Fill - Form 1: Overfishing and the disappearance of short-beaked

ENDANGERED SPECIES RESEARCH

Endang Species Res

Vol. 5: 1–12, 2008

doi: 10.3354/esr00103

Printed September 2008

Published online July 1, 2008

INTRODUCTION

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

)

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

PEN

CCESS

Endang Species Res 5: 1–12, 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,

2007b).

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.

MATERIALS AND METHODS

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

–1

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

(http://www.warnercnr.colostate.edu/~gwhite/mark/

mark.htm) was used for mark-recapture abundance

estimation.

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: 1–12, 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

months).

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

–1

, mean ± SD), 14 for beach

seiners (2.3 ± 3.78 mo

–1

), and 16 for bottom trawlers

(2.0 ± 1.31 mo

–1

). 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

–1

for boats 4 to 7 m

long, and 6.31 t yr

–1

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

–1

us-

ing IB = 0.123M

0.8

(Innes et al. 1987), where IB is the in-

gested biomass (kg d

–1

) 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.

RESULTS

Population trends

Mark-recapture estimates showed that common dol-

phin numbers declined significantly and almost

steadily between 1995 and 2007 (Fig. 4). No mark-

α=

()

∑

∑∑

ij ik

Endang Species Res 5: 1–12, 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).

DISCUSSION

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

decline.

Boat kind Boat length Mean no. Active Annual % of total

(m) of active months biomass annual

boats removal (t) biomass

removal

Purse seiner 15–25 7.5 9.5 1166.5 32.7

1.0 6

12 1.0 8

Beach seiner 8–12 1.0 4 528.7 14.8

23.0 6

Bottom trawler 20–25 6.0 8 797.8 22.3

1.0 6

Trammel 4–5 22.8 12 915.4 25.6

5–7 49.4 12

7–10 15.5 6

83.9 12

10–12 1.3 6

2.2 12

Longliner <7 15.3 12 162.7 4.6

7–9 10.9 12

9–10 4.5 12

Table 3. Composition of the fishing fleet operating in the study area and biomass

removed annually

Endang Species Res 5: 1–12, 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

population.

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-

istry of Agriculture, General Directorate of Development &

Protection of Forests and Natural Environment, Directorate of

Aesthetic Forests, Woodland and Hunting.

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Form 1: Overfishing and the disappearance of short-beaked

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