Chemosphere 54 (2004) 669–677

www.elsevier.com/locate/chemosphere

Levels of organochlorine compounds in spotted dolphinsfrom the Coiba archipelago, Panama

A. Borrell *, G. Cantos, T. Pastor, A. Aguilar

Department of Animal Biology, Parc Cient�ııfic de Barcelona, GRUMM, University of Barcelona, c/Baldiri i Reixac 4-6,

Barcelona 08071, Spain

Received 26 June 2002; received in revised form 28 July 2003; accepted 3 September 2003

Abstract

Blubber and skin samples from 63 spotted dolphins (Stenella attenuata) (18 males, 40 females and 5 of unknown sex)

were collected by biopsy techniques in the waters of the Coiba archipelago. Blubber was analyzed for organochlorine

compounds and skin for gender determination. Mean levels of HCB (hexachlorobenzene), tPCB (polychlorinated bi-

phenyls) and tDDT (dichlorodiphenyltrichloroethane) were 0.064, 2.30 and 6.4 mg kg�1, respectively. These levels are

low and are not considered to represent a threat to the S. attenuata population. No significant differences either in

concentrations of HCB, tPCB and tDDT or in PCB profiles were observed between males and females. The ratio tDDT/

tPCB was 2.69, indicating predominantly agrarian versus industrial activities in the area. The ratio ppDDE/tDDT was

0.83, a high figure that suggests both a local reduction of DDT inputs and a high rate of DDT degradation. Significant

quantitative and qualitative differences were observed between two schools, suggesting intra-population heterogeneity

in organochlorine exposure possibly due to demographic segregation.

� 2003 Elsevier Ltd. All rights reserved.

Keywords: Stenella attenuata; Central America; Pollution; PCB; DDT

1. Introduction

The Coiba archipelago off the western Pacific coast of

the Republic of Panama, together with its adjacent wa-

ters, is a National Park established since 1991 (Fig. 1).

The park hosts a rich cetacean fauna, among which the

spotted dolphin (Stenella attenuata) is the most abun-

dant species. It distributes throughout the whole archi-

pelago occupying mostly coastal waters (Aguilar et al.,

1997).

Chemical pollutants have been identified in recent

years as a major threat for certain populations of marine

mammals (Reijnders and Aguilar, 2002). In the last

three decades, the pollutants of the organochlorine

*Corresponding author. Tel.: +34-93-403-4555; fax: +34-93-

403-4554.

E-mail address: assump@bio.ub.es (A. Borrell).

0045-6535/$ - see front matter � 2003 Elsevier Ltd. All rights reserv

doi:10.1016/j.chemosphere.2003.09.011

family, all of anthropogenic origin, have become ubiq-

uitous in natural environments. These compounds had

been widely used since the mid-1940s as agricultural

pesticides (DDTs and HCB) and in a variety of indus-

trial applications (PCBs). In the 1970s, the production

and commercialization of both groups of compounds

were restricted in many countries (Edwards, 1973;

Hayes, 1975), and since then their release into the en-

vironment has decreased markedly (de Voogt and

Brinkman, 1989; Voldner and Yi-Fan, 1995; Breivik

et al., 2002). However, some compounds, particularly

pesticides, are still used in many developing countries,

including those of Central America (Borrell and Aguilar,

1999; UNEP, 1999).

Organochlorines are very lipophilic and tend to

concentrate through food webs. Dolphins are particu-

larly prone to accumulating these compounds because of

at least three reasons: (i) they are top predators, (ii) they

present a large layer of hypodermic fat, or blubber, that

ed.

Fig. 1. Location of sampling of spotted dolphins schools.

670 A. Borrell et al. / Chemosphere 54 (2004) 669–677

facilitates organochlorine storage, and (iii) they are

small-sized compared to other cetaceans and thus have a

relatively high metabolic rate (Aguilar et al., 1999). In

mammals, organochlorines have been reported to cause

immune depression (Lahvis et al., 1995; de Swart et al.,

1996; Luebke et al., 1997; Busbee et al., 1999; Ross et al.,

2000b) and the subsequent triggering of infectious dis-

eases (Aguilar and Borrell, 1994; Simmonds and Mayer,

1997; Jepson et al., 1999; Van Loveren et al., 2000),

reproductive failure (Reijnders, 1986; Addison, 1989;

Baker, 1989; Helle et al., 1990; Reijnders and Brasseur,

1992; Beland et al., 1993; Brunstrom et al., 2001;

Schwacke et al., 2002), abnormal growth and decreased

calcium deposition in bone (Zakharov and Yablokov,

1990; Mortensen et al., 1992; Zakharov et al., 1997),

induction of cancer (Martineau et al., 2002) and a

number of other physiological disorders (Olsson et al.,

1994).

Borrell and Aguilar (1999) reviewed published refer-

ences on organochlorine tissue levels in cetaceans from

Central and South America and found no information

relative to the inshore Pacific waters of Panama. The

geographically closer data are those from the surveys

carried out by O’Shea et al. (1980) on striped dolphins

(Stenella coeruleoalba) and Fraser’s dolphins (Lageno-

delphis hosei), collected in the period 1973–1976, and by

Prudente et al. (1997) on spinner dolphins (Stenella

longirostris) collected in the period 1980–1982. Speci-

mens for both studies were obtained in the offshore

waters of the Eastern Tropical Pacific and thus from a

relatively distant area. This information is therefore not

useful to assess current organochlorine levels in the ce-

tacean fauna of the coastal waters of the Coiba archi-

pelago.

Industrial activities are generally low in the Pacific

coast of Panama, so no significant local organochlorine

inputs from this source are expected. However, agri-

culture is more active and is likely to release pesticides

into the environment. Besides, potential input of or-

ganochlorines from distant regions carried by water

currents or atmospheric transport is also feasible. The

spotted dolphin is an active predator that feeds on

fish, cephalopods and crustaceans (Perrin and Hohn,

1994) and can therefore be used as a bioindicator to

assess the magnitude of the organochlorine pollu-

tion present in the area. On the other hand, tissue

levels are also indicative of the potential impact of

organochlorines on the local spotted dolphin popula-

tions.

This study presents concentrations of organochlorine

compounds (PCBs, DDTs, and HCB) found in the

blubber of spotted dolphins from the Coiba archipelago

and discusses their potential origin and sources of vari-

ation.

A. Borrell et al. / Chemosphere 54 (2004) 669–677 671

2. Methods

Blubber and skin samples were collected in the waters

of the Coiba National Park (Fig. 1) from the dorsal

region of bow ridding spotted dolphins using a biopsy

dart of the butterfly valve type, as designed by Aguilar

and Nadal (1984). This sampling method is non-

destructive and generally considered not to be long-term

harmful for the involved individuals (Aguilar and Bor-

rell, 1994). The dart, with a 1 cm diameter tip adjusted to

penetrate about 2 cm, was fired with a spear gun at a

distance of about 3–5 m from the animals, thus ren-

dering a tissue mass of about 0.5 g. In the spotted dol-

phin, the blubber of the dorsal region is relatively thin

(usually <1 cm), so the excised biopsy contained a

complete representation of all blubber layers. This al-

lowed to combine all layers into a single analytical

sample that provided a composite average of the whole

blubber thickness. Thus, any bias due to stratification of

pollutants or lipid composition, as that described to

occur in large whales (Aguilar and Borrell, 1991) or,

moderately, in harbour porpoises, Phocoena phocoena

(Koopman et al., 1996), was avoided.

Samples were collected using a small speed boat

during a six months period, extending from December

1996 to June 1997, from 63 spotted dolphins belonging

to six different schools (Table 1). Calves were never

targeted for biopsy extraction, so all collected samples

came from juveniles or adults. The excised tissues were

preserved in deep freeze until the analysis.

Sex was determined in the skin portion of the sample

by PCR amplification of ZFX and ZFY, two specific

DNA regions of the sex chromosomes with slight dif-

ferences in their nucleotide sequence (B�eerub�ee and Pals-

bøll, 1996). The DNA was extracted following Valsecchi

(1998).

For organochlorine analysis, the blubber part of the

biopsy (�0.4 g) was ground with anhydrous sodium

sulphate and extracted for 5 h with hexane (residue-free

quality) in a Soxhlet apparatus. The extract obtained

was concentrated to 40 ml. A portion (10 ml) of this

Table 1

Characteristics of the sampled schools and biopsies obtained from th

School Date Position n individ

approxim

1996 1 16/12/96 7�150N; 81�510W 300

1997 2 16/05/97 7�530N; 81�470W 15

3 20/05/97 7�300N; 81�550W 250

4 23/05/97 7�320N; 81�550W 150

5 30/05/97 7�350N; 82�010W 600

6 08/06/97 7�250N; 81�580W 500

extract was used to determine the quantity of extractable

fat per gram of blubber. A further quantity was mixed

with sulphuric acid for the clean-up. The resulting ex-

tract was concentrated to 1 ml and centrifuged for 5 min.

Chromatographic analysis was carried out on a

Hewlett-Packard 5890-II gas chromatograph equipped

with an electron capture detector set at 350 �C. A fused

silica capillary column (length 60 m, 0.25 mm ID) coated

with SPB-1 was used as the stationary phase (0.25 mm

film thickness). A splitless technique was used to inject 1

ll of the purified extract. Temperature was programmed

as follows: injection at 40 �C for 1 min and increased to

170 �C at a rate of 25 �Cmin�1; 1 min constant; increase

to 250 �C at a rate of 2 �Cmin�1, and then to 280 �C at

5 �Cmin�1.

A preliminary screening of the samples revealed that

heptachlor was not present in the tissues analyzed.

Therefore, this compound (0.1 lgml�1) was used as an

internal standard. The samples were analyzed for the

following compounds: HCB, DDTs and PCBs. Blanks

of pure n-hexane were periodically run to ensure the

purity of the system. Recoveries of organochlorine com-

pounds ranged 82–101% (n ¼ 12). The detection limit

was 1 lg kg�1 wet weight. The laboratory participated in

interlaboratory calibration exercises for organochlorine

compounds in biota, organized by Quasimeme (1998)

and NIST/NOAA (2000), and obtained satisfactory

results.

The total PCB concentration (tPCB) was calculated

as the sum of 20 individual peaks (IUPAC# 95, 101,

110, 128+183, 138, 141, 144+135, 146, 149, 151, 153,

170, 171+202, 174, 177, 180, 187, 194, 195, 196+203).

Congeners were quantified by their weight percentage in

Aroclor 1260 (Safe et al., 1987) using a standard of this

PCB mixture. From these data, we calculated the con-

tribution (measured as %) of individual PCB congeners

to the concentration of tPCB.

The total DDT concentration (tDDT) was calcu-

lated as the sum of the five DDT compounds [p,p0-DDE

and o,p0-DDE (dichlorodiphenyldichloroethylenes), p,p0-

TDE (dichlorodiphenyldichloroethane), o,p0-DDT and

em

uals

ation

Biopsies

Males Females Unknown Total

3 1 2 6

1 0 0 1

7 5 2 14

1 1 0 2

2 11 1 14

4 22 0 26

18 40 5 63

0

2

4

6

8

10

HCB tDDT PCBs DDE/tDDT tDDT/tPCB Lipid c.

Con

cent

ratio

n(m

g/K

g)or

ratio males n=17

females n=38both n=60

Fig. 2. Mean and standard deviation of organochlorine con-

centrations, grouped by molecular structural types, their asso-

ciated ratios and the lipid content of blubber split by sex and for

the sample as a whole. Concentrations are expressed as mgkg�1

on a lipid basis.

672 A. Borrell et al. / Chemosphere 54 (2004) 669–677

p,p0-DDT (dichlorodiphenyltrichloroethanes)]. Pollutant

concentrations were calculated on the basis of the weight

of extracted lipids (mg kg�1 lipid). This latter variable is

also given to allow recalculation of concentrations on a

fresh weight basis, which is needed for the comparison

with some of the previously investigated cetacean pop-

ulations from the same region.

Means and standard deviations about the mean were

calculated for concentrations of analytes (and relative

proportions of analytes) for each school of dolphins

separately. Data were tested for normality using a

Kolmogorov–Smirnov test of goodness of fit. As the

data distributed normally, we examined differences in

lipid content, organochlorine compounds, ppDDE/

tDDT, tDDT/tPCB and congener/tPCB ratios between

sexes through Student’s t-test. Differences between

schools were established by means of analyses of vari-

ance (ANOVA) and Tukey’s tests for group compari-

sons.

To test the significance of multivariate differences in

PCB patterns between schools 5 and 6 (the only schools

with large enough sample sizes), we used stepwise dis-

criminant analysis. A jackknifed calculation system was

used to determine the predictive power of discriminant

functions. This involved leaving out each of the cases in

turn, calculating the functions based on the remaining

n� 1 cases, and then classifying the left-out case. For

this analysis, congener concentrations were normalized

by dividing them by the tPCB concentration of the

sample to avoid the effect of variation in concentrations.

Data were also log-transformed to equal population

covariance matrices. All statistical tests were carried out

using the SPSS 9.0 statistical package.

3. Results

No significant differences were found between males

and females (Fig. 2). This was so both when these

variables were analyzed separately for each school and

when all individuals were pooled and treated as a single

group. Therefore, data from both genders were pooled

and treated as a single subset in subsequent tests.

Table 2 depicts the organochlorine concentrations

detailed for each compound and their associated ratios,

split by school. The ANOVA indicated that dolphin

schools presented significant (p < 0:05) differences

among them in pollutant concentrations. According to

the Tukey test, concentrations of all DDT forms, tPCB

and the tDDT/tPCB ratio were significantly higher

(p < 0:05 for all compounds) in school 5 than in school

6. School 3, whose mean levels of contaminants was

intermediate to those of schools 5 and 6, did not show

significant differences with neither of them. We did not

test potential differences between the other groups be-

cause of their reduced sample size.

The stepwise discriminant analysis was performed on

20 predictive variables (logarithm of the relative abun-

dance of the various congeners in relation to the tPCB)

and data collected only from schools 5 and 6, which

were those with a sample size large enough to be con-

sidered as representative. The profile of the PCBs, that

is, the relative contribution of the various PCB cong-

eners to the total PCB load showed, according to the

stepwise discriminant analysis, significant differences

between these schools. Fifteen variables were excluded

by the stepwise procedure. Using the remaining five

variables, corresponding to the congeners: 144+135,

153, 95, 170 and 149, the analysis was able to correctly

classified 94.4% of the 36 samples (Fig. 3).

4. Discussion

Males and females were not found to exhibit signifi-

cant differences either in organochlorine concentrations

or in related ratios. In the males of most mammals,

organochlorine tissue concentrations increase with age.

The same pattern is observed in females during the ju-

venile stage, but levels stabilize or decrease after reach-

ing sexual maturity due to pollutant transfer through

gestation and lactation (Aguilar et al., 1999). As a

consequence, tissue concentrations in adult females are

generally lower than those of adult males (Aguilar et al.,

1999). The absence of sex-related differences found in

our study is difficult to explain, but may reflect a biased

representation of age-classes. Thus, if sampled males

were predominantly juveniles, their organochlorine

concentrations would be expected to be similar to those

of mature females. However, lack of age information

from the individuals analyzed does not permit testing

this assumption.

Table 2

Mean and standard deviation of the percentage of lipid extraction, organochlorine compound concentration and associated ratios, split

by school. Concentrations are expressed as mgkg�1 on a lipid basis

School

1 (n ¼ 5) 2 (n ¼ 1) 3 (n ¼ 14) 4 (n ¼ 2) 5 (n ¼ 14) 6 (n ¼ 24)

Mean Std Mean Std Mean Std Mean Std Mean Std Mean Std

% Lipid

content

33.79 8.93 36.73 – 32.68 7.65 39.21 1.56 34.31 10.66 39.58 10.45

Pesticides

HCB 0.06 0.02 0.08 – 0.06 0.03 0.05 0.01 0.07 0.11 0.06 0.04

ppDDE 4.43 2.44 1.74 – 5.88 4.33 4.62 0.08 3.92 2.41 6.49 2.86

opDDE 0.08 0.03 0.04 – 0.10 0.04 0.08 0.03 0.07 0.04 0.11 0.04

ppTDE 0.19 0.06 0.18 – 0.23 0.14 0.18 0.07 0.14 0.04 0.23 0.10

opDDT 0.28 0.13 0.17 – 0.32 0.23 0.33 0.07 0.16 0.06 0.32 0.14

ppDDT 0.34 0.16 0.40 – 0.43 0.30 0.31 0.08 0.28 0.10 0.46 0.23

tDDT 5.31 2.81 2.52 – 6.95 4.86 5.52 0.17 4.56 2.53 7.61 3.01

%DDE/

tDDT

82.65 2.90 68.74 – 82.20 7.78 83.72 4.05 83.48 8.08 84.02 7.49

PCB congeners (IUPAC#)

95 0.073 0.042 0.080 – 0.072 0.063 0.075 0.031 0.046 0.039 0.055 0.045

101 0.055 0.015 0.055 – 0.066 0.032 0.065 0.010 0.059 0.034 0.055 0.017

110 0.041 0.009 0.038 – 0.048 0.029 0.041 0.002 0.044 0.037 0.041 0.018

128+183 0.075 0.036 0.048 – 0.079 0.047 0.098 0.028 0.056 0.023 0.077 0.027

138 0.213 0.101 0.158 – 0.229 0.129 0.284 0.069 0.179 0.064 0.239 0.080

141 0.026 0.009 0.035 – 0.025 0.010 0.019 0.013 0.018 0.007 0.023 0.011

144+135 0.057 0.019 0.043 – 0.046 0.023 0.060 0.005 0.033 0.014 0.047 0.014

146 0.039 0.013 0.034 – 0.043 0.028 0.049 0.006 0.026 0.009 0.047 0.018

149 0.120 0.052 0.107 – 0.116 0.059 0.148 0.026 0.091 0.030 0.120 0.037

151 0.086 0.033 0.061 – 0.057 0.035 0.071 0.035 0.050 0.027 0.065 0.029

153 0.442 0.205 0.355 – 0.529 0.312 0.637 0.170 0.392 0.133 0.503 0.177

170 0.157 0.119 0.085 – 0.178 0.093 0.233 0.055 0.115 0.044 0.170 0.065

171+202 0.033 0.012 0.017 – 0.039 0.017 0.043 0.005 0.022 0.007 0.030 0.010

174 0.104 0.099 0.038 – 0.058 0.032 0.068 0.017 0.040 0.012 0.065 0.037

177 0.031 0.018 0.016 – 0.031 0.015 0.040 0.002 0.021 0.007 0.031 0.010

180 0.258 0.154 0.174 – 0.310 0.190 0.392 0.095 0.223 0.082 0.316 0.124

187 0.145 0.079 0.105 – 0.169 0.102 0.202 0.045 0.126 0.047 0.176 0.062

194 0.043 0.028 0.024 – 0.059 0.029 0.060 0.007 0.050 0.023 0.055 0.027

195 0.115 0.057 0.339 – 0.136 0.158 0.049 0.013 0.143 0.064 0.271 0.153

196+203 0.124 0.053 0.733 – 0.152 0.061 0.167 0.038 0.134 0.039 0.148 0.063

tPCB 2.18 0.88 2.54 – 2.37 1.24 2.80 0.58 1.80 0.61 2.53 0.86

tDD/tPCB 2.32 0.43 0.99 – 2.68 0.79 2.02 0.48 2.44 0.56 3.05 0.70

012345678

-5 -4 -3 -2 -1 0 1 2 3Discriminant function

Freq

uenc

y

school 5 school 6

Fig. 3. Discriminant analysis plot of the variation of PCB

patterns (log PCB congeners/tPCB) in schools 5 and 6.

A. Borrell et al. / Chemosphere 54 (2004) 669–677 673

Information available on pollutant levels in spotted

dolphins worldwide is very scarce and is limited to only

two surveys. Cockcroft and Ross (1991) reported orga-

nochlorine levels in four spotted dolphins (1 female and

3 males) from the eastern coast of southern Africa. PCBs

ranged 4.77–48.3 mgkg�1 wet weight, and DDTs 7.59–

65.2. These levels are much higher than those found in

our sample. Conversely, Kemper et al. (1994) analyzed a

single individual from Australian waters and reported

very low levels (PCBs¼ 0.82 mgkg�1, DDTs¼ 1.19

mgkg�1 and HCB¼ 0.009 mg kg�1 wet weight) as

compared to the results of our survey. However, no

conclusions on geographical or time differences can be

drawn because of the small sample size of these surveys,

674 A. Borrell et al. / Chemosphere 54 (2004) 669–677

which prevent taking into account the biological traits of

the individuals sampled (sex and age). Overall, the or-

ganochlorine concentrations found in the blubber of

the spotted dolphins from Coiba are quite low, and

well below the thresholds usually associated, in other

odontocetes, with pathological alterations (Martineau

et al., 1987; Aguilar and Borrell, 1994; de Guise et al.,

1995). Therefore, although subtle physiological altera-

tions cannot be discarded, the observed levels are not

considered able to bear adverse consequences for the

persistence of the population.

The levels found in this study fall within the lower

fringe of the range commonly detected in cetaceans of

comparable body size, diet and reproductive biology

from other regions (Aguilar et al., 1999), but they are in

the same range as those found by Prudente et al. (1997)

in spinner dolphins from the central offshore waters of

the Eastern Tropical Pacific (tDDT¼ 1.2 mgkg�1 and

PCB¼ 0.8 mg kg�1). Conversely, O’Shea et al. (1980)

found remarkably higher tDDT concentrations in

striped dolphins (45.46 mg kg�1) and Fraser’s dolphins

(11.02 mg kg�1) from the offshore Eastern Tropical Pa-

cific, but only slightly higher PCB concentrations (3.96

and 5.2 mg kg�1 respectively). These differences are

likely to reflect the time trends followed by the orga-

nochlorine compounds in the region. In the period

1973–1976, when the samples analyzed by O’Shea et al.

(1980) were collected, DDT was still in use and the

western coast of the United States was a main source of

environmental pollution of this compound (Edwards,

1973). PCBs were also in use, although their release was

comparatively less significant. In the period 1980–1982,

when the dolphins examined by Prudente et al. (1997)

were collected, the use of both groups of organochlo-

rines had long been banned (Hayes, 1975; de Voogt

and Brinkman, 1989) and their environmental re-

leases discontinued. Despite the potential influence of

interspecific or small-scale geographical differences, the

decrease in tissue concentrations observed between

the pioneering work of O’Shea et al. (1980) and the

present survey and that by Prudente et al. (1997) un-

doubtedly reflects relocation and degradation of the

early organochlorine inputs. This suggests that, despite

indications that DDT is still commercially used in

the region (UNEP, 1999), such use is likely to be re-

duced and of limited significance to the marine envi-

ronment.

In our survey, HCB was always been found at very

low concentrations. Worldwide inputs of HCB in the

environment have been traditionally small. Moreover,

this compound is more degradable and less persistent

than DDTs and PCBs, particularly in warm environ-

ments, where its decomposition rates are higher (Cala-

mari et al., 1991). The combination of these factors

undoubtedly explain the low blubber concentrations

observed in the spotted dolphins, which are consistent

with previous findings in other marine mammals (Borrell

and Aguilar, 1999).

The mean tDDT/tPCB ratio observed indicates the

predominance around Coiba of agricultural contami-

nation over that of industrial origin. The ratio is rela-

tively high when compared to those commonly found in

marine mammals worldwide (Borrell and Aguilar, 1999),

but it is comparable to the few values that have been

determined in similar studies for the region.

On the other hand, the ratio ppDDE/tDDT is sub-

stantially higher than that found by O’Shea et al. (1980)

in striped and Fraser’s dolphins from the offshore waters

of the Eastern tropical Pacific, but lower than that found

by Prudente et al. (1997) in spinner dolphins from this

region. Overall, the observed value is considered high

as compared to other populations of marine mammals

(Addison et al., 1984; Aguilar, 1984; Aguilar et al.,

2002). Because DDE is the main product of the meta-

bolisation of commercial DDT, the ppDDE/tDDT ratio

reflects the aging of the environmental load of this pes-

ticide once releases are discontinued. In this study, the

high ratio observed reflects the reduction in use of DDT

compounds that has taken place in the region since 1971

due mainly to the restrictions imposed in the US (Ed-

wards, 1973).

Two of the schools sampled extensively (#5 and #6)

showed significant differences in PCB and DDT con-

centrations and in PCB congener profiles, apparently

indicating that the source of PCB and DDT pollution at

which these two schools are exposed is different. Because

pollutants are incorporated via food, such difference can

only be explained through dissimilar diet composition or

habitat. At least two situations may explain such dis-

similarities: (i) the two schools may share the same

habitat but their composition with regard to age or re-

productive condition is different and they therefore have

specific feeding behaviours or uses of the habitat (Ber-

nard and Hohn, 1989), and (ii) the two schools may

normally have allopatric distributions which would im-

ply that they are exploiting separate food resources.

Whatever the case, the results strongly suggest that the

two schools are segregated, either spatially or behavio-

urally, at least on a short- or medium-term scale. In-

vestigation into the geographical location where schools

were sampled (Fig. 1) and the date when biopsies were

collected (Table 1) does not clarify the question because

the two schools were sampled at a distance of only 20

miles and separated by a period of 9 days.

Heterogeneity in pollutant profiles between schools

apparently pertaining to the same population and shar-

ing the same habitat are not a new finding in cetaceans.

It has also been observed in long-finned pilot whales,

Globicephala melas (Aguilar et al., 1993; Caurant et al.,

1993), a species that constitutes highly stable extended

families that do not tend to disperse and thus differen-

tiate genetically; such differences do not appear to be

A. Borrell et al. / Chemosphere 54 (2004) 669–677 675

related to geographical distribution, but reflect stable

demographic fragmentation (Amos et al., 1993; Ander-

sen, 1993). A similar social organisation has been pro-

posed for other species of small cetaceans, particularly

killer whales, Orcinus orca (Bigg et al., 1990), where

intercommunity differences in pollutant profiles have

also been reported (Ross et al., 2000a), but no specific

information on this regard is available on spotted dol-

phins. Further research is needed to clarify the demo-

graphic structure and segregation patterns responsible

for the observed interschool heterogeneities in pollutant

profiles in this species.

Acknowledgements

The authors are grateful to Jaume Forcada, Enric

Badosa, Alex Monn�aa and Manel Gazo, who assisted in

the collection of the dolphin biopsies, and to C�eesar,Flores and Mali for their enthusiasm and dedication

during the fieldwork. Santiago Castroviejo and Javier

Hergueta provided invaluable support. We also thank

Javier Pav�oon, Marco Antonio Nieto, Benjam�ıın Pimentel

and the rest of the AECI and Estaci�oon Biol�oogica de

Coiba staff for the logistic support. This study was made

possible through funding granted by the Comisi�oon In-

terministerial de Ciencia y Tecnolog�ıı aCICYT (project

AMB 99-0640) and the Agencia Espa~nnola de Coope-

raci�oon Internacional (AECI). Samples for this study

were supplied by the Banco Medioambiental de Tejidos

Biol�oogicos (BMA), with the support of the Pew Fellows

Program in Marine Conservation and Earthtrust.

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