DISTRIBUTION and NUMBERS OF STRIPED DOLPHINS IN THE WESTERN MEDITERRANEAN SEA AFTER THE 1990...

MARINE MAMMAL SCIENCE, 10(2):137-150 (April 1994) 0 1994 by the Society for Marine Mammalogy

DISTRIBUTION AND NUMBERS OF STRIPED DOLPHINS IN THE WESTERN

MEDITERRANEAN SEA AFTER THE 1990 EPIZOOTIC OUTBREAK

JAUME FORCADA

ALEX AGUILAR Department of Animal Biology (Vertebrates), Faculty of Biology, University of Barcelona, Diagonal 645, 08028 Barcelona, Spain

PHILIP S. HAMMOND Sea Mammal Research Unit, Natural Environment Research Council,

High Cross, Madingley Road, Cambridge CB3 OET, U.K.

XAVIER PASTOR

RICARDO AGUILAR Greenpeace International, Mediterranean Sea Project,

Ses Rafaletes 13 lo, 07015 Palma de Mallorca, Spain

ABSTRACT

A survey was conducted in the summer of 199 1 in the western Mediterranean to map the distribution and to estimate the number of striped dolphins surviving the 1990 outbreak of the morbillivirus epizootic. The highest densities of striped dolphins were found in the Alboran Sea and the Ligurian Sea. Total numbers were estimated at 117,880 (SE: 38,962; 95% CI: 68,379-214,800). The mortality resulting from the epizootic is unknown but may have been severe because the mean size of dolphin schools observed during the epizootic outbreak was significantly smaller than that before the event. A return to larger school sizes after the 1990 outbreak is not evidence of a recovery but can be viewed as a regrouping of individuals into schools of a preferred size. Calves were observed in 24.6% of the schools. They were not seen in schools of ten or less but were present in about half the schools of 30 or more dolphins, a result which suggests segregation similar to that seen in populations of this species elsewhere in the world.

Key words: striped dolphin, Stenella coeruleoalba, Mediterranean Sea, distribution, numbers, school size, school segregation.

The striped dolphin (Stenella coeruleoalbu) is the most commonly encountered marine mammal in the western Mediterranean (Fig. l), where it has accounted for more than 60% of the recorded strandings (Duguy et al. 1988). It has been

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138 IMARINE IMAMMAL SCIENCE. VOL. IO, NO. 2. 1794

451

4Oh

35h

0 5,E 10E I

U 45N FRANCE

SICILIAN

I I 5w 0 5E IOE

Figwe 1. The western Mediterranean Sea.

suggested that the population expanded in rhe 1970s (Viale 1980), but incidental catches in drift nets and other fishing gear (Notarbartolo di Sciara 1990, Di Natale and Notarbartolo di Sciara, in press) have exerted an additional mortality of unknown proportions.

In 1990 a morbillivirus epizootic (Domingo et ul. 1990, van Bressem et al. 1991) began to affect striped dolphins in the Mediterranean Sea. From July to November 1990, hundreds of dead animals were washed up along the western Mediterranean coasts of Spain, France, and northern Italy (Aguilar and Raga 1990, Aguilar and Raga 1993, Bompar et al. 1992). In summer 1992 a new outbreak of the epizootic began off southern Italy and spread into the eastern Mediterranean (Bortolotto et al. 1992, Webb 1991). In total, over I.,000 carcasses were recovered during 1990 and 199 1 (Aguilar and Raga 1993).

The total mortality resulting from the epizootic in 1990 and 1991 cannot be determined for two main reasons. First, the striped dolphin is a mainly offshore species, and it is believed that many animals which died sank before reaching the coast and were thus never recorded in the statistics. This is supported by the numerous reports of dolphin carcasses in an advanced state of decom- position found floating far from shore from Coast Guard helicopters, fishing boats, yachts, and especially dedicated cruises (Aguilar and Raga 1993). Second, adequate monitoring of diseased dolphins washed ashore was only carried out along the Spanish, French, and Italian coasts; information from Morocco, Algeria,


FORCAUA E?’ Al..: STRIPED DOLPHINS AFTER EPIZOOTIC OUTBREAK 139

Tunisia, and Malta is fragmentary or non-existent, although substantial mortality of striped dolphins is known to have occurred also in these areas (Aguilar and Raga 1993). It seems certain, therefore, that the total number of recorded deaths is only a fraction, probably small, of the true mortality attributable to the epizootic. Even if the total mortality were known, it would still be impossible to determine the effect of the epizootic on the population because no information is available on striped dolphin population size in the western Mediterranean prior to the outbreak.

As the outbreak progressed during 1990, several short cruises and an aerial survey were conducted to investigate the effects of the mortality in some of the affected areas (Aguilar et uf. 1991, Aguilar and Raga 1993). The surveys confirmed the presence of dead animals throughout the range surveyed, and also that changes had occurred in the size of striped dolphin schools. But data from these surveys were insufficient either to estimate population size or to determine changes in the population.

In summer 1991 a survey was conducted in the western Mediterranean with the specific aims of mapping the distribution of the species in the waters affected by the epizootic in 1990 and estimating total numbers in the area. In doing so, a baseline for future population studies was to be established, enabling changes to the western Mediterranean striped dolphin population to be monitored in future years. This paper presents the results of the survey.

Data Collection 1b’fETHODS

The survey was conducted from 3 August to 3 September, 199 1. The area surveyed included the waters of the western Mediterranean affected by the 1990 outbreak of the epizootic (Aguilar and Raga 1993) from the Straits of Gibraltar in the west to northwestern Italy and the islands of Corsica and Sardinia in the east. The Tyrrhenian Sea south of Corsica and the Silican Channel, which were apparently not affected by the 1990 outbreak, were not surveyed.

The area was stratified for logistical reasons into five subareas taking account of the zoogeography of the western Mediterranean. Cruise tracks were designed to provide representative samples of data in each subarea and the whole survey area. The tracks were organized in a systematic zigzag or sawtooth pattern for efficiency, with a random element introduced to determine the starting points (Hiby and Hammond 1989). The survey was conducted from the 46-m Green- peace ship M/V Sirius. The searching platform was situated 8 m above sea level and a crow’s nest 15 m above sea level was used occasionally for the confirmation of sightings. Cruising speed was a constant 10 knots (18.5 km/h).

Searching for cetaceans was carried out by groups of 3-4 scientific observers and was maintained throughout daylight hours. Observers used binoculars of various powers (8 x - 10 x ) to search except when weather conditions were poor. Geographical positions were determined by a GPS positioning system, accurate to within a few meters. Collection of data for the estimation of numbers using


140 MARINE MAMMAL SCIENCE, VOL. 10, NO. 2, 1974

line transect methods (Burnham et dl. 1980) was carried out following the protocols developed during the North Atlantic Sightings Survey of 1987 and 1989 (e.g., Sigurjonsson et al. 1989, Oien 1991).

The problem of rounding angles to sightings to convenient values (Hammond 1986) was avoided by using angle boards located at the observation platform. Observers were trained prior to the cruise to estimate radial distance to a sighting, by checking their distance estimates of a floating buoy equipped with a radar reflector, against accurate radar measurements. During the course of the cruise, experiments to reinforce this training were carried out. Spikes in the distribution of perpendicular distances are probably due to rounding errors in the estimation of larger radial distances.

All schools sighted were approached to confirm species composition and to estimate school size. This was especially important in the southwestern Medi- terranean, south of latitude 39”N, where striped and common (Delphinus delphis) dolphins, morphologically very similar, are both present, sometimes in the same school. All the large mixed schools encountered were composed of striped and common dolphins, both of them forming tight aggregations, but never including individuals of the other species. Sightings of schools from which the species composition could not be determined were not included in the analyses of density and school size.

To avoid a stop/start effect after the confirmation of each sighting (Kishino and Kasamatsu 1987, Hiby and Hammond 1989), the track line was rejoined by following a convergent course instead of returning to the exact point where the sighting had been made. Thus, potential repeated sightings of the same schools, which would bias density estimates upwards, were avoided. !Schools sighted while off-effort were not considered for the analyses of numbers and school size.

Analysis

Following Burnham et ul. (1987), the number of striped dolphins was estimated as:

nj(O)sA fi=----- 2L

with estimated variance

vE?r(ti) = fi2 [

v%(n)

n2

+ vZ[j(O)] + vECr(?)

/I?<O)” - 72 I

where n is number of schools sighted on effort; f(O) is the probability density function of perpendicular distances to detected objects, evaluated at zero; L is the distance searched; s is mean school size, and A is the size of the area surveyed.

It follows that density of schools is estimated as:


FORCADA ET AL.: STRIPED DOLPHINS AFTER EPIZOOTIC OUTBREAK 141

and density of striped dolphins estimated as:

6, = &,

Confidence limits were calculated assuming that fi is log-normally distributed. The 95% confidence interval is then estimated as:

(fi/C, Cti)

where

C= exp[1.96 log,jl + v)].

Estimates of numbers were calculated using the computer program DIS- TANCE (Laake et al. 199 1). The Fourier Series model was fitted to unsmeared perpendicular distances. The interval widths were chosen by the analysis program, and no truncation of the data was necessary to improve the fit of the model. The best model fit to the data was a Fourier Series with three terms, as determined by the Akaike Information Criterion (Akaike 1973) used by the analysis program. A bootstrap procedure (Efron 1979) was used to estimate the variance, standard error, and confidence intervals of density and number of striped dolphins. The number of successful bootstrap runs was 1,000. For the school-size analysis, detection of dolphins was considered a function of the school size, as many small schools were sighted at small perpendicular distances and few at larger perpendicular distances (Fig. 2). A robust estimate of school size was obtained computing a size-bias regression estimate (Buckland et al. 1993), in which school size was log transformed in order to reduce the influence of the few very large schools sighted.

The searching effort was stratified by sea state as measured by the Beaufort scale. Optimal sea-state conditions were considered to be of Beaufort between 0 and 3. There was some searching effort in sea states of Beaufort 4 and above, but these conditions were considered suboptimal and these data have not been included in analyses to estimate numbers.

Sizes of the surveyed areas, which were mostly delimited by coastline, were calculated by specifically designed computer routines. To investigate changes in striped dolphin school size, data were extracted from the University of Barcelona database for the years 1982-1990, as well as from previously published ref- erences for this species in this area (Raga et al. 1985, Gannier and Gannier 1989).

RESULTS AND DISCUSSION

Survey

The area surveyed was 593,660 km* in size. A total of 6,852 km was searched on effort (Fig. 3a) out of 8,036 km sailed during the cruise. Table 1 shows the number of sightings made and the mean school size of each small


142 MARINE MAMMAL SCIENCE. VOL. IO. NO. 2. 1994

350 -

w 250 N . 5 200

.

. . . .

50-1 . . . a . . . .

. l t

---

0 500 1000 1500 2000 2500 PERPENDICULAR DISTANCE (m)

Figme 2. Plot of school size xr. perpendicular distance for each striped dolphin school seen while searching under conditions of sea state Beaufort 3 or less.

cetacean species encountered. Striped dolphins were by far the cetacean most commonly sighted, and the present paper concentrates only on this species.

Distribution

As expected, striped dolphins were the most commonly observed cetacean species over the whole western Mediterranean. A preference for open waters beyond the continental shelf and for temperate waters were the common char- acteristics of the distribution (Fig. 36). In the southwest several mixed schools of common and striped dolphins were found. Schools of unidentified species were found throughout the whole area and not considered when mapping the presence of striped dolphins,

Two areas showed a particularly high density of striped dolphins: the ALboran Sea, especially in the far west towards the Straits of Gibraltar, and the waters east and north of Corsica including the Ligurian Sea (Fig. 36). These two regions are considered to be the most productive in the western Mediterranean (Fedj 1990, Jacques 1989) and provide a rich habitat for marine life. In contrast, the striped dolphin was relatively scarce to the east of the Alboran Sea, south of the Balearic Islands, and in the waters between the latter and Sardinia (Fig. 3b), presumably because of the reduced availability of adequate food resources.

Of the 126 sightings of striped dolphin schools observed, 6 were mixed schools with common dolphins. Of this number of sightings 119 were made


FORCADA ET AL.: STRIPED DOLPHINS AFTER EPIZOOTIC OUTBREAK

, , I 5W 0 5E 10E

-45 N

40N

35N

5’w 6 5’E IdE

Apre 3. (a) Cruise tracks searched during the survey. (b) Distribution of striped dolphin schools seen during the survey.



Table 1. Number of sightings of small cetaceans in the western Mediterranean.

Schools sighted School size (all schools)

Species composition Total On-effort Range Mean SE

Striped dolphin 126 119 l-300 19.7 3.5 Striped/common dolphins 6 6 15-300 71.3 43.3 Common dolphin 23 18 3-500 64.4 11.1 Bottlenose dolphin 9 8 1-12 7.2 1.5 Risso’s dolphin

: 7 2-18 7.2 1.9

Long-finned pilot whale 5 4-44 18.2 6.7

during searching effort under sea-state conditions of Beaufort 3 or less. Figure 4 shows the probability sighting distribution based on perpendicular distances for the 119 on-effort sightings of striped dolphins.

The size-bias regression estimate was significant (P < O.Ol>, and yielded an expected mean school size of 12.92 dolphins (SE = 1.669). The total number

1.10

0.83

:: ? z 2 0 0.55

f=

5 iz w” L 0.28 n

0.00 I - I I

1000 2000 3000

PERPENDICULAR DISTANCE IN METERS

Figure 4. Probabiality sighting distribution based on perpendicular distances to the 119 schools of striped dolphins sighted while searching under conditions of sea state Beaufort 3 or less, and the fitted Fourier series model.


FORCADA ET AL.: STRIPED DOLPHINS AFTER EPIZOOTIC OUTBREAK 145

Table 2. Estimates of the parameters used in the estimation of the number of striped dolphins in the area surveyed, with estimates of their standard errors, coefficients of variation, and 95% confidence intervals,

Parameter Point

estimate Standard 95 % confidence

error % cv interval

f(O) 0.001294 0.0000919 7.1 0.001126-0.001486 Effective search width b-4 Cl/f(O)Il 773.1 54.90 7.1 672.8-888.4

Encounter rate (schools.km-‘) [n/L] 0.02376 0.004568 19.2 0.02972-0.06516

Density of schools (schools.km-2) 0.01537 0.002799 18.2 0.01078-0.02189

Mean school size 12.92 1.669 12.9 10.04-16.62 Density of dolphins

(dolphins. km-z) 0.19857 0.06563 33.0 0.1151-0.3618 Number of dolphins 117,880 38,962 33.0 68,379-214,800

of striped dolphins in the western Mediterranean was estimated as 117,880 (SE = 38,962; 95% CI = 68,379-214,800) (Table 2).

It was not possible to evaluate the effect of any movement of schools in response to the survey platform. If such movement occurs before the school is detected, it can bias estimates of population density through changes in the observed distribution of perpendicular distances. However, we are confident that this was not a problem on the survey. In good weather conditions schools were frequently detected up to 2 km from the vessel. No evidence of reaction to the vessel was observed in these cases, and animals were seen to react only when the vessel approached very close to them (less than 200 m). In these cases some dolphins approached the vessel to bow-ride or to swim next to the wake. This supports our decision to discard data collected under poor weather conditions, when schools were detected at shorter distances and could have reacted to the vessel before being detected. (For Beaufort conditions of 4 or larger, the mean perpendicular distance for striped dolphin sightings was 64 m, while for Beaufort between 0 and 3, the mean was 482 m).

Presence of Calves

The survey was carried out immediately prior to and during the peak of calving for striped dolphins in the western Mediterranean (Aguilar 1991). The calves observed, therefore, were either about one year old or newborns. Overall, calves were present in 24.6% of the observed schools, but their presence was strongly dependent on the size of the school (Fig. 5). No calves were observed in schools smaller than 10 individuals, and the percentage of calves present increased with school size up to a maximum of about 50% in schools of more than 30 animals.

Previous studies of striped dolphin school composition in Japan (Miyazaki


146 MARINE MAMMAL SCIENCE, VOL. IO, NO. 2, 1994

l-10 II-20 21-30 31-40 41-50 ’ 50

SCHOOL SIZE

Figure 3. Percentage of sighted schools containing calves as a function of school size (IZ = 24).

1977, Miyazaki and Nishiwaki 1978) have shown that individuals of this species segregate into three basic types of school: (1) “immature’‘--composed of juveniles of both sexes; (2) “reproductive’‘--composed of sexually mature individuals, calves, and a few immatures; and (3) “mature’‘-composed only of adult dolphins. The size of the schools of adults, either reproductive or not, is substantially larger than that of those composed of juveniles. The results from our survey are consistent with this general pattern of segregation. The small schools, up to 10 animals, were probably composed only of juveniles whilst many of the larger schools, especially those of more than 30 individuals., were reproductive schools. Because of this pattern of segregation, a higher percentage of schools contained calves in areas where mean school size was large. Th.is was particularly true in the Alboran Sea where 46% of all schools seen contained calves.

Variation in School Size

The size of striped dolphin schools encountered on the survey ranged from 1 to 300 animals. In the Alboran Sea schools were larger (mean size = 71.7,


% SCHOOL SIZE BEFORE THE EPIZOOTIC OUTBREAK ---.---____

5 10 15 20 25 30 35 40 45 50 55 60 65 70 r70

SCHOOL SIZE

SCHOOL SIZE DURING THE EPIZOOTIC OUTBREAK %

60

40 -

20.

O- 7 ---,__ --7- ..-. r Jip-,--,--kllpL -.-- r _.-,_ --

5 10 15 20 25 30 35 40 45 50 55 60 65 70 ~70

SCHOOL SIZE

% SCHOOL SIZE AFTER THE EPIZOOTIC OUTBREAK

80

60 -

5 10 15 20 25 30 35 40 45 50 55 60 65 70 $70

SCHOOL SIZE

Figzue 6. Distribution of striped dolphin school sizes (a) before the epizootic outbreak (Unpublished data from the University of Barcelona database), (n = 85); (b) during the 1990 outbreak (Aguilar et ccl. 1791), (n = 27); and (c) during the survey (i.e., after the 1990 outbrak), (n = 111).



SE = 22.1, n = 15) than in other regions surveyed (mean size = 13.2, SE = 1.4, n = ill), a difference significant at the 0.001 level (t-test). Because of this difference and because all data on school size before and during the 1990 outbreak of the epizootic came from outside the Alboran Sea, survey data from this region have been omitted from the investigation of changes in school size.

Figure 6 shows the distribution of striped dolphin school sizes observed before, during, and after the 1990 outbreak of the epizootic in the western Mediter- ranean. Before the epizootic outbreak, mean school size was 2 5.3 (SE == 4.7); schools up to 20 animals constituted about 70% of the sample, and groups larger than 50 animals were common (13%) (Unpublished data from the University of Barcelona database). During the 1990 outbreak the mean school size decreased to 7.0 (SE = 2.3) (Aguilar et al. 1991), a difference significant at the .05 level (t-test). Seventy percent of the schools observed had five or fewer dolphins, and groups larger than 50 dolphins were rarely reported. (3%). During the survey mean school size had increased to 13.2 (SE = 1.4) (without Alboran Sea data, as indicated above), and larger schools (> 5 0 dolphins) formed about 10% of the sample.

One explanation for the decrease in mean school size during the 1990 outbreak of the epitootic may be that it was caused by the death of individuals within schools. Alternatively, schools may have split up into smaller groups. The return to larger schools in 199 1, following the outbreak, cannot be a recovery of the population, but can be viewed as a regrouping of individuals into larger schools.

It appears that adults suffered a greater mortality during the epizootic ou.tbreak than did juveniles (Calzada et al. 199 1). Reproductive schools of striped dolphins are composed of a large number of individuals, largely adults, undoubtedly to facilitate social activities, foraging, or reproduction (Miyazaki and Nishiwaki 1978). Therefore, the reduction in school size produced by the epizootic outbreak may have compromised, to some extent, the reproductive capacity of the population. Regrouping into larger schools may be a response of the surviving dolphins to overcome this difficulty. However, it is unclear what effect reduction and subsequent regrouping of schools may have had in terms of the population dynamics of this species, although the observation of calves in the summer following the outbreak would indicate that at least some reproductive activity existed,

ACKNOWLEDGMENTS

Thanks are due to all those who participated in the cruise and especially to the crew of the M/V Sirius. Marius Tresanchez helped in the preparation of the software routines. Giuseppe Notarbartolo di Sciara reviewed a draft version of this paper. Jeff Laake assisted in the understanding of aspects of the program DISTANCE, and Dave Botchers con- tributed with suggestions to the data analysis. Participation of University of Barcelona personnel was funded by the Spanish Institute for the Conservation of Nature (ICONA) and the Spanish Inter-ministry Commission for Science and Technology (CICYT), projects NAT90-1255-E and NAT91-112%C04-02. P. S. Hammond benefitted from a British Council Integrated Actions grant no. 290A. Michael Scott and an anonymous reviewer improved the final manuscript with their suggestions.


FORCADA ET AL.: STRIPED DOLPHINS AFTER EPIZOOTiC OUTBREAK 149

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Received: 1 February 1993 Accepted: 19 October 1993


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