) examined a population of white whales in western
Hudson Bay which he later believed (Sergeant 1981) to be in a balanced though
not low state of exploitation. No differences are apparent (Table 16) in mean age
at female sexual maturity, nor in frequency of calving. It is unfortunate that thewestern Hudson Bay population was not sampled for these features around 1950
at the outset of the industrial fishery at Churchill, Manitoba.
Thus the majority of the comparative population studies show a difference inreproductive rates between lightly and intensively exploited populations. Sergeantand Brodie's populations of white whales may have been exploited at similar rates,or the parameters may not have been measured sufficiently accurately. No change
in mean age at first reproduction has yet been demonstrated in any of the withinpopulation or between-population studies; the changes are all changes in length oflactation. Probably the mechanism is between a lactation anoestrus in females
within a denser population, and ovulation during lactation in better-fed females
within a heavily exploited population. There is therefore a density-dependent
reproductive rate within an Odontocete population. It remains to be shown
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SERGEANT

whether or not a calf suckled for only 8 months by its mother has a higher mortality
rate than a calf suckled for 24 months. Probably, the length of lactation has
little significance, since later lactation is intermittent (Tavolga and Essapian 1957;
Tavolga 1966) in Tursiops truncatus; and the existence of" aunties" from among
younger or anoestrous females safeguards an older calf when the mother has a newone. Therefore, a higher reproductive rate is probably not necessarily associatedin Odontoceti with a higher calf mortality rate. It is only when the adolescents
form herds on their own account that their mortality rate may rise, as demonstrated
by the high singleton death rate of adolescent Lagenorhynchus acutus, and by Perrin
and Powers (1980) in Stenella attenuata from infections with Crassicauda (p. 31).
These mortalities might be density-dependent. However the existence of social
mechanisms maintaining calves within the herds will continue to keep calf mortality
low.
Thus in Odontocetes survival of juveniles is maximixed by a high degree ofparental care and sociality. Old animals too may benefit from care-giving behaviour. The only mechanism remaining to increase mortality among adolescentsis aggressiveness of older animals, and this mechanism disappears within the mostsocial species.
A population however must expand slowly if it is to maintain itself. Eventually it will come up against an environmental barrier which will limit its continued growth. Such barriers must in some way involve either food or space.
Mass strandings represent a high proportion of the total mortality of the social
species and must therefore constitute the main mechanism by which populations ofthese species are limited.
.
It is just possible that the true Delphinidae can be divided into two groups:
the Delphininae with about a year's gestation, and a normally long lactation
period which can be shortened in the way described; and the Orcininae with a long
pregnancy, and a long lactation not susceptible to being shortened. This dichotomy, if real, would fit with the subdivision into frequent mass-stranders and others.
Unfortunately there are not yet enough data on pregnancy length or on densitydependence in lactation length to know if the suggested division is real. For instance, the lengths of pregnancy and of lactation of Orcinus orea seem to be unknown.
The same is true for Pseudorca crassidens and Peponocephala electra though it should be
only a matter of time before a sufficient number of animals are examined in mass
strandings of these species.
PROXIMATE CAUSES OF STRANDINGS

Examination of mass-stranded Cetacea has shown no obvious bacteriological,
helminthological or other pathological findings including those of neuropathology(Hall, Gilmartin and Mattson 1971, Hall and Schimpff 1979). Yet single-stranded
animals are frequently highly infected either with trematodes in the liver and brain(Ridgway and Dailey 1972), or with nematodes in lung alveoli or head air sinuses(Delyamure 1955, Dailey and Perrin 1973, Dailey and Stroud 1978). Since
Sci. Rep. Whales Res. Inst.,
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MASS STRANDINGS OF TOOTHED WHALES

37

many delphinds are known to have heavy infections of nematodes in air-sinuses orlungs, it is important to obtain quantitative information on numbers of parasites
in normal and stranded animals. The only such data known to me are due toDelyamure (1955), who examined D. delphis from the Black Sea. Seven naturallystranded animals had heavy infections of the nematode Skjrabinalius cryptocephalus in
their tracheae and bronchi with pus blocking the small respiratory passages, together with hyperemia, and destruction of the bronchial mucosa. Examination of
the lungs of 604 D. delphis caught in the fishery showed that 175 (28.9%) had S.
cryptocephalus infections with from 3 to 227 nematodes present. Infections increased
with age, though a few adults were self-cured with healed lesions. Evidence for
acute bacterial infections tends to come more from captive animals where the watermust have high concentration, e.g. of Staphylococcus aureus (Colgrove and Migaki1976). In summary, nearly all singly-stranded animals are diseased or highly
parasitised, but mass-stranded animals are not. Geraci and St. Aubin (1979)
found that the adrenal cortex of mass-stranded adult female Lagenorhynchus acutus
showed cysts characteristic of prolonged stress. This interesting observation needs
to be followed up, and it is particularly important to examine the cortex fromfree-living controls to compare with those of the stranded animals.
Most authors (e.g. Dudok van Heel 1962) have placed emphasis on the immediate circumstances of mass-strandings, such as the configuration of the coastline,
but I do not believe these features to be important. Certainly, many strandings
occur on shoaling coasts. Thus Maigret (1979) notes a concentration of mass
strandings of Globicephala macrorhyncha, Delphinus delphis, Tursiops truncatus and Steno
bredanensis on the north side of the Cap-Vert peninsula, Senegal. Six of sevenstrandings occurred in May and June when there are trade winds from the northwest and a surface current setting from the northeast on to the shore. Anothersuch region may be the neighbourhood of Wellfleet, Cape Cod, where, according
to SEAN reports, Lagenorhynchus acutus strand repeatedly as well as other species
such as Globicephala melaena occasionally. I believe that the animals can best be
regarded as " drift bottles" which will inevitably tend to arrive on such sites.
Passivity of the animals is stressed in many observations, e.g. the pilot whales
G. macrorhyncha which stranded repeatedly along the west coast of Florida in August
1971 (Fehring and Wells, 1976); and the 30 Pseudorca crassidens which were filmed
coming ashore at the Dry Tortugas, Florida in June 1976 (Porter, 1977). Moreover,as is well known, a herd once so stranded is likely, if taken out to sea, to strand
again (Fehring and Wells, 1976). I therefore judge it more important to examine
the internal causes of the observed passivity.
MASS STRANDINGS AS EMIGRATIONSConsidering the extralimital occurrences of mass-stranding Pseudorca crassidens at
Britain and probably also in south temperate seas one would expect mass-strandings
within an increasing population to be accompanied by some range expansion.
This does not seem to have occurred as yet with Lagenorhynchus acutus which has
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SERGEANT

merely increased within its known range in the northwest Atlantic, there being noevidence of a concomitant increase of the same species within the northeast Atlantic(Evans, 1980, 1981 ). Thus for this species, the scope of a population increase so
far been has quite restricted. It will be worthwhile to continue to monitor it.
PRACTICAL APPLICATIONSSergeant (1977) used the frequency of singleton strandings of fin whales on the
coasts of Britain over time, expressed as an order of rank within all Cetacean
strandings, in order to determine changes of the population as a result of commercial
whaling.
If frequency of mass and singleton strandings varies directly with the abundance of an Odontocete species, prone to mass strand, then occurrence of mass
strandings could give additional evidence on the state of such a stock. This could beof practical value for a species such as the sperm whale.
In the eastern North Atlantic, Fairlie (1981) notes that there has been no decline in recent years in the (singleton) stranding rate of sperm whales in Ireland,though all Mysticetes have declined in their stranding rate at Britain, this correlating well with the reduction of their populations by hunting (Sergeant 1977,1979). From the evidence given I would deduce that sperm whales have not declined in the eastern North Atlantic. Mitchell (1975b) demonstrated a cross-overfrom Nova Scotia to Spain from tagging, showing that eastern and western North
atlantic sperm whales are not isolated from one another. Allen, Compton-Bishop
and Gordon (1979) demonstrated a reduction in catch and increase in catch perunit effort of sperm whales at the Azores between the 1950's and 1970's, due tomassive human emigration and reduction in number of whaling stations. Catches
there in the 1950's were about 400 per annum; in the 1970's about 200 and falling.
The main catch of sperm whales is now at Iceland (about 200 per annum) where
only adult males are available. I conclude that the North Atlantic population of
sperm whales is probably not reduced nor are present catches excessive. The mainconcern appears to be whether the falling mean size of males due to selective
catching of them(Martin 1981) could affect reproductive success.
In the eastern North Pacific there have been 4 recent mass strandings: in the
Gulf of California in 1953 and 1954 (Gilmore 1957, 1959) and in 1979 (Byrant
1979; SEAN Bull.,January 1979), and on the coast of Oregon in June, 1979 (Anon,
1979, SEAN Bull. June 1979). The Mexican strandings were all or mainly of
males; the Oregon stranding was of a harem herd. Pike and MacAskie (1979)
show that harem herds normally extend as far north as off the coast of British
Columbia between spring and autumn. Knowledge of stock identity of sperm
whales in the North Pacific Ocean is still incomplete, but Bannister and Mitchell(1980), basing themselves partly on unpublished tagging recoveries due to D. W.
Rice, and partly on discontinuous distributions of sperm whales at sea, accept theexistence of an eastern North Pacific or American stock. Horwood and Garrod(1980), examining the state of this stock from catch per unit of eftort and cornSci. Rep. Whales Res. Inst.,
No. 34, 1982

MASS STRANDINGS OF TOOTHED WHALES

39

parisons of pregancy rate over time, conclude it is in a healthy state.
At New ,Zealand, there were mass strandings of sperm whales in 1970 (Robson
and Van Bree 1970), 1972 (Robson 1978) and 1974 (Stephenson 1975)with apparently none subsequently (fide SEAN Bulls. to December 1981). Best (1976),
table 2) shows that stock reductions of sperm whales in the southern hemispheregenerally and the New Zealand area in particular (Best's division VII) were severe
for exploitable males, reducing them to about half initial stock size by 1972.
However it had been computed that the stock of mature females was reduced only
to ea 90% of initial sock. Clearly, the females are the main animals which appear
in strandings of harem herds.
I conclude that analysis of mass and single strandings of sperm whales may
throw light on the status of stocks additional to that which may be obtained byother methods, and that this finding makes important the widespread dcoumentation of their strandings.
SUMMARY

1. In eastern Newfoundland from 1950 to 1971 an abundant oceanic populationof pilot whales Globicephala melaena was hunted down to low catch levels. Its cumulative catch gave an initital population estimate of 54,000. It was protected in1972. The population was estimated in 1980 to number at least 14,000 animals.
Recovery of the population was presumably accelerated by immigration fromareas where the species had remained common. From 1975, mass-strandings
began at Newfoundland and neighbouring areas and continue at the rate of between
100 and 200 animals a year.
In the area of the northwest Atlantic extending from the Gulf of Maine to
the Gulf of St. Lawrence, white-sided dolphins Lagenorhynchus acutus have become
very common since about 1970 with at least 10,000 individuals present in the U.S.
sector of this range. During the 1970's there were two mass strandings and many
singleton strandings of this species.
At North Island, New Zealand, harem herds of sperm whales have massstranded on many occasions, and this area contains one of the highest densities ofsperm whales and especially females in the southern hemisphere.
From these examples, mass strandings can be associated with dense populations of Odontocetes.
2. From an examination of the literature, mass strandings are the main sourceof coastal mortality (perhaps exceeding 90% of deaths) in a group of species (Globicephala spp., Pseudorca crassidens, Peponocephala electra) belonging to the subfamily
Orcininae of the family Delphinidae. Mass strandings represent a lower percentage
of deaths in typical dolphins of the subfamily Delphininae.
3. The frequency of mass strandings can vary geographically, whatever the rateof overall incidence. A study of the incidence of mass strandings by latitude in
the allopatric species Globicephala melaena and G. macrorhyncha along the eastern
North American seaboard showed that each species has a wide" core" area where
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SERGEANT

both mass and singleton strandings occur, located to the northward in G. melaenaand southward in G. macrorhyncha. At the edges of the respective ranges, where
the two species are in contact, only singleton strandings occur in both species.
This evidence is taken to mean that mass strandings occur in that part of the range
where the species is abundant.
4. A comparison of life histories shows that Globicephala melaena among theOrcininae exhibits greater sociality, expressed by greater homogeneity of herds,
than do Tursiops truncatus and Lagenorhynchus acutus among the Delphininae. Pilot
whales also show absence of aggression by adult to immature a~imals. This
greater sociality occurs among member species of the Orcininae in spite of a greater
sexual dimorphism, delayed sexual maturity and higher male mortality rate,features typical of territorial marine mammals of other groups.
Physeter catodon exhibits mass mortality among harem herds, but solitarystranding mortality of subadult and adult males at high latitudes, consistent withthe known aggressive behaviour of harem bulls.
5. No evidence exists for emigration among mass-stranding populations of Odontoceti, except for the examples of Pseudorca crassidens stranding in cool seas.
6. In Mysticetes, high mortality occurs among young animals just after weaning,
and at extreme old ages. This pattern remains among Odontoceti in the weaklysocial Phocoena phocoena, but is not observed from length frequencies of stranded
Delphinus, Tursiops and Globicephala. However high adolescent mortality remains
due to aggression in Lagenorfrynchus acutus and Stenella attenuata at least. It is argued
that sociality reduces the mortality of juveniles of the social Odontoceti, probably
through such features as the long lactation and other features of parental caresuch as succorant behaviour. Since length of lactation has been demonstrated to
be density-dependent among members of the Delphininae and among sperm whales,social behaviour must be important in reducing calf mortality. When other sourcesof mortality are reduced, social mortality expressed in mass strandings becomesthe major source.
7. The main purpose of the study has been to emphasise internal factors andespecially population density as responsible for mass strandings and to discount
immediate external factors such as coastal configuration. What the internal factors
might be leading to morbidity of herds is unknown but it is suggested that studies
of stress might be valuable, with the need to study controls from among normal
animals at sea.
ACKNOWLEDGMENTS
I am greatly indebted to Jon Lien, Memorial University of Newfoundland, and to
Keith Hay, Department of Fisheries and Oceans of Canada in St. Jonh's for unpublished information on mass strandings and on net catches of humpback whales.
Michael Moore and Paul Brodie also provided useful observations. Especially I
wish to thank the compilers of the Scientific Event Alert Network (SEAN) of theSmithsonian Institution, Washington, D. C. and many observers on the United
Sci. Rep. Whales Res. Inst.,
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MASS STRANDINGS OF TOOTHED WHALES

41

States east coast for the opportunity to analyse their carefully documented observations of Cetacea.
M. Didier Marchessaux, Aix-en-Provence, France, recently a visitor to Ste.
Anne de Bellevue, Hr. Chr. Hysing-Dahl, Bergen, Norway and especially Dr.
Margaret Klinowska, Department of Anatomy, Cambridge University, provided
me with many valuable references. Robin Allen and Michael Scott of the Intertropical Tuna Commission, William F. Perrin and Douglas DeMaster of Southwest
Fisheries Center, La Jolla, and James Mead and Aleta Hohn of the National
Museum of Natural History, Washington, criticised earlier versions of the paper;
Dr. A. W. Mansfield-a later draft. They are not responsible for its imperfections.
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Duauv, R., 1978. Ibid. VII Annee 1977. Ann. Soc. Sci. Nat. Charente-Maritime, VI(5): 333-344.
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