DISTRIBUTION, SEASONAL MOVEMENTS, AND
ABUNDANCE OF PACIFIC WHITE-SIDED DOLPHINSIN THE EASTERN NORTH PACIFIC*STEPHEN LEATHERWOOD, RANDALL R. REEVES,
ANN E. BOWLES, BRENT S. STEWART, AND
KIMBERLEY R. GOODRICH
Hubbs-Sea World Research Institute, 1700 South Shores Road, San Diego, CA 92109, U.S.A.

ABSTRACTAll relevant data available through 1979 on Pacific white-sided dolphins,
Lagenorhynchus obliquidens, in the eastern North Pacific were examined for patterns of distribution, movements, and abundance. In waters east oflongitude180° white-sided dolphins occur from about latitude 20°N to latitude 61°N,
on the continental shelf and offshore. They are present in the mouth of the
Gulf of California and in inland marine waters of Washington, British Columbia, and Southeast Alaska, but apparently they do not usually enter the Bering Sea or regularly penetrate Prince William Sound. They appear to becontinuously distributed across the temperate North Pacific. Seasonal movements cannot be unequivocally demonstrated in any area, although observed
peaks in numbers suggest regular seasonal influxes into waters north oflatitude
40°N in spring and summer and between latitudes 25°N and 30°N in fall.
Evidence for seasonal presence is most convincing off Southern California,
between latitudes 30°N and 35°N, where peak numbers occur inshore in
November through April. No population estimate is possible with currently
available data. On aerial surveys in the well-studied Southern California
Bight, Pacific white-sided dolphins were observed to occur at a peak frequencyof 1.42 individuals per nm flown; because perpendicular sighting distanceswere not recorded consistently for these sightings, we are unable to estimate
survey strip-width or translate the frequency of individuals sighted into an
estimate of density. In a region off Baja California, Pacific white-sided dolphins have been estimated to occur in densities up to 0.06 individuals per
nm 2 • They appear to be the second or third most abundant delphinid inSouthern California waters in winter, after common dolphins and northern
right whale dolphins. The Pacific white-sided dolphin may be the mostabundant delphinid in the temperate eastern Pacific. The 1300 herds sighted
averaged 88 individuals (s= 366). Herds were significantly larger in southern* Information in this report was assembled for the U.S. National Marine Fisheries Service, Southwest Fisheries Center, La Jolla, California under Contract NOAA/79-ABC-00201 and was originally released as
Leatherwood, S. and W. A. Walker. 1982. Population biology and ecology of the Pacific white-sided
dolphin, Lagenorhynchus obliquidens, in the Northeastern Pacific. Part I. Distribution, seasonal movements
and abundance, with a bibliography and summary of specimen materials. NOAA, NMFS, SWFC
Administrative Report LJ-82-18C. 76 p.
Sci. Rep. Whales Res. Inst.,

No. 35, 1984, 129-157

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PACIFIC WHITE-SIDED DOLPHIN

131

since 1966 (Walker, 1975; National Marine Fisheries Service (NMFS), Washington, D. C., 1982, unpublished records; Reeves and Leatherwood, 1984). Small
numbers have been taken incidentally in fishing operations for tuna, anchovy, andsalmon in subtropical, midtemperate and northern temperate waters, respectively(U.S. Dept. of Commerce, 1981 ). These dolphins prey on commercially valuable
fish and squid (Jones, 1981; Stroud, Fiscus, and Kajimura, 1981).
In 1979, under contract from the Southwest Fisheries Center, NMFS, a study
was begun on the population biology and ecology of Pacific white-sided dolphinsin the eastern North Pacific. This paper reports on distribution, seasonal movements, and abundance of the species in North Pacific waters east of longitude
180°, as surmised from all data available through 1979. Results of the remainder of the NMFS-sponsored study, addressing morphology, geographic variation,and general biology, will be published elsewhere by Leatherwood, William A.
Walker and William F. Perrin.
BACKGROUND AND PREVIOUS LITERATURE

Gill (1865) initially described L. ohliquidens from two specimens (U.S. National
Museum Numbers 1963 and 3886) taken off the California coast. Scammon (1874),
referring to those dolphins with which he had become acquainted while whaling in
the eastern North Pacific, reported that "this species has a wider range and congregates in larger numbers than any other of the dolphin family." True (1889)
gave the Pacific white-sided dolphin's distribution as the North Pacific Ocean,
Puget Sound, and the coast of California. Subsequent accounts have added limited
regional and seasonal information while restating the more generalized descriptionsof range made by these early authors.
In the eastern North Pacific, individuals of L. obliquidens have been reported
from the southern tip of Baja California (about latitude 23°N) northward along thecoasts of California, Oregon, Washington, British Columbia, and Alaska to 61°N
(Scheffer, 1950) and westward through the Gulf of Alaska and North Pacific to
Amchitka Island (Scheffer and Slipp, 1948; Cowan and Guiguet, 1956; Brown and
Norris, 1956; Pike, 1956, 1960; Norris and Prescott, 1961; Brownell, 1964; Fiscus
and Niggol, 1965; Walker, 1975; Wahl, 1977; Barham, 1978; Leatherwood andReeves, 1978; Everitt, Fiscus, and DeLong, 1979; Dohl, Norris, Guess, Bryant, andHonig, 1980; Consiglieri and Braham, 1982).
In the western North Pacific these dolphins have been reported to occur from
Taiwan northward along the coasts of Japan and Asia to the Kurile and Commander Islands (Okada and Hanaoka, 1939; Tomilin, 1957; Sleptsov, 1961; Nishiwaki, 1967; Mitchell, 1975). They have not been reported reliably from the Bering Sea (Nishiwaki, 1967; Consiglieri and Braham, 1982).
The statement that L. obliquidens "can be seen in the center of the North
Pacific including the Hawaiian Islands" (Nishiwaki, 1967) has not been substantiated, although recent sightings at about latitude 40°N, longitude 180° (G. Naftel,
Honolulu, Hawaii, personal communication) indicate that they do occur in theSci. Rep. Whales Res. Inst.,
No. 35, 1984

132

LEATHERWOOD, REEVES, BOWLES, STEWART AND GOODRICH

pelagic mid-Pacific at temperate latitudes. Nishiwaki (1967) also claimed that
Pacific white-sided dolphins are known from as far south as Panama in the eastern
Pacific, but we are not aware of any confirmed records from south of 20°N. Theiroccurrence in partially enclosed areas, such as Osaka Bay (Kuroda, 1953), Puget
Sound (Scheffer and Slipp, 1948), and the Inside Passage along the coast of British
Columbia (Pike and MacAskie, 1969) is well documented. They have been reported to penetrate the Gulf of California to as far north as Gorda Bank (23°00'N,
109°30'W) (Leatherwood and Reeves, 1978; Leatherwood et al., 1982). Althougha skeleton was collected from Valdez, Alaska, in Prince William Sound (latitude
61°N) in 1901 (Scheffer, 1950), no Pacific white-sided dolphins were seen inside theSound during recent extensive aerial and ship surveys (Hall, 1979).
Seasonal shifts in distribution have been reported. Pacific white-sided dolphins are said to be more common in coastal waters during fall and winter, moving
offshore during spring and summer (Brown and Norris, 1956; Cowan and Guiguet,
1956; Norris and Prescott, 1961 ; Brownell, 1964; Pike and MacAskie, 1969; Barham, 1978; Leatherwood and Reeves, 1978; Everitt et al., 1979; Dohl et al., 1980).
They are thought to occur only in warmer seasons in the northeastern Gulf of
Alaska (Consiglieri and Braham, 1982) and to be more numerous in cooler seasons
south of Point Conception, California (Leatherwood and Reeves, 1978). It has
been suggested that such movements are related to changes in prey distribution(Norris and Prescott, 1961; Brownell, 1964; Barham, 1978) and water temperature(Leatherwood and Reeves, 1978; Dohl et al., 1980).
Since Scheffer's (1950) summary of records along the North American coast,
no comprehensive review of the Pacific white-sided dolphin's distribution and movements in the northeastern Pacific has appeared in the literature. The only published estimates of popluation size are Nishiwaki's (1972) statement that there are
30,000 to 50,000 in Japanese waters and Fox's (1977) estimate that about 24,000
inhabit an approximately 1.5 million km 2 area off California and Baja California.
Leatherwood and Walker (1979) considered L. obliquidens one of the three mostabundant delphinids, along with Delphinus delphis and Lissodelphis borealis, in Southern California waters during midwinter.
In this report we examine information available through 1979, from sightingsand collections made north of latitude 15°S and east of longitude 180°, for patterns of distribution, movements, and abundance of the species in the eastern North
Pacific.
MATERIALS AND METHODS

Records of sightings of Pacific white-sided dolphins in the eastern North Pacific forthe years 1949 through 1979 were available from the literature and various unpublished sources. A complete file containing all the distribution data obtained
for this report has been deposited with the Southwest Fisheries Center. Principal
contributors are listed in Table 1, and their approximate areas of coverage areshown in Fig. 2. As in three previous efforts of this kind to investigate otherSci. Rep. Whales Res. Inst.,
No. 35, 1984

PACIFIC WHITE-SIDED DOLPHIN133

TABLE I. PRINCIPAL SOURCES OF UNPUBLISHED RECORDS OF SIGHTINGS OF PACIFIC
WHITE-SIDED DOLPHINS IN THE EASTERN NORTH PACIFIC (MODIFIED FROMDAHLHEIM, LEATHERWOOD AND PERRIN, 1982). THE NOTATIONS IN THE MIDDLE
COLUMN INDICATE WHETHER, FOR PURPOSES OF THIS REPORT, THE LISTEDPROGRAM'S SURVEY EFFORT HAS BEEN CHARACTERIZED SUBJECTIVELY(*),
QUANTITATIVELY(+) OR NOT AT ALL (Q) .

Institution/Activity

Principal Area(s) Covered.......

~

Period(s) and Effort~

U.S. National Marine
Fisheries Service
A. Southwest Fisheries
Center
1. Dolphin-tuna observer
program and associatedresearch cruises, 1968
through 1973.

421

I.

Dolphin-tuna observer
program and associatedresearch cruises, 1974
through 1979.
3. Albacore observer
program and associatedresearch cruises, 1971
through 1979.
B. National Marine Mamma! Laboratory
I. Pelagic fur seal programa. 1958 through 1979
2.

Total
No.
Used

Latitudes 35°N to l 5°S, coast
to approximately longitude
160°W.

*

Latitudes 35°N to l 5°S, coast
to approximately longitude
160°W.

+

Latitudes 25°N to 46°N,
within 250 nm of coast.

*

Heavy effort nearshore JanFeb declining and moving
seaward March throughremainder of year. Somecoastward research Qtr. 4.
Same as above, but effort
data available as number ofship survey hours per 5°square (see Fig. 3).
33 vessels, May throughSeptember, generally moving
south to north through season.
410

Latitudes 32°N to 62°N, eastof USA/USSR Convention
line in Bering Sea, east of
longitude 180° in remainderof Gulf of Alaska and N.
Pacific.

0b.

1960's

Latitudes 21°N to 35°N,
within 150 nm offshore andoffshore Baja California as
far as Islas Revillagigedos.

02.

Dall porpoise program
1978 and 1979

Southern Bering Sea andnorthern Pacific Ocean, near
Aleutian Islands. Somecruises in Gulf of Alaska.

03.

Platforms of opportunity All areas.

0

Sporadic coverage, largely insummer, principally searching for and collecting fur
seals. (See various reports ofNorth Pacific Fur Seal Commission ; Kajimura et al.,
1980; Stroud et al., 1981).
Effort characterized but summary unavailable in useable
format.
Cruises working north tosouth in winter and spring,
January through April(Fiscus and Niggol, 1965;
Rice 1963a, b; 1974). Effort
characterized subjectively but
summary unavailable in useable format for this report.
Observers aboard motherships and research vessels
much of summer. Sighting
conditions often poor. Effort
summaries described by
Bouchet (1981) but unavailable in useable format for
present report.
Coverage opportunistic, noeffort data available.
Continued ..•Sci. Rep. Whales Res. Inst.,
No. 35, 1984

134

LEATHERWOOD, REEVES, BOWLES, STEWART AND GOODRICHTABLE 1.

Institution/Activity

II.

Naval Ocean Systems
Center

A.

Ship surveys 1965
through 1975

B.

Aerial surveys 1968
through 1975

III.

University of California

A.

Scripps Institution of
Oceanography-La Jolla

B.

Coastal Marine Laboratory Santa Cruz

1.

Ship surveys

2.

Aerial surveys

IV.

Smithsonian Institution
Pacific Ocean Biological
SurveyContinued.

Principal Area(s) Covered...,....

til>t:iPeriod(s) and Effort

140
Within 150 nm of coast fromlatitudes 22°N to 35°N ineluding Gulf of California.
Some cruises north to Kodiak,
Alaska in spring.
Continental Shelf waters(Shore to longitude 121°W)
between latitudes 31°30'N
and 34°N (Southern California Bight).

0Effort data not consistently
recorded. Cruises conducted
all seasons, but principal
effort winter and spring.

+

Total 29,000 nm surveyed, allmonths. Effort data retained
as transect lines by month(see Fig. 4).

Principally within 100 nm ofcoast of California and Baja
Calif. and in Gulf of California.

01950-1979-miscellaneous
cruises with no indication ofsurvey effort by area orseason.

Continental Shelf waters
between latitudes 31°N and
34°N (Southern California
Bight).
Continental Shelf waters
between latitudes 31°N and
34°N (Southern California
Bight).

+

Effort available as total nm
surveyed by month.

+

Effort available as total nm
surveyed by month.

Latitudes 27°N to 35°N, coast
to longitude 120°W.

01967-1968-unable toquantify effort by area orseason.

54

Additional sources include: The University of Southern California, the Natural History
Museum of Los Angeles County, San Diego Museum of Natural History, Marineland of
the Pacific, Sea World, and numerous colleagues.
* unpublished.

Total
No.
Used

47

33*
196**

** published.

small cetaceans of the eastern North Pacific (Leatherwood and Walker, 1979;
Leatherwood, Perrin, Kirby, Hubbs, and Dahlheim, 1980; Dahlheim et al., 1982),
we pooled all acceptable data to describe overall distribution and to search fortrends in distribution and abundance by season and latitude. Many of the sightings represented incidental observations from programs for which no measure ofsurvey effort was available; such records were of limited value. Other incidental
records came from programs for which survey effort could be subjectively described,
e.g., in terms of general periods and areas of coverage. These records were somewhat more useful. The most important records, however, were those obtained
from the following three programs which provided quantitative data on survey
effort.
Southwest Fisheries Center (SWFC), NMFS
Scientific observers aboard American tuna vessels and U.S. government reSci. Rep. Whales Res. Inst.,
No. 35, 1984

PACIFIC WHITE-SIDED DOLPHIN

135

U

NOS.C and
BLM /UCSC PROGRAMSSWFC DOLPHIN -TUNA OBSERVER PROGRAM.
SWFC ALBACORE OBSERVER PROGRAM
SMITHSONIAN SURVEY
NMML PROGRAMS

I
I
I.. . .. ..

..

United States... . ·r
I
I

Fig. 2. Approximate areas covered by the principal programs providing sightingsof Pacific white-sided dolphins (modified from Leatherwood et al., 1980).

search vessels operating in the eastern tropical Pacific have supplied sightings madein the area from about latitudes 15°S to 35°N, between the shore and about longitude l 60°W (Table 1). Sightings from this program were subjected to critical
scrutiny using methods explained in detail by Leatherwood (1978). Unverifiable
sightings were discarded. Effort (time during which a NMFS marine mammal
observer was on watch) was calculated as number of hours of watch within each5-degree block (Fig. 3). All minutes in a watch series were assigned to the 5degree block in which the watch series began. Additional information concerning
the protocol for data collection in this program is available in the NMFS Observer's
Handbook (unpubl. manuscr., SWFC, La Jolla).

Sci. Rep. Whales Res. Inst.,
No. 35, 1984

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through 1976. The lower left figure shows principal transects flown.

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lt R

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1

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2 2

2 2
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~!

San!E._ Barbara

\

119°

Los
Angeles

118°

l

l

l

""v 1
1 1 2 11 1 1 1 2 1

i 1\...

1 1 1 1 1 1 1 1 1 1

1 1
1 1

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0-

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1 2 1 1 4 4 4 2 2 1
l 1

1 2 1
1 1 l

J

l l 2

1 1 l
1 1

2
1 1 1

l

~1

,,,..._

Fig. 6. Ship survey effort by University of California, Santa Cruz (UCSC), April
1975 to March 1976. Numbers in 5-degree blocks denote number of times thatarea was surveyed by ship during the study period (Doh! et al., 1980).

1 1

l
l 1 l

1 1 2 3 3 2
1 2 2 1 1 2
1 1 2
1l 1 4 3 1

'It:-.
1 1 1 3 5 5 3 1
2 4 6 ii!-~
1 10 3 '\,l 1
1 1 4 5 2 3 2 3 3 6 6 8 4 6 2 11"-...
1 1 1 l l 1 l 4 6 8 6 2 ·2 4 2 2 1 3 4 l 3 2'l 1 l 1 1 l 4 2 1 l 3 6 ~~ 1 3 1
5 4
3 i"- ....
l 1'r--..
4 2
3 2 5 3 2 2 2 2 2 2 4 4 4 4 ,j 3 5 ) ' 4 2 J
J l
3 4 5 Js 2 2 .] l 4 2
l l
l 1

f::" 111~ 2_~ "'"'""U""> 2 3_ 2'-2 1 lj'-n1 1 1
1 2 2 2 3 3 :}.
l4 2 ~rB' 3 4 7 7 4 1 1 1\
1 1
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1 1 1 2 1
1 1 1 3 4 3 1 1 3 3 2 3 2 5 5 2 1

1 2 2 l

121°

\

'

\

Mexico1u.S.A.
r-··-··

San
Diego

117°

33°

34°

~

-

z

~

t"

tl

0

~tl

"'

>-3
~

~......

0......

>'!j~0......

140

LEATHERWOOD, REEVES, BOWLES, STEWART AND GOODRICH

Naval Ocean Systems Center (NOSC)
Aerial surveys were conducted regularly by the Naval Undersea Center (now
NOSC) from January 1968 through April 1976 over the California continental
shelf, principally between latitudes 32°N and 34°N (Table 1; Fig. 4). The
methods employed in these surveys were described in detail by Leatherwood (1974),
Evans (1975), Leatherwood and Walker (1979), and Leatherwood et al. (1980).
Sightings were critically evaluated using the review procedures of the SWFC system referred to above, and unverifiable sightings were discarded. Effort was given
as plots of transect lines (Fig. 4) and estimates of nautical miles (nm) flown.
University of California, Santa Cruz ( UCSC)
Shipboard and aerial surveys sponsored by the U.S. Department of the Interior, Bureau of Land Management (BLM), were conducted in the Southern
California Bight during 1975 and 1976 by the Coastal Marine Laboratory, University of California, Santa Cruz (Dohl et al., 1980) (Table 1). Transects were
designed to provide uniform coverage of the entire study area throughout the year.
Methods of quality control on sightings data were not reported. Therefore, because original data entries were unavailable to us for review, we have simply accepted sightings on the assumption that they were subjected to adequate quality
control by the program's internal review process. Effort was characterized as thenumber of times each 5-minute block was surveyed (Figs 5 and 6) and as thenumber of nautical miles covered (Table4). Note the substantial overlap in aerial
coverage between NOSC and UCSC surveys (Figs 4, 5, 6).
Other Sources
In addition to the unpublished sources of data, we examined scientific publications and extracted sighting records from them. For all these records, we collected, as available, date, location, number of animals, and information source.
We assembled a total of 1300 reliable records, 1104 unpublished accounts from various programs and 196 previously published. Except for obvious misidentifications
or obscure references with irreconcilable disparities, we assumed published records
reflected correct identification. Sightings published twice or more were only
entered in the data base once. Each known at-sea collection for which date, location, and number of animals was available was also considered to represent asighting of a herd. For each map and analysis we used all appropriate sightings.
For those sightings in which herd size was recorded as a range (e.g., 45-50 animals),
we used the midpoint rounded to the lowest integer (e.g., 47).
Data were grouped by various combinations of quarter, 1- and 5-degree latitude belts, and depth zones (inside or outside the approximately 1,000 fm isobath).
Maps, histograms, descriptive statistics, and frequency distributions were prepared,
and grouped or paired samples were compared to detect patterns of occurrence(Table 2). For numbers of herds and numbers of animals, we determined sample
sizes (N), sum, mean (x), standard deviation (s ), standard error (sx) and range(Table 3).
Sci. Rep. Whales Res. Inst.,.No. 35, 1984

PACIFIC WHITE'-SIDED DOLPHIN141

TABLE 2. THE VARIOUS COMBINATIONS IN WHICH AVAILABLE SIGHTINGSDATA WERE EXAMINED
Quarterly
OverallNo. Herds
No. Individuals
Herd size

MM

1° belt
H
H

D
D

F

M
M

D
D

Depth zone
by 5° belt
and quarter5° belt
H
H

D
D

H
H

D
D

H
H

D

M=maps
H=histograms
D =descriptive statistics
F=frequency distribution

TABLE 3. NUMBER OF HERDS, TOTAL NUMBER OF ANIMALS, AND HERDSIZES FOR PACIFIC WHITE-SIDED DOLPHINS, BY QUARTER AND
5° LATITUDE BELT, FROM LATITUDE 20°N TO 60°N
Latitude
belt
20-25
20-25
20-25
20-25
25-30
25-30
25-30
25-30
30-35
30-35
30-35
30-35
35-40
35-40
35-40
35-40
40-45
40-45
40-45
40-45
45-50
45-50
45-50
45-50
50-55
50-55
50-55
50-55
55-60
55-60
55-60
55-60

Quarter
I

No.
Herds
23

II
III

IV
I

II
III

IV
II
III

IV
I

II
III

IV

7
3
4

99
85
105
55
87
49
38
66
154
119
38
16

I

8

II
III

28
56

IV

8

II
III

17
47
39

IV
I

II
III

IV

6
9
8

15
11

Total no. Mean herdsize
animals269.396, 196
30
210
35
105
11.50
46
91.88
9,096
65.22
5,544
292.0530,665
119.456,570
100.25
8,722
88.24
4,324
142.76
5,425
48.67
3,217
23.15
3,565
20.54
2,444
22.39
851
62.25
996
65.25
522
61.82
1, 731
39.64
2,220
8.88
71
10.82
184
83.28
3,914
68.15
2,658
80.67
484
59.44
535
174.88
1,399
204
13.60
68
748

St. dev.

St. error

682.28
44.99
35
9.95
466.24250.72936.48321.03217.41209.02417.03115.84
59.57
64.20
49.31
125.10112.68160.69
61.58
14.67
10.31
210. 74
163.6894.09
43.41
337.8315.32
107142.27
17.00
4.98
46.86
27 .19
91.39
43.29
13.31
29.86
67.65
14.26
4.80
5.89
8.00
31.28
39.84
30.37
8.23
5.19
2.50
30.74
26.21
38.41
14.47
119.443.96
32.26

572.17258.15280.31

165.17
66.ti5161.84

Min. herd Max. herd
size
size
3,000
125
10
75
20
3,500
2,000
6,000
1, 750
1
1,000
1
1,000
2,500
750
500
500
300
450
2
300
725
1
275
2
45
30
1,000
900
2
250
250
5
3
100
1,000
50

I
111

12
I5

IV

3

II

Sci. Rep. Whales Res. Inst.,
No. 35, 1984

2,215
2,057
529

184.58137 .13176.33

2

12

2,000
1,000
500

142

LEATHERWOOD, REEVES, BOWLES, STEWART AND GOODRICH

For analysis of latitudinal effects, only groupings by 5-degree latitude belt
resulted in adequate sample sizes. One-degree belts were retained for mapping.
Maps were produced showing number of herds (Figs 7 and 8) and number ofanimals (Figs 9 and 10), by one-degree blocks for all samples combined. Charts
were made on a DEC-PDP 11/34 using the AMPS Mapping Package (Analytical
Software, Inc.). Densities for each block on the sighting charts were calculated bysumming the number of herds and number of animals sighted in each block eachquarter. For example, all the animals sighted from longitudes 117°00'W tol 17°59'W and from latitudes 32°00'N to 32°59'N were counted as being in the block
32°N by 117°W.
The following problems with data ruled out the use of significance tests on
any of the data sets: (a) sightings were not collected at random; (b) sightings could
not be assumed to be independent of one another, because of the animals' tendency
to aggregate and variability among observers in defining and scoring groups as

Fig. 7. Number of sightings (herds) of Pacific white-sided dolphins by I-degree
block from all seasons and all sources, 1949 to 1979. The inset illustrates the
species' probable range.
Sci. Rep. Whales Res. Inst.,
No. 35, 1984

~~

¥

:0

~

~"'
~

~~

....

,;.» '?

~·
<Jo ~

:>

20

30

40

170

180

180

170

N=32/

,,,,,,-

170

Ill!

"+20

160

IIl"

.

rn

150

160

lllrn

m

140

140

140

rn

130

'

130

130

\-

~

'

''

120

'

'r

120

120

~

-'

0

110

E)

20

30

40

50

60

20

30

40

50

60

20

20

40

50

60

30

40

50

60

~ 30

_...

'~

110

110

_..

~

110

-~\

1
ill

\-

120

180

~· -'-~· -'-

I

180

180

m

1

~

180

170

N=/77

,,,,,,

170

170

N=357

,,,,,,

170

160

160

160

.. .,"

150

150

140

140

III

ffi
lllllII III

Fig. 8. Number of sightings (herds) of Pacific white-sided dolphins by I-degree
block, by quarter, from all sources, 1949 to 1979.

150

rn mm

150

!SO

160

N= 407180'l~III~60

20•mm " 10-20

lllll " 5-10!/~

on " 2-5,,,,,,

160

m"I.......,, ..A-

F'O

~

il

180

30 HI

40

50

60

\-

130

130

III

120

E)

~

110

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e

120

110

I

50

60

20

20

H-130

In 40

_.. i

~

110

110

\~:'

'

120

120

30

IH<10

~ 50

• 60

'\~ ~

~~

130

""

:;;:

z

::r:
...,

"d

t"

0

l:j

l:j
t'1
l:j

~....

~.......,

0

'":rj
....

~
0
...,

~~~

~~

--~ ~

~~

~~

~

~

~

?:

180

20

180

30 Hl

40 ~

50 Hl

50 111

.,_,~

-

[]]

[Il

170

•<:>

"'-

140

[Il

mm ill[!]J [!]J

II

[]]

\-

ITll

[]]

~

lllil

™

-

Number

•

.

.&,,.

~

>1001

110

../

IDlll 501-1000[Il] 11-100
lIIIl 101-500[[] 1-10

:A

p---

I

110

SIGHTINGS KEY

I

I

~

'

120

130

120
130
140
150
150
Fig. 9. Total number of Pacific white-sided dolphins observed by I-degree block,from all sources, 1949 to 1979.

,o

N = 1121 138 Individuals

,.

..A-

;?.P

1

L ihiifiilffii i11\1i1 llliirtillit

150
3'

I
I

150
C3 \

I

0

170

20

30

40

50

hi 50

t-'

~

>-3

:i:::

""'
0

:;>::l

tl

0
0

tl
C'l

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~:;>::l

i:rj

rJJ.

i:rj
~w.

t-'

~

0

t:C

~w.

<
i:rj

i:rj
i:rj

:;>::l~tl

00:;>::l

i:rj

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i:rj

:E

~-

!"'"

~

~

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1'"

~~

.....

""'~
~""~

~

~~

180

Ill"'

20

180

30 ~

40

Iii

o.,,

[]]

170

160

....."'
140

\-

~-

130

~

,,

130

130

130

p-

110

110

~

::>1001

120·110"IDlll!t_~\•lllII 101-500[]] 11-100

Number(]] 1-10.

SIGHTINGS KEY

p-

110

120

.J

120

rrlllHll\,\~

nm 101-500

aa 11-100

Number(]] 1-10

SIGHTINGS KEY

120

40

20

20
180

30 ~

~ 30

180

180

IB

~

180

.1

50

60

20

3o

40

50

60

I,,

50

60

20

~30

~

50

60

M_.A...

e

M_.A...

0

160

150

170

,,,.P

170

N

lSO

lSO

140

140

01

140

160

150

140

=12, 656 Individuals

160

160

·--" .. <>=21, 780 Individuals

170

N,,,,,,-

170

Iii

1111

Iii

~

Fig. 10. Total number of Pacific white-sided dolphins observed by I-degree block,by quarter, from all sources, 1949 to 1979.

150[]] []]Il]

140

l!Ill!Il

140

150

140

lSO

N• 44,186 Individuals

,.

160

,,,,,,-

160

170

M_.A...

150=28,818 Individuals

170

N,,,...-

160

180

e

M..A-

0

170

180

so ~

60

20

30 ~

4J

so

60

uo

130

130

am

m

130

UJ.U
IUl-::>UU(]] 1-10

NumberSIGHTINGS KEY

p-

110

p-

110

110

l:?O

t:\

c-

~

110

.J

lllII 101-500[ill 11-100(]] 1-10

NumberSIGHTINGS KEY

120

120

.,~,

~"-

\-

120

so

60

20

30

20

~ 30

~ 40

ii

~

~ 40

~so

60

::;
(JI

z

::i::
,_.

'tl

t""

0

"'
u
ti::I
u
u

t;l
,_.

t-l

~,_.

0

~
0
,_.
""'.!
,_.

146

LEATHERWOOD, REEVES, BOWLES, STEWART AND GOODRICH

separate small herds or "superherds "; and (c) pooled effort was impossible toquantify. We feel that even if statistical significance were demonstrated, it would
not be possible to ascribe any biological significance to the results. Therefore, todefine patterns for the area overall, we depended solely on direct examination ofgraphic presentations of the data.
Data from the Southern California Bight were examined in somewhat greater
detail. For the NOSC aerial surveys, we calculated number of herds per hundred
nautical miles and number of dolphins per nautical mile flown (Table 5, I 1 and I 2,
TABLE 4. SUMMARY OF EFFORT AND SIGHTINGS OF PACIFIC WHITE-SIDEDDOLPHINS BETWEEN LATITUDES 32°N AND 35°N AND A CALCULATED INDEXOF DENSITY FROM UCSC AERIAL SURVEYS, MAY 1975-MARCH 1976
PeriodNauticalmiles flown

Number ofanimals seen

I2
Number of dolphins
per n.m.

May
July
AugustSeptember
OctoberNovember
DecemberJanuary
February
March

954
2,080
1,383
765
1,935
2,034
934
3,255
2,520
1,800

2,024
2,540
70
100
292
1,892
0
12
7
768

2.120
1.220
0.050
0.140
0.150
0.930
0.000
0.003
0.003
0.043

TABLE 5. INDICES OF DENSITY OF PACIFIC WHITE-SIDED DOLPHINS (L.o.) IN THE
SOUTHERN CALIFORNIA BIGHT (32°N TO 34°N), 1968-1976, FROM NOSC AERIALSURVEYS. THE TOT AL NUMBER OF HERDS SEEN IS FOLLOWED IN PARENTHESES
BY THE NUMBER OF HERDS FOR WHICH ESTIMATES OF NUMBERS WEREAVAILABLE. ALSO SHOWN ARE INDICES OF DENSITY FOR NORTHERNRIGHT WHALE DOLPHINS (L.b.) FOR THE SAME AREA (FROM
LEATHERWOOD AND WALKER, 1979)

Month

Effort flown
in n.m.

Total no.
of herds
L.o.

January
February
March
April
May
June
July
AugustSeptember
OctoberNovember
December
Totals

1,890
2,005
2,645
4,475
2, 1501,390
2, 1202,267
1,470
1, 795550
2,025
24,76216 (15)12 (18)7
23 (15)
3
1
4
3
3 (2)
3 (2)
19 (13)95 (72)

Ii-Herds per
100 n.m.
L.o.

L.b.

0.85
0.60
0.26
0.51
0.14
0.07
0.19
0.04
0.20
0.17
0.55
0.94
0.384

3.30
11.48
1.38
2.04
0.39
0
0
0
0
0.36
0.54
I. 77

Total no.
of indiv.
L.o.

I 2-Dolphins
per n.m.

2,687
1,626
766
3,626
90
50
268
6
68
296
159
2, 12311, 766

1.42
0.81
0.29
0.81
0.04
0.04
0.13
<0.01
0.05
0.16
0.29
1.05
0.48

L.o.

Sci. Rep. Whales Res. Inst.,
No. 35, 1984

147

PACIFIC WHITE-SIDED DOLPHIN

respectively). Because herd size was not estimated for each sighting, in calculating
the 12 values we: (a) found that no significant variability in herd sizes among months
could be demonstrated and (b) assumed that those for which herd size estimates
were available (72) were representative of all 95 herds. Therefore, in estimating
number of animals seen per month, each empty cell was filled with the overall

1
1.40

1.40

1.20

1.20

N'
II
z
c

~
~

z
0
0

1.00

1.00

0

en
a::

'TI

0

J:

CD
l'T1

;:o

.....
w

s:

z

0.80 0

0.80

<
0

u.
0

c

a::

l>

w

r

lil

(/)

0.60 z.....

~

0.60:::::>

z

s:

II

D
0.40

0.40

0.20

0
Fig. 11. Indices of abundance of Pacific white-sided dolphins off Southern California 1968 through 1976, based on NOSC aerial surveys. The * indicates 12
values calculated from Doh! et al., 1980, Table III-93, for aerial surveys of thesame area, May 1975 to March 1976.

Sci. Rep. Whales Res. Inst.,
No. 35, 1984

148

LEATHERWOOD, REEVES, BOWLES, STEWART AND GOODRICH

mean (x= 145.7, s=271.2).
For comparison, we converted indices of density for northern right whale
dolphins in the same area, reported by Leatherwood and Walker (1979) as thenumber of herds or animals per square nautical mile, to 11 and 12 values and presented those in Table 5. For the UCSC aerial survey data (Dohl et al., 1980), we
also calculated monthly 12 values (Table 4).* Trends indicated by the two programs (NOSC and UCSC) were compared graphically (Fig. 11 ), but no attempt
was made at a rigorous analysis of observed patterns.
RESULTS AND DISCUSSION

Overall Pattern of Distribution
Herds of Pacific white-sided dolphins were seen in all 5-degree latitude belts
between latitudes 20°N and 60°N in all quarters of the year, except for latitudes
55°-60°N in quarter I, January-March. The 1,300 herds (112,138 individuals)
included in the analysis were widely distributed from the continental shelf throughout the pelagic temperate eastern North Pacific, although records were more abundant within the few coastal I-degree blocks in each latitude belt (Figs 7 through10). The apparent coastward concentration as well as the areas of higher density
observed north to south along the coast probably reflect levels and centers of observational effort (Fig. 12), with sightings clustered near Seattle, near the Monterey/ Santa Cruz area, and within the 35° to 30°N and 30°N to 25°N latitudebelts.
The steady decreases in number of sighting records throughout the year, from407 herds in the first quarter to 177 herds in the fourth quarter, also probably reflect effort. In fact, for all the data combined, there were no detected trends indistribution of herds which could not be explained readily on the basis of the survey effort alone. It is only when records are examined regionally, with emphasison the three programs permitting quantitative analysis, that any patterns can bediscerned which might be considered biologically meaningful.
The probable North Pacific range of white-sided dolphins, inferred fromrecords presented herein and published records from the western North Pacific, isshown in the inset to Fig. 7. Available data suggest a continuous range acrossthe North Pacific. In the area covered by this paper, sightings decrease in thearea west of longitude 135°W and south of latitude 45°N, but there has been insufficient observational effort in pelagic areas to confirm the existence of a hiatus
between eastern and western North Pacific " populations ".
Distribution North of Latitude 40°N
Sample sizes are small and effort unquantifiable north of latitude 40°N.
Sightings do occur there throughout the year, although almost all the more north-* Estimates of density and indices of density from previous programs are presented in the units reported.
Original data were unavailable to us; so, we were unable to translate to common units.
Sci. Rep. Whales Res. Inst.,
No. 35, 1984

PACIFIC WHITE-SIDED DOLPHIN

50

VitlJ 50° N - 60°N

n = 73 herds~ 30°N - 35°N

n = 240 herds[/=I 25°N - 30°N

n = 344 herds

149

I-

z

w

(..)

a: 40
w

a_

>(..)
z

w

:::::>

0

30

w

a:

.l.L.

w

>

j:: 20

<{

_J

w

a:
10

QUARTER
Fig. 12. Relative distribution of sightings by quarter within the three areas ofgreatest interest. The number of herds comprising each class interval is shown.

ern records refer to quarters II and III (spring through fall), and almost all thesightings for quarters IV and I are in coastal regions. In the Gulf of Alaska, theseasons when Pacific white-sided dolphins are reported most frequently and widely
are also the periods of most intensive fishing and research activity. We have no
way of determining whether this apparent seasonal pattern in dolphin distribution
is real or an artifact of observational effort. Based on opportunistic sightings of 3,8, 50, and 15 herds in quarters I through IV, respectively, Consiglieri and Braham(1982) concluded that this species, indeed, occurs seasonally in the Gulf of Alaska.
They further concluded, based on the same set of sightings, that in southeasternand southern Alaskan waters Pacific white-sided dolphins occur principally on thecontinental shelf. Further evidence that Pacific white-sided dolphins are present
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LEATHERWOOD, REEVES, BOWLES, STEWART AND GOODRICH

seasonally off southern Alaska is provided by Hall and Tillman (1977) and Hall
(1979), who reported that despite extensive year-round aerial and ship surveys in
Prince William Sound and immediately adjacent to its entrances, Pacific whitesided dolphins were observed only once-in October just outside Montague Strait.
Currently available data are opportunistic in nature or limited in region, andtherefore inconclusive. The question of seasonality of occurrence of Pacific whitesided dolphins in the Gulf of Alaska and North Pacific north of latitude 40°N,
particularly in pelagic regions, cannot be resolved without systematic seasonal
sampling over a broader area than has been surveyed to date (1982).
From available data, we are unable to verify the reported occurrence of Pacificwhite-sided dolphins in the Bering Sea (Clark, 1945 ). The various programs of
NMML (see Table 1), the sources of incidental sightings used by Consiglieri andBraham (1982), and 8 one-hundred hour aerial surveys of the eastern Bering Sea
conducted between March 1982 and April 1983 (Leatherwood, 1983) likely provided adequate coverage of the waters north of the Aleutians to judge whether
these dolphins occur regularly in the Bering Sea. The absence of records is taken
to mean that they do not.

Distribution South of Latitude 23°N
Similarly, we regard the evidence that Pacific white-sided dolphins do notoccur regularly south of about latitude 23°N to be convincing (see Figs 7 and
9). Coverage of the waters south of Baja California has been extensive in botharea and season.
Although observational effort in the NMFS dolphin-tuna program generally
decreased from quarter I to quarter IV according to the fishing schedule of thetuna fleet, there was some effort in a broad area south of latitude 30°N year-round(Figs 2 and 3). In addition to the shipboard observations, three extensive aerial
surveys of the eastern tropical Pacific tuna grounds have been conducted by NMFS
(Smith, 197 5; Barham, Evans and Coe, 197 5; Barham and Powers, 1977; Powers
and Barham, 1977; Holt and Powers, 1982). Therefore, we consider the low number of sightings there-five confirmed south of latitude 23°N, the southernmost at
20°N-to be biologically meaningful. If appreciable numbers of Pacific whitesided dolphins had been present, there undoubtedly would be more records.
Depth Preferences
Northern and southern limits of the species' range are more readily defined
than are its inshore-offshore limits. There have been a few sightings in water as
shallow as 20 fathoms (Consiglieri and Braham, 1982; NOSC unpublished records;
Leatherwood, personal observations) though these appear exceptional excursions
into shallow water. Otherwise, these dolphins are encountered widely in continental shelf, continental slope, and offshore zones. Because of uncertainty inposition records, we cannot report the maximum depth of water where Pacificwhite-sided dolphins have been seen. However, the westernmost records in the
study area were plotted at locations in which surrounding water for a radius of at
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PACIFIC WHITE-SIDED DOLPHIN

151

least 100 nm was more than 3000 fms deep.
Little can be said about the relative frequency of occurrence of Pacific whitesided dolphins in waters of different depth. The fact that, overall, most sightings
were made inshore of the approximately 1000 fm isobath probably reflects effort
patterns rather than an aspect of dolphin distribution.
There does appear, however, to be a tendency for Pacific white-sided dolphins
to occur nearer to the coast south of about latitude 30°N. The effort in pelagic
waters between latitudes 20°N and 30°N was appreciable and the encounters withPacific white-sided dolphins few.

Seasonal Trends in Distribution
The only two portions of the study area in which observed distribution patterns are not likely an artifact of sampling effort are the waters off Baja California
and in the Southern California Bight. Off Baja California, the effort is tied tofishing and research activities of the tuna fishery, essentially declining steadily
throughout the calendar year with a slight increase in the fourth quarter associatedwith research cruises and early departures from San Diego by fishing vessels boundfor the eastern tropical Pacific (Fig. 3 ). Despite this declining trend in effort,
the number of sightings and number of animals per quarter off Baja California did
not decline consistently through the calendar year (Figs 8 and 10, Table 6).
In fact, for the area between latitudes 25°N and 30°N, the number of sightings andtotal number of animals observed in the third quarter were substantially greaterthan the number observed during the previous two quarters (Table 3). This sharp
increase in numbers during a period of relatively low sampling effort is of interest.
It suggests an influx of animals to this area during late summer and early fall.
There is no reason to believe such an influx would consist of animals moving inshore from the west. Rather, judging by published accounts about the seasonal
movements of Pacific white-sided dolphins south of Point Conception (e.g. Brown
and Norris, 1956; Leatherwood and Reeves, 1978; Leatherwood et al., 1982), it
seems most reasonable to us that these are dolphins moving southward from thearea off Southern California between latitudes 30°N and 35°N.
The monthly indices of density from the NOSC aerial surveys indicate that
Pacific white-sided dolphins are at their peak density and peak number off Southern
California from November through April, and are less common from late spring
TABLE 6. DETAILS OF EFFORT AND SIGHTINGS OF PACIFIC WHITE-SIDED DOLPHINS,
25°N AND 30°N, FROM NMFS DOLPHIN-TUNA PROGRAM, 1974 THROUGH 1979.
I 1 AND 12 VALUES ARE NUMBER OF HERDS AND NUMBER OFINDIVIDUALS PER SURVEY HOUR, RESPECTIVELY.
QuarterApproximate
number ofsurvey hours

Number ofsightings

11

Number ofanimals12

Meanherd size

I
II
III
IV

>375
565
250
190

99
85
105
55

<0.27
o. 15
0.44
0.29

9,096
5,544
30,6656,570<24.26
9.83
3,738.66
34.58

92
65
292
119

Sci. Rep. Whales Res. Inst.,
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ii

152

LEATHERWOOD, REEVES, BOWLES, STEWART AND GOODRICH

through late fall (Fig. 11, Table 5 ). This seasonal pattern is consistent withthat reported for the area by previous investigators, based on subjective impressions (e.g., Norris and Prescott, 1961; Leatherwood et al., 1972) or on preliminary
analysis of the NOSC data (Leatherwood and Reeves, 1978; Leatherwood et al.,1982). It is also very similar to the seasonal pattern reported by Leatherwood andWalker (1979) for the northern right whale dolphin, a sympatric species with which
Pacific white-sided dolphins have a strong affinity in coastal waters (Table 5 ).
From these data, Pacific white-sided dolphins appear to reach their lowest densities off Southern California during the periods discussed above, when they areapparently at their highest densities off Baja California. We emphasize that theargument for a relationship between these two phenomena is only circumstantial;
there is no direct evidence of a coast-wise migration. The origin of the apparent"immigrants " to Baja California waters in quarter III and the destination of the"emigrants" from Southern California in quarters II and III remain unknown.
The peak density off Southern California is 1.42 individuals per nm (Fig. 11); off
Baja California it is reported to be 0.06 individuals per nm 2 (Fox, 1977).
The trends indicated by the UCSC aerial surveys are slightly inconsistent withthose indicated by the NOSC program; the latter covered a slightly more coastal
area. Densities derived from data collected in the 10-month UCSC programfluctuate considerably, with about 1-2 individuals per nm 2 in May, July, and

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Fig. 13. Log transform of sizes of herds of Pacific white-sided dolphins observed in
the eastern North Pacific, 1949 to 1979. Number of herds comprising each classinterval is shown. The log transform was performed strictly for presentation. No
analysis was conducted on logs of data.
Sci. Rep. Whales Res. Inst.,
No. 35, 1984

PACIFIC WHITE-SIDED DOLPHIN

153

November, and much lower densities in all other months (Table 4 ). The differences may be due to small sample sizes and the short period of coverage in the
UCSC program. However, if they are not, then the results may indicate a shiftof the Southern California portion of the population offshore and northwestward
in quarters II and Ill.
Hubbs (1961) described a major transition zone for marine fauna along theouter coast of Baja California at about 24°N latitude, in the vicinity of Magdalena
Bay. Au, Perryman and Perrin (1979) noted another water-mass change at themouth of the Gulf of California, where the fauna tends to change from temperate
to tropical. As a temperate-zone species, the Pacific white-sided dolphin probably
is influenced by these oceanographic changes. In the areas of most intensive observational effort, between latitudes 20°N and 30°N, it was observed mainly shoreward of the outer margin of the California current (Owen, 1974), suggesting that
it is principally an inhabitant of rich upwelling waters.
Trends in Herd Size
The overall distribution of herd sizes is shown in log-transform as Fig. 13.

200 . . . . - - - - - - - - - - - - - - - - - - - - - - ,N=/268

177

w 150
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Fig. 14. Mean herd sizes by 5-degree latitude belts, from all sources, 1949 to 1979.
Mean values are shown above each class interval. Number of herds comprisingeach class interval is shown.

Sci. Rep. Whales Res.Inst.,
No. 35, 1984

154

LEATHERWOOD, REEVES, BOWLES, STEWART AND GOODRICH

The number of animals in the 1,269 sightings for which "group" size estimates wereavailable ranged from 1 to 6,000, about a mean of 88. Above about 20 individuals,
the apparent " pulses " in group size are probably spurious, being caused by thetendency of observers to classify estimates into rounded units. Herd sizes varied
over the entire geographic range of the sightings (Fig. 14 ). Those north oflatitude 55°N and south of latitude 30°N were significantly larger than those between 30°N and 55°N (n= 1,268, F=3.69, p<0.01 ). In the northernmost area
(north of 55°N), where sea-surface conditions are consistently inhospitable to cetacean surveys, it is possible that bad weather and observer bias resulted in a tendency to see and record only large herds. In the other two areas (south of 30°N
and between 30°N and 55°N), we assume that observers could detect herds of various sizes and that most participants were able to estimate herd size in a relatively
accurate and consistent manner. Therefore, we do not believe the observed differences are artifactual. It is unclear why herds would be larger off Baja California than elsewhere. Perhaps there are inherent differences in herd-size estimationfrom aircraft (the principal platform used for observations between latitudes 30°N
and 35°N) and ships (the platform used almost exclusively south of 30°N).
ACKNOWLEDGEMENTS

This work was sponsored by the National Marine Fisheries Service, Southwest
Fisheries Center, La Jolla, California under contract NOAA/79-ABC-00201 to S.
Leatherwood and William A. Walker. We thank Dr William F. Perrin (contracttechnical representative) and Mr Walker for invaluable help with this phase of
the study. Further, we thank them and Dr Douglas DeMaster for review of thecontract report. The Inter-American Tropical Tuna Commission kindly aided in
the analysis and preparation of figures. Chick Hayashi prepared the figures.
Angela Rowley typed the manuscript.
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