Endang Species Res
Vol. 5: 112, 2008
doi: 10.3354/esr00103
Printed September 2008
Published online July 1, 2008
Once one of the most common cetacean species in
the Mediterranean Sea, the short-beaked common dol-
phin Delphinus delphis has declined throughout the
region since the 1960s (Bearzi et al. 2003). In 2003, the
Mediterranean population was classified as Endan-
gered in the International Union for Conservation of
Nature (IUCN) Red List of Threatened Species, based
on observed declines in the number of animals and
extent of occurrence, as well as deterioration in the
quality of habitat in large portions of the Mediter-
ranean. In 2005, Mediterranean common dolphins
were listed in Appendix I and II of the Convention on
the Conservation of Migratory Species (Bonn Conven-
tion, CMS). While it would be difficult to imagine a
more favourable legal and international framework to
support action in the region, conservation of common
dolphins has remained ‘on paper’ (Bearzi 2007).
Although much of the Mediterranean basin has not
been surveyed, there is clear evidence of population
decline in portions where detailed studies have been
conducted, e.g. the northern Adriatic Sea (Bearzi et al.
2004a) and the Gulf of Vera in southern Spain
(Cañadas & Hammond 2007). Here, we document the
precipitous, ongoing decline of common dolphins in
eastern Ionian Sea coastal waters across 13 yr. Also, we
show that the decline suggested previously for a core
area of 480 km
between 1997 and 2004 (Bearzi et al.
2005, 2006) has occurred over a wider area (1050 km
© Inter-Research 2008 · www.int-res.com*Email: giovanni.bearzi@gmail.com
Overfishing and the disappearance of short-beaked
common dolphins from western Greece
Giovanni Bearzi
, Stefano Agazzi
, Joan Gonzalvo
, Marina Costa
Silvia Bonizzoni
, Elena Politi
, Chiara Piroddi
1, 2
, Randall R. Reeves
Tethys Research Institute, Viale G.B. Gadio 2, 20121 Milano, Italy
Fisheries Centre, University of British Columbia, 2202 Main Mall, Vancouver, British Columbia V6T 1Z4, Canada
Okapi Wildlife Associates, 27 Chandler Lane, Hudson, Québec J0P 1H0, Canada
ABSTRACT: Once one of the most common cetaceans in the Mediterranean Sea, the short-beaked
common dolphin has declined throughout the region since the 1960s and in 2003 this population was
classified as Endangered in the International Union for Conservation of Nature (IUCN) Red List.
Here, we document the species’ precipitous decline in eastern Ionian Sea coastal waters across 13 yr.
While 150 animals were present in the study area (1050 km
) in 1996, only 15 were observed in 2007.
A 12 mo assessment of fishing effort and catch, together with circumstantial evidence, suggests that
the decline was caused largely by prey depletion resulting from overfishing. We analyzed the
impacts of various fishing gear and estimated the degree of resource overlap between common dol-
phins and local fisheries. The total biomass removed annually by 308 fishing boats in the study area
averaged 3571 t, while that consumed by common dolphins was 17 t. Resource overlap between com-
mon dolphins and fisheries expressed as an average Pianka index of 0.5 differed according to
fishing gear, being higher for purse seiners (0.7) and beach seiners (0.4) and lower for bottom trawlers
(0.1), trammel boats (0.2) and longliners (0.0). Only about 10 active purse seiners (4% of the total
active fishing fleet) were responsible for 33% of the biomass removal, and likely had the greatest
impact on prey of common dolphins. This study indicates a high risk of local disappearance of com-
mon dolphins in the very near future, unless fishery management measures are implemented imme-
diately. Purse seining should be the main management target.
KEY WORDS: Short-beaked common dolphin · Fishing · Prey depletion · Mediterranean Sea
Resale or republication not permitted without written consent of the publisher
Endang Species Res 5: 112, 2008
and a longer time interval (1995 to 2007). Bearzi et al.
(2006) discussed possible reasons for the observed
declines of common dolphins and other megafauna
that prey on epipelagic schooling fish and concluded
that the most likely cause of the declines was prey
depletion as a result of fishing pressure. Here, we pro-
vide quantitative data on fishing effort and landings in
the Kalamos area, and we link these data to the on-
going local decline of common dolphins (and other
megafauna). Our 12 mo assessment is consistent with
the hypothesis that ecosystem damage has been
caused by unsustainable fishing (Stergiou et al. 1997,
Eastern Ionian Sea waters surrounding the island of
Kalamos have been included by the Hellenic Ministry of
the Environment in the Natura 2000 network (‘Sites of
Community Importance’) under the 9243 EEC ‘Habitats’
Directive. In 2002 the Agreement on the Conservation of
Cetaceans in the Black Sea, Mediterranean Sea and con-
tiguous Atlantic area (ACCOBAMS), ratified by
Greece, identified the area of Kalamos as one where pi-
lot conservation and management actions should be de-
veloped and implemented immediately to preserve the
habitat of common dolphins. In 2004, the Contracting
Parties to ACCOBAMS ‘welcomed’ a
Conservation Plan for Mediterranean
common dolphins that identified
Kalamos as an Area of Conservation Im-
portance (Bearzi et al. 2004b).
The study presented here (1) indi-
cates that there is a high risk that com-
mon dolphins will disappear from the
Kalamos area in the very near future
and (2) identifies fishery management
measures that must be implemented
immediately to prevent the further
decline of common dolphins and deteri-
oration of the local ecosystem.
Study area. The study area, situated
in eastern Ionian Sea coastal waters, in-
cludes the Ionian islands of Meganisi,
Kalamos and Kastos and covers 1050 km
of sea surface (Fig. 1). The sea floor is
mostly 50 to 250 m deep, with rocky
coasts and shallows covered by seagrass
meadows. Waters are transparent (Sec-
chi disk readings ranging between 10
and 30 m) and oligotrophic (Gotsis-Skre-
tas & Ignatiades 2007), and river and
agricultural runoff is insignificant (Pitta
et al. 1998).
Survey and photo-identification effort. Surveys
were conducted ad libitum from 4.7 to 5.8 m inflatable
craft with rigid hulls powered by 50 to 100 HP 4-stroke
outboard engines from 1995 to 2007. The survey cover-
age totalled 34 801 km on effort and 908 survey days,
from May to September each year (Table 1). Surveys
were conducted at speeds between 28 and 36 km h
under consistently good conditions: unimpaired visibil-
ity, sea state 1 Douglas with no swell, and at least 2
experienced observers scanning the sea surface (eye
elevation of approximately 2 m). Such conditions char-
acterize ‘on effort’ in Table 1.
Photo-identification of individuals. Colour photo-
graphs suitable for individual identification, based on
long-term natural marks on the dolphins’ dorsal fins
(Würsig & Würsig 1977, Neumann et al. 2002), were ob-
tained on 356 d. On each encounter with a group of dol-
phins, as many good photographs as possible were taken
of all individuals present. Photo-identification was per-
formed following Würsig & Jefferson (1990), with cam-
eras equipped with 70 to 200 mm f2.8 zoom lenses, using
100 ISO colour transparency film from 1995 to 2002, and
digital photography from 2003 to 2007. Considering that
digital photography substantially improves the effi-
Fig. 1. Study area (delimited by bold line) with bathymetric contour lines and
locations of fishing ports and other mooring/landing sites where local fishing
fleets were monitored on a monthly basis from November 2006 to October 2007.
The location of the study area relative to the eastern Mediterranean basin is
shown in the inset
Bearzi et al: Common dolphin disappearance in western Greece
ciency of individual dolphin identification (Markowitz et
al. 2003), transparencies were scanned at high resolution
and turned into digital images for consistency of evalua-
tion and analysis. Photos were then cropped around the
dorsal fin and visible part of the body and selected using
consistent criteria (i.e. entire dorsal fin visible, fin per-
pendicular to camera, high sharpness and resolution, no
water spray masking fin profile), based on recommenda-
tions provided by Read et al. (2003). Following such se-
lection, the catalogue included 13 319 transparencies
and 4290 digital photos, totalling 17 609 dorsal fin pho-
tos. These images were then matched, and the identified
animals were included in a database. Only individuals
with distinctive dorsal fin profiles, bearing marks suit-
able for reliable long-term identification from either side
of the fin, were used for mark-recapture analyses. The
characteristic white patch present on both sides of the fin
of some animals could be used to confirm a match (Neu-
mann et al. 2002, Bearzi et al. 2005) but not as a distinc-
tive feature. Individuals with a single tiny nick or no nick
on the dorsal fin’s trailing edge, and individuals judged
to be recognizable only based on dorsal fin shape and/or
temporary scars were not used in the
analyses, as their inclusion could result
in borderline or incorrect matches and
thus violate one or more mark-recap-
ture assumptions. Patterns of site fi-
delity implied by photo-identification
data (Fig. 2, where information from
1993 and1994 is also shown) and indi-
vidual movements indicated by recor-
dings of boat position (used as a proxy
for dolphin position) at 6 min inter-
vals during photo-identification follows
(Fig. 3), assisted in the selection of
the most appropriate model for mark-
recapture analyses.
Mark-recapture analyses. Mark-
recapture methods rely on the number
of animals marked and their propor-
tion in subsequent samples to esti-
mate population parameters, includ-
ing abundance (Seber 1982). Since the
1980s these methods have been used
widely to estimate abundance of cer-
tain cetacean species based on photo-
graphic records of naturally marked
individuals (Hammond 1990, Ham-
mond et al. 1990, Wilson et al. 1999).
Pollock’s robust design (Pollock 1982)
with Huggins estimator has been the
preferred model. Previous work in the
Kalamos area showed that this popu-
lation of common dolphins could be
regarded as open among years and
closed within each annual sampling interval of 3 to 5
mo (Table 1), based on a high degree of site fidelity,
individual movement patterns, and low rates of immi-
gration (Bearzi et al. 2005). The programme Mark 4.3
mark.htm) was used for mark-recapture abundance
Monitoring fishing effort and landings. Prior to this
study it had been difficult to document fishing pressure
in the study area due to (1) poor reliability of the land-
ing data (Watson & Pauly 2001) a well-known prob-
lem in the wider Mediterranean and in the Hellenic
Seas in particular (Stergiou et al. 1997, 1998, Briand
2000) and (2) the fact that local fishermen often
deliberately misreport their catches with the intention
of reducing taxation or avoiding stricter regulations
(Bearzi et al. 2006). Estimation of biomass removal is
essential to assess the impact of fisheries (Pitcher et al.
2002). As reliable data on fishing effort and fishery
landings were unavailable, we incorporated in our dol-
phin study a monitoring component aimed at obtaining
direct information on fishing within the study area.
Year May Jun Jul Aug Sep Subtotal
Daily surveys: common dolphin encounters
1995 6: 3 13: 6 8: 4 27: 13
1996 3: 2 17: 15 22: 11 17: 6 59: 34
1997 15: 17 25: 18 20: 16 22: 14 22: 17 104: 82
1998 14: 11 17: 10 21: 18 20: 13 24: 17 96: 69
1999 16: 7 20: 15 22: 21 22: 8 80: 51
2000 22: 13 21: 12 24: 18 21: 11 88: 54
2001 1: 1 19: 16 18: 10 22: 7 17: 2 77: 36
2002 2: 0 17: 10 16: 3 23: 5 11: 1 69: 19
2003 10: 6 11: 3 18: 4 20: 5 59: 18
2004 4: 0 16: 2 11: 2 19: 3 11: 1 61: 8
2005 3: 1 15: 2 15: 3 17: 0 16: 1 66: 7
2006 1: 0 14: 5 14: 0 16: 0 18: 0 63: 5
2007 13: 1 14: 1 17: 0 15: 2 59: 4
Total 40: 30 187: 92 204: 101 255: 102 222: 75 908: 400
Km surveyed ‘on effort’
1995 225 566 378 1169
1996 35 413 850 477 1774
1997 231 608 543 432 576 2390
1998 313 367 623 625 597 2525
1999 541 510 716 689 2456
2000 867 504 901 723 2995
2001 1 465 541 962 500 2469
2002 50 574 501 1075 463 2663
2003 1137 860 1359 936 4292
2004 247 670 298 995 755 2965
2005 155 445 514 940 883 2937
2006 3 679 642 1079 918 3321
2007 566 887 949 444 2846
Total 1000 6953 7061 11448 8339 34801
Table 1. Number of daily surveys, common dolphin encounters and km
surveyed ‘on effort’
Endang Species Res 5: 112, 20084
Fig. 2. Pattern of site fidelity by year and month for 143 common dolphins photo-identified in the study area between 1993
and 2007. Each row represents the sighting history of 1 individual
Fig. 3. Movements of com-
mon dolphins in the study
area between 1997 and 2007
Bearzi et al: Common dolphin disappearance in western Greece
In November 2006, we started a programme to mon-
itor the local fishing fleet in all 16 ports and other
mooring/landing sites (Fig. 1). All sites were monitored
once per month on days of bad sea state conditions
and/or on Sundays, i.e. on days with a relatively high
probability that the whole fishing fleet would be
moored or in port. A total of 324 fishing boats were cat-
alogued. Classification of fishing boats and gear was
based on Nedelec & Prado (1990). The activity status of
each boat was recorded visually every month based on
fishing gear on board or near the boat’s mooring place,
fishermen working on board, boat conditions, and
direct inquiries. The local fishing fleet i.e. the boats
recorded as active in any given month during the study
period included 12 purse seiners (of which 11 were
15 to 25 m and 1 was 12 m long), 24 beach seiners of
8 to 12 m, 9 bottom trawlers of 20 to 25 m, 50 longliners
(29 of 4 to 7 m and 21 of 7 to 10 m), and 213 trammel
boats (98 of 4 to 7 m and 115 of 7 to 12 m). Boats shorter
than 4 m were not considered, as their impact was
assumed negligible. For the same reason a total of 16
boats 4 to 7 m long equipped with gas-powered lamps
and tridents/harpoons were also excluded from the
analyses. Some boats had multiple gears and switched
from one fishing method to another depending on
seasonal closures (Kapantagakis 2007). For instance,
most beach seiners operated as trammel boats during
months of beach seining closure. Therefore, active
boats were classified according to the gear used in
each month of sampling (e.g. a boat could be scored as
a purse seiner in September and as a bottom trawler in
October, depending on the gear used during those
Landings of purse seiners, beach seiners and bottom
trawlers were monitored between November 2006 and
October 2007. A total of 26 landings were recorded for
purse seiners (2.9 ± 1.9 mo
, mean ± SD), 14 for beach
seiners (2.3 ± 3.78 mo
), and 16 for bottom trawlers
(2.0 ± 1.31 mo
). Total catch by species was recorded
visually by trained researchers based on the number of
full boxes landed (boxes were routinely divided by
species before landing). Full boxes were estimated to
average 10 kg irrespective of species (an assumption
routinely used for market purposes at landing sites,
confirmed by direct observations). Days of activity per
month per boat for purse seiners, beach seiners and
bottom trawlers were based on Kapantagakis et al.
(2001), also taking into account periods of seasonal
fishing closure. Average catch for trammel boats and
longliners was estimated as 3.42 t yr
for boats 4 to 7 m
long, and 6.31 t yr
for boats 8 to 11 m long (Stergiou
et al. 2007c). Percentage catch contribution of the most
abundant species for longliners and trammel boats
operating in the Ionian Sea was based on Stergiou et
al. (2007a). Discard rates were assumed to be 14% for
purse seiners (Tsimenides et al. 1995 a value
reported as likely to be an underestimate), 28% for
beach seiners (Stergiou et al. 1996), 39% for bottom
trawlers (Machias et al. 2001) and 9.8% for netters and
longliners (Stergiou et al. 2002, Tzanatos et al. 2007).
Biomass removed annually by fisheries calculated
separately according to fishing gear—was the product
of average daily catch per boat, days of activity per
month, number of boats recorded as active in each
month of sampling, and months of fishing activity.
Resource overlap. To assess the similarity of biomass
composition between common dolphin prey and fish-
ery catches, we used Pianka’s niche overlap index
(a measure of resource overlap between 2 species;
Pianka 1973, Pusineri et al. 2004):
where Pij is the percentage of prey item i of predator j,
and Pik is the percentage of prey item i of predator k.
The index ranges between 0 and 1, and the similarity is
higher the closer the index is to 1.
While the precise diet composition of common dol-
phins in the study area is not known, diet preferences
were inferred from (1) stomach contents of dead ani-
mals in the study area (Bearzi 2000) and in neritic habi-
tat elsewhere (reviewed in Bearzi et al. 2003, Cañadas
2006), (2) observations of common dolphin feeding be-
haviour and diving patterns in the study area (Bearzi et
al. 2005), (3) direct observations of feeding in neritic
Mediterranean waters (Mussi & Miragliuolo 2003,
Cañadas 2006), and (4) analysis of fish scales sampled
during surface feeding by common dolphins in the
study area (Agazzi et al. 2004, Bearzi et al. 2006). Based
on this information, we assumed an average diet to be
composed of 80% Clupeidae and Engraulidae, 10%
Belonidae, Gadidae and small Carangidae, 5% Lolig-
inidae, and 5% other families (Table 2). Average daily
food consumption was estimated as 4.2 kg animal
ing IB = 0.123M
(Innes et al. 1987), where IB is the in-
gested biomass (kg d
) and M the body mass in kg.
Body mass of common dolphins in the central Mediter-
ranean averages 82.5 kg according to Cagnolaro et al.
(1983). The body mass of juveniles was estimated as
50% of adult mass; nursing calves were not considered.
Population trends
Mark-recapture estimates showed that common dol-
phin numbers declined significantly and almost
steadily between 1995 and 2007 (Fig. 4). No mark-
ij ik
Endang Species Res 5: 112, 2008
recapture estimates could be obtained in 2006 (5 en-
counters) and 2007 (4 encounters) due to insufficient
recapture events in different months. However, owing
to the small group sizes and small number of encoun-
ters, all animals sighted during those years were
photo-identified. There were 12 marked animals in
2006 and 9 in 2007 (Fig. 4).
As mark-recapture estimates relied on natural mark-
ings to identify individuals, they refer exclusively to
the population of marked animals. To include the
unmarked portion and estimate total abundance, the
proportion of unmarked individuals (which also
included subadult classes) was computed based on
the number of photographs of marked and unmarked
dorsal fins obtained daily (Williams et al. 1993, Bearzi
et al. 2008). Based on 1858 high-quality dorsal fin
photos taken in 1997 (the year with greatest photo-
identification effort), the mean proportion of unmarked
animals in the population was 0.44 (95% CI = 0.382 to
0.494, n = 74). By adding this mean proportion of
unmarked individuals to the mark-recapture estimate,
we obtained estimates of total population size (Fig. 4).
Even though mark-recapture analyses were pre-
cluded in 2006 and 2007 by insufficient recaptures in
those years, encounters were so few (Table 1), groups
so small (range 2 to 9), and photo-identification effort
so extensive that we had confidence in our ability to
count and photograph all individuals present during
the sightings. Direct counts of total animals photo-
graphed (18 individuals in 2006, 15 in 2007) were con-
sistent with values obtained by adding the estimated
unmarked animals to the number of marked animals
(21 individuals in 2006, 16 in 2007), but field counts
appeared to be the most accurate and were therefore
used to complement information obtained through
mark-recapture (Fig. 4).
The observed patterns of site fidelity (Fig. 2) and
dolphin movement (Fig. 3) suggest that the decline
indicated by the mark-recapture analysis applies to the
entire local population of common dolphins.
Impact of fishing and resource overlap
Landings by purse seiners averaged 706.2 kg (SD =
967.91, n = 26, range 0 to 4030). Species landed were
predominantly horse mackerel Trachurus trachurus
(26.2% of the total catch), European pilchard Sardina
pilchardus (23.1%), bogue Boops boops (15.8%), Euro-
pean mackerel Scomber scombrus (11.2%), round sar-
dinella Sardinella aurita (10.6%; sometimes discarded
before or during landing operations due to their low
market value), picarel Spicara smaris (7.5%), Atlantic
bonito Sarda sarda (1.9%), European squid Loligo vul-
garis (1.7%), European barracuda Sphyraena sphy-
raena (1.0%) and skipjack tuna Katsuwonus pelamis
(0.8%). Landings by beach seiners averaged 125.6 kg
(SD = 101.55, n = 14, range 29 to 410). Species landed
were predominantly picarel (56.1%), European pil-
chard (26.5%), bogue (6.83%), European squid
(3.58%), European barracuda (1.8%), striped red mul-
let Mullus surmuletus (1.3%), greater amberjack Seri-
ola dumerili (0.9%), European mackerel (0.6%) and
annular seabream Diplodus annularis (0.6%). Other
Beach Purse Bottom Trammel Longliners Common
seiners seiners trawlers dolphins
Clupeidae, Engraulidae 26 34 0 13 0 80
Belonidae, Gadidae, small Carangidae 0 26 17 0 0 10
Loliginidae 4 2 15 0 0 5
Large Carangidae, all other Families 70 38 68 87 100 5
Table 2. Estimated percentage composition of catches by fishing gear and inferred diet composition of local common dolphins
(see ‘Resource overlap’ section in ‘Materials and methods’)
Fig. 4. Number of common dolphins in the study area
between 1995 and 2007, estimated through photographic
mark-recapture. h: total number of marked animals photo-
identified; j: mark-recapture estimates; d: total population
estimates (marked and unmarked animals)
Bearzi et al: Common dolphin disappearance in western Greece
species caught in small quantities (0.4 to 0.1%) in-
cluded gilthead seabream Sparus aurata, Octopus spp.,
salema Sarpa salpa, saddled seabream Oblada mela-
nura, common pandora Pagellus erythrinus, European
seabass Dicentrarchus labrax and common cuttlefish
Sepia officinalis. Landings by bottom trawlers averaged
495.3 kg (SD = 350.40, n = 16, range 195 to 1510). Spe-
cies landed included predominantly European hake
Merluccius merluccius (36.6%), horse mackerel
(17.0%), European squid (14.9%), red shrimp Aristeus
antennatus (11.4%), striped red mullet (7.7%), bogue
(4.1%), angler Lophius piscatorius (1.7%), thornback
ray Raja clavata (1.4%), common pandora (1.1%), silver
scabbardfish Lepidopus caudatus (1.0%), spottail man-
tis shrimp Squilla mantis (0.6%), European seabass
(0.5%), and Octopus spp. (0.5%). Other species
caught in small quantities (0.4 to 0.1%) included pi-
carel, small-spotted catshark Scyliorhinus canicula,
common cuttlefish, gilthead seabream, flathead mullet
Mugil cephalus, and common sole Solea solea.
Estimates of the total biomass removed by the local
fishing fleet between November 2006 and October
2007 and a comparison of the relative impact of differ-
ent fishing boats are shown in Table 3, taking into
account days of inactivity and discards (see ‘Materials
and methods’). The estimated total biomass removed
by local fisheries was 3571 t. A Pianka’s index of 0.5
reflects the likely present overall degree of resource
overlap between common dolphins and fisheries in the
study area (Table 2). Resource overlap differed accord-
ing to fishing gear, being 0.7 for purse seiners, 0.4 for
beach seiners, 0.1 for bottom trawlers, 0.2 for trammel
boats, and 0.0 for longliners (Fig. 5). The apparent lack
of resource overlap between common dolphins and
longliners is consistent with the catch composition
observed in the Cyclades Islands (Stergiou et al. 2002)
and in the Patraikos Gulf (Tzanatos et al. 2006). The
biomass consumed by 15 common dolphins estimated
to be present in the study area during the same time
interval was about 17 t (Fig. 5).
Once common and relatively abundant in the area of
Kalamos, common dolphins have declined dramati-
cally over the past decade. While approximately 150
animals used the study area in 1996, only 15 were ob-
served in 2007. Tuna and swordfish also have declined
(Bearzi et al. 2006). Large tuna, in particular, were
commonly encountered in the early years of the study
but were rarely seen after 2001. The decline of high-
order marine predators that feed on epipelagic prey is
consistent with the hypothesis that intensive exploita-
tion of epipelagic fish stocks reduced the availability of
key prey, making this coastal habitat less able to sus-
tain populations of large predators (Bearzi et al. 2006).
Other factors that might have played a significant role
include unrecognized oceanic changes, tissue contam-
ination by xenobiotic chemicals and incidental mortal-
ity in fishing gear, but only the last is likely to have
been relevant. A local worker reported that 5 common
dolphins with mutilations were found stranded near
Paleros between December 2004 and April 2005. Of
those, only 1 animal could be exam-
ined and based on mutilation of its tail
flukes, it likely died in fishing gear
(Bearzi 2006). No other reports were
received of common dolphin bycatch
during the entire 15 yr study (1993 to
2007) or during the 12 mo of fishery
monitoring. Also, floating carcasses of
common dolphins (possibly indicative
of bycatch) were never observed over
a total of 1163 d spent at sea within
and around the study area since 1991,
totalling >67 000 km of navigation. By-
catch is an important cause of mortal-
ity of common dolphins in some parts
of the world (Tregenza et al. 1997, Tre-
genza & Collet 1998, Julian & Beeson
1998) and it may have contributed to
the observed decline in common dol-
phins around Kalamos. However, we
consider it unlikely that bycatch was
the sole or even the main cause of the
Boat kind Boat length Mean no. Active Annual % of total
(m) of active months biomass annual
boats removal (t) biomass
Purse seiner 1525 7.5 9.5 1166.5 32.7
1.0 6
12 1.0 8
Beach seiner 812 1.0 4 528.7 14.8
23.0 6
Bottom trawler 2025 6.0 8 797.8 22.3
1.0 6
Trammel 45 22.8 12 915.4 25.6
57 49.4 12
710 15.5 6
83.9 12
1012 1.3 6
2.2 12
Longliner <7 15.3 12 162.7 4.6
79 10.9 12
910 4.5 12
Table 3. Composition of the fishing fleet operating in the study area and biomass
removed annually
Endang Species Res 5: 112, 2008
As an alternative hypothesis, it is necessary to con-
sider the possibility that the observed trends were a
consequence of emigration or long-range movements.
However, surveys outside the study area have pro-
vided no evidence to support the suggestion that com-
mon dolphins have shifted their distribution away from
the Kalamos area (Bearzi et al. 2005). This area has
been one of the few remaining pockets of high-density
occurrence in the central and eastern Mediterranean,
and there are few reports of common dolphins in sur-
rounding areas. Dedicated cetacean surveys con-
ducted by the Tethys Research Institute in the Ionian
Sea between 1991 and 2007, covering more than
8200 km outside the Kalamos study area, produced no
sightings of common dolphins. Notwithstanding sub-
stantial dedicated effort by several research groups,
only 3 sightings have been reported to date from east-
ern Ionian Sea waters outside the study area (1 in each
of 1992, 1995 and 1998; Angelici & Marini 1992, Politi
et al. 1992, Frantzis et al. 2003, Gannier 2005). Be-
tween 1999 and 2007, common dolphins were never
seen north, west or south of the study area during
cetacean surveys totalling tens of thousands of kilo-
metres of dedicated effort and covering large portions
of the eastern Ionian Sea, as well as waters west of the
Peloponnese (Frantzis et al. 2003, Lacey et al. 2005,
IFAW 2007, A. Frantzis pers. comm.). More generally,
common dolphins appear to be rare in, or absent from,
the adjacent central Mediterranean areas explored so
far, including the Hellenic Trench, other neritic and
pelagic portions of the Ionian Sea, and the Adriatic Sea
(Notarbartolo di Sciara et al. 1993, Bearzi et al. 2003,
Frantzis et al. 2003, Gannier 2005, Lacey et al. 2005,
IFAW 2007), the only exception being the inner Gulf of
Corinth, where a few common dolphins remain
(Frantzis & Herzing 2002). Recent genetic evidence
(Natoli et al. 2008) indicates a relatively high degree of
differentiation among common dolphins in the Medi-
terranean and shows that common dolphins in the
eastern part of the basin, including the Ionian Sea, are
largely isolated from the western Mediterranean
Prey depletion caused by overfishing is thought to be
one of the main reasons behind the overall decline of
Mediterranean common dolphins (Bearzi et al. 2003,
2004a, Cañadas & Hammond 2007). The declines in
fish stocks and loss of marine biodiversity caused by
fishing pressure are a growing concern worldwide
(Pauly et al. 1998, 2002, Jackson et al. 2001, Pitcher
2001, Csirke 2005, Worm et al. 2006). Unsustainable
fishing has been implicated in dramatic ecological
changes in the Mediterranean Sea (Sala 2004), where
it has caused the decline of many fish stocks (Caddy &
Griffiths 1990, De Walle et al. 1993, Stanners & Bour-
deau 1995, Caddy 1997, Garcia et al. 2005). Some of
the Mediterranean fish stocks that have been over-
exploited include important prey species of common
dolphins (Lleonart 2005).
While the impact of fishing remains to be assessed at
the regional scale, there is ample and growing evi-
dence of overfishing in the Hellenic Seas, including
the eastern Ionian Sea. Around the island of Kalamos,
Fig. 5. Estimate of total biomass removed by common dolphins and fisheries in the last year of the study. The darker part of the
fisheries bars shows removal of species representing key prey for common dolphins, as suggested by the index of resource overlap
Bearzi et al: Common dolphin disappearance in western Greece
total landings have decreased since the mid 1980s
(Papaconstantinou et al. 1985, 1988, Papaconstantinou
& Stergiou 1995, Stergiou et al. 1997). European Com-
mission (EC) figures indicate that catch per day of both
demersal and pelagic resources has declined steeply in
the Hellenic Ionian Sea over the last decade (EC 2004).
In the eastern Ionian Sea, in particular, purse seine
catch per day of both anchovies and sardines declined
since the mid 1990s (EC 2004), especially for vessels
larger than 15 m, which account for the majority of the
catch of small pelagic fish species (Stergiou et al.
2007b). The long-term increasing trends in Hellenic
marine landings from 1964 to 1994, attributed to fleet
modernization and geographic expansion of the fish-
eries over this period, have been followed since the
mid 1990s by rapidly declining trends in landings and
yields, suggesting that the fishing has been overly
intense, i.e. unsustainable (Stergiou et al. 1997, 2007b,
Stergiou 2005).
Overlap between dolphin prey species and fishery
target species as indicated by the Pianka’s index
does not prove direct competition (Briand 2004). How-
ever, it is reasonable to infer competitive effects when
key prey becomes scarce and remains subject to heavy
fishing pressure (Trites et al. 1997, Kaschner & Pauly
2004, Pusineri et al. 2004). Around Kalamos, the poten-
tial for exploitative competition (Keddy 1989) between
high-order predators such as common dolphins and
local fisheries targeting their prey is apparent (Fig. 5).
Overfishing and destructive fishing methods affect not
only top predators, but also the fishery and in turn the
fishing communities. Though it was not possible to
quantify this aspect as part of the present study (cf.
Saenz-Arroyo et al. 2005), local artisanal fishermen
consistently lamented the declines in catches and re-
ported to our research team that the local marine eco-
system had become less productive in recent decades.
The waters around Kalamos are an important
spawning area for European pilchard (Somarakis et al.
2000, 2006a,b, Machias et al. 2007) and hake (Politou
et al. 2006, Politou 2007), making this Natura 2000 Site
of Community Importance a candidate for special pro-
tection based on EC Regulations for the sustainable
exploitation of fishery resources in the Mediterranean
(EC 2006). In addition to common dolphins, the area is
home to species included in Annex II to the Habitats
Directive, including a resident community of common
bottlenose dolphins Tursiops truncatus and endan-
gered species such as the Mediterranean monk seal
Monachus monachus and the loggerhead sea turtle
Caretta caretta.
In view of the evidence provided here, and consider-
ing the existing political and legal commitments to
conserve cetaceans and preserve marine biodiversity
(see ‘Introduction’) (Owen 2004), relevant local, na-
tional and regional governmental bodies are obliged to
take action to reduce fishing pressure and limit the use
of fishing gears that can have unintended harmful ef-
fects on the marine environment. Fishery management
measures are urgently needed to reduce current over-
exploitation and allow for the recovery of endangered
marine megafauna. Such measures should include the
following: (1) an immediate moratorium on purse sein-
ing, as advocated by the large local community of arti-
sanal fishermen; (2) restrictions on bottom trawling;
(3) strict enforcement and appropriate penalties for il-
legal fishing; and (4) full implementation of the Coun-
cil Regulation 1967/2006, which also demands a ban
on beach seining by May 31, 2010 (beach seining is
considered harmful to fish stocks and habitats and has
been banned in most EU Countries). As EC funding
tools exist to compensate the affected fisheries, this
may be a ‘win-win’ situation, where existing regula-
tions can be effectively implemented to solve a specific
conservation problem, with the added value of protect-
ing marine biodiversity, ensuring continued ecosystem
services, preserving artisanal fisheries, and bringing
long-term benefits to human society.
Acknowledgements. This work was supported in part by a
Pew Marine Conservation Fellowship (a programme of the
Pew Institute for Ocean Science), by OceanCare, by The
Whale and Dolphin Conservation Society, by the UNEP/CMS
Agreement on the Conservation of Cetaceans of the Black
Sea, Mediterranean Sea and Contiguous Atlantic Area, and
by UNEP’s Regional Activity Centre for Specially Protected
Areas. Thanks to S. Bruno, S. Ferretti, A. Frantzis, A. Natoli,
A. Petroselli and many other collaborators for contributing to
field data collection and data analysis. Our gratitude also goes
to all the volunteers who made this research possible through
their financial support and help in the field. A. Frantzis, T.
Lewis, D. Moutopoulos, G. Paximadis, E. Tryfon and A. I.
Tsikliras contributed relevant information. Constructive com-
ments were provided by C. M. Fortuna, G. Notarbartolo di
Sciara, D. Pauly and 2 anonymous reviewers. R. Gramolini
developed a dedicated tool to ease ArcView GIS analyses.
The Milan Civic Aquarium and Hydrobiological Station pro-
vided logistical support. The work presented here was con-
ducted under research permits issued by the Hellenic Min-
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Editorial responsibility: Rebecca Lewison,
San Diego, California, USA
Submitted: January 2, 2008; Accepted: May 1, 2008
Proofs received from author(s): June 25, 2008