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NOAA Technical Report NMFS 28

Review of Geographical

Stocks of Tropical Dolphins

(Stene//a spp. and

De/phinus de/phis)

in the Eastern Pacific

William F. Perrin, Michael D. Scott,

G. Jay Walker, and Virginia L. Cass

March 1985.

y

7t I 'L MAMMAL LAB v v t'l-, r WAY NE BLDG 4

S 11. IL ~vA ~o115.0070 .

U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Marine Fisheries Service
NOAA TECHNICAL REPORTS NMFS

The major responsibilities of the National Marine Fisheries Service (NMFS) are to monitor and assess the abundance and

geographic distribution of fishery resources, to understand and predict !luctuations in the quantity and rlistribution of these

resources, and to establish levels for optimum use of the resources. NMFS is also charged with ihe development and implementation of policies for managing national fishing grounds, development and enforcement 01 domestic fISheries regulations,

surveillance of foreign fishing off United States coastal waters, and the development and enforcement of international fishery

agreements and policies. NMFS also assists the fishing industry through marketing sen·ice and economic analysis programs,

and mongage insurance and vessel construction subsidies. It collects, analyzes, and publishes statistics on various phases of the

industry.

The NOAA Technical Repon NMFS series was established in 1Y8J to replace two subcategories of the Technical Reports

series: "Special Scientific Repon-Fisheries" and "Clrcular.·· The series contains the following types of repons: Scientific

investigations that document long-term continuing programs of NMFS, intensive sCIentific reports on studies of restricted

scope, papers on applied fishery problems, technical rcpons of general Interest intended to aid conservation and management.

repons that review in considerable detail and at a high technical level cenain broarl areas of research, and technical papers

origi.na~ing in economics studies and from management investigations.

Copies of NOAA Technical Report NMFS are a,·ailable free in limited numbers 10 governmental agencies. both Federal

and State. They are also available in exchange for other scientific and technical publications in the marine sciences. Individual

copies may be obtained from: U.S. Depanment of Commerce, National Technical Information Service, 5285 Port Royal Road,

Springfield, VA 22161.
NOAA Technical Report NMFS 28

Review of Geographical

Stocks of Tropical Dolphins

(Stenella spp. and

Delphinus'delphis)

in the Eastern Pacific

William F. Perrin, Michael D. Scott,

G. Jay Walker, and Virginia L. Cass

March 1985

U.S. DEPARTMENT OF COMMERCE

Malcolm Baldrige, Secretary

National Oceanic and Atmospheric Administration

John V. Byrne, Administrator

National Marine Fisheries Service

William G. Gordon, Assistant Administrator for Fisheries
The National Marine Fisheries Service (NMFS) does not approve, recommend or endorse any propriety product or proprietary material mentioned

in this publication. No reference shall be made to NMFS, or to this publication furnished by NMFS, in any advertising or sales promotion which

would indicate or imply that NMFS approves, recommends or endorses any

proprietary product or proprietary material mentioned herein, or which has

as its purpose an intent to cause directly or indirectly the advertised product to be used or purchased because of this NMFS publication.
CONTENTS

Introduction .

Materials and methods .

Morphological analyses .

Distribution data .

Stenella aflenuata .

Stenella longirostris .

Tagging .

Species accounts .

Stenella attenuata .

Stenella longirostris .

Stenella coeruleoalba .

Delphinus delphis .

Origins of the geographical forms .

Recommendations .

Stenella attenuata .

Stenella longirostris .

Stenella coeruleoalba .

Delphinus delphis .

Acknowledgments .

Literature cited .

Appendix 1. Statistical reporting blocks for eastern tropical Pacific .

Appendix 2. Stock composition of records of Stenella longirostris by 50 block .

Figures

1. Known distribution of Stenella attenuata in the eastern Pacific, showing SOPS '79 species range and stock boundaries, and

new species range .

2. Known distribution of Stenella longirostris in the eastern Pacific, showing SOPS '79 species range and stock boundaries, and

new species range .

3. Known distribution of Stenella coeruleoalba in the eastern Pacific, showing SOPS '79 boundaries and new species range .

4. Known distribution of Delphinus delphis in the eastern tropical Pacific, showing SOPS '79 species range and stock boundaries, and new species range .

5. Collection localities for adult specimens of Stenella attenuata included in body-length analyses .

6. Collection localities for adult specimens of Stenella longirostris included in body-length analyses .

7. Collection localities for adult specimens of Stenella coeruleoalba included in body-length analyses .

8. Collection localities for adult specimens ofDelphinus delphis included in body-length analyses .

9. Records of offshore and coastal spotted dolphins in the far-eastern Pacific .

10. Records of offshore and coastal spotted dolphins off A) North America, B) Central America, and C) South America .

II. Relative abundance of records of offshore and coastal spotted dolphins by distance from coast .

12. Frequency distributions, ranges, means, and standard deviations of body length for male and female spotted dolphins,

Stenella attenuata, from four geographical areas .

13. Frequency distributions of ovarian-corpus count for northern and southern offshore samples of adult female spotted

dolphins .

14. Distribution of eastern and Costa Rican spinner dolphins in the eastern tropical Pacific .

15. Distribution of eastern and Costa Rican spinner dolphins off Central America .

16. Frequency distributions, ranges, means, and standard deviations of body length in samples of adult specimens of Stenella

longirostris of four stocks .

17. Distribution of whitebelly spinner dolphins in the eastern Pacific .

18. Racial composition by 50 square of sightings ofStenella longirostris identified to geographical race .

19. Frequency distributions of ovarian-corpus counts in northern and southern samples of adult female spotted dolphins .

20. Records ofStenella coeruleoalba in the eastern Pacific .

21. Length-frequency distribution and means of body length for northern tropical and southern tropical adult males and females

of Stenella coeruleoalba .

22. Frequency distributions, ranges, means, and standard deviations of length in two stocks of Delphinus delphis .

23. Records ofDelphinus delphis in the eastern tropical Pacific .

24. Conceptual model of radial pattern of geographical variation in pelagic populations of Stenella in the eastern tropical

Pacific .

25. Recommended management units for Stenella coeruleoalba .

26. Recommended management units for Delphinus delphis in the eastern tropical Pacific .

iii

I

2

2

4

4

5

6

8

8

14

16

17

21

23

23

23

23

23

23

23

26

27-28

2

3

3

4

5

6

7

8

9

10-11

II

12

13

14

15

16

17

18

18

19

19

20

20

22

24

24
Tables

I. Releases of spaghetti tags by Martini and Nilson aboard the private vessel Serenity in 1975 and 1976 7

2. Estimated school sizes for sightings of coastal and offshore spotted dolphins within 100 nmi of the coast 13

3. Records of Costa Rican and eastern forms of Stenella longirostris within 50 nmi of the coast, north and south of Gulf of

Tehuantepec 15

4. Distances from coast of specimen records ofDelphinus delphis in Square 189 and Squares 168-169 . . . . . . . . . . . . . . . . . . . . . 21

iv
Review of Geographical Stocks of Tropical Dolphins

(Stenella spp. and Delphinus delphis)

in the Eastern Pacific

WILLIAM F. PERRIN,! MICHAEL D. SCOTT,2 G. JAYWALKER,!

and VIRGINIA L. CASS·

ABSTRACT

Information on geographical variation is reviewed for Stenella attenuata, S. longirostris, S. coeruleoalba, and

Delphinus delphis in the eastern tropical Pacific, and boundaries for potential management units are proposed.

National Marine Fisheries Service and Inter-American Tropical Tuna Commission sighting records made from

1979 to 1983 which were outside boundaries used in a 1979 assessment were examined for validity. Tagging

returns and morphological data were also analyzed. Several stock ranges are expanded or combined. Three

management units are proposed for S. attenuata: the coastal, northern offshore, and southern offshore spoiled

dolphins. Four management units are proposed for S. longirostris: the Costa Rican, eastern, northern whitebelly, and southern whitebelly spinner dolphins. Two provisional management units are proposed for S. coeru·

leoalba: the northern and southern striped dolphins. Five management units (two of which are provisional) are

proposed for D. delphis: the Baja neritic, northern, central, southern, and Guerrero common dolphins. Division

into management units was based on morphological stock differences and distributional breaks.

INTRODUCTION

Tuna fishermen make use of four species of dolphins in the

international purse-seine fishery for yellowfin tuna in the eastern

tropical Pacific in finding and capturing the tuna (Smith 1983).

These are (in order of importance to the fishery): the spotted

dolphin, Stenella attenuata; the spinner dolphin, S. longirostris; the

common dolphin, Delphinus delphis, and the striped dolphin, S.

coeruleoalba. Perrin et al. (1983) delineated the known ranges of

the species in the eastern Pacific in a recent review. The purpose

of this paper is to review available information on geographical

variation of the four species in the eastern Pacific and to propose

boundaries for potential management units.

Geographical vai ;"tion in S. attenuata and S. longirostris was

first described by Perrin (I 972a, b, 1975a). He placed the

nominal endemic S. graffmani (Lonnberg 1934) (described from

Acapulco, Mexico) in the synonymy of S. attenuata (Gray 1846)

and described three subspecies from the eastern and Central

Pacific: 1) the coastal spotted dolphin, S. a. graffmani (formerly S.

graffmani), a relatively large and heavily spotted form thought to

occur within about 20-25 km of the shore and in gulfs and embayments from southern Mexico to Colombia; 2) the offshore spotted

dolphin, S. a. subsp. "A" (unnamed), a smaller and more lightly

spotted pelagic form thought to occur from about 25 km offshore

from Cabo San Lucas, Mexico, to Colombia and west to at least

long. 145°W in tropical water; and 3) the Hawaiian spotted

dolphin S. a. subsp. "8" (unnamed) from Hawaiian waters,

similar to the offshore form but less spotted and with a larger

skull. In the case of the spinner dolphins, specimens from the

eastern tropical Pacific had been referred (by Hester et al. 1963)

to S. microps (Gray 1846; described from an unknown locality)

and animals from Hawaii referred (by Morris and Mowbray

'Southwest Fisheries Center La Jolla Laboratory. National Marine Fisheries Service, NOAA, P.O. Box 271, La Jolla, CA 92038.

'Inter-American Tropical Tuna Commission, La Jolla. CA 92038.

1966) to S. roseiventris (Wagner 1846; described from the western

Pacific). Perrin (l975a) considered the spinner dolphins to belong

to a single species, S. longirostris (Gray 1848) (described from an

unknown locality) and described four subspecies. The Costa Rican

spinner dolphin, S. I. subsp. "A" (unnamed), is a relatively long

and slender form of overall gray color and with a very long

rostrum, from nearshore waters of Costa Rica and El Salvador.

The eastern spinner dolphin, S. l. subsp. "8" (unnamed; formerly

identified as S. microps), is a smaller but similarly colored form

with a small skull and (in many adult males) with the dorsal fin

canted forward almost so as to appear reversed and occurs westward from the coast of Mexico out to about 800 km. The whitebelly spinner dolphin (so called by tuna fishermen), S. I. subsp.

"C" (unnamed), is similar to the eastern spinner dolphin in size

and skull but has a ventrolateral white area extending from the

flipper to the genital region and a less triangular dorsal fin; it

ranges from about 800 km offshore to long. 140 oW. The

Hawaiian spinner dolphin, S. I. subsp. "D" (unnamed; formerly

identified as S. roseiventris), is a relatively large form with large

skull, sharply defined three-part color pattern and slightly falcate

dorsal fin and occurs in Hawaiian waters. The first and last of

these forms ("A" and "D") were at that time each known from

only a few specimens, as were the coastal and Hawaiian forms of

the spotted dolphin.

Perrin (l972a) described geographical variation in the color

pattern of S. longirostris in detail and proposed a generalized

model for variation in this and other delphinids. Under this

scheme, the variation in the spinner dolphin is seen as variation in

extent and density of a color-pattern component called the "dorsal

overlay," with the overlay being modally darkest and most extensive in the Costa Rican and eastern forms and lightest and least

extensive in the Hawaiian form. The color patterns of the various

forms are thus perceived as variations on a theme rather than as

discretely different patterns.

As more distributional data have accumulated, the known

ranges of the several forms of S. attenuata and S. longirostris have
expanded (Perrin 1975b; Perrin et al. 1982). Studies of material

from below the Equator led to division of the offshore spotted

dolphin and the whitebelly spinner dolphin each into northern and

southern stocks by Perrin, Sloan, and Henderson (1979). They

found the southern offshore spotted dolphin to be slightly smaller

than the northern animal, with a larger skull and more distinct

color pattern. The southern whitebelly spinner dolphin they found

to be larger than the northern form, with coloration more resembling that of the Hawaiian form. These findings and additional

distribution records were taken into account in the most recent

delineation of the ranges, which was used in an exercise referred

to below as "SOPS 1979" (for Status of Porpoise Stocks [Workshop], 1979) in 1979 (Smith 1983) (Figs. 1,2). Scott (1981) and

Alverson (1981) reviewed the ranges, the former incorporating

data through 1980.

The range of S. coeruleoalba in the eastern Pacific has been

divided into management units based solely on apparent latitudinal gaps in distribution (Smith 1983), yielding five units:

"northern temperate," "northern tropical," "central tropical,"

"southern tropical" and "southern temperate" (Fig. 3). To date,

geographical variation in this species in the eastern Pacific has not

been studied.

Delphinus delphis varies morphologically a great deal in the

eastern Pacific. Banks and Brownell (1969) referred a shortbeaked offshore form and a long-beaked neritic form occurring

off southern California and Baja California to D. delphis L. and D.

bairdii Dall 1873, respectively. Van Bree and Purves (1972)

reported material that showed intergradation in skull morphology

between the two forms and concluded that they belong to the

single species D. delphis. Evans (1975,1982) described five forms

in the eastern Pacific: I) a relatively large form inhabiting waters

inside the 1DO-fathom curve off southern California and Baja

California, with a very long rostrum and a muted color pattern

(later called the "Baja neritic" form); 2) and 3) small, shortbeaked northern temperate and northern tropical forms; and 4)

and 5) large Short-beaked central and southern tropical forms

delineated by apparent latitudinal gaps in distribution. He also

suggested the possible existence of an "equatorial-oceanic"

population inhabiting waters west of long. 95 oW. On the basis of

additional distribution data, Au et aP modified the boundaries of

the stocks and added a "southern temperate" stock (Fig. 4). They

included all animals between lat. 3° and 15 ON in the "central

tropical" stock, not recognizing Evans' tentative "equatorialoceanic" stock. In the 1979 stock assessment (Smith 1983), the

Baja neritic and northern temperate stocks were combined into a

single management unit. The stock structure of D. delphis was

most recently reviewed by Scott (1981).

In this paper we review developments and data collected for the

four species since the 1979 stock assessment.

MATERIALS AND METHODS

Morphological Analyses

The samples and methods used in analyses of variation in skull

and body length in S. attenuata and S. longirostris are described in

Schnell et al. 4 and Douglas et al. (1984). In addition to considera-

'Au, D. K. W.. W. L. Perryman. and W. F. Perrin. 1979. Dolphin distribution

and the relationship to environmentJI features in the eastern tropical Pacific.

Soulhwesl Fisheries Center Admin. Rep. LJ-79-43. 59 p.

'Schnell, G. D., M. E. Douglas, and 1. D. Hough. 1982. Geographic variation in

morphology of spotted and spinner dolphins (Stenella attennala and S lonfiiroslris)

from the eastern tropical Pacific. Southwest Fisheries Center Admin. Rep. LJ-82-

15C.213p.

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Figure I.-Known distribution of 51<nella allenuala in the eastern Pacific, showing SOPS '79 species range and stock boundaries, and new species

range (from Perrin et al. 1983).

2
1\ (/

~,~~ ["'~

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20'

10'

N

O'

s

10'

20'

160'W 150' 140' 1300 1200 1100 100' 90' ao' W

Figure 2.-Known distribution of51enella longiroslris in the eastern Pacific, showing SOPS '79 species range and stock boundaries, and new species

range (from Perrin et al. 1983).

(..... l'\

30' I-----I---+_+-----+-'-_J-t--t--t-'-·"'--tt-I ---:-\.. \D~ ./ 0 >~. ,,-"NO THERN "-..' TEM ERATE 'I

10'

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Figure 3.-Known distribution of 51enella coeruleoalba in the eastern Pacific, showing SOPS '79 boundaries and new species range (from Perrin et

01. 1983).

3
100' 90' 80' W

, , •

Tahiti

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IV ~I----/ C," ••"" M 10' 1----+--+--+--Ht---+--+--I---+--+--~=:.:::j:~-__1_--_l_-___'__II-- 'lL ~L'---- CENTR L TROPI AL COM' ON DOLP IN ~~

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20' ,';;;-;:;---'----:-;;:;:--L-_~:::---L-~::_::_::__---l--~~_-.l-----l_:_:_-....L--..L--L-...J.--..L--L---1 160' w 150' 140' 130' 120' 110'

Figure 4.-Known distribution or Delphinus delphis in the eastern tropical Pacific, showing SOPS '79 species range and stock boundaries, and new

species range (frnm Perrin et al. 1983).

tion of the results obtained by those authors, we reviewed all

body-length data collected through 1981 (Figs. 5-8), including

some data for the inshore forms of S. attenuata and S. longiroslris

not included in the contracted analyses, and data for S.

coeruleoalba and D. delphis. We also examined data on frequencies of ovarian corpora for S. attenuata and S. longiroslris. Sources

of the data are described in Perrin and Oliver (1982); statistical

reporting areas are defined in Appendix I.

Distribution Data

The sources and nature of the distribution data are described in

Perrin et al. (1983). Data were included for all NMFS and IATTC

(Inter-American Tropical Tuna Commission) tuna/dolphin

observer cruises from 1979 through most of 1983.

Stenella attenuata.-We reviewed all records of spotted

dolphins from within 100 nmi of the coast (or a coastal island)

that were identified to stock ("coastal" or "offshore"). These

comprised 1,650 sightings and collection records. Of 502

"coastal" records, we concurred with the identifications for 218;

67 were changed to "offshore," and 217 were downgraded to

unidentified spotted dolphin. Of 1,147 "offshore" records, we accepted 456, (;hanged 44 to "coastal," 640 to unidentified spotted

dolphin, and 7 to unidentified dolphin. The large number of

changes to unidentified spotted dolphin was occasioned by the

fact that in recent years NMFS observers aboard commercial

purse seiners have largely identified spotted dolphins to stock

based only on locality without regard to morphological information, all those within 50 nmi of the coast being designated as

"coastal" and all those outside 50 nmi as "offshore." In point of

fact, the coastal form was described on the basis of skull

characters, body size, and coloration (Perrin 1975a).

In reviewing a sighting, we considered the morphological

observations recorded by the observer and any photographs, bodylength data, life history data, and osteological specimens collected

in association with the sighting. If the observer noted that the

animals seemed larger and/or more heavily spotted than those

seen far offshore, or, better, if the photos, length data, or specimens indicated this (based on the published information for the

two forms), we considered the record to be a confirmed record of

the coastal form. Because the question is one of modal differences

between forms within a species, the identification was judgmental

in many cases, especially if only a few animals were measured or

if the observer's recorded comments were sketchy or ambiguous

and voucher data and specimens were absent. We tried to err on

the side of conservatism, relegating the more weakly supported

sightings to the unidentified category, but undoubtedly we classified some sightings of the coastal form as "offshore" and vice

versa.

In evaluating the observer's recorded information, we also

considered such factors as sighting distance and conditions,

experience of the observer, and quality of documentation and

identifications for other sightings made by the same observer. In

4
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LONGITUDE

Figure 5.-Collection localities ror adult specimens or5tenella attenuata included in body-length analyses. Adults defined as males with testis.epididymis weight or 100 g

or more and remales with at least one ovarian corpus. Open dots are small islands.

addition, we considered the context of the sighting record. If, for

example, one net set (a sighting record) was made on the same

school or near the same time and in the same area as a previous set

assignable to stock based on recorded observations or specimens,

we considered the second record to be of the same form.

Stenella longirostris.-To examine the distribution of the

Costa Rican spinner, we reviewed all records of S. longirostris

within 50 nmi of the coast. The number of confirmable records

was small, mainly because the Costa Rican form differs externally

from the eastern form only in total body length and relative length

of the rostrum and apparently not in coloration. Also, the Costa

Rican spinner dolphi~ is only very rarely captured by tuna fisher·

men (Perrin 1975b). Of 42 "Costa Rican" records, we confirmed

33, assigning the rest to unidentified spinner. Of these confirmed

records, 24 were based on aerial photographs taken during a

photogrammetric experiment by the IATTC in 1980. One of 196

"eastern" records was judged to be "Costa Rican"; 28 were confirmed as "eastern," 157 changed to unidentified spinner, and 10

were accepted. One of 118 unidentified records was changed to

"eastern."

We also reviewed all records of eastern spinner dolphins and

whitebelly spinner dolphins outside the respective SOPS '79

boundaries (see Fig. 2). We again adopted a conservative stance

because of the necessarily judgemental nature of the identifications in many cases. Of 198 "eastern" records, we confirmed 115

(8 supported by specimen data), assigned 40 to "whitebelly," 4 I

to unidentified spinner, and 2 to S. attenuata. Of 280 outlying

"whitebelly" records, 184 were confirmed (1 with specimen

data), 35 assigned to "eastern," 49 to unidentified spinner, I to S.

attenuata, I to Delphinus delphis, and 10 to unidentified dolphin.

In addition to considering the sighting factors discussed above,

we found it necessary to "second-guess" the observer's understanding of the modal nature of the differences between the two

forms, as a large number of sightings of "mixed" schools of

eastern and whitebelly spinner dolphins were reported. In some

cases, this designation was likely valid, because the observer noted

separate groups of the two forms in the net. In other cases we

assigned the "mixed" sighting to one or the other of the two forms

because only a very few individuals of the "other" type were seen,

as would be expected in the case of subspecific-Ievel modal difference. There were a large number of "mixed" sightings, however,

where it was unclear from the information recorded if the record

should be assigned to both forms. In some cases, color-pattern

"intergrades" were reported, again something to be expected in

the case of subspecific variation but difficult to evaluate when

5
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1\ I -

SPIN NER D ~LPHIN V

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160' W155' W 150' W liS' W liO O W 135' W 130' W 125' W 120' W lIS' 110' W lOS' W 100' W 95' W 90' W BS' W BO' W 75' W

LONGITUDE

Figure 6.-Collection localities for adult specimens ofStenella longirostris included in body·length analyses, Adults defined as forS. allenuata. Specimens not identified to

stock are not included.

assigning a school to one mode or the other or both. In these situations, we made judgmental decisions based on other aspects of the

sightings, giving heavy weight to the experience and apparent

knowledgability of the observer.

We noted three additional problems primarily encountered in

sightings made by inexperienced observers, usually during the first

few days of a first cruise:

I) Juvenile eastern spinner dolphins appear whitebelly-like in

having a light ventrum (Perrin I972a), and some were clearly

misidentified in the sighting records. Where specimens were collected, we were able to detect the error; other instances probably

remain in the whitebelly spinner dolphin records that we accepted.

2) Adult eastern spinner dolphins when seen at a distance,

especially when spinning, appear slightly lighter ventrally then

dorsally. In some cases it was clear that the observer had not yet

seen whitebelly spinners and was mistaking this muted countershading for the sharply demarcated dark-gray and white pattern of

the whitebelly form. Again, some instances of this have probably

gone undetected in our review.

3) New observers in some cases clearly confused the similar

fishermen's terms "whitebelly" (Delphinus delphis, S. coeruleoalba or Lagenodelphis hosei) and "whitebelly spinner" (S.

longirostris). In an earlier review of species ranges (Perrin et al.

1983), we found that this had led to obviously spurious records of

S. longirostris far north of the tropical habitat. Again, in the present review we found sightings (made at a distance) which

members of the crew identified as "whitebellies" but the inexperienced observer identified as "whitebelly spinners," most likely misunderstanding the crewman's identification.

In view of these difficulties, and because of the rather unusual

broad overlap of the ranges of geographical forms (which usually,

by definition, are allopatric but may not be in these highly mobile

and socially complex cetaceans), we consider the allocation of

outlying sightings to whitebelly and eastern spinners on balance to

be lefM> reliable than the allocations for the other stocks and

species reviewed in this study.

Tagging.-The tagging methods and operations (except for the

tag releases listed in Table 1) have been described previously

6
z

I r--j 1\ -~-S~RIP~;;- ;OU;H N

---,---,--::::;

I (/ I I I f'""- . Lit" -histc ry sample ...........

! .

'~ (

v~ \\ \. I -)

~~ \ 0 Q

... '. ,

, h I~~~ \,. Hawa" R.vllia GloedO'1f \. L 'C:>

·1 ------..... """ '- )

I Clipperton (

COCOliO .~ . I .

)1 MaJpeio

Ct1rJ:tmas I I .

Galap~ :

. (

I

I <:

I : Ma~~... f\ I

i I \ .! . . ·1 I i'Tahiti i

z

z

o

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z

z

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o

N

lill60 , HISS' H ISO' H US' H 110' H 135' H 130' H 125' H 120' HilS' H 110' H lOS' H 100' H 9Sa H 90' HaS' H BO' H 75' H

LONGITUDE

.

'"

n

z W

0 0

::::J

I--<

I--< z CI:

-I .",

·0

If)

.",

If)

If)

If)

Figure 7.-Collection localities for adult specimens of Stenella coeruleoalba included in body-length analyses. Adults defined as for S. attenuata.

Table I.-Releases of spaghetti tags (metal dart, type d-Perrin, Evans, and Holts 1~9) by Martini and Nilson aboard the private vessel Serenity in

1975 and 1976. The species identifications are tentative.

Position Position

Species Tag no. Date (lat., long.) Species Tag no. Date (lat., long.)

S. attenuata 05002 27DEC75 8°20'N, 78°45'W Tursiops truncalusl 05009 3JAN76 0042'S, 89°30'W

05004 27DEC75 8°IYN, 78°47'W 05010 3JAN76 0042'S, 89°30'W

05006 27DEC75 8°08'N, 78°50'W 05011 3JAN76 0046'S, 89°30'W

05007 27DEC75 8°07'N, 78°50'W 05012 3JAN76 0046'S, 89°30'W

05008 27DEC75 8°06'N, 78°50'W 05013 3JAN76 0046'S, 89°30'W

05017 27DEC75 8°07'N, 78°50'W 05015 3JAN76 0046'S, 89°30'W

S. longiroslris 05034 I1MAR76 8°23'S, 137°36'W 05019 3JAN76 0046'S, 89°30'W

05036 12MAR76 8°42'S, 139°20'W 05021 3JAN76 0046'S, 89°30'W

05038 12MAR76 8°42'S, 139°40'W 05025 22JAN76 1016'S, 90022'W

05040 20MAR76 8°58'S, 140009'W 05028 24JAN76 0050'S, 89°41'W

05041 21MAR76 8°57'S, 140003'W 05030 5FEB76 0006'S, 90052'W

05043 21MAR76 8°56'S, 140 001'W 05033 16FEB76 0038'S, 91°33'W

05044 21MAR76 8°56'S, 140001'W Unidentified dolphin 05022 7JAN76 0048'S, 90011'W

05046 22MAR76 8°55'S, 140 001'W (T. Iruncatus or 05029 24JAN76 0050'S, 89°41'W

05047 23MAR76 8°57 'S, 140 006'W Delphinus delphis) 05031 16FEB76 0038'S, 91°32'W

'Included here for compleleness; lhese tagging data have not been published elsewhere.

7
1 co~MON POLPH N

(

/

IItl -histo y samJ Ie

I 1 I "

"~ '- \\ I (

i, -)

,

i -.~

~~ (I <J

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I .~ ,~ h8ll 188 V Ha••,1 I I , r~ i·-J L Re.yllla (\ ..... :--... . 148 "- "- '- )

134 CIlPIMrt IlOX 128 :.1126'" .(:\ (

.

. I

I

, lOll

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.

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,

~.~" .

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LONGITUDE

en

"'

en

Figure S.-Collection localities for adult specimens of Delphinus delphis included in body-length analyses. Adults defined as for Slenella attenuata. See Appendix 2 for

square-numbering guide.

(Perrin, Evans, and Holts 1979; White et al. 1981; Jennings and

Stives5 • Powers ,~t aI. 6 ). Part of the results (tag recapture and

resights) were presented and analyzed in Perrin. Evans, and Holts

(1979) and Jennings and Stives (footnote 5).

SPECIES ACCOUNTS

Stenella attenuata

Our results for this species agrec with those of similar earlier

studies (Perrin 1975a, b; Perrin, Sloan, and Henderson 1979) and

the results of the multivariate analyses by Schnell et al. (footnote

4). Our review of records within 100 nmi of the coast confirmed

the existence of a relatively large-bodied, heavily spotted form

living near the coast from northern Mexico to Ecuador (Fig. 9).

The nearshore zone is not inhabited exclusively by the coastal

sJen'1ings. J. G.• and R. K. Stives. 1979. Summary of preliminary results 01 a

dolphin tagging feasibility study. npublished meeting document 79/32. Status of

Porpoise Stocks Workshop, August 29-31. 1079. L. Jolla. California, 22 p.

·Powcrs.1. E., R. W. Butler. 1. G. Jennings. R. McLain. C. B. Peters. and 1. DeBeer.

1979. Sumr.1ary of research resuils from the fourth cruise of the Dedicated Vessel,

Septe'l1ue, 12 to October 31st. 1978. Southwest Fisheries Center Admin. Rep.

U-79-14, 46 p.

form, however. There are well·supported records of the smaller,

less-spotted offshore form within 20 nmi of the coast, in close

proximity to well-supported records of the coastal form (Fig. lOA,

B, C). Conversely, there are some we!!-supportt::d records of the

coastal form fairly well offshore, surrounded by records of the offshore form. The greatest distance from shore for a "coastal"

record supported by body-length data was 70 nmi (off central

Mexico), and the supported offshore record closest to the coast

was at 16 nmi (also off central Mexico) (Figs. 10, 11). The transition from "coastal" to "offshore" is relatively regular out to about

60 nmi, with a continuing low f.equency of sightings of the

coastal form out to 90 nmi (fig. 11). The distance at which the

number of included records east of the line which are "offshore"

is equal to the number of "coastal" records excluded (offshore of

the line) is about 35 nmi. This, of course, is based on the distancefrom-shore frequency distribution in the sample cotlsidered here.

Within 50 nmi, the boundary used in the past in stock assess:nent

and management, the composition (based on the stratification in

Figure 11) was 52.7% "coastal" and 47.3% "offshore." A Ene

drawn at 100 nmi from shore could safely be said to encortlpas~ all

of the coastal range.

The coastC\1 form may also occur around some of the oceanic

islands in the eastern Pacific (ReviEa Gigedos, Clipperton Island,

8
60'W

• Offshore

• Coastal

.. '

COASTAL & OFFSHORE

100'W 90'W

MEXICO

/

Continuation of offshore !ra!nlg!e!c!p!O!in!ts!nolt!pl,!o!tt!e!d>llllillllillll

110'W 5'S

Figure 9.-Records of offshore (dots) and coastal (x's) spoiled dolphins in the far-eastern Pacific.

Cocos Island, and MalpeIo Island), but there is only one sighting

record indicating this. An IATTC technician aboard a commercial

seiner appr0ximately 60 nmi off Malpelo Island in 1983 observed

a school of 15-30 robust and heavily spotted dolphins that "looked like coastals." The distance from the mainland was approximately 240 nmi. Without more records, it is difficult to say

whether the observer just happened to see a few heavily spotted

offshore animals (which would be expected under the racial

paradigm, given enough observation effort) or the animals around

the island are similar to those close to the mainland. We assume

the former, pending availability of more information.

Adult male coastal spotted dolphins average about 23 cm longer

than offshore males (those north of SOPS '79 north/south boundary-Fig. I-and east of long. l20 0 W) and coastal females

about 19 cm longer than offshore females (Fig. 12), although there

is broad overlap in length. The standard deviations for the coastal

~amples are apprecillbly larger than those for the offshore samples

(CY = 5.07 and 3.91, respectively). This and the right-skew in the

coastal female distribution may mean that some animals classified

as "coastal" should have been identified as "offshore," or that

intergrades are more common in "coastal" schools than in "offshore" schools, i.e., that the coastal form is more variable, perhaps

reflecting proportionately greater introgression of offshore genes

into the coastal population than the reverse. Another possible

explanation IS that coastal females continue to grow longer after

attainment of sexual maturity than do offshore females.

The coastal form has been characterized as occurring in relatively " ...small groups or schools or (sic) fewer than I 00 individuals" (Perrin I975a:I27). In our review of sighting records, we

found that while most records were of small groups of animals,

there were also several records of very large schools of the coastal

form (Table 2). Estimated school sizes ranged to 20,000 animals.

Most of the very large schools were seen off Costa Rica, south of

Puntarenas (lat. 8°_lOON and long. 83°-85°W), while some were

seen off central Mexico (lat. I5°N).

Schnell et al. (footnote 4) concluded that the results of their

multivariate analyses of skull data "fully warrant(s) the recognition of the southern S. attentuata as a distinct stock." They found

the skulls of spotted dolphins from south of the Equator to be, on

the average, larger and to have a smaller temporal fossa than those

of animals from north of the Equator. They reached the same conclusion from analysis of body-length data; they found the southern

animals to be, on the average, shorter than the northern animals.

The results of our review of body-length data are the same (Fig.

12). Males from the eastern portion of the SOPS '79 "northern

offshore" range were modally about 3.5 cm longer than those

from the "southern offshore" range (this difference and others

between the offshore samples are statistically significant at P <

0.00 I); the difference for females is 2.5 cm. When the southern

sample is compared with the portion of the northern sample from

west of long. 120 0 W, the difference is smaller, 2.0 cm for males

and 1.3 cm for females, a result consistent with the finding by

9
MEXICO 20'N

25'N =----'-;__--~o::::::_---""-r_;__O!"-"""'-------_,_-----------,__-----_r_-----....,

, ... :.... ...., COASTAL & OFFSHORE /

/, ' Coastal • ':-- '. & Coastal with voucher data

• ' ",&'> "-s: &'> Offshore

?/~-0 . ., .i. Offshore with voucher data

'1'/ /'~~ .i> &. :" 0&.

.•a.. 00 ...... : &.

m ... "~'.' '.

': . .' ". .'l ':.....'t::~',: .

"" '.:- .:.::::.:-,:.,: .:. :::i.~~~;:;,. ., .' ::. JIo4 "",.

I • °

0 • • 'l' •sOo" ...

• 0. • ~;- 0° •

• , " '&, .~•• :'. "

" .'. . "I1!J' " •••' :::':;' '. I ',l:\.. «: '. .' ...... !._+

I

« "" :r": "i'~ ','•• 4. '.~ Acapulco

~~ /A -:.,'. '1' ·.:·'.'.···....-..,.·t·'~ '.::' ",i.. 4Ik /.;: r////;o" .,••,° 0 •• 0°:' .:i~. ,. °1 0

•• I•• '////, ::-:~::.t·:. ','.,>:~ " '~",: . "" ",': , 'l; 0 // 0~ '/~ .. :. ) .. ,.... 0_.,.... .• .

l5'N /'////, W///. /'/// / //// /'///. /'///,/% W/1/" I\·!~\'.:.....<'::f~.::·~:;;:·~;(~·:~,t:~~:~'t~~.: :.,~.-. Continuation of offshore range (points not Plotted~)~ "~·••:.t'·:i'J .,.~~,.:,.;,'4':~~:::~ .... I '. I~"

~~.~/''/~/'//••.//// '/~-«:.fI~/~ ...'.'.-. ,.'.'1 ~o'//~' .,,'. '. • :,,:.;,:.:.~.;;~,~.>~:: . //// ///0 ':\"'':J{~:' . //:/, '/0 • 'l'/. 1~ . :["~'l~i // • -,;. :i..c{:."....~:--': ~:.:.'~: •.:i, \(~.~.. ////////'///'0~ ,;0 all//./:/~· _ . .' ./, ff%z /'////, '/, ':.::-

:,;0 ~~;0 'l'/. all. «:

W#&//&, ////0,/;«:

l15'W 110'W 105'W 100'W 95'W

20'N r------------,..---,-------,------,------------,------------.....,

COASTAL & OFFSHORE

, Coastal

& Coastal with voucher data

Offshore

'" Offshore with voucher data

"

CENTRAL AMERICA

75'W

..

~,..

'.

.. l''. ,

, ,

1111111111~i~~~~~~~~~~~~~~t~'~·~·~·~~~~~~l~~~_:l J .. : a. I

'~ ..

.. SON ': ••~.

95'W 90'W 85'W 80'W

.'

10'N

10
SON

0°5

5°S

•

','. :. ! , • &

'. . -.1 . , ,.-:. .. . . , : '. .. .... , " .. , .' '. · , . . .. . .. . . .. ., .... , .... , . ,- .. .'

·:·~·I . & SOUTH AMERICA

-

, .

COASTAL & OFFSHORE

• Coastal

& Coastal with voucher data

I • Offshore

£ Offshore with voucher data

-

Continuation of offshore range (points not plolled)

• ,

.'

900 W 800 W 75°W

Figure 10.-Records of offshore (small dots) and coaslal (x's) spotted dolphins off A) North America (upper left), B) Central America (lower left), and C) South America

(above), with records that are supported by photos, measurements, or osteological specimens indicated (triangles).

SAMPLE SIZES (sightlngs or collections)

(51) (86) (110) (112) (87) (79) (60) (48) (71) (68)

100

OFFSHORE FORM

In=461 )

DISTANCE FROM COAST OR ISLAND (nm~

10 20

L86~1 t

S82 I

COASTAL

(n=242)

FORM1: 27.7

I L 17.21 C

I ,-6.7 i 63 I 3.4--' (I' ---c...- 2.8--,

100

90

'"

0:-

o 80 ... 0

..J z 70 '" u ... .!!

~g 60

u .. ... 0 50 o g' w;:

u" 40

z.!!'

w 0

30 0: _

0: 0

"iii u_ 20 u

0 10

0

0

Figure 11.-Relative abundance of records of offshore and coastal

spotted dolphins by distance from coast. "C" is most offshore

"coastal" record supported by photos, measurements, or

osteological specimens; "0" is most inshore "offshore" record

with such support. "A" is distance from coast at which the proportion of included records that are offshore equals the proportion of

coastal records that are excluded.

11
186.6I

175-220cm 168-206cm

SOUTHERN

OFFSHORE

9(314)

8=7.12

185.3

I

SOUTHERN

OFFSHORE

r:! (86)

8=7.52

30

10

20

0

250 18?6 200 250

177-228cm NORTHERN 165- I

215cm I NORTHERN

20 OFFSHORE I

I OFFSHORE

r:! I 9

- I

- West of 1200 I West of 1200

C

Q) (200) I (499) CJ I .. 10 8=9.10 I

Q) I 8=7.34

Q. I

- I

> I

0 I

0 I

Z 250 W 187,8 200 250

:;:)

0 166-240cm NORTHERN 163-215cm NORTHERN w

a: 20 OFFSHORE OFFSHORE

u. r:! 9

East of 1200 East of 1200

(994) (3077)

10 5=9.46 8=7.28

180 200 220 240250

197-246cm 179-227cm

I

I

I

o......--+-...l-~ ...........L.........L.4~4-~..:.I.I.....4........-+-..L.....4

150160 180 200 220 240250 150160

10

BODY LENGTH (em)

Figure 12.-Frequency distributions, ranges, means, and standard deviations of body length for male and female spotted dolphins, 51enella attenuata, from

four geographical areas. Sample sizes in parentheses.

12
10

Figure 13.-Frequency distributions of ovarian-corpus count for northern and

southern otTshore samples of adult female spoiled dolphins.

N. OFFSHORE

SPOTTEDEAST OF 1200 W

("=3,097)

NUMBER OF CORPORA

Avg. 7.1 corpora I

I

I

5

t- O

Z 0 10 20 30 40 50

LIJ

0

a:

LIJ 10

0- Avg. 7.8 corpora I

I

I

I

I

S.OFFSHORE

SPOTTED

5 ("=317)

not recovered) at lat. 10037'S, some 4-5 mo later. This result, if

confirmed, would suggest that the apparent north-south morphological differences may actually reflect a c1inal situation similar to

that (east-west) which may exist in the northern offshore portion

of the range, rather than a substantial separation of populations

north and south of the Equator.

The results of a study of breeding seasonality (Barlow 1984)

show very different distributions of birth dates in southern and

northern offshore spotted dolphins (SOPS '79 boundaries). In the

south the single calving season is quite short, March to May,

whereas in the north there are two diffuse breeding peaks, in the

spring and in the autumn. Barlow stratified the northern sample

into two subsamples, from lat. 0°_S ON and from north oflat. SON,

and found an intermediate pattern in the equatorial stratum of a

single but diffuse calving peak from March to June, statistically

different from the patterns both to the south and the north. He

concluded that these results support the view of a separate population south of the Equator but that the transition from a diffuse

bimodal breeding pattern in the north to a single sharp peak in the

south may not be abrupt. He offered two possible explanations for

the apparent patterns: I) if seasonality in reproduction is determined by the timing of oceanographic or trophic factors, the difSchnell et al. (footnote 4) of affinities between southern skulls and

a very small sample of skulls from the far-western part of the

northern range. The samples used in our body-length comparison

covered the geographic range of the species more completely than

did the samples of skulls available to Schnell et al. (Fig. 5).

Two additional lines of evidence suggest that the southern

animals are smaller than the northern animals: I) Hohn and Hammond7 estimated length at birth from northern offshore samples at

85.4 cm. Their estimate from southern offshore samples was 83.2

cm. Their conclusion that the southern form is about 2 cm shorter

than the northern form at birth is tentative because of the rather

small sample size for the southern form. 2) Average length at

attainment of sexual maturity was estimated at 177-180 cm

(1974-82) and 175 cm (1973-82) for northern and southern

samples, respectively (Barlow in press).

Because both females and males can continue to grow after

attaining sexual maturity (Perrin et al. 1976), we considered the

possibility that the differences between the northern and southern

samples in average lengths of adults reflect not inherent racial difference in size but difference in age structure in the two regions.

Samples from the southern region have not been aged, so direct

comparison of age structures was not possible. We were able,

however, to compare the frequency distributions of ovariancorpus count for the two samples (Fig. 13). If ovulation rates are

assumed to be similar in the two regions, it is clear that the age

structures for adult females are not radically different. In any case,

the difference is in a direction opposite that which would be expected were the adult southern animals smaller because of being,

on the average, younger.

The results of tagging studies (Perrin, Evans, and Holts 1979;

Jennings and Stives footnote 5, and unpublished data-J.G. Jennings8 ) indicate considerable east-west movement within the

northern offshore range. Of 206 tag recoveries or resights of tags

or fin notches, 9 indicated movement from east to west of the

120th meridian; three involved movement of at least 1,300 nmi,

over a period of 9-10 mo. While this inference of long east-west

movement must remain provisional (none of the 3 cases involved

the actual recovery of a tag), when it is considered in the context

of a possible east-west cline in morphology (body-length, coloration, and skull measurements), it suggests that such morphological

differentiation is maintained despite at least temporary long-range

movements of individuals and perhaps schools. One tag released

north of lat. lOON was resighted by a tuna-seiner crewman (but

'Hahn, A. A., and P. S. Hammond. 1983. Growth in the first year of the offshore spOiled dolphin, Slenella attenuata, in the eastern tropical Pacific. Southwest

Fisheries Center Admin. Rep. LJ-83-08, 33 p.

8J. G Jennings, Fishery Biologist, Southwest Fisheries Center La Jolla Laboratory,

National Marine Fisheries Service, NOAA, P.O. Box 271, La Jolla, CA 92038, pers.

commun. January 1983.

School size Coastal form Offshore form

(no. of dolphios) n % %

<100 144 63.2 59 13.2

(min. <10) (min. <10)

100-1,000 64 28.1 270 60.3

>1,000 20 8.8 119 26.6

(max. >20,000) (max. >20,000)

Total 228 448

Table 2.-Estimated school sizes for sightings of coastal and otTshore spoiled

dolphins within 100 nml of the coast. Ranges of school sizes in parentheses.

13
ference could reflect geographical change in the timing of the

environmental determinants, or 2) the area between lat. 0° and

5 ON could represent an area where two populations can occur.

with the apparent distribution of birthdates actually being a composite of two different distributions. He noted that if a previously

noted relative hiatus in sightings in the vicinity of the Equator

(Perrin, Sloan, and Henderson 1979) is real, the latter explanation

would be the least likely. The band of low density of sightings for

2°_3° south of the Equator is still present (Perrin et a!. 1983) but

not as well defined as previously.

Hohn and Hammond (footnote 7) have confirmed Barlow's

results with modal-progression analysis of monthly distributions

of length (after adjusting for use of different growth curves in the

two studies), finding birth of two annual seasonal cohorts in the

north but only one in the south. They also found a pattern of variation within the northern range; data from the far west and from

along the Equator do not fit well with the pattern in data from the

more easterly and northerly major part of the range. They concluded that while spotted dolphins may have different calving

seasons depending on area of capture, this seasonality is probably

-'not actually a function of area but of schools or groups of schools

which tend to inhabit different areas with different environmental

conditions. "

Resolution of the nature of the east-west and north-south variation mllst await study of more material from west of long. 120 0 W

and from near the Equator (see also the discussion below on

origins of the geographical forms).

Stenella longirostris

In our review of records within 50 nmi of the coast, we found

only 33 convincing "Costa Rican" records, all south of the Gulfof

Tehuantepec (Fig. 14) and north of the Bay of Panama. Of these,

16 were supported by photos or measurements of specimens (Fig.

15, Table 3). We found no coastal records of eastern spinner

dolphins with such support below the Gulf of Tehuantepec and

only six unsupported records there. It appears that the eastern

spinner dolphin is largely, or possibly totally, replaced on the

coast below about lat. 15°N by the larger, longer beaked Costa

Rican form. We ascribe the relatively small number of convincing

records of either form to the difficulty of estimating size of

animals at sea; the two forms are not known to differ in coloration. The sample of body lengths for the Costa Rican form taken

directly from specimens is very small (Fig. 16), but it is probable

that modal adult length differs from that of the eastern form by

about 30-40 em in both sexes. A very large sample of photogrammetric lengths for both forms supports this conclusion (un pub!.

data, M. D. Scott).

We found several well-supported record, of the eastern form

outside the SOPS '79 boundary (Fig. 14); the range of this form

-t---+---l-J

90' W BS' W BO' W 75' W

\

\

\

-r-~-~~=---T--~--i--r--+--t-lrr----i

,

o M.rqu.... • 0

11\1:1- --+--J--~I~'~:....----+-----+---+--"=------'f~~---1 I ! . ,

+---+---+---+---+---+---+---+--~-'--Y-----<~k>"'-+---+--+-+---+---+-----'-'+--rt-~

!

V"> +---+-----4----l----+---I--'=--!---+-

z

.",

l"l

Z

'0

l"l

Z

.",

'"

z ., '0

·0

N

"'I

i cr.[

·~r--

:1 l

·u)+I---+---+--<~-+---+---+---+---+--_+--_+--_+--_+---_+---i"~L--t_-O>"_.;;;:_+-_j'-+---'-<i

:I I·,

.?d 1-- J__-+__T_Oh_It_'-+---+---+----I---I---I~--I~--I~--t:--- 16u' lSS' lSO' W 115' W 110' W 135' W 130' W 125' W 120' W llS' W llO' W lOS' W 100' W 95' W

~I

i

.;t--';--I---I------iI------iI----'---l'---~_r/lf_L_:~~~~~'"":~'.._oIi'"I"- ::J L "

S-: Ichrl-:-,m-..+--'----=t::::==F=-d----+--=--+..:....--+---+--=:::!::"+=--+---.:··~+-.-· ~--+--"--+.:.-~-+~a.;r:¥~"7-rt-++---l

,

z

LONGITUDE

Figure 14.-Distribution of eastern and Costa Rican s!,inner dolphins in the eastern tropical Pacific, showing records outlying the SOPS '79 boundary for the eastern

form. Outlying records enclosed in squares are supported by specimen data.

14
Table 3.-Records of Cos'a Rican and eastern forms of Slenella longiroslris

within 50 nmi of the coast (or a coastal island), north and south of Gulf of

Tehuantepec ('V lat. ISON). Number of records supported by photos, bodylength data, or skull specimens in parentheses.

Gulf of Distance from Costa Rican form Eastern form

Tehuantepec coast (nmi) (records) (records) Total

North of Gulf 0- 9 0 0 0

10-19 0 8(1) 8(1 )

20-29 0 13(2) 13(2)

30-39 0 7(0) 7(0)

40-49 0 11(4) 11(4)

Subtotal 0 39(7) 39(7)

South of Gulf 0- 9 3(2) 0 3(2)

10-19 1(1) 1(0) 2(1)

20-29 7(3) 5(0) 12(3)

30-39 '19(9) 0 '19(9)

40-49 3(1 ) 1(0) 4(1)

Subtotal 33(16) 7(0) 40(16)

Total 33(16) 46(7) 79(23)

'Includes 17 sightings made in a small area off southern Mexico during an aerial

photogrammetric experiment by the Inter-American Tropical Tuna Commission in·

1980 (see Fig. 15). Other intervals contain records from this source as follows: 20-29

nmi, 6; 40-49 nmi. I (all Costa Rican).

extends farther to the west than previously documented, to at least

long. 134 oW, albeit in apparent very low density relative to that

of the whitebelly form (Fig. 17). We found only one wellsupported record of the whitebelly form to the east of the SOPS

'79 boundary (Fig. 17). We attribute the large number of unsupported whitebelly sightings to the identification difficulties

discussed in the Materials and Methods section and believe that

many of them probably represent sightings of misidentified

eastern spinners. In any case, the inshore density of whitebelly

spinner sightings is very low relative to that of the eastern form

(Fig. 14).

Rather than draw boundaries around what are believed to be the

major ranges of the two forms, as has been done in the past, we instead present a summary of relative frequency of records by 5° square (Fig. 18; values in Appendix 2). The sample sizes on the

fringe of the continous range of the species in the eastern tropical

Pacific are small, but those in the major zone of overlap are very

large, and the proportions likely reflect the true relative abundances of schools of the two forms. As discussed above, the real

levels of whitebelly-spinner-dolphin records in the northeastern

reaches of the eastern range are likely lower than indicated, e.g.,

we believe that the anomalously high proportion in the square at

Eastern

Eastern with voucher data

, Costa Rican

4:. Costa Rican with voucher data

EASTERN & COSTA RICAN

~"

: . /;--(.". • $:.~ • ~.,.\..... ' I . ....~::.~.:.:::\- ..,. . ·e.· .

• h ".; .:.a·r:rt~,I:.::P~':~;r~:...:' li: I

;.: :'·~::·'~1·· l"l;~~"."..$. ·'I.'!.:· :(. " S..~··:i·:· ~·,:···~,,~~t: .• -::.,: .=*. ~.i .~.: '. I .' f'" ',,"'\ (1)" , ;.L',:y; 'C ":,' ~"'O: ":,' ..,'.,' .... .~'.:: l·itz:. " .. ...Y;I-:": # .(1:'1': •.... t ....'.. ,

..,.,1':".,." .. ' ";"':'~·"f ...'\j:.·~i· '1~'\"":' ~'. :':,.;/ .'t-...... '1! ,., w.' c '-, • "',. ) ... .... ,.' ..... :"-:-1(-;.•, ",_,'1:'1 :,.· ',·';1~: .. ·I•• ·: ••",y ..

~., 7- .,. • ,', "~:. \:J.', " If':;" •• '.-••:" ..' '•• '. ., •••~~ ':':-: ...... ~, : ···C:· ,· ,~,l:.·,· r~' .{~I·•• ~' .'. . "'•. : '.r·;': ,,;•• L' 't. •• .. ,.••• :\;' •

." .'1":' '. I" ••'\ ,.' 'I' . .' ,,,\ _Y'!. .". r" .. ~i:':'..4\1,,' ..' .". .~ , ':' .,)" • ' '... .. •.' "~t':"": ..... ' '-;"::--;;fj.-i",:. "-,:.,": •-£ •,~,»+.0-'. '(----'" . . ". ..' .-, -L•••,.·t-'.'----,,--------'hld.._

'··~r:·:~.;. ~:.... ~~: ..~~... ... ~:.:': . ',",,:. ~::'"

~~~:.. ,: I::.::,::.':,.J '~~':'~'~ ·~~'::'~·=;~"Cl~. .:...... .. . ... :. •• s ••••• :: ••:. • •••••• •.... , •• • ... . .. , .. :..: .[ : \ ,~ '" .. .: .~, ":"'f ~.;\:- ..' '~ ,..I

f .. :...~' .'.' . .•. . . .... ::·::·~.l·:· . ... .. ..... ... .'..... : . ': ... . t---------+---------j------;----+------.•.--.-.-,~..-.-.------'.'----.';..----.---+---'-\

,',.. '.. :

..;-

'.

o

Figure IS.-D1stribulion of eastern (dots) and Costa Rican (x's) spinner dolphins off Central America, with records inside 50 nmi supported by photos, measurements,

or osteological specimens indicated (triangles).

15
BODY LENGTH (em)

forms. Our comparison of body length for northern and southern

whitebelly spinner dolphins using the SOPS '79 ranges showed a

difference of about 2-3 cm (significant at P < 0.0 I) in males and

females (Fig. 15). No new information is available on color pattern of the southern spinner dolphins beyond that given in Perrin,

Sloan, and Henderson (1979).

Because of the possibility that the difference in average body

length could be caused by a difference in age structure (Le.,

relatively more younger and smaller adult females in the northern

region), we examined the frequency distributions of ovariancorpus count for the two samples (Fig. 19), again assuming similar

ovulation rates. The distributions do indeed differ, with a higher

proportion having fewer than three corpora in the northern sample than in the southern sample. This may reflect differing age

structures or differing ovulation rates; resolution of the question

will require comparison of aged samples (not yet available for the

southern form).

Tag returns for S. longirostris from the eastern tropical Pacific

have been few and suggest less extensive movement than do those

for S. attenuata (Perrin, Evans, and Holts 1979). There has been

one sighting (by a yachtsman) of a tagged animal in the Marquesas

(a spaghetti tag of unspecified type; J. G. Jennings footnote 8),

where nine spinner dolphins were tagged in 1976 (Table I). There

have been no tag returns indicating movement across the Equator

or between the far-eastern and far-western parts of the eastern

tropical Pacific above the Equator.

Barlow (in press a) found a single sharp seasonal breeding peak

in the eastern spinner dolphin and at least two modes in the northern whitebelly spinner dolphin. He also found a regional difference in timing of the peak in the eastern spinner dolphin, with

offshore animals calving earlier in the year than inshore animals.

He noted that distributional studies indicated that there are inshore and offshore population centers for eastern spinner dolphins

with a decreased density of sightings in between (Fig. 14 and Au

et al. footnote 3) and suggested that a further subdivision of the

eastern spinner into two populations/stocks would be meaningful.

He did not examine seasonality in spinner dolphins south of the

Equator.

Stenella coeruleoalba

198.8

I

I

r

I

I

I

193-211cm

COSTA RICAN

(12)

240 140 160 180 200 220 240

217.7

COSTA RICAN I

I

30 cf (3) I

I

I

I

212-216em I

I

20 II

I

I

I

I

I

'0 I

I

I

I

I

I

0

'40 '60 180 200 220

182.4

30 , SOUTHERN SOUTHERN

WHITEBELLY WHITEBELLY

cf (147) ('54)

20 5"8.36 5:].08

165-235cm 162-198em

'0

0 160 200 240 17,5.6 240

30

NORTHERN NORTHERN

WHITEBELLY WHITEBELLY

20 cf (382) (483)

5"7.31 5"6.82

160-203em 157-196em

'0

'E.,

u

e '60 1~6.1 200 240 200 240

>

0

Z

w 30 EASTERN EASTERN :::l cf (431) (697) a

w 5'=6.12 5=5.99 a:

... 20 l6o-l9gem 152-l93cm

'0

0 200 240 '80 200 240

40

Figure 16.-Frequency distributions, ranges, means, and standard deviations of

bo<ly length in samples of adult specimens of 51enella tongiroslris of four stocks.

Sample sizes in parentheses.

the tip of Baja California may be the result of unfamiliarity with

the two kinds of spinner dolphins by south-bound first-time

observers.

Eastern spinner dolphins are, on the average, about 3-4 cm

smaller (significant at P < 0.00 I) than (northern) whitebelly spinner dolphins (Fig. 16). The length-frequency distributions for the

two forms are very similar in shape, and the variances are small

compared with those for other samples examined here, suggesting

that the two populations are well delineated and accurately represented by the samples.

Schnell et al. (footnote 4) found "marked differentiation of a

southern form from populations to the north" based on

multivariate analysis of skull measurements. They found skulls

from the south to be larger, with a proportionately larger temporal

fossa, than skulls from the north, a situation exactly the opposite

of that found in S. attenuata (smaller skulls, with larger fossa, in

the south). As was the case of S. attenuata, they noted morphological affinities, in some of the analyses, between southern

specimens and those from west of long. 120 0 W. They also found

southern spinners to be longer than northern specimens. However,

they did not distinguish between the eastern and whitebelly color

Very little new information is available for thi, species.

Delineation of the three tropical stocks was based on apparent

latitudinal gaps in distribution, pending availability of adequate

series of morphometric data (Smith 1983; Au et al. footnote 3).

The band of relatively low density of sightings between lat. 10°

and 15 ON is apparent in the data (Fig. 20), but the distribution

now appears relati vely continuous south of lat. 15 ON and east of

about long. 115°W, with a region of lower apparent density

around the Galapagos Islands (which may only reflect the lesser

sighting effort there-Perrin et al. 1983, Fig. l). It would seem

that there is now little or no justification for dividing the animals

in this area (south of lat. 15 ON and eaSt of long. 115 OW) into two

stocks based on distribution data alone. The same applies to the

eastern and western concentrations within the "central tropical"

range; sighting effort has been relatively lower in the region between them. Only the low-density band between lat. 10° and

15°N at this point can be said with some confidence to be real.

There is a 200-300 nmi wide swath extending northeastward from

long. 105 oW where no striped dolphins have been sighted despite

extremely high levels of sighting effort. We conclude that direct

interchange between the population centers to the north and south

probably occurs so seldom as to be negligible. In the region im16
i-T II

----t--

M.r~ ••••

. . i-f-I I --J--1 ---+---+- 1- -~-- 0"' J" 'Tahiti' I I ; I

l'l - -- --J--f--------,f----f- -/---/-- - -i----J--f-------J--+--t-------J 160' WISS' W ISO' W 115' W 110' W 135' W 130' W 125' W IZO' W liS' WilD' W lOS' W 100' W 95' W 90' W 85' W BO' W 75' W

LONGITUDE

z

0.,

'"

:;11

,

zl

z

z

.,

W , 0 °0

:::J ~,

(-< I

I "-

z! "-........ (-<

a:

-..J 0.,

Chrl:tmu

l-~: "' I I

0

Figure 17.-Distribution of whitebelly spinner dolphins (northern and southern) in the eastern Pacific, showing records east of the SOPS '79 boundary. Record enclosed

in square is supported by specimen dala.

mediately to the west (between about lat. 10° and 15 ON and long.

105° to 120 0W), where sighting effort has also been extremely

heavy, there are only a few records of the species; the density here

is apparently also very low.

In the area north of lat. 25°N, where sighting effort has been

extremely heavy to at least lat. 31 oN, scattered sightings of striped

dolphins have continued to accumulate (Perrin et al. 1983). but it

would appear that the animal occurs rarely in this warm-temperate part of its range in the eastern Pacific. It seems to us unlikely

that this represents a separate population center. Similarly, the

deduction of the existence of a "southern temperate" stock south

of lat. 10 0S is probably also not justified.

The very limited adult body-length data (most of the animals

killed have been juveniles) do not suggest a modal difference in

length between animals from the .northern and central tropical

ranges (Fig. 21). Only one adult (a 204 cm female) fro!ll the

southern tropical range has been measured.

Delphinus delphis

Although "Baja neritic" and "northern tropical" stocks were

defined by Evans (1975,1982) based on morphology and color

patterns (Fig. 4), these have been pooled in assessment and

management (Holt and Powers 1982; Smith 1983). Common

dolphins sampled in the fishery and tropical sightings have been

identified only as "Delphinus delphis" and allocated by position to

the "northern tropical" (3 °N-100S) latitudinal management units.

This pooling is evident in the "northern tropical" (between lat.

15 ° and 28 ON), "central tropical" (lat. 3 °-15 ON), or "southern

tropical" body-length data (Fig. 22); the Baja neritic form is about

30 cm longer than the northern tropical form (Evans 1982). The

animals included in the "northern tropical" sample came from

two areas: Baja California (Squares 168, 169, and 189 in Figure

8), and off southern Mexico (Square 146). Those from north of

Magdalena Bay (Square 189) appear from their lengths (shaded

portions of hIstograms in Figure 20) to have been of the Baja

neritic form, whereas those from south of Magdalena Bay and

around Cabo San Lucas into the Gulf of California (Squares 168

and 169 iii Figure 8) appear to have been of the short-beaked

northern tropical form. Evans (1982) showed the range of the Baja

neritic form as including the Gulf of California. Some of the Baja

neritic common dolphins on the outer coast were taken more than

50 nmi offshore, and some of the short-beaked "offshore"

animals were captured within 10 nmi of the coast (Table 4).

When this is considered in the context of the pooled distribution

data (Fig. 23), it is clear that common dolphins off Baja California

cannot be identified to stock on the basis of an inshore-offshore

criterion alone. The two forms are, however, fairly easily identi17
30° 11----.-jI----+--+---I---------1---+--+--+---'--...\c\

Hawaiian Is.

20°

0° 1---+-----1----1----1---

10°1 f---+----+---

Tahiti'

20°1 f---+----+---+--.+---+---------1---+--.+---+----+--+--+---I----+--

300~=~=:!::::::=:!::::::===~=!:===:!~==:!=:=::!:=~===::!:=~=====~=~==!:::==!:::==:!.::::!.J 160° 150° 140° 1300 1200 1100 100° 900 800 70°

Figure IS.-Racial composition by S° square of sightings of 51enella longiroslris identified to geographical race. Open areas of squares represent eastern form (and, for

four squares, Costa Rican); shaded areas represent whitebelly form (northern and southern). Sample size given for each square. Number of Costa Rican records in

parentheses.

15

Avg. 7.1 corpora

SOUTHERN

WHITEBELLY

SPINNER

("=155)

o 10 20 30

NORTHERN

WHITEBELLY

SPINNER

(n=487)

>

U

Z

W

::J

o

W

a: 5 LL

CQI

CJ 4i 10

.e

Figure 19.-Frequency distributions of ovarian-corpus counts in northern and

southern samples of adult female spoiled dolphins.

NUMBER OF CORPORA

18
·0

N

Z

III

Z

W

0

:=J '" f-< I / ..:- ... :::~'~:" .~'::,:.., I' .:,' . '. '. ·,"C.::'·.·: ::.:·::..~.:;:'~:·il~~.~;.~· ..~, t..~~

I' .' °l;o., . '. °0 "0' ... J .: ~: ..;.~ ·~'~';~~:'.;r:-;~_~~ .. z l---~/~---+---+---d_"""'=t===:j:::="""'~;;::---+~.-:--+-;-/-----J~.'-;---+----:-~...::+-~-+.----:=:: ..:....-"'t-'~':",:,c::~~osf.i::=;':":::'-A~.:-t-:'~"~-1 -J ·1Il I ,.....- " .' . .:'-: I •••• ;.t;. >.:. )',It:;':.,.~

chds:mai ,/ .. .J'" " ". '.. ~::.: '.: ....;.. r.'~~:r --,/ ,.,/ V . eO'. • • ,_ •.,,- :0

·0 ~_.__'~_- ~. :._. 'J. .,., ",_~"~ .: ,'. ::.. (

. '0- ......nP.oo .. : '0'·"

-: ..--4---+---1-1 - "-....---=t=::::.:-=-iL------1------4--·.....::.... ::,..~--_+--_+--__1f____-::..·;_: '-,'t':-,..:::-.~.:.~..::.:=:.~T'~'::':"";,-;" ..... :.:.:"~:..j';..~:.!~•..L~i_·-\()rl----j

...... _--. 0° • .. : •• '~'. ' ••• ' • \"",

.,.---..Lr---...- .... ....~... .j\ o MarQueaas I ,

0+---+----1---+---"1-'--"-'-+-+---+-_I----j-----+-+----P~'_\-:-. :..'+-----.~.,. J-'-.</,-",;- .•-1.Vf----7"--+--'\;--!

lIl+---t---t---+--+--+--+---+----If---t---+--+--+--t-="'---r=---I-·--+--~

Tahiti

l<l+-:---+;----+:---+---+c---I-,---+:---+--+---+:----l:---I-;---+:---+---+---+----1:-----I 160' ISS' ISO' liS' 140' W 135' W 130' W 125' W 120' W liS' W 110' W lOS' W lOa' W 95' W 90' W BS' W BO' W 7S c W

LONGITUDE

Figure 20.-Records of Slenella coeruleaalba in the eastern Pacific. Boundary from Perrin et al. (1983).

205.4 I

I

60 60

NORTHERN NORTHERN

TROPICAL TROPICAL

cf (9) 1971 40 40 9 (21)

20 20

0 0

200 240 240

CENTRAL 214.9 198.3

TROPICAL I I CENTRAL 20 I 20 I cf( 10) I TROPICAL

I 9 (22) I

I

0 0 160 200 240 160 200 240

BODY LENGTH (em)

19

>

()

Z

W

o

w

a:

LL.

....

-C

Q)

(,)

...

Q)

a.

--

Figure 21.-Length·frequency distribution and means of body

length for nothern tropical and central tropical adult males and

females of Slenella coeruleoalba. Sample sizes in ,.rentheses.
30 189.3 30 ,

NORTHERN NORTHERN

20 TROPICAL TROPICAL c! (62) 20 9 (90)

-.. ...... SQ.168 & 169 c

CIl 10 10

CIl

Eo

> 0 0 (,) 200 250 250 Z

W CENTRAL CENTRAL

0

w 30 TROPICAL 30 TROPICAL cr. c!(1771 9(306) u. 206.8 5"9.25 I 5=6.80

20 I

! 20

10 10

0 0 150 250 150 200 250

BODY LENGTH (em)

Figure 22.-Frequency distributions, ranges, means and standard

deviations of length in two stocks of Delphinus delphis. Sample sizes

in parentheses.

. "..:.r

~~::.' • ~: ~. eo' ':e o :.:'

+---+---1-----+--+-+...0---+.--.;:.. : '. I .

----rC."~I"'·II

-,-------,-- -----r---r------,-~____r---T

I

';~~ I t---+---r-T-'--+---+----+- - --i---

o<f'~-~ -_. ~-. t~--+------+----+----+----+-~-=-------.,.--"'" '-,'- __~i "---+-.---j-----j

0;.1 . "T••II , . ----'-.---.f---+- -------:- ,.. ---;---

160' W155' W 150' W 145' W 140' W 135' W 130' W 125' W 120' W 115' W 110' W 105' W 100' W gs' W 90' W 85' W 80' W 75' W

0<f'

'"

Z

°0

'"

Z

J)

N

Z

°0

N

Z

J)

Z

W "0 0

:::J

f-<

f-<

a:

-.J J)

Figure 23.-Records of Delphinus delphis in the eastern tropical Pacific. Boundary from Perrin et al. (1983).

20
Table 4.-Distances from coast ofspecimen rewrds of Delphinus delphis

in Square 189 (Baja neritic form; north of lat. 25°N) and Squares

168-169 (northern tropical form; south of lat. 25°N). Most offshore

record of Baja neritic at 51 nm;; most inshore northern tropical at 51

nmi. See Figure 8 for locations of squares and Figure 22 for specimen

data.

fied at sea by size, shape, and coloration (Leatherwood et al.

1982).

In addition to the specimens indicated in Figures 8 and 2, there

were 21 animals measured from Square 189 for which gonadal

material was not collected that would allow them to be classified

as adult or immature. The lengths of II males were 153, 164, 195,

208, 218, 225(2), 227, 228, 230, and 236 cm. The lengths of 10

females were 181, 197(2), 199,205,206(2),209,213, and 215

cm. These lengths more likely belong to the same frequency distributions as the adult animals from Square 189 than to those of the

animals from Squares 168 and 169 just to the south. Therefore,

maximum length of the Baja neritic form can be assumed to be at

least 236 cm.

The sample from off the state of Guerrero in southern Mexico

(Square 146) is too small (2 males and 3 females, cross-hatched in

Figure 22) to allow a firm conclusion to be made, but it apparently

does not belong to the same distribution as the animals from

Squares 168 and 169.

Evans initially (1975) did not subdivide the short-beaked

"temperate-subtropical" animals north of lat. 20 0 N into

temperate and tropical stocks. He later (1982) defined a "northern" stock in the eastern tropical Pacific extending from about tat.

16°.18 ON to just below Cedros island (about lat. 28 ON) but did

no! delineate a "temperate" stock to the north. Tl:e 1979 assessment used "northern temperate" and "northern tropical" management units divided at lat. 28°N. The very extensive distribution

data do not now suggest a gap or transition of any sort down the

entire length of Baja California, and the "northern tropical" bodylength data (Fig. 22) accord precisely with Evans' (1975) pooled

sample of all short-beaked "temperate-subtropical" animals from

north of lat. 20 0 N. Thus no evidence exists to support dividing the

short-beaked common dolphins at lat. 28°N.

Evans (1975) found short-beaked common dolphins from south

of lat. 15 ON ("central tropical") to be modally longer than those

from north of lat. 20 0 N as well as differing from them in skull

features and possibly in color pattern. The present larger sample

of central-tropical length data (177 males and 306 females, vs. 40

rnales and 48 females in Evans (1975» confirms this; the difference is approximately 18 cm in males and 16 cm in females. With

the exception of the local concentration of records off southern

Mexico between about long. 97 ° and 103 ow (Fig. 23), the

central-tropical concentration (centered around lat. 8°_1 OON) is

separated from the northern-tropical concentration by an 800 nmi

Distance from

coast (nmi)

0- 9

10-19

20-29

30-39

40-49

50-59

60-69

70-79

80-89

90-99

Total

Baja neritic in

Square 189 (no.)

1

2

2

4

1

10

Records

Northern tropical in

Squares 168 & 169 (no.)

I

7

7

10

12

12

8

9

4

6

76

21

wide zone in which sighting effort has been heavy and sightings of

common dolphins rare (only a dozen or so, compared with several

thousand sightings of other species). This separation and the morphological differences speak strongly for treating the two populations as separate stocks. The coastal animals from between long.

97 ° and 103 ow are problematical; their lengths do not accord

well with the distributions for either the "northern tropical" or the

"central tropical" samples, although they fit best with the latter.

Determination of their affinities with any confidence must await

study of osteological material and accumulation of more length

data. Provisionally, they could be included in the central-tropical

unit. They may prove to comprise a separate population, however,

and it would be equally reasonable to treat them separately at this

time.

Length was available for only one (female) common dolphin

from the far-western portion of the central-tropical range (Square

134 in Figure 8); it is within the central-tropical range of length

(shaded in Figure 22). The number of records of the species from

west of long. 1200 W is small (Fig. 23), despite heavy sighting effort (Perrin et al. 1983), and the distances separating the area from

the denser population centers to the east are large. Determination

of affinities must await more information.

Evans (1975,1982) did not examine material from south of lat.

3°N, the "southern tropical" range. Length data are available for

only 6 adults (females: 180, 182, 188, 189, 194, and 198 cm).

These accord about equally well with the length distributions for

the central tropical and the northern tropical samples (Fig. 22).

Since the distribution of the common dolphin in this area exhibits

a largely reciprocal relationship with that of the spotted and spinner dolphins, being abundant in regions likely to have lower seasurface temperatures (near the Galapagos Islands and near the

coast of Peru at lat. 0°-5 OS) than do the areas where those species

are abundant (Perrin et al. 1983), it is possible that the concentration near and below the Equator represents a temperate-tropical

stock analogous to the short-beaked northern temperate-tropical

stock. There is fairly good separation from the central-tropical

center by a band of heavy sighting effort and few sightings between lat. 0° and 5°N.

ORIGINS OF THE GEOGRAPHICAL FORMS

The results for Slenella spp. suggest that a concept of radially

patterned geographical variation in the eastern tropical Pacific

may be more appropriate than the primarily latitudinal approach

that has held sway to date. The radial pattern also meshes better

with our developing understanding of the ecology of pelagic

dolphins in the eastern tropical Pacific. Perrin (in press) has suggested that the variation between central Pacific and eastern

Pacific races of Slenella allenu<11a and S. longiroslris may actually

constitute ecological character displacement between the two

species in the eastern tropical Pacific, where they school and feed

together. (They school and feed separately elsewhere.) Under this

hypothesis, the animals forage together because of mutual advantage gained in finding prey or in avoiding predators9 in the special

cond:tions of the oceanographically anomalous eastern tropical

Pacific,lo and their divergence in coloration, tooth size, jaw

'Norris and Dohl (1980) have suggested that Eastern Tropical Pacific spinner

dolphins may use the shelter of spotted dolph:n schools as a surrogate for the shallow

bays in which they spend their daytime resting hours in the central Pacific.

loThe eastern Pacific is peculiar in having a relatively thin, warm mixed layer (50

m thick) above a stable steep thermocline and a thick oxygen minimum layer. This

establishes an effective <hallow habitat much like the nearshore habitat in other

tropical waters (see Perrin el al 1976 and Au et al. footnote 3).
muscles, etc. reflects divergent specializations in feeding (species,

size, and depth of prey) that minimize direct competition between

the components of the multispecies "school" (which also often includes yellowfin tuna). The two species (and the tuna) do indeed

have different feeding habits in the eastern tropical Pacific, even

in the same "school" (Perrin et al. 1973). In addition, Schnell et

al. (footnote 4) found that expression of characters of the skull in

S. attenuata that are associated with feeding is correlated with environmental parameters over long distances in the eastern tropical

Pacific. For example, a correlation exists between size of the temporal fossa (through which the main jaw-closing muscle passes)

and sea-surface temperature, solar insulation, and thickness of the

oxygen-minimum layer. Stenella longirostris varies in a similar

way. Schnell et al. concluded that "the rather marked morphological covariation between S. attenuata and S. longirostris involving

a number of characters could reflect common causes associated

with environmental spatial variation."

A conceptual model of the radial nature of the pattern of variation can be based on what we now know in a general way (Fig.

24). In this view, the animals in the far eastern Pacific (the eastern

or Costa Rican spinner dolphins and, for the sake of argument, the

offshore spotted dolphins north of the Equator and east of long.

l20 0 W), and the dolphins in the central Pacific (Polynesia and

points west) comprise the purest populations of the ecologically

differentiated forms. Those in between (whitebelly spinner and

offshore spotted dolphins south of the Equator and west of long.

80 150' 1300 120' 110' lOa' 90' 0 W

S. ATTENUATA

D. Size of teeth & ~L_ • ~emporal fOSS'Y'~ r\ Q

.

\... H .....

~v--/0 ( l-----,l ~~ ~-- V l/,..,,,;: o,••~~ \::t::=1 L

/' SMA L --- ""

L (

~~~ SMA l ,,,,,.n. (

\ ME[ pE """ IUM LAR CO;OI l\~

"" I--- "'\P:IO

Ch,," ) .......... - . \

Sf,1ALL

------

,y••

~~

-- ,lIp_go,"

- /'

,- - "QU'" /' . . --

SM ALL /'

/

.

T,MII

20.

180'W

10

A

s

O·

N

10'

20

30

1\ S. LONGIROSTRIS

,D. Density & extent 'f"'=. ~L- • ~f dorsal OYerlr~ r\ ---~

~~ I~ \: -, "--,",, Q

-------- .

\,; H....II V ··f··,..··· ~------? L "" L1G HT &

HRE -PA ~T ( & \ k &'-1\ ( L1GHT & MEblUM ~IlPP.t10 DAR

HRE E-PA T\ TWp-PA ~T

"'"

o10Ne TON

i CO"OI

..............r--- ----I-- ..,113110 ) Ch,l"

-- 1--.

- p.m.;;'.:

L1GH & -- > /-

-- ------1--- - nREE ART /'

,- t,

~"Q"'" /'

/' \

/ \ . . t'I'I!t1

B

30'

20

'0

N

o

s

10

20

160'W 150' 140' 130' 1200 1100 90' 80 W

Figure 24.-Conceptual model of radial pattern of geographical variation in pelagic populations of Slenelta A) S. allenuala and B) S. longiroslris in the eastern tropical Pacific. Color pattern information from Perrin (1972a) and Perrin,

Sloan, and Henderson (1979).

22
120 0 W) would thus comprise "hybrid" swarms. At least two

historical scenarios can be suggested: differentiation during isolation of populations in the far-eastern Pacific by cool-water communication between the hemispheres during the glacial epoch (as

has been suggested for the tropical Atlantic-Perrin et al. 1978),

or existence, now or in the past, of selection pressures in the fareastern Pacific sufficiently strong to give rise to differentiation

without isolation by a physical barrier. In either case, the present

situation of pronounced polymorphism could be stable or could be

dynamic, e.g., introgression of central Pacific genes from the west

could be neutralized by continuing strong selection pressures in

the ecologically different eastern-Pacific, or the Indo-Pacific pool

could be in the process of swamping out the eastern-Pacific differentiation. Lacking historical data, this of course is only speculation. Analysis of more osteological material from Polynesia may

shed some light in relationships between animals there and those

in the outer reaches of the eastern tropical Pacific.

RECOMMENDAnONS

Stenella attenuata

I) Schools should not be identified as "coastal" or "offshore"

on the basis of position alone, because the ranges of the two forms

overlap. Any school within 100 nmi of the coast for which size

and degree of spotting of the animals is not determined should be

relegated to unidentified status. If a management boundary is set

inside 100 nmi to balance errors of identification based exclusi velyon position, it should take into account relative school sizes for

the two forms as well as relative abundance of schools, in order to

know what proportion of the population would be protected at

various distances from the coast.

2) The northern and southern samples are clearly morphologically distinct. Although the spotted dolphins west of long. 120 0 W

show affinities with those south of the Equator, and though the

east-west and north-south variation may be broadly rather than

sharply clinal, the present division of offshore populations into

nOithern and southern units should be retained pending study of

material from west of long. 120 oW, from the equatorial region,

and from Hawaii. Consideration should also be given to seasonal

and year-to-year variation in oceanographic regimes north and

south of the Equator and in the area between the two apparent

centers of distribution.

Stenella longirostris

I) South of lat. l6°N, north of lat. 7°N, and within 50 nmi of

the coast, schools of spinner dolphins should be assigned to "Costa

Rican" or "eastern" based on modal body length and relative beak

length. If this is not possible, they should be called unidentified

spinner.

2) Identification of spinner schools as "eastern" (or "Costa

Rican") or (northern or southern) "whitebelly" should be based

on modal adult color pattern and modal dorsal-fin shape in adult

males. Assessment and management of the two forms in the overlapping portions of their range should be an area-by-area basis

(e.g., 5 ° squares, as in Figure 18).

3) As in the case of the spotted dolphin, the division of whitebelly spinners into northern and southern management units

should be retained pending study of material from areas poorly

represented in previous studies (near the Equator and in the farwestern area) and comparison of the age structures of the northern

23

and southern samples. Osteological material from Hawaii should

be analyzed to determine relationships with animals in the eastern

tropical Pacific. Seasonal and year-to-year oceanographic variation should also be considered in future definition of units.

Stenella coeruleoalba

A single provisional division into northern and southern stocks

is recommended (Fig. 25). This is based solely on a hiatus in distribution, and existing osteological material should be analyzed.

Delphinus delphis

I) Within 100 nmi of the Pacific coast of Baja California and in

the Gulf of California, schools of common dolphins should be

identified as "Baja neritic" or "northern" (temperateltropical)

based on modal length, coloration, and relative beak length. If

these features are not observable, the school should be classified

as unidentified common dolphin.

2) Division into northern, central, and southern units is recommended (in addition to the Baja neritic stock, which is contained

within the northern range) (Fig. 26). As the central-southern division is based on distribution alone, material from the southern

area should be collected and compared with existing material

from the central area.

3) Delineation of a "Guerrero" unit of common dolphins off

southern Mexico is marginally justified pending study of material

from the area.

ACKNOWLEDGMENTS

Nearly all of the data and specimens analyzed here were collected by government-employed biologists and technicians aboard

commercial tuna seiners. Individual cruises lasted as long as 5 mo,

and there have been more than 800 cruises. The labors and dedication of these people and the cooperation and assistance by the

owners, operators, and crews of the tuna boats made this study

possible.

We thank the following for critical review of the manuscript

and the useful suggestions for its improvement: J. G. Mead, K. S.

Norris, F. G. Alverson, G. D. Schnell, D. W. K. Au, A. E. Dizon, G.

T. Sakagawa, and I. Barrett.

LITERATURE CITED

AL VERSON. F. G.

1981. Comments on the distribution of spotted, spinner. common and striped

dolphin in Ihe Iropical Pacific Ocean. 'n P. S. Hammond (edilor), Report on

the Workshop on Tuna-Dolphin Interactions, Managua. Nicaragua, April 6-

10, 1981. Inter-American Tropical Tuna Commission, La Jolla, CA, 259 p.

BANKS, R. C., and R. L. BROWNELL.

1969. Taxonomy of the common dolphins of the easlern Pacific Ocean. J.

Mammal. 50:262-271.

BARLOW, J.

1984. Reproductive seasonalily in pelagic dolphins (S'enella spp.): implications in measuring rales. 'n W. F. Perrin, R. L. Brownell, Jr., and D. P.

DeMaster teditors), Reproduction in whales, dolphins and porpoises. Rep.

Int. Whaling Comm. (Spec. Issue 6), p. 191-198.

In press. Variability, trends and biases in reproductive rates of spOiled

dolphins (S'enella allenua,a). Fish. Bull., U.S.

DOUGLAS, M. E., G. D. SCHNELL, and D. J. HOUGH

1984. Differenliation belween inshore and offshore spOiled dolphins in the

eastern tropical Pacific Ocean. J. Mammal. 65:375-387.

EVANS, W. E.

1975. Distribution, differentiation of populations, and other aspects of the
/-

"- \ (/ \ _/ "-" .~~ / ,/'~40 \\ /

//

10-..- .... i " 1\ I\. \ -;

"-!:: Ha.all

~~ " Revilla Glgado.

~~~L~ '0>

--- --- 16°N,122 ORTHE N -~- I--

/ IPED 0 LPHIN / / IZoN,1l5OW

~Ilpp.no

12°N, 105° r\[

I

I

I Cocoa . ~......- D soun ERN S RIPED OLPHlr . ' / V..,Ipalo

// , l-chd~," , ,/ / 0

----- '0', • p.'ap,gol'

------ -- -....

-----r----...., )1\ ~.,qu•••

. \ V \ \

"-

-- , .. , , tahiti ,

30'

20'

10'

N

0'

S

10'

20'

160' W 150' 140' 130' 120' 110' 100' 90' 80' W

Figure 25.-Recommended management units for Slenella coeruleoalba.

", "....::

\~o,", ...". C,-.-,..;r-'

(/ ,

l'

''\ BAJA NERITIC ) \\ J\.. ~oooo~" \ NORTHEI N au RRERO l~ 10 .. CO:C~:~>~' ( \ 0< MMON >; 0 h>.. "\, H••• 11

I ON.121~ Revill."Glgadoen'II ~CJ L '0>

'" 170 N 1040 160 N

- . - ........ ~r-... ""

V "- -/

'--~ 14o N,97oW

130N .ISoW, 13ON,I .'W / ~IIPP"IO

C CENTRA COMMON DOlPHI~

Cocoa . ---r--........ . 2

V oN, O'W ..,lpalo

.

ell'.a'" a r-...... 0

----- '~ IT' , palapago••

SOl fnERN 0< ptA10N [) lPHlN

r---1\ t.

~Iw':~'" 1\ \ .... -" , /\ . .. . . '-hili .

30'

20'

10'

N

O'

S

10'

20'

160' W 150' 140' 130' 120' 110' 100' 90' 80' W

Figure 26.-Recommended management units for Delphinus delphis in the eastern tropical Pacific.

24
natural history of Delphinus delphis Linnaeus in the northeastern Pacific.

Ph.D. Thesis, Univ. California, Los Angeles, J45 p.

1982. Distribution and differentiation of stock of Delphinus delphis Llnnaeus

in the northeastern Pacific. In Mammals in the seas, Vol. IV, p. 45-66.

FAO, Rome.

HESTER, F. J., J. R. HUNTER, and R. R. WHITNEY.

1963. Jumping and spinning behavior in the spinner porpoise. J. Mammal.

44:586-588.

HOLT, R. S., and J. E. POWERS.

1982. Abundance estimation of dolphin stocks involved in the eastern tropical Pacific yellowfin tuna fishery determined from aeriai and ship surveys to

1979. NOAA Tech. Memo. NMFS-SWFC-23, 95 p.

LEATHERWOOD, S., R. R REEVES, W. F. PERRIN, and W. E. EVANS.

1982. Whales, dolphins, and !,orpoises of the eastern North Pacific and adjacent Arctic waters. U.S. Dep. Cammer., NOAA Tech. Rep. NMFS Circ.

444,245 p.

MORRIS, R. A., and L. S MOW BRAY.

1966. An unusual barnacle attachment on the teeth of the Hawaiian spinner

dolphin. Norsk Hvalfangst-tidende 55:15-16.

NORRIS, K. S., and T. P. DOHL.

1980. Behavior of the HawaIian spinner dolphin, S,ellello /ollgisroslris.

Fish. Bull., U.S. 77:821-849.

PERRIN, W. F.

I972a. Color patterns of spinner porpoises (S,ellella cf. 5 IOllgiroslris) of the

eastern Pacific and Hawaii, with comments on delphinid pigmentation.

Fish. Bull., U.S. 70:983-1003.

1972b. Variation and taxonomy of spotted and spinner porpoises (genus

Slellella) of the eastern tropical Pacific and Hawaii. Ph.D. Thesis, Univ.

California, Los Angeles, 490 p.

J975a. Variation of spotted and spinner porpoise (genus S,elldla) in the

eastern Pacific and Hewaii. Bull. Scripps Inst. Oceanogr. 21, 206 p.

1975b. Distribution and differentiation of populations of dolphins of Ihe

genus Slellella in the eastern tropical Pacific. J. Fish. Res. Board Can.

32:1059-1067.

In press. Patterns of variation in small cetaceans. Acta Zool. Fenni.::a.

PERRIN, W. F., J. M. COE, and J. R. ZWEIFEL.

1976. Growth and reproduction of the spotted porpoise, Slellell" allelluala, in

the offshore easlern tropical Pacific. Fish. Bull., U.S. 74:229-269.

PERRIN, W. F., W. E. EVANS, &nd D. B. HOLTS.

1979. Movements of pelagic dolphin, (S,ellella spp.) in ,he eastern tropical

/

25

Pacific as indicated by results of tagging, with summary of tagging operations, 1969-76. U.S. Dep. Commer., NOAA Tech. Rep. NMFS SSRF-737,

14 p.

PERRIN, W. F., E. D. MITCHELL, and P. J. H. van BREE.

1978. Historical zoogeography of tropical pelagic dolphins. [Abslr.] II.

Congr. Theriol. Int., Hrno, Czechoslovakia, June 20-27, 1978, p. 73.

PERRIN W. F, and C. W. OLIVER.

1982. Time/area distribution and composition of the incidental kill of

dolphins and small whales in the U.S. purse-seine fishery for tuna in the

eastern tropical hcific, 1979-80. Rep. Int. Whaling Comm. 32:429-444.

PERRIN, W. F., M D. SCOTT, G. J. WALKER, F. M. RALSTON, and D. W. K. AU.

1983. Distribution of four dolphins (Slellella spp. and Delphillus delphis) in

the eastern tropical Pacific. with an annotated catalog of data sources.

NOAA Tech. Memo. NMFS-SWFC-38, 65 p.

PERRIN, W. F., P. A. SLOAN, and J. R. HENDERSON.

1979. Taxonomic status of the 'south-western stocks of spinner dolphin

Slenella IOIlRiroslris and spotted dolphin S. allellua/a.' Rep. Int. Whaling

Comm.29:175-184.

PERRIN, W. F., T. D. SMITH, and G. T. SAKAGAWA.

1982. Status of populations of spotted dolphin, S,ellella allelluala, and spinner

dolphir, S. Ivng;rosrris, in the eastern tropical Pacific. In Mammals in the

'eas. Vol. IV, p 67-83. FAO, Rome.

PERRIN, W. F., R. R WARNER, C. H. FISCUS, and D. B. HOLTS.

1973. St0mach conlents of porpoise, S,ellella spp., and yellowfin luna,

ThulIlI"s albacares, in mixed-species aggregations. Fish. Bull., U.S. 71:

1077-1092.

SCOTT, M. D

1981. Dolphin stocks in the eastern tro?ical Pacific. III P.S. Hammond

(editor), Report on the Workshop on TunJ-Dolphin Interactions, Managua,

Nicaragua, Ap'iI 6-10, 1981. Inter-American Tlopical Tuna Commission,

La Jolla, CA, 259 p.

SMITH, T. D.

1983. Changes in 51" of three dolphin (Stellell,. spp.) populations in the

eastern tropical Pacific. Fish. Bull., U.S. 81:1 -13.

van BREE, P. J. H., and P. E. PURVES.

1972. Remarks on the validity of Deiphlllus bairdii (Cetacea, Delphinidae).

J. Mammal. 53:372-374.

WHITE, M. J., Jr., J. G. JENNINGS, W. F. GANDY, and L. H. CORNELL.

1981. ".n evol'.nion 0f tagging, marking, and tattooing techniques for small

delphinids. NOAA Tech Memo. NMFS-SWFC-16, 142 p.
140°

~4~ 3371336 3351

.,.:~ I .«~ I

"'\"W l{ &.- I~ I

320 3191318 ~17 316 315 314~ 313

~.,:2/ '_>_~l .~ . ~~ 300 299 298 297 296 295 294 293 292

50°~.+--+--+-t--t--+---+-'+-~­

280 279 278 277 276 275 274 273 272 :>71

f--+--+--+-+---t--+--t----+--+~I

260 259 258 257 256 255 254 253 252 251

400'..--1--+---+-+--t--!-----i--+--t'\---i

240 239 238 237 236 235 234 233 232 11

\

120 119 118 I 17 116 115 114 I 13 I 12 I I 1

220 2.9 2.. 2.7 2.6 2.5 2.4 213 2.2 211 1';;','\ 1\'9

30" r\--+----,

~. \.\ ~2-0-0_+_19-9_+1-9_8_+_19-7_+1_9-6_+_19-5_+1-9-4_+_19-3_____r'-9-2_+_1-9_I -t-19_0-;-~811 J

180 179 .178 177 176 175 174 173 172 171 170 169 ~I~ I

W '60 .59 ,~:l'57 '56 .55 .54 '53 .52 .5. '50 .49 '4.\-:T:~ r--t--t----+-+----+--+---+---t----+--+---+--+---i-~

140 139 138 137 136 135 134 133 132 131 130 129 128 127 126 125 124 A

1001----+---+---+---+--+--+---+--+--+--+----t--+--t------+--+---+--~~

110 109 108 107 106 105 104 103~2

r---t-+---+--t--+---+--t---+----t--+-+--+-~-+----t--+----+-l100 99 98 97 96 95 94 93 92 91 90 89 88 87 86 85 84 83 ,/82

0°

80 79 78 77 76 75 74 73 72 71 70 69 68 67 66 65 64 63( 62

I

12 II 10 89 7 6 5 4 3 20 19 18 17 16 15 14 13

,

60 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 44 43 ~2

~~ \

40 39 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22

t--+--t--+--+-----+---+-+----+---+---+--+---+---+--+--+--+------+--___+_~ _

2 I '\ 20°L---l-......L.-~-.........-oI...----'~---'--......L._~_...J...._ ........._J....__.L...__oI...____''_____''___.L__J....__..l....__J

170° 160° 150° 140° 130° 120° 110° 1000 90° 800 700

Appendix I.-Statistical reporting blocks (5°) for eastern tropical Pacific.

26
Appendix 2.-Stock composition of records of 51enella longiroslris by 5° block (see Appendix I for locations).

Costa Rican Eastern Whitebelly Total Unidentified

BlOCK % of % of % of iden- % of

no. No. lO'ed No. lO'ed No. lO'ed tified No. total Total

3 0 0 I 100 0 I

23 0 0 26 100 26 1 4 27

24 .J 0 37 92 40 2 5 42

2:; 0 0 16 100 10 0 16

26 0 0 4 100 ,1 0 4

43 0 0 13 tOC 13 5 28 18

44 0 4 ·167 98 171 16 9 187

45 0 0 97 100 97 5 5 102

46 0 l! 31 100 31 I 3 32

47 0 (1 I 100 I 0 I

48 0 0 I 100 0 1

54 0 0 2 100 2 I 33 3

55 0 0 1 100 I 10 90 II

63 0 I 3 29 98 30 29 49 59

64 0 J 7 41 93 44 13 23 57

65 0 0 102 100 102 22 18 124

66 0 0 32 100 32 6 16 38

67 C 0 57 100 57 6 10 63

68 0 0 21 100 21 I 5 22

69 0 0 13 100 13 0 13

70 0 0 9 100 9 0 9

71 0 0 I 100 I 0 I

72 0 0 I 100 I 0 I

73 0 0 100 2 0 2

82 f) 1 13 7 87 8 2 20 10

83 0 18 37 31 63 49 37 43 86

84 0 24 26 67 74 91 49 35 140

85 0 15 8 183 92 198 55 22 253

86 ') 9 5 162 95 17 I 31 15 202

87 0 22 12 155 88 ~77 24 12 201

88 f) 17 5 310 95 327 23 7 350

89 .J 2 2 131 98 133 13 9 146

90 0 4 5 82 95 86 17 17 103

91 C 0 70 100 ~O 4 5 74

92 a 0 29 100 29 2 6 31

93 0 0 2 100 2 2 50 4

102 0 6 67 3 33 9 14 61 23

103 9 6 75 56 49 37 133 82 38 215

104 2 2 60 51 55 47 i 17 80 41 197

105 0 51 47 57 53 108 70 39 178

106 0 236 70 99 30 335 164 33 499

107 0 220 61 142 39 362 167 32 529

108 0 169 43 220 57 389 172 31 561

109 0 42 31 95 69 137 46 25 183

110 0 25 24 81 76 106 34 24 140

111 0 26 10 233 90 259 62 19 321

112 0 21 5 378 95 399 57 13 456

113 0 12 3 345 97 357 57 14 414

114 0 8 3 294 97 302 28 9 330

115 0 I 1 81 99 82 7 8 89

116 0 0 I 100 1 0

117 0 0 2 100 2 0 2

124 3 9 25 74 6 17 34 36 51 70

125 0 653 92 54 7 707 494 41 1,201

126 0 789 91 74 9 863 294 25 1,157

127 0 664 93 53 7 717 190 21 907

128 0 932 88 132 12 1.064 297 22 1,361

129 0 481 86 78 14 559 132 19 691

130 0 229 76 74 24 303 88 23 391

131 0 70 35 129 65 199 58 23 257

132 0 60 16 312 84 372 73 16 445

133 0 27 8 306 92 333 55 14 338

134 0 6 6 101 94 107 6 6 113

27
Appendix 2.- Continued.

Costa Rican Eastern Whitebelly Total Unidentified

Block % of % of . % of iden- % of

no, No. m'ed No. m'ed No. m'ed tified No. total Total

135 0 0 23 100 23 4 15 27

136 0 0 5 100 5 I 17 6

137 0 0 I 100 I 0 I

138 0 0 2 100 2 0 2

145 0 0 I 100 I 3 75 4

146 0 65 94 4 6 69 44 39 113

147 0 203 96 8 4 211 132 38 343

148 0 86 90 10 10 96 45 32 141

149 0 192 93 15 . 7 207 104 33 311

150 0 97 89 12 11 109 46 30 155

151 0 4 67 2 33 6 2 45 8

152 0 I 17 5 83 6 3 33 9

153 0 0 9 100 9 0 9

168 0 0 5 8 5 46 44 51

169 0 0 6 30 6 9 31 15

Total 14 0.1 5,659 50.7 5,496 49.3 11,169 3,579 24.1 14,748

"MINE MAMMAL LABORATORY, NWAfC.

NAnONAL m

7600 SANO POINT WAY N Eo, BUILDING 112

SEADlE. WASHINGTON 981 Iii •• _

28