) in the eastern ionian seaionian sea coastal waters. the area, including the islands of meganisi,...

AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS

Aquatic Conserv: Mar. Freshw. Ecosyst. 15: 243-257 (2005)

Published online 18 January 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/aqc.667

Occurrence and present status o f coastal dolphins (Delphinus delphis and Tursiops truncatus) in the eastern Ionian Sea

G IO V A N N I BEA RZI*, ELEN A PO LITI, STEFA N O A G A ZZI, SEBASTIANO B R U N O , M A R IN A COSTA and SILVIA BO N IZZO N I

Tethys Research Institute, Viale G.B. Gadio 2, M ilan, Italy

A BSTR A C T

1. B oat surveys aim ed a t studying short-beaked com m on dolphins and com m on bottlenose dolphins in eastern Ion ian Sea coastal w aters were conducted between 1993 and 2003. D uring 835 survey days, 24 771 km o f to ta l effort was d istributed w ithin an area o f 480 km 2, resulting in 428 com m on dolphin and 235 bottlenose dolphin sightings.

2. Individual photo-identification was perform ed extensively th roughou t this study, m aking it possible to m on ito r the num ber o f anim als seen in the study area each year and their long-term residency patterns.

3. C om m on dolphins declined across the study period, from 2.18 encounters/100 km in 1997 to 0.40 encounters/100 km in 2003. In contrast, there was a relatively stable presence o f bottlenose dolphins, some individuals showing high levels o f site fidelity and others using the area only occasionally.

4. The local decline o f com m on dolphins and the low density o f bottlenose dolphins appeared to reflect the general status o f these cetacean species in the w ider M editerranean region, where com m on dolphins were classified as endangered in the IU C N R ed List in 2003.

5. Based on the available evidence, we infer th a t the present unfavourable status o f com m on dolphins in eastern Ion ian Sea coastal w aters is largely a consequence o f prey depletion.C opyright © 2005 John W iley & Sons, Ltd.

k e y w o r d s : cetaceans; Ionian Sea; M editerranean Sea; Delphinus delphis; Tursiops truncatus; ecology; anthropogenic impact; prey depletion

INTRODUCTION

Once one o f the com m onest species in the M editerranean Sea, the short-beaked com m on dolphin (.Delphinus delphis; hereinafter referred to as com m on dolphin) has experienced a generalized and m ajor decline th roughout the region during the last 30^10 yr, for reasons tha t rem ain poorly understood (Bearzi et al., 2003). In 2003 the M editerranean com m on dolphin ‘subpopulation’ was classified as endangered in the IU C N Red List o f Threatened Animals, based on an inferred decline in abundance o f ^ 5 0 % over the

“"Correspondence to: G. Bearzi, Tethys Research Institute, Viale G.B. Gadio 2, 20121 Milan, Italy. E-mail: [email protected]

C opyright © 2005 John W iley & Sons, Ltd. Received 6 A pril 2004Accepted 21 Septem ber 2004

http://www.interscience.wiley.com

mailto:[email protected]

244 G. BEARZI E T AL.

last three generations (approxim ately 30-45 yr), the causes o f which ‘m ay no t have ceased or m ay no t be understood or m ay no t be reversible’ (http://w w w .iucnredlist.org).

The com m on bottlenose dolphin (Tursiops truncatus', hereinafter referred to as bottlenose dolphin) is the com m onest cetacean over the M editerranean Sea continental shelf (N otarbarto lo di Sciara and Dem m a, 1997). However, the distribution o f coastal groups appears to be increasingly scattered and fragm ented into small isolated units (N atoli et al., 2004).

In the eastern Ionian Sea, two communities (sensu Wells et al., 1987) o f com m on dolphins and bottlenose dolphins have been the subjects o f a study th a t started in 1993, which was designed to investigate the ecology, social organization and behaviour o f two species sharing the same coastal waters (Politi et al., 1994, 1999). The area offered unique opportunities to observe com m on dolphin groups in a portion o f the M editerranean where the species is generally rare, and its sheltered waters enabled daily surveys to be conducted under optim al sea conditions. A t the beginning o f the study, com m on dolphins were abundant and could be found every day with little navigation effort. However, the results o f this study indicate that since a t least 1997 there has been a substantial decline in com m on dolphin num bers. Here, we discuss the potential reasons for the com m on dolphin decline in eastern Ionian Sea coastal waters and we propose some m anagem ent measures tha t could be used to reverse this trend.

M ETHODS

Study area

The survey and photo-identification effort was concentrated w ithin a core study area situated in eastern Ionian Sea coastal waters. The area, including the islands o f M eganisi, K alam os and K astos, covers approxim ately 480 km 2 o f sea surface (Figure 1). The sea floor is m ostly 50-150 m deep, w ith rocky coasts and shallows covered by seagrass meadows. W aters are transparen t (Secchi disk readings ranging between 10 and 30m ), oligotrophic and unaffected by significant river and agricultural runoffs (Pitta et al., 1998). Surface w ater tem peratures ranged between 15 and 30°C during the study period (A pril-O ctober).

Survey effort and encounter rates

Surveys were conducted from a 15 m sailing vessel in years 1993-1994, from 4.7 m inflatable craft with pneum atic keels powered by 40 H P outboard engines in years 1995-1996, and from 4.7-5.8 m inflatable craft w ith rigid hulls powered by 50-80 H P four-stroke outboard engines in years 1997-2003. The survey coverage between 1993 and 2003 totalled 24 771km ‘on effort’ and 835 daily surveys, encom passing three seasons. The num ber o f observers, eye elevation and vessel speed varied inconsistently in the first 4 yr o f the study, m aking the da ta unsuitable for the com parative analysis o f encounter rates. Therefore, da ta collected in 1993-1996 were used only for group size and photo-identification analyses unaffected by the survey m ethod, and excluded from encounter rate analyses. Conversely, survey m ethods rem ained consistent between 1997 and 2003. Encounter rates were com puted based on 18 447 km o f survey effort during 572 daily surveys conducted in June-Septem ber between 1997 and 2003 (Table 1, Figure 2). D ata were defined as ‘on effort’ when the following conditions applied: (1) daylight and good visibility; (2) sea state ^ 1 D ouglas with no swell (including either completely flat sea, flat sea w ith capillary waves or wavelets less than 10 cm high w ith no foam or breaking crests); (3) at least two experienced observers scanning the sea surface in search of dolphins; (4) eye elevation o f approxim ately 2 m for bo th observers; and (5) survey speeds between 28 and 3 6 k m h _1. All the daily surveys started from the port o f Episkopi (island of K alam os, Figure 1) and returned there. A lthough routes varied depending on w eather conditions and other factors, an attem pt was m ade to provide an even coverage o f the study area (Figure 2).

Copyright © 2005 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 15: 243-257 (2005)

http://www.iucnredlist.org

COASTAL D O LPH IN S IN T H E EASTERN IO N IA N SEA 245

20°30‘ 20°40' 20°50' 21°00'

200 m

mainland Greece

Lefkada

Me

Kalamos,

10 km

Figure 1. Study area (delimited by bold line) showing the locations cited in the text and bathymetric contour lines. The location of thestudy area in the M editerranean Sea is shown in the inset.

The m ain study area was subdivided into a to tal o f 222 cells o f 1' x 1' (1852 x 1460 m). Encounter rates were calculated for each year by the ratio n jL , where n is the to tal num ber o f sightings and L is the total num ber o f kilometres spent on elfort. This is equivalent to com puting the weighted m ean o f the encounter rates («,;//,;) recorded w ithin each cell, where n¡ and are respectively the num ber o f sightings and the num ber o f kilometres spent on elfort in each cell, and the weights are given by the ra tio ¡¡JL. The sampling variance o f the encounter rate was then calculated using the form ula (Buckland el aí., 1993)

Ek im a ¿ii= i l\ i¿ L'

vari n/L.) = ------ ~ ^ \ ------

where k is the num ber o f cells surveyed.Cells that received a to tal survey elfort lower than a cell’s diagonal (2358 m) in any given year were

excluded from the analysis o f encounter rates for all years (Figure 2).

Group size and individual photo-identification

The sizes o f 428 com m on dolphin and 235 bottlenose dolphin groups observed in 1993-2003 were estim ated in the field by m eans of independent counts perform ed by different crew members over



Table 1. Survey effort in 1993-2003: number o f daily surveys and total sightings per species (above) and kilometres surveyed ‘on effort’ (below). The boxed subset indicates the survey effort on which encounter rates were computed

Y ear A pr M ay Jun Jul A ug Sep Oct Subtotal Sightings3

D d T t

D aily surveys1993 2 13 11 10 36 15 41994 7 15 18 18 2 60 26 61995 6 13 8 27 9 41996 3 18 22 17 8 68 36 251997 4 17 25 22 23 23 11 125 89 251998 7 15 17 22 21 26 4 112 76 231999 16 23 22 22 2 85 52 312000 22 23 24 21 90 54 262001 1 21 20 22 18 82 35 282002 2 19 16 24 11 72 17 322003 9 18 11 19 21 78 19 31

Total 20 35 150 189 219 195 27 835 428 235

K ilom etres surveyed ‘on effort’1993 41 236 290 273 8411994 157 371 296 269 38 11311995 131 334 263 7271996 32 413 823 465 203 19361997 11 230 626 522 428 581 375 27731998 158 328 361 597 543 545 74 26061999 549 548 689 662 13 24612000 831 511 896 699 29372001 1 472 552 934 478 24382002 50 529 451 1021 454 25042003 448 1059 783 1205 921 4416

Total 617 609 4657 5133 7460 5610 704 24771

aDd: Delphinus delphis; Tt: Tursiops truncatus.

protracted observation sessions. G roups were defined as ‘dolphins observed in apparent association, m oving in the same direction and often, but no t always, engaged in the same activity’ (Shane, 1990). M embers o f the focal group usually rem ained w ithin approxim ately 100 m of each other and were all potentially photo-identifiable. The num ber o f dolphins in the group and their m ajor age classes were assessed visually in situ, and corrected a posteriori whenever photo-identification analyses provided additional inform ation, e.g. when the num ber of individuals photo-identified was larger than the group size estim ated in the field. G roups were followed for an average o f 129min (June-Septem ber subset; SD = 76.86, SE = 3.18, « = 586, range 1-495; m edian= 120 , IQ R = 1 0 3 ). W hen a group split, one o f the daughter groups was followed, based on a random choice th a t was independent of group size and /o r activity (M ann, 1999). A change in group size and/or com position occurred on average every 59m in (SD = 52.22, SE =1.56, « = 1142 , range 3-354; m edian = 42, IQ R = 57). The size (and behaviour) o f a focal group was sampled at 3 m in intervals th roughout the duration o f the sighting. Yearly m ean group sizes were then obtained by extracting from the database a group size sample every 60 m in (i.e. a t 7:00, 8:00, 9:00, etc.), i.e. an arbitrary time interval greater than the m ean duration of


COASTAL D OLPHINS IN THE EASTERN IO N IA N SEA 247

Figure 2. Survey effort in the study area between 1993 and 2003. Cells used for the analysis of encounter rates (see Methods)are shown in the inset.

associations and smaller than the m ean time taken to encounter a new group (W hitehead, 2004). G roups other than the focal group observed at a distance (i.e. dolphins in sight sensu Bearzi el aí. (1997)) were not considered. Only groups observed between June and September were considered, to allow for consistency w ith encounter rate analyses and eliminate any variability potentially caused by seasonal changes in m ean group size. G roups observed outside the study area (Figure 1) were excluded from the analysis.

Individual photo-identification was perform ed consistently th roughout 1993-2003, based on long-term natural m arks, such as nicks and notches on the dolphins’ dorsal fins (Wiirsig and Jefferson, 1990). F o r some individual com m on dolphins the identification was based on a com bination of natural m arks and dorsal fin pigm entation patterns (N eum ann el aí., 2002). Only individuals having obvious long-term m arks on their dorsal fins were included in the catalogue. Photographs suitable for photo-identification analyses were obtained for a to ta l o f 575 days, using A F reflex cameras equipped with 80-200 m m f2.8 APO lenses. This elfort produced a catalogue o f 18 547 com m on dolphin and 8245 bottlenose dolphin images. Excluding non-adult size classes (i.e. newborns, calves and juveniles), an average of 48% o f all com m on dolphins and 59% of all bottlenose dolphins encountered were positively identified.

N on-param etric m ultiple com parisons followed Z ar (1984).



RESULTS

Encounter rates

Encounter rates o f com m on dolphin groups showed a significant decrease between 1997 and 2003 (K ruskal-W allis rank test, 77=29.61, d f = 6 , P < 0.001; Figure 3). Com m on dolphin encounter rates declined continuously during the course o f this study, from a m ean o f 2.18 groups/100 km recorded in 1997 to a m ean o f 0.40 groups/100 km in 2003. Encounter rates o f bottlenose dolphin groups (meaniggv 2 0 0 3 = 0.61 groups/100 km) did not show significant variations (K ruskal-W allis rank test, 77=8.36, d f = 6, P = 0.213; Figure 4).

Group size

C om m on dolphin group sizes showed significant differences am ong years 1993-2003 (June-Septem ber subset; K ruskal-W allis rank test, 77=102.84, d f= 10, P < 0 .0 0 1 ). N on param etric m ultiple com parisons (PcO.OOl) showed tha t group sizes recorded in years 1993-1994 were significantly higher than those recorded in 1997-2001. G roup sizes in 1995-1996 were significantly higher than in 1997 and 1999. M oreover, group sizes in 1999 were significantly lower than in all other years except 1997 and 2000-2001. The m ean group size was 13.1 between 1993 and 1996 (SD = 9.28, SE = 0.62, « = 223, range 2-40; m edian = 10, I Q R = 12), 7.2 in 1997-2001 (SD = 4.70, SE = 0.20, « = 555, range 1-26; m edian = 6, IQ R = 6) and 9.2 in 2002-2003 (SD = 4.65, SE = 0.51, « = 83, range 1-23; m edian = 8, IQ R = 6; Figure 5).

Bottlenose dolphin group sizes also showed significant differences am ong years 1993-2003 (K ruskal- Wallis rank test, 77=65.61, d f = 10, P < 0 .0 0 1 ). However, no consistent longitudinal trend was apparent (Figure 6). G roup sizes in the years 1994 and 2003 were significantly higher than those recorded in both 1996-1998 and 2000-2002 (non-param etric m ultiple com parisons, P < 0.001). The m ean size o f bottlenose dolphin groups observed in 1993-2003 was 6.8 (June-Septem ber subset; SD = 4.16, SE = 0.22, « = 364, range 1-24; m edian = 6, IQ R = 5).

0.025

0.02

k-0•*->C3OÜC0Cto02

0.015

0.01

0.005

1997 1998 1999 2000 2001 2002 2003Years

Figure 3. Common dolphin mean encounter rate by year, 1997-2003. Error bars represent 95% confidence intervals.


COASTAL D O LPH IN S IN T H E EASTERN IO N IA N SEA 249

0 . 025

0.02

a>•*->coi— 0.015

C3Ooc0) 0.01ctod)2

0.005

1997 1998 1999 2000 2001 2002 2003Years

Figure 4. Bottlenose dolphin mean encounter rate by year, 1997-2003. Error bars represent 95% confidence intervals.

17

16

15

14

13

12

11a.10

U)9

8

7

6

5

4

31993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Years

Figure 5. Mean group size by year, 1993-2003: common dolphins. Error bars represent 95% confidence intervals.

There was no correlation between the num ber o f kilometres surveyed on effort before sighting any dolphin group and the size o f the group sighted, both for com m on dolphins (Spearm an’s r = 0.048, n = 202, P = 0.493) and for bottlenose dolphins (Spearm an’s r = 0.152, n = 104, P = 0.123), indicating tha t the capability o f detecting dolphins in the study area was independent o f the num ber o f individuals comprising a group.



17

16

15

14

13

12

10U)

9

8

7

6

5

4

31993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Years

Figure 6. M ean group size by year, 1993-2003: bottlenose dolphins. Error bars represent 95% confidence intervals.

Individual photo-identification

The rates at which individual dolphins were identified during the study are shown in Figures 7 and 8. The curve w ith the rate o f discovery for com m on dolphins evened out by 1997, indicating tha t m ost o f the potentially identifiable individuals had been catalogued by tha t year, and that rates o f im m igration were low. The continued discovery o f a few individuals each year long after the curve has flattened is to be expected even in the absence o f im m igration. Individuals previously present in the study area can become photo-identifiable as they develop natural m arks on their dorsal fins (e.g. in the case o f sub-adults as they grow older; Bearzi et al., 1997). Conversely, there was a continuous discovery o f bottlenose dolphins across the study period (Figure 8), indicative o f a high proportion o f transient individuals. The alternative hypothesis o f a closed but very large com m unity was rejected based on further evidence provided by pho to identification data (see below).

A to ta l o f 72 com m on dolphins w ith natu ral m arks on their dorsal fins suitable for reliable, long-term identification were catalogued. M ost individual com m on dolphins were consistently resighted across the study period, although there was a progressive reduction in the num ber o f animals sighted in the second half o f the study (Figure 9). Bottlenose dolphins showed a rem arkably different pattern. O f a to tal o f 48 bottlenose dolphins catalogued, only about 12 individuals were found consistently in the study area, whereas m ost others were seen sporadically or only once.

The to tal num ber o f individual com m on dolphins identified each year decreased after 1996 (low num bers in 1993-1995 reflect poor effort in those years). Conversely, m ore bottlenose dolphins were identified in the last years o f this study (Figure 9).

DISCUSSIO N

This study highlights a continuous decline o f com m on dolphins since 1997, which is consistently shown by (1) decreasing encounter rates (Figure 3); (2) decreasing m ean group sizes, indicating tha t decreasing


COASTAL D OLPHINS IN TH E EASTERN IO N IA N SEA 251

93 9-1 95 1996 1999 2000 2001 2002 200380

70

COJ■O 6 0

wIO□ 50

'O

>"5 40

o30ft

U>20

n3E3u

50 100 150 200 250 300 350

Daily photo-id sessionsFigure 7. Discovery curve for common dolphins in 1993-2003.

93 94 95 1996 1997 1998 1999 20ÛÛ 2001 2002 2003

' S 4 5

«■o

40

« 35 «HQ 30

T} 25

20

15

50 100 150 200

Daily photo-id sessionsFigure 8. Discovery curve for bottlenose dolphins in 1993-2003.



5 0 -

■ua? c a ■a cm ra3■a>coi—a •QE

1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003Years

Figure 9. Number of individual common dolphins (light grey) and bottlenose dolphins (black) photo-identified in 1993-2003.

encounter rates resulted from the rarefaction o f dolphins in the study area rather than from their aggregation in progressively larger groups (Figure 5); (3) a decreasing to tal num ber o f recognizable individuals encountered each year (Figure 9); and (4) a flattening o f the rate o f discovery curve since 1997, indicative o f very low rates o f im m igration (Figure 7). A t the beginning o f the study, com m on dolphins were encountered at high densities and resighting rates between years were high. However, during the course o f the study, some individuals either died or abandoned the study area, and others were seen much less regularly than before, suggesting tha t they were utilizing a wider geographical area.

N o obvious tem poral changes in encounter rate (Figure 4) and group size (Figure 6) were recorded for bottlenose dolphins, which were present at low but relatively stable densities th roughout this study. Politi el aí. (2000) noted signs o f em aciation in a high p roportion o f bottlenose dolphins photo-identified in the eastern Ionian Sea, and this was tentatively attributed to a scarcity o f prey caused by intensive trawling. Such fishing activity was dem onstrated, inter alia, to have depleted the local stock o f E uropean hake Merluccius merluccius (Papaconstantinou el aí., 1985). However, no evidence o f a decline in bottlenose dolphins was recorded across this study. Bottlenose dolphins are resilient animals, and they often m anage to survive under unfavourable circumstances (e.g. see Bearzi el aí. (2004a)), largely owing to adaptive strategies involving a high degree o f flexibility o f diet, social organization and behaviour (Shane el aí., 1986; Shane, 1990; Bearzi el aí., 1999; Blanco el aí., 2001), including opportunistic foraging behind trawlers (Bearzi el aí., 1999) or near fish farm cages, as observed in eastern Ionian Sea coastal waters (Bearzi el aí., 2004b). Only around 25% o f the bottlenose dolphins identified were found consistently in the study area across the study period, whereas m ost others appeared to be transient, as indicated by their resighting patterns and by the discovery curve (Figure 8). This predom inantly transient pattern differs from that of bottlenose dolphins studied in northern A driatic Sea coastal waters, where higher degrees o f site fidelity were observed for m ost com m unity members (Bearzi el aí., 1997).

Both bottlenose dolphins and com m on dolphins were m ostly seen in small groups. A m ean group size of seven for bottlenose dolphins is typical in coastal M editerranean waters, where the species is typically found in groups o f 5-10 individuals (Bearzi el aí., 1997; N otarbarto lo di Sciara and Dem m a, 1997). Conversely,



com m on dolphin groups observed in the eastern Ionian Sea were unusually small. C om m on dolphins worldwide are am ong the m ost gregarious o f all cetaceans (Evans, 1994; Reeves et al., 2002), w ith average school sizes ranging between 50 and 400 animals, and m axim um school sizes o f 650^1000 (Acevedo- Gutiérrez, 2002). In the M editerranean, com m on dolphins are m ostly found in groups o f 50-70 animals w ith aggregations o f 100-600 animals recorded occasionally (N otarbarto lo di Sciara et al., 1993; F orcada and H am m ond, 1998; Cañadas et al., 2002; Bearzi et al., 2003). A lthough relatively smaller group sizes can be expected in coastal habitats w ith low predation pressure (Acevedo-Gutiérrez, 2002), the small m ean sizes o f groups observed in the eastern Ionian Sea (decreasing from about 13 individuals in 1993-1996 to seven to nine individuals in 1997-2003) suggest th a t the com m on dolphins in our study area may be facing some degree o f social disruption.

Bearzi et al. (2003) reviewed the status of M editerranean com m on dolphins and concluded tha t the factors potentially contributing to their generalized decline in the region included: (1) reduced availability o f prey caused by overfishing and habita t degradation; (2) contam ination by xenobiotic chemicals, resulting in im m unosuppression and reproductive im pairm ent; (3) environm ental changes, such as increased water tem peratures, affecting ecosystem structure or dynamics; (4) incidental m ortality in fishing gear, especially gili nets. These and other hypotheses may also explain the decline observed in the eastern Ionian Sea. However, some o f the potential threats should be regarded as m inor or unlikely causal factors.

The current study recorded no evidence (e.g. carcasses w ith telltale signs on their bodies) to suggest that dolphins in our study area are being killed deliberately or are dying from entanglem ent in fishing gear. A lthough organochlorine levels in com m on dolphins sampled around K alam os were relatively high (Fossi et al., 2003), there is no evidence th a t contam inant levels are higher in the eastern Ionian Sea than in the few M editerranean areas where com m on dolphins are still abundant. On the contrary, eastern Ionian Sea coastal waters are considered to be relatively unpolluted (Pitta et al., 1998), and no evidence exists tha t local contam inant levels have increased significantly in recent years. A lthough exposure to xenobiotic contam inants is a potentially im portan t th reat (Fossi et al., 2003), it is unlikely to be a prim ary cause for the recent decline in com m on dolphins recorded in our study area.

Changes in oceanographic param eters can have large-scale impacts on the m arine environm ent, and the distribution o f com m on dolphins m ay be related to shifting environm ental conditions, such as sea tem perature (Gaskin, 1968; Forney, 1999; N eum ann, 2001). These kinds o f fluctuation affect dolphin distribution and/or abundance prim arily by influencing the availability o f their prey (Selzer and Payne, 1988). I t is difficult, therefore, to discrim inate between the effects o f climate change and other factors affecting dolphin prey, such as overfishing. In the study area, sea-surface tem peratures recorded by us across the 11 yr o f study did no t show any obvious trend (Tethys Research Institute, unpublished data), and gradual or small-scale environm ental fluctuations would in any event not be especially good candidates for explaining the rapid decline o f com m on dolphins in the eastern Ionian Sea.

W hether the observed com m on dolphin decline is simply a consequence o f m igratory behaviour or long- range movem ents by some or m ost com m unity members is no t known, and this hypothesis certainly deserves further investigation. To date, several surveys conducted outside the study area between 1997 and 2003 have provided no evidence to suggest tha t the core o f this com m unity has shifted elsewhere.

The patterns observed in the eastern Ionian Sea are consistent w ith the hypothesis o f decreased availability o f key prey. As stressed by C hapm an and Reiss (1999) ‘the lack o f sufficient food to maximise reproductive potential m ay be the m ost im portant regulator o f population size in anim als’. D ispersion over a wider foraging range and reduced group sizes represent behavioural strategies typically adopted by anim al communities when food resources are declining and/or distributed in small and low-density patches (N orris and D ohl, 1980; Wiirsig, 1986; Chapm an and Chapm an, 2000). C om m on dolphins in the study area have a particularly fluid fission-fusion social system (Bruno, 2001; Bruno et al., 2004) tha t was likely prom oted by levels o f predation th a t m ay be assumed as low, and by patchy prey resources (Shane et al., 1986; W ürsig, 1986; Shane, 1990; C lapham , 1993). A lthough such a social system allows for a certain degree



o f flexibility, living in progressively smaller and scattered groups (e.g. as a response to decreasing prey resources) m ay bring the com m unity to a po in t o f social collapse (Slooten et al., 1993; Bruno, 2001). This problem may be particularly acute in a prom iscuous species such as the com m on dolphin, for which sperm com petition (Stockley and Purvis, 1993) is likely a central feature o f its reproductive strategy (Cockcroft, 1993; M urphy et al., 2004). The im portance o f living in groups o f an appropriate size has been highlighted in the nearby G ulf o f C orinth, Greece, where Frantzis and H erzing (2002) found a high occurrence o f mixed groups th a t included com m on dolphins, striped dolphins Stenella coeruleoalba and Risso’s dolphins Grampus griseus. Those authors suggested th a t com m on dolphins increasingly associated w ith striped dolphins when they could no longer find enough conspecifics to form large single-species groups.

Despite high levels o f sym patry, associations between com m on dolphins and bottlenose dolphins were rarely observed and no evidence of m utual avoidance or m utual a ttraction was recorded over the course of this study. In 1365 h o f observation, members o f the two species were seen within 100 m o f one another for a cumulative to tal o f 180 m in on four non-consecutive days (Tethys Research Institute, unpublished data). D uring those short periods when com m on and bottlenose dolphins either deliberately approached each other or just happened to be in the same area, close associations were recorded on only two occasions, one in which the animals were feeding together near the surface (6 min) and another in which they were swimming in a tight form ation (6 min). W hether this was a consequence of different prey preferences, an attem pt to reduce com petition, or a com bination o f bo th factors is no t known. However, systematic observations between 1997 and 2003 indicated th a t the two species have rem arkably different behaviours and exhibit specialized feeding strategies to target different prey (Ferretti et al., 1999). Bottlenose dolphins spent m ost o f their time perform ing long dives (up to 8 min), apparently targeting demersal prey, whereas surface foraging was a rare event. Conversely, com m on dolphins typically perform ed dives shorter than 2 m in and were often seen chasing epipelagic schooling fish near the surface. Analyses o f floating fish scales collected after bouts o f near-surface feeding by com m on dolphins showed th a t their prey included European anchovies Engraulis encrasicholus and sardines (European pilchard Sardina pilchardus and round sardinella Sardinella aurita) in similar proportions (Agazzi et al., 2004).

M uch o f the fish fauna o f the eastern Ionian Sea is reduced because o f overfishing, and the potential exists for exploitative com petition (Keddy, 1989) between com m on dolphins and local m id-w ater fisheries targeting their prey, particularly anchovies and sardines (Bearzi et al., 2003; Agazzi et al., 2004; Pusineri et al., 2004). The m ean trophic level o f fishery catches in the eastern Ionian Sea coastal waters encompassing the study area has been decreasing over the past 20 years (Stergiou and K oulouris, 2000), indicating tha t the effect o f fishing down the food web (Pauly et al., 1998) is at play in this p art o f the M editerranean. Since the m id 1980s, decreased to tal landings were recorded around K alam os (Papaconstantinou et al., 1988; Papaconstantinou and Stergiou, 1995; Stergiou et al., 1997), an area that has been subjected to intensive bottom trawling (Papaconstantinou et al., 1985). Furtherm ore, the available da ta for the entire G reek part o f the Ionian Sea show th a t catch per day for bo th anchovies and sardines declined significantly (P < 0 .0 5 ) during 1996-2001, despite the fact th a t the fishing effort (num ber o f days at sea) rem ained constant across this period (Anonym ous, 2001). A lthough prey overlap between cetaceans and fisheries does no t necessarily m ean th a t there is direct com petition (Briand, 2004), it is reasonable to assume th a t such com petition m ight occur when key dolphin prey are scarce and subjected to heavy exploitation (Trites et al., 1997). I t is also im portan t to consider tha t prey quality is at least as im portant as quantity when evaluating the ecological interactions between fisheries and m arine m am m al populations (Trites and Donnelly, 2003).

Largely based on the local presence o f relict com m on dolphin groups, the G reek M inistry o f the Environm ent included the eastern Ionian area around the island o f K alam os in the N atu ra 2000 netw ork (‘Site o f C om m unity Im portance’) under the 9243 EEC H abitats Directive. However, this designation has no t yet provided protection to this dolphin community. In light o f the negative trends reported here, m anagem ent strategies are urgently needed. In the absence o f clear evidence on cause-effect relationships,



action aimed at controlling overfishing is certainly a m anagem ent obligation, as well as an appropriate precautionary m easure to prevent a continued decline o f com m on dolphins in this area. Fishery m anagem ent measures aimed to reduce the heavy exploitation of epipelagic fish stocks would be likely to produce benefits. In particular, the stocks o f European anchovy and E uropean pilchard, representing im portan t com m on dolphin prey, are heavily exploited by the local purse seiners, and catches are know n to have declined dram atically in the study area in recent years. The use o f networks o f m arine reserves encompassing com m on dolphin critical habitats and o f no-take zones aimed at protecting resident and m igratory fish stocks also represent appropriate m anagem ent tools to improve the feeding situation for dolphins and to create the conditions for their recovery, while providing benefits for the adjacent fisheries and the m arine environm ent a t large (Agardy, 1997; R oberts et al., 2001; Stergiou, 2002; H ooker and Gerber, 2004).

ACKNOWLEDGEMENTS

This w ork was co-funded by a Pew M arine C onservation Fellowship — a program m e of the Pew Institu te for O cean Science. F u rther support was received by the Agreement on the Conservation o f Cetaceans o f the Black Sea, Mediterranean Sea and Contiguous A tlantic Area (ACCOBAM S), by The W hale and D olphin C onservation Society (W DCS), and by ASM S O ceanCare. K onstan tinos I. Stergiou (U niversity o f Thessaloniki, D epartm ent o f Zoology, L aboratory o f Ichthyology) provided relevant inform ation on fishery trends in Greece. W e are grateful to John H arw ood, G iuseppe N o tarbarto lo di Sciara, R andall R. Reeves, Simone Panigada, M addalena Bearzi and an anonym ous referee for their insightful com m ents on early drafts. John H arw ood also helped us choose a m ethod for the analysis o f encounter rates. R oberto G ram olini developed a dedicated tool to ease ArcView G IS analyses. T hanks to Sabina A iroldi, M auro Bastianini, Enrico C abras, Sabrina Ferretti, A lexandros Frantzis, Joan G onzalvo, C ristiana M iglio, A da N atoli, Simone Panigada, G eorge Paxim adis, G iovanna Pesante, C hiara P iroddi, Francesco Q uondam and A viad Scheinin for contributing to field da ta collection and /o r da ta analysis. O ur gratitude also goes to all the volunteers who m ade this research possible th rough their financial contributions and help in the field. The Venice N atu ra l H istory M useum and the M ilan Civic A quarium and H ydrobiological S tation provided invaluable logistic support.

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