NOAA Technical Report NMFS 119

Stocks of Dolphins

(Stenella spp. and Delphinus delphis)

in the Eastern Tropical Pacific:

A Phylogeographic Classification

Andrew E. Dizon

William F. Perrin

Priscilla A. Akin

u.s. Department of Commerce

June 1994
U.S. Deparnnent

of Conunerce

Ronald H. Brown

Secretary

National Oceanic and

Atmospheric AdIninistration

D. James Baker

Under Secretary for Oceans and

Atmosphere

National Marine

Fisheries Service

Rolland A. Schmitten

Assistant Administrator for

Fisheries

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NOAA Technical Report NMFS 119

A Technical Report of the Fishery Bulletin

Stocks of Dolphins

(Stenella spp. and Delphinus delphis)

in the Eastern Tropical Pacific:

A Phylogeographic Classification

Andrew E. Dizon

William F. Perrin

Priscilla A. Akin

June 1994

U.S. Department of Commerce

Seattle, Washington

Stocks of Dolphins (Stenella spp. and Delphinus delphis)

in the Eastern Tropical Pacific: A Phylogeographic Classification

ANDREW E. DIZON, WILLIAM F. PERRIN, and PRISCILLA A. AKIN

Southwest Fisheries Science Center, NOAA/NMFS

La Jolla, California 82037

ABSTRACT

Current information is reviewed that provides clues to the intraspecific structure of

dolphin species incidently killed in the yellowfin tuna purse-seine fishery of the eastern

tropical Pacific (ETP). Current law requires that management efforts are focused on the

intraspecific level, attempting to preserve local and presumably locally adapted populations. Four species are reviewed: pantropical spotted, Stenella attenuata; spinner, S. longirostTis; striped, S. coeruleoalba; and common, Delphinus delphis, dolphins. For each species,

distributional, demographic, phenotypic, and genotypic data are summarized, and the

putative stocks are categorized based on four hierarchal phylogeographic criteria relative to

their probability of being evolutionarily significant units.

For spotted dolphins, the morphological similarity of animals from the south and the west

argues that stock designations (and boundaries) be changed from the current northern

offshore and southern offshore to northeastern offshore and a combined western and

southern offshore. For the striped dolphin, we find little reason to continue the present

division into geographical stocks. For common dolphins, we reiterate an earlier recommendation that the long-beaked form (Baja neritic) and the northern short-beaked form be

managed separately; recent morphological and genetic work provides evidence that they

are probably separate species. Finally, we note that the stock structure of ETP spinner

dolphins is complex, with the whitebelly form exhibiting characteristics of a hybrid swarm

between the eastern and pantropical subspecies. There is little morphological basis at

present for division of the whitebelly spinner dolphin into northern and southern stocks.

However, we recommend continued separate management of the pooled whitebelly forms,

despite their hybrid/intergrade status. Steps should be taken to ensure that management

practices do not reduce the abundance of eastern relative to whitebelly spinner dolphins.

To do so may lead to increased invasion of the eastern's stock range and possible replacement of the eastern spinner dolphin genome.

Introduction

Management ofdolphins incidentally killed in the purse

seines of the eastern tropical Pacific (ETP) yellowfin

tuna fishery is directed at the stock or population level.

Realistic stock delineations are necessary both for estimating the impact ofthe fishery on dolphin species and

for ensuring conservation of locally adapted genetic

variation. Incidental mortality has primarily affected

four species of dolphins, listed in order of exploitation

(DeMaster et aI., 1992): the pantropical spotted dolphin, Stenella attenuata; the spinner dolphin, S.

longirostris; the common dolphin, Delphinus delphis; and

the striped dolphin, S. coeruleoalba. Over the years, two

Status ofPorpoise1 Stocks (SOPS) workshops have been

convened to review the status of the impacted stocks

(Anon.2; Smith3). In 1983, for a planned third workshop,4 Perrin et al. (1985) prepared a review of evi1 Convention in the fishing community uses the term "porpoise"

for dolphin.

2 Anon. 1976. Report of the workshop on stock assessment of

porpoises involved in the eastern Pacific yellowfin tuna fishery.

Southwest Fisheries Center, National Marine Fisheries Service,

NOAA, Admin. Rep. LJ-76-29, 53 p.

3 Smith, T. D. 1979. Report of the status of porpoise stocks workshop (August 27-31, 1979, La Jolla, California). Southwest Fisheries Center, National Marine Fisheries Service, NOAA, Admin.

Rep. LJ-79-41, 120 p.

4 Although panels of experts were convened to review components of the assessment, including stock identity, the full workContinued
2 NOAA Technical Report NMFS 119

dence regarding the geographic forms ofETP dolphins

and recommended how intraspecific divisions should

be made for status-of-stock assessments and management. Our present review includes new information

collected since then and updates recommendations regarding stock boundaries.

Intraspecific structure of dolphin species is characterized by pronounced geographical variation probably

reflecting local adaptation and mechanisms that limit

gene flow between neighboring populations (Evans,

1982; Perrin, 1984; Perrin et aI., 1991). Either relatively

limited movement (perhaps hundreds of kilometers in

spinner dolphins), socially mediated barriers, reproductive timing differences, or some combination of

these allow genetic variation to accumulate, differentiating local populations. However, this abundance of

variation can obscure rather than facilitate the interpretation of stock structure. Surveys are required that

are large, in terms ofsample size and area and time period

sampled, and well designed, in terms of eliminating bias.

Pronounced geographic variability also dictates that care

be taken in circumscribing stocks so that local variation is

preserved. Differential exploitation oflocal stocks coupled

with a stock designation that is too broad could lead to

destruction oflocally adapted populations.

The intent of the Endangered Species Act of 1973

and the Marine Mammal Protection Act of 1972 is to

manage exploitation of living marine resources so that

locally adapted populations are not irreversibly reduced

to the point where they no longer are functioning

entities in the ecosystem. Although management-bystock is a basic principle of the conservation acts, no

meaningful attempts to define the term "stock" were

made within the legislation, save to refer to the stock as

a "distinct population" that "shares a common space"

and that "interbreeds." Unfortunately, stock has no

universally accepted definition among biologists because it is not immediately obvious how to define the

extent of "sharing" or "interbreeding." Recently, Waples

(1991, a and b) for Pacific salmon (Oncorhynchus spp.)

and Dizon et aI. (1992) for marine mammals have

developed guidelines that, although not totally removing subjectivity in the process of stock designation,

focus attention on important criteria. Both reviews considered that, for the purposes of management, a population will be considered distinct if it is an "evolutionarily significant unit" (ESU; Ryder, 1986). Waples suggested that for a population to be an ESU it must be

"substantially reproductively isolated" and "represent

an important component of the evolutionary legacy of

the species." If reproductive isolation is sufficient, evoContinued

shop was never held due to re-authorization of the Marine

Mammal Protection Act, which permitted interim kill for five

more years and required a five-year survey of trends in abundance.

lutionary differences can accrue within a population,

increasing fitness within the local habitat. This increase

in genetic variation then becomes an "important component in the evolutionary legacy of the species"

(Waples, 1991, a and b).

Although the ESU concept is an improvement over

that of "sharing a common space and interbreeding,"

which after all encompasses any arbitrary division of a

panmictic population, it is still subjective in its use of

"substantial" and "important." Existing measures of fitness, such as viability and growth rate (Leberg, 1990;

Pastene et aI., 1991), have little utility in defining specific fitness to a local environment in animals for which

test breeding programs cannot be conducted. In practice,

locally adapted populations are subjectively identified using a variety of proxy data. These data allow inferences to

be made regarding isolation and genetic uniqueness and

can be classified as distributional, populational (demographic) response, phenotypical, and genotypicaI. When

differences between populations in one or more of the

proxies are apparent, it is inferred that two separate biological populations, and potentially two ESU's, exist.

Dizon et aI. (1992) argued that the degree to which a

population is probably an ESU should be made part of

the stock designation. In brief, their point for doing so

was to ensure that the relative "worth" of a population

in the management arena is made apparent and that

due consideration is taken ofthe criteria of "significant

isolation" and "important component of the evolutionary legacy." Dizon et aI. (1992) proposed that a hierarchal classification scheme be applied to stock designations

to create a matrix of Waples' (1991, a and b) criteria of

isolation and genetic uniqueness.

Category I populations, having the highest probability of being ESU's, are characterized by a discontinuous genetic divergence pattern where a

locally adapted and closely related population is

geographically separated (i.e. allopatric or parapatric with physical barrier). Geographic separation increases the probability that habitat differences exist between the isolated populations, resulting in different selection pressures.

Category II is similarly characterized by significant genetic distances between assemblages but

with weak geographic partitioning (i.e. partial syrnpatry or parapatry with no barrier).

Category III is the converse of II, having little

genetic differentiation between assemblages that

are clearly geographically separate (as in Category

I) and probably reproductively isolated.

Category IV assemblages, having the lowest probability of being ESU's, are characterized by weak

geographic partitioning (as in Category II) and

no evidence of genetic uniqueness.
Dizon et al.: Dolphin Stocks in the Eastern Tropical Pacific 3

They noted that under this scheme, a stock designation is always relative to some other reference population. In general, the reference population will be its

nearest conspecific neighbor or neighbors, and itshould

be listed unless obvious.

In the categorizations, it is important to realize that

geographic partitioning is used as a proxy for amount

of gene flow. It is a statement regarding the probability

of reproductive isolation. However, if fixed genetic differences exist between sympatric or parapatric populations, thus establishing that no gene flow occurs (Hey,

1991) and that the separation is presumably at the

species level, it would be misleading to try to categorize

the populations; the system is designed for intra- rather

than interspecific classification. For instance, two sympatric populations that are totally isolated genetically

would be classified mistakenly as Category II, that is,

having a lower probability of being an ESU than a

Category I population. In fact, rather than being two

evolutionarily significant components of a single species, the populations in this example would represent

two separate species. Long- and short-beaked common

dolphins are probably an example of this situation and

are discussed below.

In addition to phylogeographic designation, Dizon

et al. (1992) proposed that the information used to

classiry be included by using the following single letter

abbreviations: distributional (a), population response

(b), phenotypic (c), and genotypic (d) information. To

be included was both evidence for and against. For

example in ''Type IIbc/a," information to the left ofthe

slash is evidence for "lumping," that to the right is

evidence for "splitting"; missing letter abbreviations signiry lack of reliable data.

When using this system to categorize populations,

biological judgement is required to decide the significance of available data to determine which phylogeographic type is most appropriate. In the following,

we attempt to apply this approach to categorize stocks

of the impacted dolphin species. We use the Perrin et

al. (1985) review as a starting point and consider what

new information is available. Each ofthe four species is

considered separately, and the treatment is organized

by the four data types: distribution, population response,

phenotype, and genotype. At the end of each species

section, we offer our classifications of the geographic

stocks of the tropical dolphins in the ETP. Note that each

stock is classified relative to some other population.
4 NOAA Technical Report NMFS 119

Pantropical Spotted Dolphin

Stenella attenuata

Current U.S. Management Situation _

In the eastern and central Pacific Ocean, four stocks

are recognized: coastal, Stenella attenuata graffmani;

northern offshore, southern offshore, and Hawaiian.

Quotas have been established for the U.S. fleet for

northern and southern offshore as well as coastal spotted dolphins (Anon., 1986). These divisions were recommended by the SOPS meetings in 1983 and are

shown in Figure 1. Procedures for assigning spotted

dolphins in the ETP to stock type were based on either

geographic location, morphology, or both. The coastal

and northern offshore forms cannot be discriminated

solely on the basis of coastal location, because occasionally the offshore form is found inshore. The coastal

type, formerly known as Stenella graffmani, can be distinguished by its relatively larger body and heavier spotting. In contrast, the northern and southern offshore

forms have to be discriminated strictly on a geographical location, the division being near the Equator, although modal morphological differences exist. The

Hawaiian form is discriminated based on its proximity

to the Hawaiian Islands and will not be discussed further.

Evidence of Stock Divisions _

Distribution

Range - Pantropical spotted dolphins are distributed

globally in tropical waters and in some warmer temperate waters. In the Atlantic, they range off the coasts of

England and NewJersey southward, including the Caribbean and Gulf of Mexico, through the tropical South

Atlantic. In the Indian Ocean, they are found from the

Red Sea and the Seychelles offshore to the southern

SOUTHERN

OFFSHORE

35N

30N

25N

20N ......

'. D

15N

10N

5N

.. 0

55

105

155

".

205

160W 150W

.",,0

140W 130W 120W 110W 100W 90W 80W

Figure I

Distribution of the pantropical spotted dolphin, Stenella attenuata. stocks in the eastern tropical Pacific Ocean based

on sighting data collected between 1971 and 1991 by observers on tuna purse-seining vessels and research vessels.

Stock boundaries are from Perrin et al. (1985).
Dizon et al.: Dolphin Stocks in the Eastern Tropical Pacific 5

coast of Australia. In the Pacific, they range north off

the coast ofJapan, east to the South China Sea, and

south to New Zealand (Gilpatrick et ai., 1987; Perrin

and Hohn, in press).

Details of the distribution of the pantropical spotted

dolphin are best known for the eastern and central

Pacific. A near hiatus in distribution exists at about 2°S

(Perrin et ai., 1983) reflecting a preference for tropical

surface water (Au and Perryman, 1985; Reilly, 1990;

Reilly and Fiedler5). The hiatus, together with demographic and morphological criteria, justified segregating the offshore form into northern and southern stocks

with a boundary first set at lOS. The equatorial front,

dividing the ETP into northern and southern habitats

(Pak and Zaneveld, 1974), may be instrumental in the

development of the two populations (Reilly and

Fiedler5). To better reflect actual distribution of the

animals, the SOPS 1979 workshop modified the onedegree north-south boundary by slanting it southeasterly at 900

W to intersect the coastline of South America

at 3°S (Fig. 1; Smith3).

The northern offshore form occurs from near the

coast ofsouthern Mexico at 25°N, offCentral and South

America, to lOS and west to 145°W. The coastal form

occurs within about 200 km of the coastline and off

coastal islands from northern Mexico to Ecuador. The

offshore and coastal forms are sympatric throughout

much ofthe coastal form's range, but the two forms are

not known to school together.

Spotted dolphins also occur in the waters surrounding the Hawaiian Islands and are considered a separate

stock. There is an apparent hiatus in distribution of

about 800 km between the westernmost range of the

northern offshore form (about 15TW) and the Hawaiian area (Perrin, 1975; Perrin et ai., 1983).

Movements - Between 1969 and 1976, almost 3000

northern offshore spotted dolphins were tagged with

spaghetti tags, and by the beginning of 1977, 97 tags

had been returned (Perrin et ai., 1979a). Tagging effort extended to just north of the Galapagos Islands,

and none of the 97 recaptures occurred south of the

Equator. Minimum distance traveled (defined as the

distance between release point and recapture point)

ranged from 12.8 to 1,071 km; time at liberty ranged

from less than 1 day to 1,478 days. Average short-term

movement was 55-92 km/day with a circular distribution 400-600 km in diameter. No evidence of movement across the lOS boundary is consistent with some

degree ofisolation ofthe northern and southern stocks.

5 Reilly, S. B., and P. C. Fiedler. 1991. Interannual variability in

dolphin habitats in the eastern tropical Pacific, 1986-1990. Southwest Fisheries Science Center. National Marine Fisheries Service, NOAA, Admin. Rep. LJ-91-42, 41 p.

Rather than north-south movements, the authors suggested that onshore migrations in the fall and offshore

migrations in the spring may occur.

Tagging, tag sightings without recoveries, and recoveries continued after 1977, and 642 more spotted dolphins were tagged during a single cruise in 1978

(Hedgepeth6). Perrin et ai. (1985) reported that the

total of206 tag recoveries or sightings made by the time

the manuscript was written allowed greater confidence

to be placed in the east-west movement ofthe northern

offshore form. One individual, tagged and released at

about WON, was reportedly sighted by a fisherman (but

not recovered) at 10037'S after 4 or 5 months. This

unconfirmed record is the only indication of movement across the stock boundary at the Equator.

Population Response

Reproduction - The attempt to use life history parameters to ascertain stock differences in the spotted dolphin of the ETP is confounded by differences in the

history of exploitation and potential differences in carrying capacity over the course of the time series. Barlow

(1985) compared reproductive parameters of the putative northern and southern offshore populations, which

have been subjected to very different levels of fishingrelated mortality. Because the northern offshore stock

has been so heavily exploited, one would expect density-dependent increases in reproductive rates for the

northern offshore population relative to the southern

one. He found the proportion of females mature, the

proportion pregnant, and the proportion simultaneously pregnant and lactating higher in the less exploited southern offshore population-the reverse of

what was expected.

Chivers and DeMaster (in press) found the proportion of pregnant females significantly higher in the

southern offshore population compared with the northern, and the proportion of lactating ones significantly

lower, also opposite to that expected under a densitycompensatory modei. Similar differences were found

when northern offshore samples were compared with

coastal samples; however, the magnitude of the difference was greater.

Perrin et ai. (1976) noted seasonality of breeding in

northern offshore spotted dolphins, with two or more

peak periods annually. Barlow (1984) and Hohn and

Hammond (1985) found geographic differences in patterns of seasonality. Although births were found all

year, the northern offshore form exhibited distinct

6 Hedgepeth,]. B. 1985. Database for dolphin tagging operations

in the eastern tropical Pacific, 1969-1978, with discussion of

1978 tagging results. Southwest Fisheries Center, National Marine Fisheries Service, NOAA, Admin. Rep. LJ-85-03, 40 p.
6 NOAA Technical Report NMFS 119

modes in spring and autumn, whereas the southern

offshore form had a much more strongly pulsed reproductive period occurring only in the spring.

Spermatozoa scores elevate in the northern offshore

form during the predicted breeding season in May and

September (Barlow, 1984; Hohn et aI., 1985), but testes

weight peaks halfway between the calving periods (Hohn

et aI., 1985). In the southern offshore form, seasonal

change in testes weight and development is the same,

yet only one calving season (January) has been identified. An unidentified second season may occur six

months later.

Marked seasonal difference in reproduction times

between the northern and southern offshore populations may indicate reproductive isolation. However, the

transition from the diffuse bimodal pattern in the north

to the strongly unimodal pattern in the south may not

be abrupt. Dolphins in the area from 0' to 5'N show a

single peak reproductive mode that is synChronized

with animals to the south but also show a diffuse pattern of seasonality similar to that in the northern offshore form. This could be a region of overlap in the

seasonal distributions of the northern and southern

populations or could be a clinal change.

Age and growth - Perrin et aL (1976) estimated, by

linear regression of fetal and neonatal length, an average length at birth of 82.5 cm in the northern offshore

population. Hohn and Hammond (1985) claimed that

this figure was biased because animals were sampled

mainly from small-kill sets (:<:::40 individuals killed per

set) in which calves are over represented, causing an

underestimation of length-at-birth. They eliminated

small-kill sets, used a logistic model fitted to the fraction postnatal at various lengths, and used a 50% criterion to produce an estimate of 85.4 cm. If not stratified

to kill-per-set, their value was 82.0 cm. They estimated

the southern offshore form to be 83.2 cm at birth but

used only unstratified data in this case. Recently, Bright

and Chivers7 reestimated length at birth for females

from both the southern and northern offshore populations (northern sampled west of 120'W). Their result of

83.1 cm for the northern samples was similar to that of

Perrin et aL (1976). For the southern samples, length at

birth was 86.4 cm, somewhat larger than Hohn and

Hammond's (1985) value of 82.9 cm. The difference

between the northern and southern length-at-birth values was significant at the 0.05 level.

Length-at-birth measurements establish the origin of

the growth curve, although work by Bright and Chivers7

demonstrates that the Laird-Gompertz growth model

7 Bright, A. M., and S. J. Chivers. 1991. Post-natal growth rates:

comparison of northern and southern stocks of the offshore

spotted dolphin. Southwest Fisheries Science Center, National

Marine Fisheries Service, NOAA, Admin. Rep. LJ-91-30, 24 p.

was not particularly sensitive to a wide range of initial

length-at-birth values. No significant differences between

the northern and southern offshore samples were observed in predicted length at age one.

Average length at attainment ofsexual maturity (LSM)

in the female northern offshore form increased from 176

em in 1974 to 181 cm in 1983 (Barlow, 1985), although

the adoption of a different method for measuring animals in the interim period may be responsible for the

trend. From pooled data (1974-83) for the southern

offshore form, the estimated LSM was 175 cm.

Myrick et al. (1985) estimated average age of attainment of sexual maturity (ASM) in the northern offshore females to be from 10.7 to 12.2 years (mean=11.4).

They also found a correlation between color phase and

state ofsexual maturity. In a blind study (samples aged

without knowledge of stock origins) of 546 northern

and 319 southern offshore animals sampled between

1974 and 1988, Chivers and Myrick (1993) demonstrated significant differences in ASM between the northern and southern populations (11.12 years versus 9.78

years, respectively; P<O.OOl). No definitive conclusions

regarding stock differences between the northern and

southern offshore forms can be drawn from their age

and growth data. Several reasons plague all such comparisons: (1) the confounding problems of bias (Chivers

and Myrick, 1993), (2) possible changes in carrying

capacity due to environmental changes (Chivers and

Myrick, 1993), (3) niche replacemen t in the north versus the south (Chivers and Myrick, 1993), or (4) potential stress effects (Myrick, 1988).

Phenotype

Coastal spotted dolphins are the largest of the four

forms in the ETP: males average 223.2 cm; females,

206.5 cm (Perrin et aI., 1985). Offshore animals are

generally smaller and more slender and have lighter

skulls and smaller teeth than the more massive coastal

spotted form. This general impression was quantified

in a multivariate study of total body length and 36

measures of skull morphology (Schnell et aL8). These

differences include portions ofthe skull associated with

feeding (Perrin, 1975); the coastal form appears to be

adapted to feeding on larger and more substantial prey

than does the offshore form.

The southern offshore form is shorter than the northern: males, 186.6 cm versus 200.1 cm (east of 120'W);

females, 185.3 cm versus 187.8 cm (eastofl20'W; Perrin

et aI., 1985). Schnell et aL (1986) and Perrin et al.

8 Schnell, G. D., M. E. Douglas, and D. J. Hough. 1982. Geographic variation in morphology of spotted and spinner dolphins (Stenella attenuata and S. longirostris) from the eastern

tropical Pacific. Southwest Fisheries Center, National Marine

Fisheries Service, NOAA, Admin. Rep. LJ-82-15C, 213 p.
Dizon et al.: Dolphin Stocks in the Eastern Tropical Pacific 7

(1994) found that skulls, excluding the temporal fossa,

of the southern offshore form were relatively more

robust than those ofthe northern. Therefore, the southern animals are smaller but have relatively larger heads.

The temporal fossa of the southern offshore group was

most like that in the Hawaiian stock, which is also

separable on the basis of skull measurements (Perrin,

1975). Samples from westerly locations differed substantially from samples from the northern and more

easterly locations, which were studied in the multivariate analyses, but were similar to those from the south.

Coloration differs markedly among the various groups

as well. Overall coloration in spotted dolphins follows a

predictable pattern of development: no spots at birth,

then dark spots first appear on the ventrum, followed

by light spots on the dorsum at maturity. The ventral

spots enlarge and merge until the adult is uniform gray

below (Perrin, 1975). In offshore animals, those from

the east are the most heavily spotted; those from the far

west, the Hawaiian form, are the least-nearly unspotted as adults. An east-west or concentric cline in color

patterns, and skull characteristics, may exist because

the coloration of southern animals is similar to that of

Hawaiian animals (Perrin, 1975). The southern offshore form has lighter spotting than the northern offshore one; it also has a distinctly contrasted dark cape

and a white lateral field pattern. This pattern is similar

to that in Hawaiian spotted dolphins and the far-offshore

populations at the western end of the ETP distribution.

Recent work based on more extensive sampling reinforces the earlier tentative conclusions; the earliersimple

notion of a north-south division ofthe offshore population now has been discarded (Perrin et al., 1994). Variation in skulls was examined using 26 morphometric

measures and 4 tooth counts from 611 specimens. As in

previous studies (Schnell et al., 1986), northern offshore spotted dolphins from the eastern portion of

their range were distinct from southern animals, while

animals from the northwestern portion of the range

(including Hawaii) were more similar to animals from

the southern portion. Specific morphotypes were significantly correlated with specific environmental parameters, which also argues for ecological segregating

mechanisms. The authors suggested a concentric pattern of variation, similar to that found in the spinner

dolphin (Douglas et al., 1992). Based on this analysis,

they recommended altered stock boundaries to divide

the offshore populations into a northeastern unit and a

combined western-southern management unit, with

boundaries at 120'W longitude and 5'N latitude.

Genotype

Although karyotype (Worthen9), isozyme (Sharp, 1975;

Shimura and Numachi, 1987; Savin, 1988), and mitachondrial DNA (mtDNA; Dizon et al., 1991) studies

have been done on a few spotted dolphins, they were

not useful for discriminating northern and southern

stocks. In general, comparative isozyme studies on cetaceans have revealed low genetic divergence for a given

taxonomic level when compared to other highly mobile

animals (Shimura and Numachi, 1987). The potential

for effective use ofkarotyping and isozyme information

to separate the two populations is low because there is

probably some gene flow between the populations. Perhaps the application of other techniques, such as sequencing of mtDNA or nuclear DNA, which have the

ability to resolve low levels of genetic variation between

closely related populations, would be more likely to

demonstrate differences.

Classification _

The spotted dolphin is the most heavily exploited of all

target species of dolphins (DeMaster et al., 1992). Quotas for the U.S. fleet have been established for all three

ETP stocks. The recommendations ofPerrin et al. (1985)

and current management practice are fairly concordant: they emphasize that a decision regarding school

identification of coastal or, the then referred to, northern offshore forms should not be based solely on geographic location. Fortunately, the coastal and northeastern stocks are reasonably easy to discriminate because ofstrong morphological differences. Although the

stock pairs are partially sympatric, some isolating mechanism must operate to reduce gene flow between the groups.

Because of the strong morphological differentiation (a

proxy for presumably adaptive genetic variation), we classify the population division as IIa/c (Table 1). We use

Category II because the populations are partially sympatric and use a/ c because of the lack of significant

geographic separation, which argues for potential gene

flow, and because of morphological differences, which

argues for genetic uniqueness. We use the same reasons to categorize the comparison between the coastal

and western-southern offshore stocks.

The currently designated northern and southern offshore stocks have been the subject of a greater degree

of controversy. Although the populations are clearly

morphologically distinct based on modal differences

(see Schnell et al., 1986; Perrin et al., 1994), distinguishing the two forms at sea is difficult, and schools

are currently designated as northern or southern based

on geographic position, Le. either north or south of

1's. Some disagreement has occurred because the evi9 Worthen, G. L. 1981. Preliminary analysis of the potential for

stock assessment of Pacific Ocean delphinids by G and C chromosome banding. Southwest Fisheries Center, National Marine

Fisheries Service, NOAA, Admin. Rep. L]-81-02C, 99 p.
8 NOAA Technical Report NMFS 119

---------------------

dence of hiatus in distibution is based on lowered densities rather than the absence of sightings, giving the

Table I

Recommended stock structure and stock classification of pantropical spotted dolphins, Stenella

attenuata, in the eastern tropical Pacific Ocean. Proxy

data to the left of the slash argue against the existence

of a population division; data to the right argue for the

existence oftwo stocks (a = distribution, b = population

response, c = phenotype, and d = genotype).

Stock

Coastal

Coastal

Northeastern

Offshore

Classification

of division

between stocks

Hale

Hale

Ha/abc

Stock

Northeastern Offshore

Western-Southern

Offshore

Western-Southern

Offshore

appearance of a lack of a firm boundary. However, the

small amount of tagging data still suggests no movement across boundaries. The morphological evidence

is compelling, and the life history data are supportive,

although suspect because of problems such as sampling

bias. For these reasons, we categorize this stock division

as IIa/abc and strongly recommend that present management practices be retained with altered stock boundaries (Table 1, Fig. 2). The morphological similarity of

animals from far south to those from far west north of

the Equator argues that spotted dolphins from west of

1200W should not be grouped with those to the east but

rather with those in the south. In addition, the present

north-south boundary should be moved from lOS to

SON to better reflect the distributional hiatus and distribution of morphotypes. Current morphological data

do not warrant managing southern and western offshore populations separately, but because there is some

evidence of restricted movements between population

centers (based on life history data), we recommend that

the pooled management unit be considered provisional.

,-------------------------_.__._---_._----------------,

35N

30N

25N

20N ......

'D

15N

10N

5N

0

55

105

155

205

160W 150W 140W 130W 120W 110W lOOW 90W BOW

Figure 2

Recommended stock division boundaries for the pantropic-al spotted dolphin, Stenella attenuata. Note that the coastal

and the northeastern offshore stocks exist in sympatry within the northern range (above SON) of the former, and the

coastal and the western-southern offshore stocks, within the southern range (below SON) of the former.
Dizon et al.: Dolphin Stocks in the Eastern Tropical Pacific 9

Spinner Dolphin

Stenella longirostris

Current U.S. Management Situation _

In the eastern and central Pacific Ocean, five stocks are

recognized: Central American (also known as Costa

Rican, S. l. centroamericanus; Perrin, 1990), eastern (S. l.

orientalis; Perrin, 1990), northern whitebelly, southern

whitebelly, and Hawaiian (also known as pantropical, S.

l. longirostris; Perrin, 1990). Take is allowed by U.S.

fishermen for northern and southern whitebelly spinner and eastern spinner dolphins (Anon., 1986). The

stock division boundaries recommended for the SOPS

workshop in 1983 (Perrin et ai., 1985) are shown in

Figure 3. It was recommended that the spinner dolphins in the ETP be assigned to stock type as follows:

(1) Animals encountered south of 16°N, north of 7°N,

and within 92 km (50 nmi) of the coast should be

assigned to Central American or eastern stocks based

on relative body length and rostral length. (2) Identification of eastern versus whitebelly forms should be

based on modal color pattern and dorsal-fin shape in

adult males. (3) Identification of northern and southern whitebelly forms should be based on geographic

location of sightings.

Evidence of Stock Divisions

Distribution

Range - Spinner dolphins occur throughout the tropics and subtropics (Perrin and Gilpatrick, in press).

They are considered pelagic, although in some areas

3SNr-----------------.:::::::---------------_=o

.....{> WHITEBELLY

'.

OVERLAP BETWEEN

EASTERN & WHITEBELLY

OLD BOUNDARY BETWEEN

NORTHERN & SOUTHERN WHITEBELLIES

.. SN

SS

o

lON

l5N

lOS

lSS

20N

2SN

30N

BOW

20S =---:::=:-:-:---~::-:-:----.:~~-~~--~~-~~-- ..........---.......---I l60W lS0W l40W 130W l20W ll0W 100W 90W

Figure 3

Distribution of the spinner dolphin, Stenella longirostris, in the eastern tropical Pacific Ocean based on sightings data

collected between 1971 and 1991 by observers on tuna purse-seining vessels and research vessels. No boundary in the

northern Gulf of California has been established. Stock boundaries are from Perrin et al. (1985), except that our

current recommendation is to drop the northern and southern whitebelly stock division (dotted line) in favor of a

single whitebelly spinner designation.
10 NOAA Technical Report NMFS 119

they are found in a neritic (Leatherwood and Reeves,

1983) or coral reef habitat (Perrin et aI., 1989). Their

distribution is well described only for the eastern and

central Pacific (Perrin et aI., 1983 and 1985), although

records are slowly accumulating for the western Pacific

(west of 1600

W) and Indian Oceans (as well as the Red

and Arabian Seas). In the western Pacific Ocean, they

range north to about 35°N and south to about 200S; in

the Indian Ocean, to about 35°S along the western

coast of Africa (Gilpatrick et aI., 1987). In the Atlantic,

spinner dolphins are distributed between 35°N and

200S in the west and 200N and the Equator in the east.

There they are sympatric with the closely related clymene

dolphin, S. clymene (Perrin et aI., 1981).

Central American spinner and eastern spinner dolphins are likely parapatric or partially sympatric within

92 km (50 nmi) of the coastline between the Gulf of

Tehuantepec and Panama. There are no confirmed

records of eastern spinner dolphin much more inshore

of 92 km and no records of Central American spinner

dolphins much farther offshore (Perrin et aI., 1985).

The known ranges of spinner dolphins in the ETP

are illustrated in Figure 3. A weak distributional hiatus

exists that could be used to divide the whitebelly spinner dolphins into northern and southern hemisphere

stocks. The hiatus is thought to represent the preference of spinner dolphins for warm, low-saline tropical

waters with intermediate-to-shallow thermocline structure, which are centered off southern Mexico and extend westward along lOoN. This type of water appears

seasonally in the tropical waters south of the Galapagos

Islands (Au and Perryman, 1985; Reilly, 1990). In the

eastern portion of the ETP, upwelling-modified waters,

favored by the common dolphins, are interposed. However, while there exist clear northern and southern

population centers, the southeast centers of high density are connected by areas oflow, but not zero, density.

Perhaps of greater significance for separation of spinner stocks is the equatorial front that exists from 2° to

5°N and divides the ETP into two clearly defined ecosystems (Pak and Zaneveld, 1974; Reilly and Fiedler5).

From the hiatus in distribution, but relying more on

demographic and morphological evidence, Perrin et

ai. (1985) divided the whitebelly spinner dolphins provisionally into northern and southern stocks. The results of recent more extensive morphological studies

have reversed this, leading to a recommendation that

the two putative populations be pooled into a single

stock (Perrin et aI., 1991). A similar distributional hiatus separates Hawaiian spinner dolphins from those in

the ETP; again, this and morphological criteria led to

the separation of the Hawaiian stock (Perrin et aI.,

1985). Subsequent work linked the Hawaiian form with

a more generalized, pantropical type that is found in all

of the tropical and subtropical oceans except where it is

replaced in the ETP by the eastern and Central American subspecies (Perrin, 1990).

Presumed preferences for a particular habitat may

also argue for separate stock status of whitebelly and

eastern spinner dolphin populations. Whitebelly spinner dolphins that co-school with spotted dolphins can

statistically be placed in habitats with a deeper thermocline compared to eastern spinner dolphins that

also co-school with spotted dolphins (Reilly and

Fiedler5 ).

Movements - Little information is available from tag

returns to assess movement across the north-south stock

boundary. Perrin et ai. (l979a) conducted tagging experiments between 1969 and 1976: 324 spinner dolphins were tagged but only 7 were recovered by the

time of the report. Minimum distances traveled ranged

22-506 km; days at liberty, from less than one day to

776 days. Hedgepeth6 described a 1978 Rototag experiment that mostly focused on spotted dolphins, but during the experiment 16 spinner dolphins were tagged.

Only one spinner dolphin was resighted (not recovered). Little can be concluded from such limited data

save that spinner dolphins, compared with spotted dolphins, may disperse less. Studies ofmorphological variation reinforce the notion of more limited home ranges

in the spinner dolphin compared with the spotted dolphin (Perrin et aI., 1991; Douglas et aI., 1992).

Population Response

Reproduction - Sex ratio in the eastern form was the

same as in the northern and southern whitebelly forms

(Perrin and Henderson, 1984; Henderson et aI.lO). In

all three cases, the fraction of females was not significantly different from 0.5. However, there was a smaller

proportion of reproductively active females in the eastern form (0.43) compared to the northern whitebelly

form (0.53, Table 2).

Using an extensive data set that spanned 1974 to

1990, Chivers and DeMaster (in press) found that the

proportion of females mature varied among the eastern, northern whitebelly, and southern whitebelly populations: 0.455, 0.556, and 0.681, respectively. However,

these data are likely biased and probably not useful as a

proxy for stock difference.

Initial analyses indicated that eastern spinner dolphins had lower ovulation/pregnancy rates than northern whitebelly spinner dolphins (Perrin and Henderson,

10 Henderson,J. R., W. F. Perrin, and R. B. Miller. 1980. Rate of

gross annual production in dolphin populations (Stenella spp.

and Delphinus delphis) in the eastern tropical Pacific, 1973-

1978. Southwest Fisheries Center, National Marine Fisheries

Service, NOAA, Admin. Rep. LJ-80-02, 51 p.
Dizon et al.: Dolphin Stocks in the Eastern Tropical Pacific 11

Table 2

Summary of parameters for two populations ofspinner

dolphins, Stenella longirostris (from table 5 in Perrin and

Henderson, 1984).

Northern

Parameters Eastern whitebelly

Fraction 'i' 0.510 0.509

Annual pregnancy

rate 0.339 0.356

Fraction 'i' mature 0.432 0.533

Gross annual

reproductive rate 0.075 0.094

Length-at-birth 77 em 76 em

Length-at-one year 132.5 em, 'i' 137.2 em, 'i' 0

LSMJ 164.1 em, 'i' 175.6 em, 'i'

ASM2 -5 years, 'i' -6 years, 'i'

J LSM = Length at attainment of sexual maturity.

2ASM = Average age of attainment of sexual maturity.

1984). Using the larger data set, Chivers and DeMaster

(in press) observed that the proportion offemales pregnant and lactating was higher in the eastern form, while

the proportion of females resting was higher in the

northern whitebelly form.

Perrin et aI. (1977) reported that eastern spinner

dolphins tend toward a single mode in the timing of

breeding. Barlow (1984) reported that whitebelly spinner dolphins have two modes separated by about six

months. However, he found regional differences in

timing of breeding for the eastern form, with an earlier

calving season for the more offshore animals. Gross

annual reproductive rates were estimated at 0.075 for

the eastern (Perrin and Henderson, 1984), 0.094 for

the northern whitebelly (Perrin and Henderson, 1984),

and 0.067 for the southern whitebelly (Henderson et

aI. 10) forms.

Age and growth - Average length at birth was similar

in both the eastern form and northern whitebelly form

(Table 2); northern whitebelly calves grew somewhat

faster. Although ageing and determining absolute ASM

is subject to some disagreements, comparisons between

stocks using these measures can be useful as long as the

same methodologies were used. For instance, the ASM

of eastern spinner females was about 5 years; the LSM

was about 164 em; ASM was about one year earlier than

in the female northern whitebelly form, which matures

at 167 em (males at 165-170 em) (Perrin et aI., 1977;

Perrin and Henderson, 1984). Chivers and DeMaster

(in press), using data from 1974 to 1990, found the

same LSM differences, albeit smaller in magnitude than

found in the previous studies. The difference in LSM's

in the two populations may reflect a density-dependent

response in a recovering population versus a stable one.

Northern whitebelly males and eastern males differ

markedly in morphological indices of maturity (Perrin

and Henderson, 1984); whitebelly spinner dolphins have

larger testes than eastern spinner dolphins. Four alternative explanations were proposed by the authors: (1)

seasonal fluctuations occur, and the differences reflect

seasonal bias; (2) inherent genetic differences in populations result in different reproductive morphogenesis;

(3) age-specific sampling biases exist; and (4) testis size

and possibly male fertility is depressed in the eastern

population, conceivably because of more intense exploitation. Perrin and Henderson (1984) favored the

last explanation but could not unequivocally rule out

genetic differences or sampling biases. Seasonal fluctuations were eliminated as an explanation.

Phenotype

Eastern males have an average length of 176.1 em;

females, 171.3 cm (Perrin et aI., 1977). Northern whitebelly males have an average length of 179.8 em; females,

175.6 em (Perrin et aI, 1985).

Schnell et aI.8, Perrin et aI. (1985 and 1991), and

Douglas et aI. (1992) reported significant external and

internal morphological differentiation between Central American, eastern, whitebelly, and Hawaiian populations. Perrin et aI. (1979b) reported modal differences in color pattern, body size, and skull measurements between geographically widely separated samples

of the southern (called southwestern) form and the

northern whitebelly form. In all characteristics, the values for these southern animals fell between those for

the northern whitebelly form and the Hawaiian form.

Akin (1988) reported significant differentiation between

northern and southern forms (as well as between the

eastern and whitebelly forms) in tooth morphology,

which included length of tooth and other dimensions,

as well as internal layering characteristics. In later work

that assumed no a priori stock or subspecies stratifications, Douglas et aI. (1992), besides finding that skull

measurements discriminated subspecies, found that

many of the characteristics of the Hawaiian, or pantropical, form were more similar to those of the extreme southern samples than to those samples from

adjacent regions to the east. This supported the Perrin

et al. (1991) observation ofa radial pattern of variation ,

in which animals from the far west, southwest, and

south differed markedly from those to the northeast.

Variation in color pattern, dorsal-fin shape, and total

body length were graded steeply in the north-south

direction at about lOON and 125°W (Perrin et aI., 1991).

Outside this boundary, complex and geographically
Table 3

Recommended stock structure and stock classification of spinner dolphins, Stenella longirostTis, in the

eastern tropical Pacific Ocean. Proxy data to the left

of the slash argue against the existence of a population division; data to the right argue for the existence of two stocks (a = distribution, b = population

response, c = phenotype, and d = genotype).

divisions between the whitebelly and eastern forms or

the whitebelly and pantropical forms are somewhat

artificial; a more meaningful division would be between

the eastern and pantropical forms. Hybridization probably occurs between virtually all intraspecific parapatric

populations. The pantropical-whitebelly-eastern situation represents an extreme because of the large number of hybrid individuals and the broad zone of hybridization. Under the hybrid swarm hypothesis, we categorized the eastern-pantropical division as I/abc (Table

3). We judged the population division to be Category I

because ofthe relative distance between the two groups,

although the whitebelly hybrids may provide an avenue

for gene flow.

Given the problematic status of the whitebelly form,

it makes little sense to attempt to divide the group into

two forms. Morphological studies show a lack of geographically concordant variation within the group, although some demographic indices differ. The lack of

clear geographic concordance, absence of mtDNA difference, weak evidence of a distributional hiatus, and

presence of demographic evidence (albeit potentially

biased) would lead us to categorize the stock division as

IVacd/ab. We see little reason to manage the northern

and southern areas as separate units, and hence the

division is absent from Table 3. We also recommend

management of the pooled whitebelly forms as a separate stock, despite their hybrid/intergrade status. There

is a danger that heavier exploitation of the eastern

form may potentiate unsymmetrical gene flow from the

numerically vast pantropical populations to the west via

the whitebelly populations, perhaps swamping out the

eastern genotype. Perrin et aI. (1991) proposed a conservation zone for the eastern spinner that would facilitate management and protection of the subspecies.

12 NOAA Technical Report NMFS 119

discordant variation as well as higher standard deviations of body length existed. This discordant variation,

suggestive of contact and partial interbreeding, and the

radial nature of the variation within the ETP, has

prompted the suggestion that the whitebelly stock is, in

reality, a hybrid swarm bridging the eastern and Hawaiian, or pantropical, subspecies (Perrin, 1990; Perrin et

aI., 1991) and that the management of this form as

southern and northern populations is not justified on

morphological grounds alone (Perrin et aI., 1991).

Genotype

Allozyme studies on eastern and whitebelly populations

did not detect population differences (Sharp, 1981;

Landino, 1987). Dizon et al. (1991), employing a restriction fragment (RFLP) analysis using six different

restriction enzymes, found a unique mtDNA haplotype

in the Timor Sea off northern Australia, distinguishing

the specimens there from ETP spinner dolphins. However, the two morphologically distinct ETP spinner dolphins (whitebelly and eastern) were not genetically

distinct at the level of resolution of the analysis (Dizon

et aI., 1991). The mtDNA homogeneity of whitebelly

and eastern samples suggests a degree of genetic exchange or mtDNA introgression and supports the hypothesis of Perrin et al. (1991) that the whitebelly spinner is a hybrid/intergrade.

Classification _

Quotas have been established based on the existence of

more-or-Iess discrete stocks separated into latitudinal

bands (Anon., 1986). Current management practice

and the recommendations of Perrin et al. (1985) are

reasonably concordant.

Perrin (1990) has proposed the Central American,

eastern, and pantropical forms for separate subspecies

status, and there has been general agreement that these

populations be considered separate for assessment and

management. We categorize the parapatric Central

American versus eastern or whitebelly division as Ha/ c

(Table 3). Category H is chosen because of the strong

reliance we place on morphological data as a proxy for

adaptive genetic divergence. For the Central American

versus pantropical division, allopatry elevates the classification to Category 1.

The whitebelly and eastern stock division is problematic. Recent work by Perrin et aI. (1991) argues that the

whitebelly form is a hybrid swarm between the eastern

form and the pantropical subspecies. In that case, stock

Stock

Central American

Central American

Central American

Eastern

Eastern

Classification

of division

between stocks

Hille

!laic

I/ac

see text

I/abc

Stock

Eastern

Whitebelly

PantropieaI

Whitebelly

PantropieaI
Dizon et al.: Dolphin Stocks in the Eastern Tropical Pacific 13

Common Dolphin

Delphinus delphis

Current U.S. Management Situation _

Despite the recommendations of Perrin et aI. (1985),

current management practice uses five units in the

eastern and central Pacific Ocean: northern temperate,

northern tropical, central tropical, southern tropical,

and southern temperate. U.S. quotas are established

for the tropical units (Anon., 1986). The Baja neritic or

long-beaked form, a morphologically distinct coastal

population, is managed with the northern tropical population, despite the recommendations ofthe SOPS meetings in 1983 (Perrin et aI., 1985). At that meeting, it was

recommended that common dolphins in the ETP be

assigned to stocks as follows (Fig. 4; see also Holt and

Powers, 1982; Smith, 1983): (1) Within 184 km (100

nmi) of the Pacific coast of southern California and

Baja California, as well as in the Gulf of California,

long-beaked and northern forms should be distinguished based on modal length, coloration, and relative rostral length. (2) The temperate units should be

combined with the adjacent tropical ones, and these

new units (northern, central, and southern) distinguished based on distribution.

Evidence of Stock Divisions _

Distribution

Range - Common dolphins are distributed worldwide

in temperate, tropical, and subtropical waters in both

coastal and offshore areas (Evans, 1982). In the eastern

Pacific, they range from 36°N, including the Gulf of

California (Evans, 1982), to at least 13°S (Fig. 4). Both

the northern and the long-beaked forms are sympatric

from the coast to perhaps 184 km (100 nmi) offshore

NORTHERN

SHORT-BEAKED

CENTRAL

SHORT-BEAKED

110W 100W 90W BOW

SOUTHERN

SHORT-BEAKED

120W

';-;;,:;,'.a~- OVERLAP BETWEEN LONG-BEAKED

& NORTHERN

SHORT-BEAKED

": :."

140W 130W

'.

150W

35N

30N

25N

20N ....,.{;l

15N

10N

5N

0

55

105

155

20S

160W

Figure 4

Distribution and boundaries of the stocks of the common dolphin, Delphinus delphis, in the eastern tropical Pacific

Ocean based on sightings data collected between 1971 and 1991 by observers on tuna purse-seining vessels and

research vessels. Stock boundaries are from Perrin et al. (1985).
14 NOAA Technical Report NMFS 119

(Perrin et a!., 1985). Because of this, an "offshore"

appellation is inappropriate, and Heyning and Perrin

(in press) suggested that "short-beaked" be used for all

of the offshore stocks (northern, central, and southern) and, for consistency, that "long-beaked" instead of

"Baja neritic" be used for the inshore form.

The long-beaked common dolphin occurs within 184

km (100 nmi) of the Pacific coast of Baja California and

southern California and in the GulfofCalifornia (Perrin

et a!., 1985). As mentioned above, there is no geographic separation between the long-beaked and northern forms. The range of the former is completely contained within that of the latter, so these populations

must be distinguished based on morphology and not

on the basis of distance from shore alone. The hiatus at

13-20oN and another at about 3°N is used to divide the

offshore populations into northern, central, and southern stocks (Fig. 4). The central form occurs at 3-18°N

(Evans, 1982; Perrin et a!., 1985). Based on lowered

density at 105-120oW, it was proposed that this stock be

divided into two, an eastern and western one (Au et

a!. II). This proposal was never accepted for lack of

adequate data (Perrin et a!., 1985; Smith3). The southern common dolphin is found from 3°N to at least 13°S.

Based on 1986-90 research vessel cruises, a region of

low density separates the central and southern populations. However, some sightings have shown animals in

the area of low density (Perrin et a!., 1983), and there

are no clearly definable habitat differences isolating

the population center areas (Reilly and Fiedler5).

Morphologically distinct common dolphins may be

found offsouthern Mexico at about 15°N (Evans, 1982),

the so-called Guerrero stock. Perrin et a!. (1985) considered them "marginally justified" as a separate stock

based on their larger sizes but recommended that more

specimens be examined. However, since then no additional samples have been obtained. It is conceivable

that these are the long-beaked form, which would extend the range beyond the Baja California coast. It is

also possible that this form inhabits all coastal waters in

the eastern Pacific from California to Peru.

Movements - During tagging studies conducted from

1969 to 1976, 193 common dolphins were tagged, probably all of the northern form, but none were recovered

or resighted. However, information from radiotelemetry studies indicates that common dolphins are quite

mobile (Evans, 1971, 1974, and 1982; Perrin et a!.,

1979a). Net movements of 1.4-5.84 km/hr (0.77-3.20

nmi/hr) were recorded. Although no evidence is avail11 Au, D. W. K, W. L. Perryman, and W. F. Perrin. 1979. Dolphin

distribution and the relationship to environmental features in

the eastern tropical Pacific. Southwest Fisheries Center, National Marine Fisheries Service, NOAA, Admin. Rep. LJ-79-43,

59 p.

able for common dolphins crossing the putative stock

boundaries, the potential vagility of this species suggests that the development and maintenance of population differences involves more than simple isolation by

distance. Evans' studies (1971, 1974, and 1982) showed

that the movements of common dolphins in the ETP

were correlated with stable physiographic features (e.g.

banks and seamounts) suggesting a knowledge by the

dolphins of a home range. He suggested that this,

coupled with his observation of subtle morphological

differences between schools, argued for a degree ofsite

fidelity and lack of long-range movements; this view is

not generally accepted by other workers.

Population Response

Reproduction - Evans (1975) found differences in the

timing of reproduction between northern and central

stock areas but did not have sufficient data to define

patterns in either area. The proportion of females that

were neither pregnant nor lactating was higher in the

central population. Recently, Perryman and Lynn (1993)

used aerial photogrammetry techniques to measure body

lengths of calves in herds of common dolphins. Assuming linear growth in the first year, back calculating from

current body length gives approximate birth dates. Preliminary results showed monthly birthrates to be bimodal, peaking in spring and early summer, for both the

northern and southern forms; while for the central

form births seem to be fairly evenly distributed throughout the year.

Chivers and DeMaster (in press) examined life history parameters estimated from data collected between

1974 and 1980. No significant differences in the proportion ofmature females in various reproductive states

(i.e. pregnant, lactating, resting, and simultaneously

pregnant and lactating) were found between the three

short-beaked and one long-beaked populations. However, there were significant differences among the stocks

in proportion mature. They also noted a predictable

difference (under a density-compensatory model) in

temporal [rends of reproductive parameters in the

pooled short-beaked populations and the long-beaked

population.

Other - Walker and Cowan12 studied parasites as indicators of population groups in common dolphins in

the ETP and found the nematode Crassicauda sp. in

34.9% of the central specimens but none in the long12 Walker, W., and D. F. Cowan. 1981. Air sinus parasitism and

pathology in free-ranging common dvlphins (Delphinus delphis)

in the eastern tropical Pacific. Southwest Fisheries Center, National Marine Fisheries Service, OAA, Admin. Rep. LJ-81-23C,

19 p.
Dizon et al.: Dolphin Stocks in the Eastern Tropical Pacific 15

beaked or northern ones. The opposite pattern was

found for Nasitrema sp. infections.

Phenotype

Based on morphological evidence from examination of

small series ofspecimens, over two dozen nominal species have been put forth for the genus world-wide, although, in general, authorities recognize one or two species (Heyning and Perrin, in press). Banks and Brownell

(1969) examined a fairly lengthy series and concluded

that two sympatric species existed in the northeast Pacific: the long-beaked common dolphin, Delphinus bairdii,

and the short-beaked nominate, D. delphis. They found

no indication of overlap in the two forms in the ratio of

zygomatic width to rostrum length. After examining 70

specimens from widely divergent locations, van Bree

and Purves (1972) disagreed, claiming intergrades existed in specimens collected outside of the study area of

Banks and Brownell (1969). They suggested that

interschool variation was responsible for the clear separation demonstrated previously. Current work seems to

have resolved the argument, in so far as concerns the

animals in the northeastern Pacific. Heyning and Perrin

(in press) have obtained results consistent with a hypothesis ofseparate species. Reexamination of318 specimens, for which complete information on sex and age

existed, allowed examination of variation between and

within the forms stratified by sex and age class. In

adults, color pattern, total length, and all measures of

rostral length separated the two forms. The specieslevel taxonomic question must await morphological studies of series from other areas.

In addition to distributional hiatuses and evidence of

differential breeding seasons, the northern, central,

and southern populations can be discriminated on the

basis of total length of adult animals. Evans (1982)

separated the northern into two stocks, one off southern California (the so-called northern temperate stock)

and one offBaja (the northern tropical stock) based on

total length and various cranial features; this stock division corresponded to his finding of a hiatus in distribution. However, this division was abandoned after further work (Perrin et al., 1985). Evans (1982) also found

a size difference between adults of northern populations and those further south, which he called the "southern short-beaked form." That difference has been supported by further study (Perrin et al., 1985; Perryman

and Lynn, 1993). The form, now called the central

form, is significantly larger than the northern one

(Perrin et al., 1985; females, 194.3 versus 178.5 cm,

respectively; males, 206.8 versus 189.3 cm, respectively).

The differences obtained by Perrin et al. (1985) were

confirmed using measurements from aerial photographic images (Perryman and Lynn, 1993).

Less is known about differentiation of the central

and southern populations. Perrin et al. (1985) reported

the sizes of six adult southern females that constituted

the entire sample set through 1983. Including data

through the present only adds individuals from one

additional school, which were collected from about the

same location as the Perrin et al. (1985) samples. The

averages ofthe data sets are 190.4 cm (sd=5.3, n=16) for

females and 192.8 cm (sd=6.6, n=5) for males. These

averages can be contrasted with an aerial photographic

average of 184.3 cm (sd=6.8, n=20) for southern females with calves (Perryman and Lynn, 1993) and the

Perrin et al. (1985) average of 194.3 cm for central and

178.5 cm for northern females.

Although it is tempting to use size differences to

establish the existence of two, three, or four separate

stocks in the central and southern areas, caution is

necessary because data from the far-west region of the

central population and the southern populations are

based mostly on information from single schools. Evans

(1982) asserted that schools within only 73.6 km (40

nmi) of each other differed in color patterns and skull

measurements.

Genotype

Evans (1975) suggested that local subpopulations represent single, limited gene pools because certain elements of the color pattern, such as white blazing in the

dorsal fin, serve as a herd identification character and

possibly an isolating mechanism. Amos (1988) used

DNA fingerprinting to examine its potential to differentiate stocks. He found high variability between two

common dolphins whose stock origins, however, were

not specified.

Recently Rosel (1992) sequenced the PCR-amplified

control loop region (D-Ioop) and the cytochrome b

gene of mtDNA in an attempt to differentiate samples

of short-beaked and long-beaked common dolphins.

The northern and long-beaked samples came from some

ofthe same specimens employed by Heyning and Perrin

(in press). In approximately 750 bp sequenced, fixed

and significant frequency differences segregated the

long-beaked samples from the offshore ones. Of particular significance was the discovery of differences in

the cytochrome b gene that resulted in an amino acid

substitution in the protein. Because the cytochrome b

gene evolves more slowly than the control region, these

differences are evidence of a long genetic separation

consistent with the status of separate, albeit closely related, species.

Mitochondrial DNA sequence analysis failed to discriminate northern and central forms, although only

six central specimens were sequenced and available for
16 NOAA Technical Report NMFS 119

analysis. Clear differences, however, were apparent between the Pacific short-beaked forms and four dolphins

from the Black Sea. Interestingly, the genetic distance

between the Pacific short-beaked forms and the Black

Sea form was smaller than the genetic distance between

the Pacific short-beaked forms and the sympatric longbeaked form.

Table 4

Recommended stock structure and stock classification of short-beaked comon dolphins, Delphinus delphis. in the eastern tropical Pacific Ocean. Proxy

data to the left of the slash argue against the existence

of a population division; data to the right argue for the

existence oftwo stocks (a =distribution, b =population

response, c = phenotype, and d = genotype).

-----~-------_._--------

the northern and central stocks is fairly clear. Allopatric distribution coupled with significant size and breeding seasonality differences, supported by differences in

proportion of mature females, suggest categorization

of the northern-central division as IUd/abc (Table 4).

Genetic information is available, albeit based on a small

sample of central forms.

The southern-central division is clouded by the paucity ofsamples and some concern over the existence of

an "inshore corridor" allowing the exchange of individuals between the two regions. In addition, there is

the equivocal evidence from size measurements of animals sampled near the Galapagos. Their size range

places them in an intermediate position between the

far offshore southern animals measured by Perryman

and Lynn (1993) in an aerial photographic study. We

categorized the division as IlIac/abc.

Classification _

Although the common dolphin is not as heavily exploited as spotted and spinner dolphins, U.S. mortality

quotas have been set for the three offshore stocks. Over

the years, management has pooled the northern and

long-beaked forms. Clearly this is a problem, because

stock status is hardly in question. Obviously the two

forms should be assessed and managed separately because they show species-level separation in the eastern

Pacific. No categorization is given in Table 4; the classification scheme notation is inappropriate for specieslevel comparisons because proxy information is unnecessary when one assumes no meaningful gene flow and

significant genetic distinctiveness. In addition, the possibly morphologically distinct common dolphin, the

Guerrero stock found off southern Mexico at about

15°N, which Perrin et al. (1985) recommended be provisionally considered a separate stock, must still be considered provisional because of lack of data.

The northern-central-southern divisions are conceptually straightforward. Because common dolphins prefer waters modified by upwelling, the hiatus between

Stock

Northern

Central

Classification

of division

between stocks

IlId/abc

IlIac/abc

Stock

Central

Southern
Dizon et al.: Dolphin Stocks in the Eastern Tropical Pacific 17

Striped Dolphin

Stenella coeruleoalba

Current U.S. Management Situation _

In the eastern and central Pacific ocean, current U.S.

management units include northern temperate, northern tropical, central tropical, southern tropical, and

southern temperate stocks. U.S. quotas have been established for the three tropical populations (Anon.,

1986). This management scheme is in conflict with that

recommended by Perrin et al. (1985), who recommended that striped dolphins in the eastern Pacific be

assigned to either the northern or southern stock, depending on geographic location (Fig. 5).

Evidence of Stock Divisions _

Distribution

Range - The striped dolphin is widely distributed in

tropical, subtropical, and temperate oceans ofthe world.

It occurs in the Atlantic, Pacific, and Indian Oceans

and in the Mediterranean and Red Seas (Perrin et aI.,

in press). Because the hiatus in distribution between

lOoN and 15°N may inhibit direct interchange between

the north and south, Perrin et al. (1985) suggested a

single provisional division into northern and southern

stocks, pending the examination of existing osteological material.

Movements - No tagging-recapture studies have been

conducted on striped dolphins, and nothing is known

about their movements in the ETP. However, this species is known to be highly migratory in the western

Pacific (Miyazaki et aI., 1974).

Population Response

Chivers and DeMaster (in press) estimated life history

parameters from data collected between 1974 and 1979

3SN

30N

2SN

20N ......l>

1SN

10N

5N

0

5S

10S

15S

20S

160W 150W 140W 130W 120W 110W mow 90W BOW

Figure 5

Distribution and stock boundaries of the striped dolphin, Stenella coeruleoalba, in the eastern tropical Pacific Oceansightings data collected between 1971 and 1991 by observers on tuna purse-seining vessels and research vessels. No

boundary in the northern Gulf of California has been established. We recommend that the northern and southern

division (dotted line) be dropped in favor of a single stock.
18 NOAA Technical Report NMFS 119

and detected no significant differences between north

and south in mean proportions of mature females in

various reproductive conditions. However, the comparison was based on only 21 mature females from the

northern stock and 25 from the southern stock and

must be viewed with caution.

Phenotype

Gurevich and Stewart13 estimated maximum body length

at 220 cm in females and 225 cm in males from a

sample of70 pooled from both the northern and southern populations. Perri n et al. (1985) reported mean

lengths from 9 northern and 10 central males and 21

northern and 22 central females; the means were not

significantly different between regions. More data are

currently being made available from aerial photographs.

Using the Perrin et al. (1985) stock boundaries (Fig. 5),

no significant differences in average size of individuals

(n=197, northern; n=608, southern) or in average size

of females with calves (n=19, northern; n=66, southern) were found (Perryman and Lynn, 1994). Additional studies of the morphology of the skull and postcranial skeleton are underway (Archer 14 ).

Genotype

Electrophoretic heterozygosity at specific loci was tested

inJapanese striped dolphins by Wada (1983), who found

less genetic variability (average heterozygosity = 0.021

at 15 loci tested) than for 48 other mammalian species.

He attributed this to either population subdivision or

inbreeding over long periods of time. The first genetic

work on striped dolphin in the eastern Pacific is underway (Archer14 ).

Classification _

The most recent distribution data show several apparent foci of abundance and little discontinuity (Fig. 5).

Because the length-frequency data comparing northern and southern samples show no difference, because

the distributional gap separating the northern and

southern population centers is very small, and because

the species is known to be highly migratory in other

regions, we classify the division as Category IVac/a and

recommend that all striped dolphins in the eastern

13 Gurevich, V. S., and B. S. Stewart. 1979. A study of growth and

reproduction of the striped dolphin (Stenella coeruleoalba). ational Marine Fisheries Service, Contract No. 03-78-027-1079

to Hubbs/Sea World Research Institute, San Diego, CA. 21 p.

14 Archer, E., Scripps Institution of Oceanography, La Jolla, CA

92038, personal commun. January 1993.

Pacific be provisionally managed as a single unit. The

results of ongoing studies of geographic variation in

morphology and mtDNA may require reconsideration

of this classification. Because the demographic data

were based on very small samples ofspecimens, we have

chosen to exclude the data as evidence for lumping the

two populations, so a "b" does not appear to the left of

the slash.

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