Rapp. Comm. int. Mer Médit., 36,2001
306
Introduction
The Ligurian Sea, the core of a future international Sanctuary (fig.1), repre-
sents an area densely populated by Cetaceans : Stenella coeruleoalbais the
most abundant species, with a density of population estimated at 0.43-0.56 indi-
viduals/km
2
(1, 2), that is,
in the figured study area
(
fig. 1), a total of about
16000 dolphins. Many field
o
b s
e r
vations concerning
this population are ava
i
l-
able, but the assessment of
the main population param-
eters in the area is still lack-
ing. In non-harvested popu-
lations, stranding is a pre-
cious source of information
and its use, in Italy, was
planned since the eighties.
Our aim is to describe some
biological parameters of the
striped dolphin in the future
Sanctuary on the basis of stranding data.
Materials and methods
Since the eighties the Centro Studi Cetacei (Società Italiana di Scienze
Naturali) established a network along the coasts of Italy to recovery stranded
carcasses of Cetaceans for study; from 1986 annual reports are available (3). In
the period 1986-1997, an average of about 65 S. coeruleoalbaper year have
been recorded in Italy and in the years 1990 and 1991 this figure raised to 106
and 329, due to a morbillivirus epizootic. In 12 years stranded dolphins were
1054; 908 measures of length without notation of uncertainty are available; 663
individuals were sexed and measured according to a more detailed protocol.
More than 200 individuals were collected in our study area. Length was mea-
sured in a straight line from the tip of the lower jaw to the caudal notch.
Frequency distributions of total length per sex were prepared for : 1) the total
obtained on Italian shores; 2) the individuals collected on the Ligurian coast.
Stranded specimens were classified as babies (up to 130cm TL), juveniles (131
- 179 cm TL) and adults (from 180cm TL onward).
Results and discussion
The Ligurian sample is composed of 197 individuals, 61 females, 91 males and
45 of unknown sex or imprecise measure (fig. 2a). This yields a male/female ratio
of 1.5, with a clear dominance of males. Considering the total stranded along the
Italian coast (N = 663), the sex ratio was 1.24. A more relevant mortality aff
e
c
t-
ing males was also recorded in the French Mediterranean (male/female ratio =
1.21 in the GECEM reports) and on the coasts of Valencia (Catalonia) Balearic
Islands (4) during the morbillivirus epizootic (1.13). Plotting the sex ratio aga
i
n
s
t
size, it was shown that males outnumber females from birth to the size of 210cm
TL, where females prevail, given their longevity (5).
Why in the Ligurian area the male mortality is so important remains
unknown. Probably when a danger occurs, males are more exposed, to protect
the weaker members of the group. The Ligurian sample include those years
(1986-1990) in which several instances of killing of marine mammals were
recorded in relation to the swordfish fishery by gillnets.
The size composition of the two groups of striped dolphins presented in fig.2
is remarkably similar.A main gaussian group occurs on the right side : these are
the adult individuals, with a mode, in the largest sample, at 190cm TL in
females and at 200 cm TL in males, a sexual dimorphism typical of those mam-
malian species in which the male has a defensive role. A small gaussian group
occurs on the left at 90cm TL : this point represents the neonatal mortality.A
second small mode can be appreciated at 150cm TL. According to the growth
models proposed for the Mediterranean, this size corresponds to age 2 (6,5), or
to age 3 (7). Therefore it could represent the end of the lactating period or sim-
ply the end of the link between the juvenile dolphin and the adults and their pro-
tection.
In the Ligurian sample adult females are represented with the same size
recorded along the total Italian shores. On the contrary the largest males (200cm
TL) are under-represented. The prevalence of young adult males in respect of
full grown individuals, suggest the possibility that the rich trophic area of the
Ligurian Sea is preferentially left to females and young individuals. In any case
the growth model proposed for Italian waters (5) appears to be acceptable for
the Ligurian Sea : it is
Males: L(t) = 200-[(200-90) e
(-0.375 t)
];
Females: L(t) = 190-[(190-90) e
(-0.430 t
)]
In the total measured stranded dolphins (N = 908), 171 individuals were
babies and 221 juveniles. The 19% of both these young individuals were found
in our study area. If the stranding events would have been evenly distributed the
Ligurian coast would have produced, in respect of the total Italian coasts, the
4.5% of stranding. Of course, the causes of such concentration can be of differ-
ent origin (oceanographical, biological or related to human activities, including
the intensity of monitoring the shore). A combination of causes can be also
hypothesised. Only during spring the Ligurian percentage of babies (Table 1) is
close to that expected. During summer it becomes three times more important,
suggesting that the Ligurian Sea is an important calving ground. Newborn
babies were collected from July to October, with a maximum during August, in
coincidence with the highest annual surface temperatures. The calving period
results therefore anticipated in respect to the Spanish coast, where the maximum
of births (95%) was located in October (8).
References
1.Notarbartolo di Sciara G., Forcada J., Acquarone M. and Fabbri F., 1993.
Population estimates of fin whales and striped dolphins summering in the Corso-
Ligurian Basin. European Research on Cetaceans, 7 : 135-138.
2
.Gannier A., 1998. Une estimation de l’abondance estivale du dauphin bleu et blanc
Stenella coeruleoalba(
M
eyen, 1833) dans le future Sanctuaire Marin International de
Mediterranée Nord –Occidentale. R
ev. Ecol. (Te
r
re Vi
e
), 53 : 255-272.
3.Centro Studi Cetacei, 1987-1998. Cetacei spiaggiati lungo le coste italiane. I-XII
Rendiconti 1986-1997. Atti Soc.ital.Sci.nat.Museo civ.Stor.nat.Milano, n°128-n°139.
4
.Calzada N., Lockyer C. and Aguilar A., 1994. Age and sex composition of the
striped dolphin die-off in the Western Mediterranean. M
a
r. Mamm. Sci., 10
: 299-310.
5
.Orsi Relini L., Cima C., Palandri G., Garibaldi F., Relini M. and Torchia G., 2000.
Population parameters of the Mediterranean striped dolphin (Stenella coeruleoalba)
d
e
r
ived by stranding data. European R
e
s
e
a
rch on Cetaceans, 14 : in press.
6.Calzada N., Aguilar A., Lockyer C. and Grau E., 1997. Patterns of growth and
physical maturity in the Western Mediterranean striped dolphin, Stenella
coeruleoalba(Cetacea : Odontoceti). Can. J. Zool., 75 : 632–637.
7.Monaci F. and Leonzio C., 1997. Elementi in tracce in esemplari di Stenella
coeruleoalbaspiaggiati sulle coste del Mediterraneo. S.It.E. Atti,18 : 453-546.
8.Aguilar A., 1991. Calving and early mortality in the Western Mediterranean
striped dolphin, Stenella coeruleoalba.Can. J. Zool., 69: 1408–1412
.
THE STRIPED DOLPHINS, STENELLA COERULEOALBA, OF THE LIGURIAN PELAGIC SANCTUARY:
MAIN BIOLOGICAL CHARACTERISTICS.
Orsi Relini L.*, Cima C., Palandri G., Garibaldi F.
Laboratori di Biologia Marina ed Ecologia Animale, DIP.TE.RIS., Università di Genova, Italy - largepel@unige.it
Abstract
Stranding data (1986-1997) were used to assess the population parameters of Stenella coeruleoalbain the Ligurian Sea, in comparison
with those regarding all Italian coasts. Sizes and growth characteristics resulted similar; male mortality appeared higher; density of
population, in particular of young individuals, was remarkable in the Ligurian Sea.
Keywords : Stenella coeruleoalba, stranding, Ligurian Sea, population parameters
Figure 2. Size/frequency distributions of striped dolphins stranded
along the Ligurian and Italian coasts
Figure 1. The Ligurian sanctuary and, in
the middle, the study area.
SPRINGSUMMERFALLWINTER
Total in Italy (N)
213406139160
Ligurian babies (%)
4.915.650.033.3
Ligurian juveniles (%)
14.110.136.038.9
Ligurian adults (%)
20.821.334.822.5
Table 1. Ligurian records of stranded S. coeruleoalbaas percentage of
total Italian records.
