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Mammal Rev
. 2003, Volume 33, No. 3, 224–252.
Printed in Great Britain
.
© 2003 Mammal Society,
Mammal Review
,
33,
224– 252
Blackwell Science, LtdOxford, UKMAMMammal Review0305-1838Blackwell Publishing Ltd, 2003
? 2003
33
3224252
Review Article
Common dolphins in the MediterraneanG. Bearzi
et al.
Correspondence: G. Bearzi, Tethys Research Institute, c/o Natural History Museum, 30135 Venezia, Italy.
E-mail: bearzi@inwind.it
Ecology, status and conservation of short-beaked common
dolphins
Delphinus delphis
in the Mediterranean Sea
GIOVANNI BEARZI*, RANDALL R. REEVES†, GIUSEPPE
NOTARBARTOLO-DI-SCIARA*, ELENA POLITI*, ANA CAÑADAS‡,
ALEXANDROS FRANTZIS§ and BARBARA MUSSI¶
*
Tethys Research Institute, c/o Acquario Civico, Viale G.B. Gadio 2, 20121 Milano, Italy,
†
Okapi Wildlife Associates, 27 Chandler Lane, Hudson, Quebec J0P 1H0, Canada,
‡
Alnitak
Marine Environment Research and Education Centre, Nalon 16, 28240 Hoyo de Manzanares,
Spain,
§
Pelagos Cetacean Research Institute, Terpsichoris 21, 16671 Vouliagmeni, Greece, and
¶
StudioMare, via P.L. D’Abundo 82, 80075 Forio d’Ischia (NA), Italy
ABSTRACT
1.
The recent decline in the Mediterranean population of short-beaked common dol-
phins
Delphinus delphis
has been the subject of scientific controversy and political
indifference. Research on these animals has been very limited and there has been no
large-scale, systematic effort to assess and monitor their abundance and distribution. The
consequent lack of data has prevented a good understanding of historical and ongoing
trends.
2.
Nonetheless, literature and osteological collections confirm that common dolphins were
widespread and abundant in much of the Mediterranean Sea until the late 1960s and that
their decline occurred relatively quickly. Today, common dolphins remain relatively abun-
dant only in the westernmost portion of the basin (Alboràn Sea), with sparse records off
Algeria and Tunisia, concentrations around the Maltese islands and in parts of the Aegean
Sea, and relict groups in the south-eastern Tyrrhenian and eastern Ionian Seas. Otherwise,
these dolphins are rare in, or completely absent from, Mediterranean areas where informa-
tion is available.
3.
Circumstantial evidence and qualitative judgements by the authors suggest that the fol-
lowing factors may have contributed to the decline of common dolphins: reduced avail-
ability of prey caused by overfishing and habitat degradation; contamination by
xenobiotic chemicals resulting in immunosuppression and reproductive impairment; envi-
ronmental changes such as increased water temperatures affecting ecosystem dynamics;
and incidental mortality in fishing gear, especially gillnets. The cumulative importance of
these factors is poorly understood, and as a result, few conservation measures have been
implemented.
4.
This paper reviews current knowledge and suggests priorities for action aimed at identi-
fying and mitigating the main threats to common dolphins in the Mediterranean, with the
ultimate goal of restoring the species’ favourable conservation status in the region.
Keywords
:cetaceans, conservation, gill-netting, marine mammals
Common dolphins in the Mediterranean
225
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Mammal Review
,
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224– 252
CONTENTS
Introduction 225
Key areas of distribution 227
Ecology and behaviour 229
Past and present trends in abundance 231
Factors implicated in the species’ decline 234
Environmental fluctuations and global changes 234
Prey depletion 236
Xenobiotic contamination 238
Direct takes and bycatch 238
Priorities for action 239
Research recommendations 239
Recommended conservation measures 241
Acknowledgements 242
References 242
INTRODUCTION
The short-beaked common dolphin
Delphinus delphis
(Fig. 1) is a small cetacean species with
a wide distribution. Like most other cetaceans, however, it is not panmictic and occurs as a
series of geographically separate populations (Heyning & Perrin, 1994; Perrin & Brownell,
1994; Jefferson & Van Waerebeek, 2002). On a global scale, the systematics and zoogeography
of the genus
Delphinus
are subjects of ongoing investigation (e.g. Jefferson & Van Waerebeek,
2002). At present, two species are recognized unanimously – the short-beaked common
dolphin
D. delphis
and the long-beaked common dolphin
D. capensis
(Heyning & Perrin,
Fig. 1.
Two short-beaked common dolphins photographed in the eastern Ionian Sea show the characteristic
morphology of the species: short beak, narrow dark flipper stripe, occasional white patch in dorsal fin. Photo
by G. Bearzi.
226
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et al.
© 2003 Mammal Society,
Mammal Review
,
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1994; Rosel, Dizon & Heyning, 1994). Only short-beaked common dolphins inhabit the
Mediterranean Sea (Fig. 2) and adjacent water bodies, and therefore throughout this paper
references to ‘common dolphins’ can be understood to mean this species.
The short-beaked common dolphin was listed as lower risk ‘conservation dependent’ in
the 1996 IUCN Red List of Threatened Animals (Baillie & Groombridge, 1996). This desig-
nation reflected that although some populations had declined from historical levels, the
aggregate world population remained in the low millions (e.g. Yukhov, Petukhov & Korkhov,
1986; Gaskin, 1992; Wade & Gerrodette, 1993; LeDuc, 2002). Therefore, the species as a
whole did not appear to fit the Red List classification criteria for vulnerable or endangered.
The ‘conservation dependent’ caveat was included as a way of acknowledging the importance
of maintaining conservation measures to minimize incidental mortality of dolphins in the
eastern tropical Pacific tuna fishery (see Gosliner, 1999), as well as other measures taken in
national waters to limit the numbers taken deliberately and incidentally.
By contrast, in the Mediterranean Sea, conservation problems for the species have been
recognized since the 1970s. The UNEP Mediterranean Action Plan (Barcelona, 1975) recom-
mended strong conservation measures to protect the species but without specifying what these
should be. Determining the conservation status of Mediterranean common dolphins was
cited as a priority in past cetacean action plans of the IUCN Species Survival Commission
(Perrin, 1988; Reeves & Leatherwood, 1994) and the latest such plan notes that they have
declined dramatically in the central and eastern Mediterranean and that conservation action
is urgently needed to prevent extirpation in this portion of the species’ range (Reeves
et al
.,
2003). In 2003 the Mediterranean common dolphin ‘subpopulation’ was listed as endangered
in the IUCN Red List of Threatened Animals, based on criterion A2, which refers to a 50%
decline in abundance over the last three generations, the causes of which ‘may not have ceased
or
may not be understood
or
may not be reversible’ (http://www.redlist.org).
Although both public and institutional awareness of the importance of protecting the
natural environment has increased in several Mediterranean countries during the last few
Fig. 2.
Map of the Mediterranean Sea showing the locations cited in the text. Numbers indicate the following
localities: (1) Estepona; (2) Málaga; (3) Almería; (4) Gulf of Vera; (5) Principality of Monaco; (6) island
of Ischia and Campanian Archipelago; (7) Naples; (8) island of Kalamos; (9) Gulf of Corinth.
Common dolphins in the Mediterranean
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Mammal Review
,
33,
224– 252
decades, little progress has been made towards understanding the causes of the common
dolphin’s regional decline. Perhaps at least partly because of this dearth of understanding,
no specific conservation measures have been taken to address the problem (Notarbartolo di
Sciara & Demma, 1997). The
Agreement on the Conservation of Cetaceans of the Black Sea,
Mediterranean Sea and Contiguous Atlantic Area
(ACCOBAMS), which came into effect in
2001, has proposed that the status of common dolphins in the Mediterranean be evaluated
in a comprehensive manner, with the goals of estimating distribution and abundance through-
out the basin, identifying critical habitat and characterizing threats. Such an evaluation would
entail a series of localized surveys to locate any concentrations of animals that might remain,
with a priority in the eastern Mediterranean (ACCOBAMS, 2002).
A major hindrance to determining the status of common dolphins in the Mediterranean
is the fragmentary character of the literature, which is composed almost exclusively of
unpublished reports, academic theses or dissertations, conference proceedings and other non-
refereed publications. Although some of these studies are of high scientific quality and have
been long running, only a small proportion of the relevant available data has been published
in peer-reviewed scientific journals. This situation makes it difficult to evaluate what is known
even for many of the areas where focused research on the species has been carried out.
Questions about the past and present occurrence of common dolphins in Mediterranean
areas where no cetacean surveys have been conducted (particularly along much of the north
African coasts and in far-eastern portions of the basin) remain completely open, and nothing
is known about current abundance and trends in those areas.
In this article, we review and summarize information on common dolphins in the Medi-
terranean, with particular emphasis on their conservation. We discuss potential threats, both
‘natural’ and anthropogenic, and attempt to define the most urgent research and management
needs for the species in this region.
KEY AREAS OF DISTRIBUTION
There is no basin-wide estimate of abundance for common dolphins in the Mediterranean
Sea. Line transect ship surveys of the Alboràn Sea in 1991–92 produced an estimate of 14 736
(CV
=
0.38; 95% CI
=
6923–31 366), with a density of 0.16 dolphins/km
2
, but no estimates
were made for this species elsewhere in the western Mediterranean due to the low number of
sightings (Forcada & Hammond, 1998). Vella (1998, in press) combined data from ship and
aerial strip-transect surveys conducted 1997–2002, and obtained a density estimate of 0.135
dolphins/km
2
(CV
=
0.28; 95% CI
=
0.066–0.290) in the area around the Maltese islands.
Apart from these studies, the presence of common dolphins, and in some instances a
qualitative assessment of their relative abundance, can be inferred for other portions of the
basin on the basis of evidence from more general cetacean surveys and a few longitudinal
investigations.
Groups containing several hundred individuals are occasionally observed in the Alboràn
Sea and in the Gulf of Vera (southern Spain), in contrast with the smaller groups recorded
elsewhere in the Mediterranean (Cañadas, Sagarminaga & García-Tiscar, 2002). There are
sparse records off the coast of Algeria and Tunisia where, however, survey coverage has been
limited (Boutiba, 1994; Boutiba & Abdelghani, 1995; Zanardelli, Panigada & Bearzi, in
press). Possibly isolated groups are present around Sardinia and Corsica, particularly off their
western coasts (Notarbartolo di Sciara
et al
., 1993; Gannier, 1995; Lauriano & Notarbartolo
di Sciara, 1995; Forcada, 1998; A. Gannier, personal communication). Common dolphins
are seen in the early summer in the south-eastern Tyrrhenian Sea off the island of Ischia
(Mussi, Miragliuolo & Bearzi, in press a). The species is also present in the Sicily Channel
228
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et al.
© 2003 Mammal Society,
Mammal Review
,
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(Cavalloni, 1988; Arcangeli
et al
., 2001; Zanardelli
et al
., in press), with larger groups being
observed around Malta (Vella, 1998, 1999, in press). Common dolphins can be found in
portions of the eastern Ionian Sea, particularly around the island of Kalamos (Politi, 1998;
Politi & Bearzi, in press), and in the Gulf of Corinth (Frantzis & Herzing, 2002). Sighting
and stranding data indicate a regular presence of common dolphins in the Aegean Sea,
particularly in the Thracian Sea, Northern Sporades, the southern Evvoikos Gulf, the Saronic
Gulf and the Dodekanese (Frantzis, 1996; Öztürk & Öztürk, 1998; Carpentieri, Corsini &
Marini, 1999; Casale, Milani & Kallianiotis, 1999; Zafiropoulos, Verriopoulos & Merlini,
1999; Frantzis
et al
., in press). Apart from the eastern Ionian Sea and Aegean Sea, no reliable
data exist for most of the eastern Mediterranean basin, except for a rare occurrence of
common dolphins off the Israeli coastline (Goffman
et al
., 2000; Scheinin, 2003). Figure 3
attempts to outline the approximate species’ distribution and relative density in the Mediter-
ranean, based on a review of recent literature.
Coastal groups in western Greece seem to exhibit relatively high levels of site fidelity (Politi,
1998), but little is known about the movements and ranging patterns of animals living
offshore. The case for regarding Mediterranean common dolphins as a distinct population
is not perfect, and admittedly rests upon a somewhat complicated chain of inference. Genetic
studies indicate a significant level of divergence between Mediterranean and Atlantic popu-
lations (Natoli
et al
., in press). Differences in contaminant levels between dolphins from the
Alboràn Sea and Atlantic Ocean also suggest a certain degree of isolation. Organochlorine
concentrations in Alboràn Sea dolphins were about double those typical of dolphins in
neighbouring North Atlantic waters and showed a completely different profile [proportions
between polychlorinated biphenyl (PCB) congeners, the DDE/tDDT ratio, etc.] (Borrell
et al
.,
2001). Genetic exchange between common dolphins from the Mediterranean Sea and Atlan-
tic Ocean, to the extent that it occurs, appears to involve only animals from the Alboràn Sea
(Natoli
et al
., in press), possibly due to oceanographic features such as the Almería-Orán
thermohaline front (Tintoré
et al
., 1988) that has been shown to function as an ecological
Fig. 3.
Approximate distribution and relative density map of short-beaked common dolphins in the
Mediterranean Sea. Atlantic Ocean, Marmara Sea and Black Sea have not been considered. Drawing by
Massimo Demma.
Common dolphins in the Mediterranean
229
© 2003 Mammal Society,
Mammal Review
,
33,
224– 252
barrier for some species (Sanjuan, Zapata & Alvarez, 1994). There is little indication of
movement by common dolphins through the narrow Dardanelles Strait between the Aegean
and the Marmara and Black Seas. Intrusions or migrations to and from the Aegean Sea
cannot be excluded, since common dolphins are known to occur in the western part of the
Marmara Sea (Topaloglu, Öztürk & Colak, 1990; Öztürk & Öztürk, 1997). Therefore, genetic
mixing may occur between Aegean Sea and Black Sea common dolphins due to movements
through the Turkish Straits System (Barabasch, 1935; Kleinenberg, 1956). Black Sea common
dolphins are considered by some Russian investigators to constitute an endemic subspecies,
D. delphis ponticus
(Barabasch, 1935; Tomilin, 1957; Heptner
et al
., 1996). A preliminary
study of skull morphometrics (Amaha, 1994) suggested differences between Black Sea and
Mediterranean common dolphins. In contrast, a genetic comparison of relatively small sam-
ples (eight Black Sea, 20 central Mediterranean) revealed no significant differences between
them (
P
>
0.05; Natoli
et al.
, in press). Clearly, further work based on larger samples is needed
to characterize the relationship between Black Sea and Mediterranean common dolphins. It
is acknowledged that some genetic exchange might occur in portions of the Aegean Sea where
favourable habitat still exists (e.g. in the Thracian Sea; Frantzis
et al.
, in press). However,
what remains between the Aegean and Alboràn sectors of the Mediterranean seems to be
only isolated, remnant groups (possibly indicative of further population substructure).
ECOLOGY AND BEHAVIOUR
The short-beaked common dolphin is a poorly known species. Outside the Mediterranean,
it has been studied in a few areas, mostly in the context of abundance and distribution studies
(Evans, 1971, 1975; Hui, 1979, 1985, 1994; Dohl, Bonnell & Ford, 1986; Selzer & Payne,
1988; Holt & Sexton, 1990; Reilly, 1990; Scott & Perryman, 1991; Gaskin, 1992; Perryman
& Lynn, 1993; Wade & Gerrodette, 1993; Chivers & DeMaster, 1994; Dizon, Perrin, & Akin,
1994; Fiedler & Reilly, 1994; Ferrero & Walker, 1995; Gowans & Whitehead, 1995; Forney
& Barlow, 1998; Brereton, Williams & Williams, 1999; M. Bearzi, 2001; Neumann, 2001a,b;
Neumann, Leitenberger & Orams, 2002). Relatively little is known about groups living near
or on the continental shelf edge, and the ecology and behaviour of offshore populations
remain largely unknown (Evans, 1994).
In the Mediterranean, common dolphins are found in both pelagic and neritic environ-
ments (Notarbartolo di Sciara
et al
., 1993; Notarbartolo di Sciara & Demma, 1997; Cañadas
et al
., 2002), occasionally sharing the former with striped dolphins
Stenella coeruleoalba
(Viale, 1985; Fabbri & Lauriano, 1992; Forcada
et al
., 1994; Sagarminaga & Cañadas, 1995;
Mussi
et al
., 1998; Airoldi
et al
., 1999) and the latter with common bottlenose dolphins
Tursiops truncatus
(Politi, Airoldi & Notarbartolo di Sciara, 1994; Bearzi & Notarbartolo di
Sciara, 1995; Cañadas
et al
., 2002). Mixed-species groups of common, striped and Risso’s
dolphins
Grampus griseus
have been consistently observed in the pelagic waters of the Gulf
of Corinth, Greece (Frantzis & Herzing, 2002). Frequent associations with striped dolphins
also have been recorded in the Alboràn Sea (García-Tiscar
et al
., 2000) and near the Cam-
panian Archipelago (Mussi
et al
., in press a) while occasional associations with bottlenose
dolphins have been observed in the Sicily Channel (Cavalloni, 1988; Pace, Pulcini & Triossi,
1998) and north-eastern Adriatic Sea (Bearzi, 1996). Frantzis & Herzing (2002) compared
the occurrence of mixed groups in three Mediterranean Sea areas where common and striped
dolphins are sympatric, and noticed that the incidence of mixed-species sightings increased
as the relative abundance of common dolphins decreased. Determining why common dol-
phins have different patterns of association with other cetacean species in different Mediter-
ranean areas will require further investigation.
230
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et al.
© 2003 Mammal Society,
Mammal Review
,
33,
224– 252
Mediterranean common dolphins are typically found in groups of 50–70 animals, with
aggregations of 100–600 animals occasionally recorded (Notarbartolo di Sciara
et al
., 1993;
Mussi
et al
., in press a; Vella, in press; Cañadas
et al
., 2002). In the eastern Ionian Sea coastal
waters, however, groups rarely include more than 15 individuals, and groups greater than 40
were never observed (Politi & Bearzi, in press).
The sparse information available on the foraging ecology of common dolphins in the
Mediterranean indicates relatively flexible feeding habits, with a preference for epipelagic
and mesopelagic fish, similar to what has been observed outside the basin (e.g. Evans, 1975;
Collet, 1981; Overholtz & Waring, 1991; Berrow & Rogan, 1995; Silva & Sequeira, 1996;
Ohizumi
et al
., 1998; Birkun, 2002). The stomach contents of stranded individuals from the
Ligurian Sea and western Mediterranean indicate a diet based primarily on shoaling fish such
as European anchovy
Engraulis encrasicolus
, European pilchard
Sardina pilchardus
, round
sardinella
Sardinella aurita
and garpike
Belone belone
, but also on eurybathic cephalopod
and crustacean species (Orsi Relini & Relini, 1993; Boutiba & Abdelghani, 1995; Cañadas &
Sagarminaga, 1996). In coastal waters of the eastern Ionian Sea, shoaling fish including
anchovies and sardines are key prey (Bearzi, 2000; Agazzi, Bearzi & Politi, in press).
Recent evidence of direct interactions between common dolphins and fishing operations
in the Mediterranean is scarce, possibly reflecting the species’ low current abundance. How-
ever, such interactions were said to have been frequent in the early 20th century (e.g. Brunelli,
1932), when common dolphins – reportedly present in very large numbers – were regarded
by fishermen either as vermin or as useful indicators of fish schools around which the nets
could be set. Barone (1895) reported severe, frequent depredation and gear damage suffered
by Ligurian fishermen who targeted anchovies with gillnets set during the night. In the Gulf
of Naples, interactions between common dolphins and fishermen have been reported both
historically and recently. Local fishermen claim that cooperative fishing occurs, with the
fishermen taking advantage of fish aggregations actively chased towards the surface by
common dolphins. In the past, fish rewards were reportedly offered to the dolphins in
reciprocation (Mussi & Miragliuolo, in press). These kinds of interactions between common
dolphins and local fisheries in the Gulf of Naples have been reported since the beginning of
the 20th century (Brunelli, 1932; Police, 1932). Near Málaga and Estepona, Spain, common
dolphins follow purse-seine boats at night, surround the net when it is set and feed from
outside the net on small pelagic fish that escape from the net or protrude from the mesh
(Abad
et al
., in press; X. Valeiras, personal communication) As a result of these interactions,
some fishermen from Estepona consider common dolphins as a ‘plague’, while in the area of
Málaga fishermen also consider the benefits of having the dolphins concentrate prey.
Most of the information concerning the ecology and behaviour of common dolphins in
the Mediterranean comes from longitudinal studies conducted in and around the Alboràn
Sea, Sicily Channel, south-eastern Tyrrhenian Sea and eastern Ionian Sea. In the Alboràn
Sea, where this species has been studied since 1992, the average group size is very large
(mean
=
68.4, SD
=
102.39,
n
=
534, range 1–600) while in the Gulf of Vera (situated further
north-east on the Spanish coast), it is much smaller (mean
=
47.5, SD
=
50.17,
n
=
123, range
1–300; Universidad Autónoma de Madrid & Alnitak, 2002). Sighting frequencies for com-
mon dolphins are higher in the Alboràn Sea (0.023 groups/km, or 1.74 dolphins/km) than in
the Gulf of Vera (0.007 groups/km, or 0.36 dolphins/km). Common dolphins are sighted
mostly in and near the Bay of Almería and around Málaga and Estepona, areas known to
contain high concentrations of sardines (Gil, 1992). Data collected during the past decade
suggest that the Alboràn Sea is an important feeding and breeding ground for common
dolphins. In this area, surface feeding was observed during 11.2% of all sightings, and 46.4%
Common dolphins in the Mediterranean
231
© 2003 Mammal Society,
Mammal Review
,
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of all groups included calves. Calves were seen year round but especially between April and
July (Universidad Autónoma de Madrid & Alnitak, 2002).
In the waters around Malta, where research started in 1997 (Vella, 1998), common dolphin
groups average 26 individuals (SD
=
33,
n
=
85). Larger groups were observed in September
and October, when 75% of common dolphin sightings in Malta’s territorial waters ranged
between 150 and 250 individuals (A. Vella, personal communication). In this area, common
dolphins reportedly associate with bluefin tuna
Thunnus thynnus
between May and July (35%
of sightings), and with dorado
Coryphaena hippurus
between August and January (40% of
sightings; Vella, in press).
In the south-eastern Tyrrhenian Sea, the presence of common dolphins off the northern
coast of the island of Ischia, Italy, has been consistently documented since 1997 (Mussi
et al
.,
in press a). The animals are sighted mostly in the summer over the submarine canyon of
Cuma, a highly productive marine area characterized by high pelagic biodiversity and multi-
species associations (Mussi
et al
., in press b). Based on preliminary photo-identification data,
common dolphins may be using the area on a seasonal basis. Groups observed around Ischia
are relatively large (mean
=
65.5, SD
=
23.94,
n
=
41, range 35–100 individuals) and often
observed in association with striped dolphins, particularly during surface feeding targeting
shoaling prey. Surface feeding occurs frequently and the Atlantic saury
Scomberesox saurus
(a seasonal fish that is highly valued on local markets) is a typical prey of common dolphins
(Mussi
et al., in press a).
Around the island of Kalamos, in the eastern Ionian Sea, a community (sensu Wells, Scott
& Irvine, 1987) of approximately 100 common dolphins has exhibited a high degree of site
fidelity since studies began in 1993 (Politi, 1998). Common dolphin groups were observed a
total of 882 times in the springs and summers 1993–2002. Group sizes decreased significantly
after 1996 (Student’s t = 9.66, P < 0.001). The mean group size was 12 in 1993–96 (median = 9,
SD = 9.08, n = 157, range 1–40) and dropped to seven in 1997–2002 (median = 6, SD = 4.42,
n = 725, range 1–32). In the years 1993–2000, the mean sighting frequency was 0.016 groups/
km (or 0.11 dolphins/km), but in 2001–02, there was a significant decrease in the sighting
frequency, that dropped to 0.007 groups/km (or 0.04 dolphins/km; Student’s t = 4.88,
P < 0.001). The number of individuals encountered in the study area has decreased continu-
ally, and many individuals that used to be seen regularly until 1996 have disappeared (Politi
& Bearzi, in press; Tethys Research Institute, unpublished data). Common dolphins are often
seen feeding on shoaling prey near the surface and have never been observed to interact with
sympatric bottlenose dolphins, that seem to focus on demersal prey (Ferretti, Bearzi & Politi,
1998). Common dolphins around Kalamos exhibit a highly fluid fission-fusion social system,
and it has been suggested that this flexibility may enable the animals to adapt to environmen-
tal shifts and fluctuating prey availability (Bruno, 2001; Bruno, Politi & Bearzi, in press).
PAST AND PRESENT TRENDS IN ABUNDANCE
Delphinus delphis may have been one of the most abundant cetacean species in the Mediter-
ranean basin until at least the early 20th century. Although a certain number of misidentifi-
cations exist in past accounts (e.g. Richard, 1936, plates VI.1, VII.4; Tortonese, 1965, p. 183),
in which striped dolphins were mistakenly labelled as common dolphins (Poggi, 1982), liter-
ature, photographic records and osteological collections unambiguously indicate that com-
mon dolphins used to be abundant in many parts of the Mediterranean where they are now
absent or extremely rare (Giglioli, 1880; Barone, 1895; Arbocco, 1969; Pilleri, 1970; Duguy
& Cyrus, 1973; Casinos & Vericad, 1976; Pilleri & Gihr, 1977; Casinos, 1982; Cagnolaro, Di
Natale & Notarbartolo di Sciara, 1983; Pilleri & Pilleri, 1982, 1983; Poggi, 1986; Cagnolaro,
232 G. Bearzi et al.
© 2003 Mammal Society, Mammal Review, 33, 224–252
1996). Brunelli (1928, 1932) reported D. delphis to be a common species in Mediterranean
waters off Spain, France, Italy, former Yugoslavia (Slovenia, Croatia, Bosnia and Herzegov-
ina, and Montenegro), and Turkey. Of the dolphins killed as a result of conflicts with
fishermen off Liguria and Sardinia, Italy, 1914–17, and ending up in osteological collections,
29 (64.4%) were positively identified as D. delphis and 16 (35.6%) as T. truncatus, whereas
none were S. coeruleoalba (Poggi, 1986). Bompar (2000), reviewing historical information
and quoting literature published between 1863 and 1929 to document the presence of com-
mon dolphins along the coasts of France, concluded that the species used to be abundant
from the region of Roussillon to the islands of Hyères, and that bycatch of common dolphins
in fishing gear was a common occurrence. The same author also provided evidence that, at
least in some of the quoted sources, common dolphins were not confused with other cetacean
species. A popular book by Cousteau & Diolé (1975) included photographs of large schools
of common dolphins, reportedly taken in the years 1957–58 off the Mediterranean coasts of
France and the Principality of Monaco, where the species apparently was common. Strand-
ings data provide unambiguous evidence for declines of common dolphins in various Medi-
terranean areas, e.g. along the Spanish (Grau et al., 1986; Borrell et al., 2000) and French
Mediterranean coasts (data from Duguy & Budker, 1972; Duguy, 1973, 1974, 1975, 1976,
1977, 1978, 1979, 1980, 1981, 1982, 1983, 1984, 1985, 1986, 1987, 1988, 1989a,b, 1990, 1992;
Van Canneyt, Dabin & Collet, 1998; Van Canneyt et al., 1999; Van Canneyt, Leniere &
Collet, 1999; Van Canneyt, Heintz & Poncelet, 2000; Van Canneyt, 2001, 2002; Fig. 4). The
decline in the French sector has been matched by a concurrent increase of striped dolphins,
with the trajectories of the two species crossing in the early 1970s (Fig. 4). However, the most
compelling evidence of such a shift is provided by a comprehensive review of acquisitions of
cetacean specimens by museums and zoological collections in Italy, 1601– 1993 (Cagnolaro,
1996). In that review, the trends in acquisitions of D. delphis (n = 56), T. truncatus (n = 109)
and S. coeruleoalba (n = 243) from 1851 to 1993 show a steep decline of common dolphins,
in stark contrast with an equally steep, simultaneous increase of striped dolphins (Fig. 5).
Even though quantitative documentation of trends rests on indirect indicators, such as
stranding data and museum acquisitions, many authors concur that the aggregate population
of common dolphins in the Mediterranean has declined dramatically during the past three
decades (Casinos & Filella, 1977; Casinos, 1982; Duguy et al., 1983; Viale, 1985; Aguilar,
Fig. 4. Strandings of dolphins
along the Mediterranean coasts of
France between 1971 and 2001
(see text for references). Delphinus
delphis (n = 25), Stenella
coeruleoalba (n = 700), Tursiops
truncatus (n = 96).
Common dolphins in the Mediterranean 233
© 2003 Mammal Society, Mammal Review, 33, 224–252
1986; Cagnolaro & Notarbartolo di Sciara, 1992; Notarbartolo di Sciara et al., 1993; UNEP/
IUCN, 1994; Gannier, 1995; Notarbartolo di Sciara & Demma, 1997; Notarbartolo di Sciara
& Gordon, 1997; Forcada & Hammond, 1998; Borrell et al., 2001). Areas where the decline
has been confirmed by extensive cruises, where common dolphins were sighted only spora-
dically, include the Balearic Sea, Provençal Basin and Ligurian Sea (Forcada, Notarbartolo
& Fabbri, 1995; Forcada & Hammond, 1998; Notarbartolo di Sciara et al., 1993).
The northern Adriatic Sea represents an interesting case study. In that area, the regular
presence of common dolphins until the 1970s was well documented (Kolombatovic, 1882;
Brusina, 1889; Trois, 1894; Ninni, 1901, 1904; Brunelli, 1932; Vatova, 1932; cf. Dathe, 1934
and Dathe, 1972; Pilleri & Gihr, 1977; Pilleri & Pilleri, 1982, 1983). For unknown reasons,
in the last three decades, they have declined and almost completely disappeared there (Notar-
bartolo di Sciara & Bearzi, 1992; Notarbartolo di Sciara et al., 1993; Azzali, Casini &
Lamberti, 1994; Bearzi & Notarbartolo di Sciara, 1995; Stanzani, Bonomi & Bortolotto,
1997; Gomercic et al., 1998; Francese et al., 1999; Bearzi et al., 2000). It is now very difficult
to conduct meaningful studies on the species in the northern Adriatic because only rare,
scattered individuals remain (Bearzi & Notarbartolo di Sciara, 1995; Bearzi, 1996). During
the last 30 years, the food webs in the northern Adriatic Sea have suffered from severe
contamination by noxious manmade compounds, dramatic shifts of biotic communities,
persistent eutrophication phenomena, anoxia, and sea-floor degradation (Degobbis, 1989; De
Walle, Nikolopoulou-Tamvakli & Heinen, 1993; Corsolini et al., 1995; Nasci et al., 1999;
Dulcic & Grbec, 2000). Trends in commercial fish stocks in the last 25 years imply acute shifts
in dolphin prey type and density, probably the result of large-scale environmental changes
that were both natural and man-induced (Bombace, 1992; Solic et al., 1997; Degobbis et al.,
2000). Most significantly, given the well-known importance of small epipelagic fishes to
common dolphins, the biomass of anchovies and sardines reportedly has fluctuated widely,
and the Adriatic anchovy stock collapsed in 1987 (Bombace, 1992; Cingolani, Giannetti &
Arneri, 1996; Santojanni et al., 2001). Demersal fish catches have also declined dramatically
(Bombace, 1992). The northern Adriatic Sea has become a difficult environment for the
survival of any marine mammal species, as demonstrated by the current condition of bottle-
Fig. 5. Acquisitions of cetacean
specimens by museums and
zoological collections in Italy
between 1851 and 1993, based on
Cagnolaro (1996). Delphinus
delphis (n = 56), Stenella
coeruleoalba (n = 243), Tursiops
truncatus (n = 109).
234 G. Bearzi et al.
© 2003 Mammal Society, Mammal Review, 33, 224–252
nose dolphins in that area. Although some groups of T. truncatus survive in the northern
Adriatic, their numbers are now rather low (mean sighting frequency between 0.003 and 0.014
sightings/km of survey depending upon subarea; Bearzi, Notarbartolo di Sciara & Politi,
1997; Tethys Research Institute, unpublished data). Contaminant levels in their tissues are
high (Corsolini et al., 1995), and prey depletion has been suggested as a factor to explain the
unusually high percentage (i.e. around 80%) of the dolphins’ time budget being devoted to
food search and foraging (Bearzi, Politi & Notarbartolo di Sciara, 1999). Both dolphin species
in the Adriatic were intensively culled during the 1950s (Holcer, 1994), and this likely started
the decline of common dolphins in the area. However, there is no evidence of significant levels
of direct takes or bycatch of common dolphins in the northern Adriatic Sea that would
account for the decline observed since the 1970s. It therefore seems reasonable to propose as
a working hypothesis that the virtual disappearance of this species in the last 30 years is
related to large-scale changes in habitat quality and/or prey availability, possibly adding to
the problems caused by deliberate culling in the past. The marine environment in the area
has changed during the last decades from highly productive and relatively pristine to
degraded and overfished (Grubisic, 1974; Bombace, 1992; Nasci et al., 1999).
In the Alboràn Sea, as opposed to other Mediterranean areas, historical and recent data
suggest a quite constant presence of common dolphins, the most frequently sighted cetaceans
in the area based on both sighting and stranding data (Casinos, 1982; Grau et al., 1986; Bayed
& Beaubrun, 1987; Duguy et al., 1988; Boutiba, 1989; Laurent, 1991; Boutiba, 1994; Uni-
versitat de Barcelona, 1994; Bayed, 1996; Forcada & Hammond, 1998; Cañadas et al., 2002;
Universidad Autónoma de Madrid & Alnitak, 2002). The abundance of common dolphins
(and several other cetacean species) in the Alboràn Sea has been attributed to the area’s
biogeographical and oceanographic characteristics, which enhance primary productivity and
in turn provide ample prey for cetaceans, and to the area’s less degraded state when compared
to most other parts of the Mediterranean (Casinos, 1982; Universidad Autónoma de Madrid
& Alnitak, 2002).
FACTORS IMPLICATED IN THE SPECIES’ DECLINE
A number of interacting factors may have played a role in the decline of common dolphins
in the Mediterranean, ranging from natural fluctuations to the impact of human activities.
In this section, we discuss some of the human-induced threats that – based on the available
evidence – are most likely to be implicated in the species’ decline. These include factors as
diverse as prey depletion, contamination by xenobiotics, direct killing, fishery bycatch and
global climate change.
Other potential threats to Mediterranean common dolphins include disturbance by recre-
ational vessel traffic, noise from shipping, mineral prospecting (seismic) and military sonar
(Notarbartolo di Sciara & Gordon, 1997; Gisiner, 1998; Jasny, 1999), and oil pollution
(Geraci & St. Aubin, 1990; Engelhardt, 1987; Würsig, 1990). Although potentially pervasive,
these threats remain poorly characterized or have yet to be linked with specific effects on
common dolphins in the Mediterranean or elsewhere (Notarbartolo di Sciara et al., 2002).
Environmental fluctuations and global changes
It has been speculated that striped dolphins – that have apparently increased in numbers in
the western Mediterranean in recent decades (Aguilar, 2000) – progressively occupied the
niche of the common dolphin (Viale, 1985). Genetic studies of Mediterranean striped
dolphins indicate differences from north-eastern Atlantic striped dolphins (Archer, 1996;
García-Martinez, Raga & Latorre, 1997), thus dispelling the hypothesis that common dol-
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phins declined in the Mediterranean as a result of a recent invasion of striped dolphins from
the Atlantic Ocean. Competition with the striped dolphin has been listed among the possible
causes of the common dolphin’s decline in the Mediterranean (Casinos, 1982; Viale,
1985; Di Natale, 1987; Perrin, 1988; Cagnolaro & Notarbartolo di Sciara, 1992; Gannier,
1995; Sagarminaga & Cañadas, 1995; Notarbartolo di Sciara & Demma, 1997), but this
would be difficult to corroborate with scientific evidence, as is true of most claims concerning
interspecies competition for prey resources (e.g. see Clapham & Brownell, 1996). Although
the two species share a common habitat in portions of their range (Sagarminaga & Cañadas,
1995; Forcada & Hammond, 1998; Frantzis & Herzing, 2002), no evidence exists that striped
dolphins are competing with common dolphins, e.g. for food. It must be noted that the diet
of the striped dolphin – a species that feeds predominantly on mesopelagic cephalopods and
fishes (Casinos, 1982; Wurtz & Marrale, 1991; Perrin, Wilson & Archer, 1994) – overlaps only
slightly with the diet of common dolphins. In any event, competition would not be an issue
in areas such as the northern Adriatic Sea, where the common dolphin has disappeared while
the striped dolphin rarely occurs.
Several cases of faunal change have been documented in North America over the last few
decades that could be instructive. Reciprocal increases and decreases in the relative abundance
and distribution of pairs of small cetaceans have occurred off the north-eastern USA (white-
beaked dolphins Lagenorhynchus albirostris and Atlantic white-sided dolphins L. acutus;
Katona, Rough & Richardson, 1993; Kenney et al., 1996), in the southern California bight
(short-finned pilot whales Globicephala macrorhynchus and Risso’s dolphins; Shane, 1994)
and possibly in the Gulf of Mexico (again, short-finned pilot whales and Risso’s dolphins;
Jefferson & Schiro, 1997). The dramatic switch between the two species of Lagenorhynchus
dolphins was thought to have been related to changes in prey abundance and distribution
(e.g. sand lance Ammodytes sp., and Atlantic herring Clupea harengus), which in turn may
have been driven by large-scale changes in water temperature (Palka, Read & Potter, 1997).
Similarly, shifts in squid species dominance have been linked to the switch between short-
finned pilot whales and Risso’s dolphins in California (Shane, 1994). Episodic shifts in the
inshore occurrence of Pacific white-sided dolphins L. obliquidens in British Columbia (west-
ern Canada) have been correlated with changes in sea temperature as well as major fluctua-
tions in the local abundance of capelin Mallotus villosus, South American pilchard Sardinops
sagax and Californian anchovy Engraulis mordax (Morton, 2000). Caldwell & Caldwell
(1978) reported that common dolphins were once common along the north-eastern coast of
Florida, but disappeared from these waters since 1960. Based on the absence of known
interactions with fisheries or other human-caused mortality events involving the species, the
authors concluded that its disappearance was probably the result of natural fluctuations in
numbers or distribution, probably associated with oceanographic changes. The possibility
that similar processes occurred in the Mediterranean, with conditions improving for striped
dolphins while deteriorating for common dolphins, is something that cannot be ruled out
(e.g. see Aguilar, 2000).
Mediterranean biodiversity is undergoing rapid alteration under the combined pressure of
human impact and climate change (Bianchi & Morri, 2000). Sanford (1999) showed that
small changes in climate may generate large changes in marine communities through regula-
tion of keystone predation, and Petchey et al. (1999) demonstrated that environmental warm-
ing alters food-web structure and function of aquatic ecosystems. There is increasing evidence
of change in Mediterranean biodiversity patterns related to increasing seawater temperature
(Francour et al., 1994). According to Bethoux & Gentili (1995), temperature and salinity
changes in the deep and intermediate waters of the western basin signify changes in heat and/
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or water budgets at the surface. The warming trend in western Mediterranean deep waters
was assumed by Béthoux et al. (1990) to represent some of the earliest evidence of the
greenhouse effect (Graham, 1995).
It must be stressed that while the distribution of common dolphins may be related to
shifting environmental conditions such as sea temperature (Gaskin, 1968; Neumann, 2001a),
these kinds of ‘fluctuations’ affect dolphin distribution and/or abundance primarily by influ-
encing the distribution of their prey. In other words, factors that concentrate or disperse prey
may secondarily affect the distribution and abundance of cetaceans (Selzer & Payne, 1988).
Therefore, it may be difficult to discriminate between the effects of environmental shifts due
to climate change, whether ‘natural’ or a result of the greenhouse effect, and other factors
affecting the availability of dolphin prey, such as overfishing and habitat degradation.
In conclusion, it cannot be excluded that recent temperature changes in Mediterranean
waters may have had a negative impact on common dolphins, largely through their effects
on food-web dynamics. However, environmental fluctuations (whether ‘natural’ or related to
the greenhouse effect) are not the best candidates to explain the rapid decline of these top
predators. Rather, such a rapid decline would appear to be the result of pervasive habitat
degradation caused by overfishing, pollution, or a combination of both, which have become
increasingly serious problems in the Mediterranean concurrently with the decline in common
dolphin abundance.
Prey depletion
Jackson et al. (2001) argued that ‘ecological extinction caused by overfishing precedes all
other pervasive human disturbance to coastal ecosystems, including pollution, degradation
of water quality and anthropogenic climate change’. This lesson likely also applies to the
Mediterranean, where fisheries have had major direct and indirect impacts on ecosystem
dynamics (e.g. Briand, 2000; FAO, 2000). However difficult it may be to establish a clear,
mechanistic link between fisheries and the decline of common dolphins, such a link provides
one of the most plausible contending hypotheses.
As stressed by Chapman & Reiss (1999) ‘the lack of sufficient food to maximize reproduc-
tive potential may be the most important regulator of population size in animals’. Overfishing
and habitat degradation may have contributed to the decline of common dolphins by affecting
the availability of key prey. Although Mediterranean fisheries statistics are incomplete and
unreliable, and there is an acute lack of historical data (Briand, 2000), the available evidence
indicates that unsustainable harvesting has led to the decline of many fish stocks (Caddy &
Griffiths, 1990; De Walle et al., 1993; Stanners & Bourdeau, 1995; Briand, 2000; FAO, 2000),
with potentially serious ecological consequences (cf. Dayton et al., 1995; Jackson et al., 2001).
The mean trophic level of Mediterranean catches has declined significantly and quite steadily
since the late 1950s, although aggregate fishery landings have increased (e.g. Pauly &
Palomares, 2000; Stergiou & Koulouris, 2000). Such a pervasive and large-scale ‘fishing down’
impact on food-web dynamics (sensu Pauly et al., 1998) is likely to have a profound impact
on ecosystem dynamics, ultimately affecting top predators.
The eastern Ionian Sea is one of the Mediterranean areas where a potential for ‘exploitative
competition’ (Keddy, 1989) exists between common dolphins and local mid-water fisheries
targeting sardines and anchovies. Much of the fish fauna of the area is reduced because of
overfishing. Decreased total landings were reported (Papaconstantinou, Mytilineou & Panos,
1988; Papaconstantinou & Stergiou, 1995; Stergiou et al., 1997), and the area has been
subjected to intensive trawling (Papaconstantinou, Caragitsou & Panos, 1985; Papaconstan-
tinou, Stergiou & Petrakis, 1985). As a result of the ‘fishing down’ phenomenon, Stergiou &
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Koulouris (2000) report decreased mean trophic levels along the Greek Ionian coasts. Top
predators such as bottlenose dolphins, which are locally sympatric with common dolphins in
the eastern Ionian Sea, exhibit obvious signs of malnutrition (around 40% of photo-identified
individuals described as emaciated; Politi, Bearzi & Airoldi, 2000). Although common dol-
phins, unlike bottlenose dolphins, feed mostly on small epipelagic schooling fish, they can be
affected by overfishing when it causes a disruption of the interrelationships among the many
components of marine ecosystems. The complexity of marine food webs makes it difficult to
provide quantitative evidence that overfishing represents a threat to common dolphins. How-
ever, photo-identification and survey data showed that the total number of common dolphins
using the study area has decreased since 1996, and reduced prey availability remains the most
likely proximate cause to account for the observed trends (Politi & Bearzi, in press).
In all Mediterranean areas where common dolphins have been studied consistently, exploit-
ative competition with fisheries represents a source of concern. In the south-eastern Tyrrhe-
nian Sea, fishermen claim that the fleet targeting Atlantic saury Scomberesox saurus (locally
a key prey species for common dolphins) has decreased by an order of magnitude due to the
decline in fish stocks (Mussi et al., in press a). Moreover, purse seiners reportedly do not
comply with the regulations intended to prevent overfishing (Mussi & Miragliuolo, in press).
In the Alboràn Sea, purse seining targeting small pelagic fishes has increased dramatically in
recent years, casting doubts on the extant common dolphin population’s ability to persist at
current levels of abundance. Until recently, fishermen targeted only anchovies and sardines,
but depletion of these stocks and increased demand for low-value small pelagic fish (e.g.
round sardinella Sardinella aurita and garpike Belone belone) by the growing aquaculture
industry has led to the intensive commercial exploitation of most of the common dolphin’s
prey species (Universidad Autónoma de Madrid & Alnitak, 2002). The impacts of such trends
in fisheries on the local groups of common dolphins are unknown, but are unlikely to be
beneficial.
Prey depletion may represent a subtle and scarcely noticeable threat, and the impacts may
go unnoticed owing to inadequate research effort (e.g. monitoring changes in reproductive
success or survival rates). When mass mortality events occur, prey depletion and xenobiotic
contamination are often mentioned as potentially contributing factors. For example, inade-
quate nutrition may have compromised animal health and made Mediterranean striped
dolphins more susceptible to the epizootic that caused a large die-off in 1990–92 (Aguilar &
Raga, 1993; Aguilar, 2000).
In the Black Sea, reduced prey availability has been cited as a factor affecting the abun-
dance of common dolphins and harbour porpoises Phocoena phocoena (Bushuyev, 2000). Of
two mass mortality events involving Black Sea common dolphins in 1990 and 1994
(Krivokhizhin & Birkun, 1999), only one was recognized as being the result of a morbillivirus
epizootic (Birkun et al., 1999). Most stranded animals (dead and alive) examined during both
die-offs were emaciated (A. Birkun, personal communication). Although such emaciation
could be a result of the disease, both die-offs coincided with steep declines of European
anchovy Engraulis encrasicolus and European sprat Sprattus sprattus stocks, the main prey
of Black Sea common dolphins (Birkun, 2002). Overfishing, combined with the consequences
of eutrophication (e.g. water hypoxia) and the concurrent irruption of the introduced cteno-
phore Mnemiopsis leidyi, has been blamed for the rapid decline in anchovy and sprat stocks
(Zaitsev & Mamaev, 1997). The total commercial catch of anchovies experienced a 12-fold
decline (from an absolute maximum of 468 800 tonnes in the 1987–88 fishing season to
39 100 tonnes in 1990–91), while landings of sprat fell by a factor of nearly eight (from
105 200 tonnes in 1989 to 13 800 tonnes in 1993; Prodanov et al., 1997). This suggests a close
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relationship between large die-offs of Black Sea common dolphins and prey scarcity
(A. Birkun, personal communication).
Xenobiotic contamination
The role of xenobiotic contamination is controversial, but likely significant. High levels of
PCBs in Mediterranean dolphins, compared to levels in dolphins from other areas (Fossi
et al., 2000, in press; Aguilar, Borrell & Reijnders, 2002), represent a major concern, as toxic
contaminants such as PCBs, that accumulate in dolphin tissues through food-chain biomag-
nification, are known to cause immunosuppression and reproductive impairment in mam-
mals. PCB levels in common dolphins from the Mediterranean Sea are close to the range at
which adverse effects could be expected (Borrell et al., 2001; Fossi et al., in press). Fossi et al.
(in press) found a significant correlation between benzo(a)pyrene monooxygenase activity
and organochlorine levels in common dolphin skin biopsies, indicative of potential toxico-
logical stress in this species, even though total organochlorine levels were lower than those
found in striped dolphins and common bottlenose dolphins.
In the Mediterranean, epizootics and reproductive disorders related to high contaminant
loads appear to have affected striped dolphins primarily (Aguilar & Raga, 1993; Van Bressem
et al., 1993; Aguilar & Borrell, 1994; Munson et al., 1998), but common dolphins could also
be at risk (Fossi et al., 2000, in press). In fact, as noted above, Birkun et al. (1999) suspected
a possible role of xenobiotic contamination in a mass die-off of common dolphins in the
Black Sea, although an epizootic outbreak appeared to be the proximate cause.
The Alboràn Sea is less contaminated than the rest of the western Mediterranean (UNEP,
1984). This is consistent with the results presented by Borrell et al. (2001), who found
relatively low organochlorine levels in common dolphins from the Atlantic and westernmost
Mediterranean waters off Spain. It is unclear, however, if relatively low contaminant levels
can be related directly to the species’ abundance in the far western Mediterranean. Compar-
isons among toxicological information obtained from common dolphins sampled in different
Mediterranean areas may show whether contaminant levels are higher in those areas where
the species has declined.
Direct takes and bycatch
Through the 1950s, the deliberate catching or killing of dolphins occurred in several Medi-
terranean areas, and still occurs in portions of the basin. In the past, common dolphins were
culled because of their reputation as competitors with fisheries, and because their flesh was
valued for human consumption or for use as bait (Notarbartolo di Sciara & Bearzi, 2002).
Research methods were occasionally lethal (e.g. Richard & Neuville, 1897; Richard, 1936;
Pilleri & Knuckey, 1969; Cousteau & Diolé, 1975), reflecting the prevailing ethics of an earlier
time when Western societies had a different view of dolphins than today (cf. Lavigne, Scheffer
& Kellert, 1999). Past exploitation may have had an impact on coastal dolphin populations,
but it is unclear whether Delphinus would have been more affected than Tursiops. In fact, in
most areas where ‘predator control’ killing occurred, bottlenose dolphins were viewed as
greater threats to fisheries than common dolphins. The latter were often considered beneficial
to various fisheries (e.g. Brunelli, 1932; Police, 1932), although in some areas common
dolphins were definitely regarded as vermin (e.g. Brusina, 1889; Barone, 1895).
Fishery bycatch is a major threat to many cetacean populations, and it could well have
played a role in the decline of common dolphins in at least some Mediterranean areas (Di
Natale & Notarbartolo di Sciara, 1994; IWC, 1994; UNEP/IUCN, 1994; Aguilar & Silvani,
1995; Forcada & Hammond, 1998; Silvani, Gazo & Aguilar, 1999). In the Alboràn Sea, for
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example, it was estimated that Spanish drift gillnets caught a couple of hundred common
dolphins per year during the early 1990s. No account was taken in this estimate for the catches
by Moroccan and Italian vessels, the former four times more numerous but smaller than the
Spanish boats and the latter of unknown number but working with substantially larger nets
(Silvani et al., 1999). The Spanish fishery finally stopped in 1994 (Aguilar, 2000), but it
operated for many years and undoubtedly had some impact on the population. If driftnets
were taking common dolphins in the Alboràn Sea, it is reasonable to assume that they were
(and are) doing so in other parts of the Mediterranean where driftnet fishing and common
dolphin occurrence overlap. The lack of quantitative bycatch estimates must not be inter-
preted as evidence of insignificant impact. Pelagic driftnets were banned by the European
Union (EU) starting from 2002. Nevertheless, driftnet fishing by non-EU Mediterranean
fleets, as well as the illegal continuation of driftnet fishing within EU waters (e.g. in the south-
eastern Tyrrhenian Sea; Miragliuolo, Mussi & Bearzi, in press a), represent potentially
important ongoing threats to common dolphins.
Although fishery bycatch may threaten common dolphins in some Mediterranean areas, it
remains unclear to what extent their decline in the basin overall is related to past or present
levels of bycatch. There is no scientific evidence to suggest that bycatch has selectively reduced
common dolphins. In the aforementioned Spanish study (Silvani et al., 1999), it was noted
that roughly similar numbers of common and striped dolphins were being caught, and that
the abundance of these two species in the Alboràn Sea was similar. The number of striped
dolphins was ‘greatly reduced’ by an epizootic in 1990–92, yet this species remains widespread
and relatively abundant throughout its Mediterranean range (Aguilar, 2000). We conclude
that although bycatch may have played a significant role in some areas in the past, it is unlikely
to be the factor most responsible for the decline of common dolphins in the Mediterranean
region. Unfortunately, however, there is no prospect of obtaining reliable or complete retro-
spective data on cetacean bycatch in most Mediterranean driftnet fisheries and, as a result,
we will probably never be able to achieve a satisfactory resolution of this issue.
PRIORITIES FOR ACTION
The relative importance and interplay of the potential threats listed above are not well
understood, so designing and implementing appropriate measures to counteract negative
trends is a daunting task. Continued inaction, however, is unacceptable if there is to be any
hope of preserving viable numbers of common dolphins throughout much of their historic
range in the Mediterranean basin. The following recommendations are proposed as a basis
for addressing scientific uncertainty while moving forward with precautionary management
efforts without delay. In some areas, it may already be too late to prevent these dolphins’
disappearance from the local marine fauna, so the goals in such locations should be to
understand the causes of decline and facilitate immigration from adjacent waters, hopefully
leading to recovery. In other areas, the fate of remaining animals will likely depend upon
precautionary action and the adoption of precise conservation and management measures
to prevent further decline. The ACCOBAMS and the Barcelona Convention Protocol on
Specially Protected Areas and Mediterranean Biodiversity, which have both recently come into
force, provide an ideal framework to coordinate research efforts and design appropriate
conservation strategies for cetaceans in the region (Notarbartolo di Sciara et al., 2003).
Research recommendations
Below, we provide a series of research initiatives that should be implemented to increase
understanding of the species’ past and ongoing trends. We believe that it is important that
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work be initiated without further delay, and that results be conveyed to managers and
incorporated into the design of conservation actions as quickly and efficiently as possible.
1. Field surveys are clearly needed to determine the current distribution and abundance of
common dolphins in the Mediterranean, particularly along the entire northern African
coastline, in the Aegean Sea (especially in its northern part, the Thracian Sea) and in far
eastern Mediterranean areas where little information exists. Such surveys should be designed
to identify hot spots of occurrence that can be accorded priority for intensive research and
management. Standard methods such as vessel-based and/or aerial line transect surveys
should be used so that results can be compared over time and from one region to another.
2. A better understanding is needed of the genetic characteristics of Mediterranean common
dolphins. The risks of local or regional extinction from stochastic processes might be reduced
by preserving as much genetic diversity as possible (cf. Shaffer, 1987; Lande, 1988). Ongoing
genetic studies may provide some insight concerning rates of gene flow between what appear
to be ‘isolated’ groups of dolphins (e.g. Universidad Autónoma de Madrid & Alnitak, 2002;
A. Natoli, personal communication). However, more genetic material is needed from groups
living in different portions of the region. In addition, further comparisons should be made
between Mediterranean groups and groups from adjacent basins (i.e. Black Sea, Atlantic
Ocean). Biopsies should be collected for genetic and other analyses with minimal intrusive-
ness (e.g. Harlin et al., 1999), while recognizing that the darting is not without risk to free-
ranging dolphins (Bearzi, 2000). Samples should be archived in a central repository (e.g.
Aguilar & Borrell, in press; Anfuso et al., in press). Similarly, collaborative photo-identifica-
tion studies (e.g. see http://www.europhlukes.net) should be initiated to better understand
habitat use, the relationship between coastal and pelagic groups, and long-range movement
patterns.
3. Contaminants analyses should be conducted to identify regional differences in exposure,
and relate them to population abundance and trends. In addition, comparative analyses of
contaminant loads and evaluation of interspecies susceptibility to organic pollutants (e.g.
Fossi et al., 2000, in press) may shed light on the relative impact of xenobiotic contamination
on common dolphins as compared to other cetacean species living in the Mediterranean.
4. Sighting surveys, stranding networks and related activities will require collaboration
among individual scientists, government agencies and non-governmental organizations from
the various range states. For instance, rigorous investigations should be conducted to assess
the scale of bycatch and intentional killings of common dolphins, with a focus on areas where
evidence of conflict between dolphins and fisheries exists. This will require improved com-
munication and exchange of information at the regional level. Common dolphin conservation
workshops would provide opportunities for experts to discuss available evidence with one
another and with experts from different disciplines, local stakeholders and managers. The
goal of these efforts should be to develop and modify, on an adaptive basis, a comprehensive
programme for the assessment and monitoring of the status of Mediterranean common
dolphins, closely coupled with the implementation of measures for their conservation.
5. Comprehensive analyses of existing datasets assembled by several independent research
groups over the last decade should be promoted and supported. Such analyses, together with
the publication of results, have often been delayed because of insufficient funding and other
resources, including expertise (e.g. in the field of statistics), to deal with massive datasets.
Comparative analyses among different Mediterranean habitats would provide further insight
on why common dolphins have persisted in some areas but disappeared in others. The far-
western Mediterranean – where common dolphins are still relatively abundant – might be
regarded as a ‘control’ area for such comparisons. It may also be informative to compare
Common dolphins in the Mediterranean 241
© 2003 Mammal Society, Mammal Review, 33, 224–252
features of the central and eastern Mediterranean with those of non-Mediterranean areas
where common dolphins are thought to have declined.
6. Elucidation of ecosystem dynamics, and specifically the possible role of prey depletion and
regime shifts as factors contributing to the decline of common dolphins in the Mediterranean,
is an important, but challenging, area of research. Investigations of the spatial and temporal
variability in Mediterranean fish stocks, when correlated with common dolphin abundance
and movements, could be informative, as could output obtained from ecosystem models (e.g.
Christensen & Pauly, 1992) and analyses of food-web dynamics.
Recommended conservation measures
A large marine sanctuary for cetaceans in the Corso-Ligurian Basin has been declared by the
Governments of Italy, France and Monaco (Notarbartolo di Sciara, in press). Other smaller
marine protected areas exist or have been proposed throughout the Mediterranean Sea (e.g.
Fayos, Cañadas & Sagarminaga, in press; Raga et al., in press). In 1999, the Spanish Ministry
for the Environment included the common dolphin in its National Endangered Species Act
as ‘vulnerable’. The following year, a programme was initiated to identify important areas
for the conservation of cetaceans in the Spanish Mediterranean with the aim of implementing
the EU’s ‘Habitats’ Directive, the Barcelona Convention and the Bonn Convention (Conven-
tion on Migratory Species, or CMS) through the creation of marine protected areas (Univer-
sidad Autónoma de Madrid & Alnitak, 2002). A follow-up of this project started in the year
2002 to develop the management schemes required for these areas. Based on the presence of
a relict group of common dolphins, the eastern Ionian area around the island of Kalamos
has been included by the Greek Ministry of the Environment in the Natura 2000 network
(‘Site of Community Importance’) under the 9243 EEC ‘Habitats’ Directive (Frantzis, 1996).
The area around the island of Kalamos has also been identified by the ACCOBAMS (2002)
as one where pilot conservation and management actions should be developed and imple-
mented immediately to preserve critical habitat for common dolphins. In the waters around
Ischia, south-eastern Tyrrhenian Sea, the creation of a marine reserve dedicated to the rich
cetacean fauna was proposed recently by the Italian Ministry of the Environment, which –
if finalized – may lead to mitigation of at least some obvious threats such as harassment by
pleasure boaters (e.g. Miragliuolo, Mussi & Bearzi, in press b) and uncontrolled fishing.
However, few specific measures have been adopted that would directly benefit common
dolphin conservation in any of those areas. In fact, the 9243 EEC ‘Habitats’ Directive includes
only the bottlenose dolphin and the harbour porpoise in its Annex II (‘Animal and plant
species of Community interest whose conservation requires the designation of special areas
of conservation’), and although the Convention on the Conservation of Migratory Species
includes the Mediterranean common dolphin in its Appendix 2 (‘Migratory species that have
an unfavourable conservation status or would benefit significantly from international coop-
eration’), that status is inexplicably limited to a ‘western population’. We find it distressing
that so little has been accomplished towards the goal of conserving Mediterranean common
dolphins, and that so little success has been realized in conveying the message about this
species’ decline in the region to policy makers and, apparently, the general public.
Although the creation of an internationally coordinated network of marine protected areas
may represent an important step (e.g. Agardy, 1997; Bianchi & Morri, 2000), this is unlikely
to be sufficient for conserving the species unless specific precautionary measures are taken to
prevent further decline and, hopefully, facilitate population recovery. These measures should
be aimed primarily at reducing overfishing and habitat degradation in areas where relict
groups of common dolphins are known to reside, particularly in the central and eastern
242 G. Bearzi et al.
© 2003 Mammal Society, Mammal Review, 33, 224–252
Mediterranean Sea. At the same time, habitats where the species is still abundant should be
granted special conservation status, and actions should be taken to mitigate the existing
threats if common dolphins are to persist beyond the next few decades in the central and
eastern Mediterranean Sea.
The authors recognize that the forces causing climate change and chemical contamination
are unlikely to be influenced in a major way by concern for common dolphins in the Medi-
terranean. Lifestyle choices, entrenched patterns of overconsumption, human overpopulation
and political gamesmanship militate strongly against the types of changes needed to reverse
what are essentially global trends. However, in a moment in which the stark evidence of wide-
scale overfishing and the consequent need for immediate and decisive measures to reduce
fishing pressure is finally capturing the attention of European decision makers, the goal of
conserving common dolphins may converge with, and in fact add to, the momentum building
in the direction of improved ecological conditions for the benefit of both humans and wildlife.
In this context, the decline of common dolphins provides a further signal that our collective
actions can have large-scale, unforeseen, unintended and intractable consequences.
ACKNOWLEDGEMENTS
This work has been supported by a Pew Marine Conservation Fellowship (a programme of
the Pew Charitable Trusts operated in partnership with the New England Aquarium), by
ACCOBAMS, by WDCS (the Whale and Dolphin Conservation Society) and by ASMS
(Swiss Marine Mammal Protection). Luigi Cagnolaro is gratefully acknowledged for pro-
viding the actual data from which graphs in his paper (1996) were derived, thus allowing the
construction of our Fig. 5. Maddalena Bearzi, Peter G. H. Evans, Thomas A. Jefferson and
Bernd Würsig provided editorial comments on early drafts. Alexei Birkun Jr, Drasko Holcer,
Ada Natoli and Thomas A. Jefferson added useful bits of background information. We are
grateful to Silvia Bonizzoni, Mauro Bastianini and Sabina Airoldi for their help with litera-
ture search, and to Massimo Demma for drawing the maps (Figs 2 and 3).
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Submitted 5 February 2003; returned for revision 8 April 2003; revision accepted 11 June 2003
Editor: RM
