INTRODUCTION Download full-text
Aïssi, M.1*& Daly Yahia, M.N.1
1stBiennal Conference on Cetacean Conservation in South Mediterranean Countries October 12th– 14th2009 Tabarka - TUNISIA
Is the bay of Bizerte a school ground for juvenile bottlenose
dolphin (Tursiops truncatus) to learn hunting techniques?
(1)Department of Life Sciences, Faculty of Sciences of Bizerte, 7021, Zarzouna, Bizerte, Tunisia.
* firstname.lastname@example.org ; email@example.com
The bottlenose dolphin Tursiops truncatus (Montagu, 1821) is one of the most
worldwide abundant marine mammals. It has a cosmopolitan distribution,
widely frequent along coastal and continental shelf waters. However,
insufficient published information is available about the status of this free-
ranging species in Tunisian waters.
We propose therefore to create baseline information including the species
distribution and habitat range as first step in a long term project on the
cetacean diversity in the northern Tunisian waters. The occurrence, distribution,
group size and behaviour of bottlenose dolphins were assessed during an
autumnal short-term study conducted from September to October 2008 in the
bay of Bizerte (northern Tunisia).
MATERIALS AND METHODS
From September to November 2009, northern Tunisian coasts were monitored
for bottlenose dolphins. This area is limited to the continental shelf of the
Bizerte’s coast, connecting both Mediterranean sub-basins. It extends from
coastline to 37° 25’ N and from 009° 31’ E to 010° 16’ E, representing 1,424 Km2
Surveys were conducted aboard a 4 m long vessel cruising at maximum speed
of 20 Km h-1. This small observation platform had only one searching deck with
an eye height of 2.5 m above sea level. Transect lines did not follow a
systematic design but were defined according to a random design, and
sometimes to some indications supplied by fishing boats in the area.
Observations were made in favourable working weather conditions, defined as
calm sea, wind speeds never exceeding 3 on the Beaufort scale and the sky
not totally cloudy. Cetaceans were approached upon detection to estimate
their group size, and their behaviour.
The focal group was defined as “any group of dolphins observed in apparent
association, moving in the same direction and often, but not always, engaged
in the same activity” (Shane, 1990). Members of the focal group usually remained
within approximately 100 m of each other.
Behavioural sampling of animals encountered was based on a scan sampling
protocol in which the predominant group activity and other behavioural data
were collected for the first five minutes of an encounter (Mann, 1999).
Of the 1,000 km covered, a total of 12 sightings were recorded
corresponding at least to 75 bottlenose dolphins. All groups were
composed of juveniles that are more or less 1.5 m length, manifesting
feeding activity. This adopted behaviour using synchronic collaboration in
shallow waters displayed learning techniques for juveniles.
Fig.1. Geographical map of the study area with sighting positions of bottlenose dolphins.
Mann, J. (1999) Behavioural sampling of methods for cetaceans: a review and a critique. Marine Mammal Science, 15: 102-122.
In this study, dolphins were usually divided into small groups of five to six
individuals composed by a mixture of adults and juveniles. The number of
adults was always higher than that of calves throughout the study.
During hunting for shoal fish, dolphins surround the school in a circular form
and then dive in the middle of it to swallow the fish. During this process,
juveniles move exactly after adults. Sometimes adults slap fishes with their
flukes; launch them into the air in order for juveniles to hunt them easily.
Due to the lack of some facilities and equipment, we have not got the
opportunity to perform the photo-ID protocol, but based on visual sighting
with naked eyes, one group was recognized as seen a week ago using
the same feeding techniques.
Shane, S.H. (1990) Behaviour and ecology of bottlenose dolphin at Sanibel Island, Florida. In The bottlenose dolphin, (ed. Leatherwood, S. & Reeves, R.R.), pp 245-265. Academic Press, SanDiego, CA.
Tursiops truncatus are known to be affected by a number of anthropogenic
problems deriving from various human activities particularly severe in Tunisian
waters, including incidental captures in fishing nets and prey depletion. Here,
the conservation of Mediterranean populations of bottlenose dolphin has
received considerable attention.
This small area monitored during a short period pointed out a
probably school ground for juvenile bottlenose dolphins to learn
hunting techniques. This study must be developed to adopt a new
cetacean protection measurement in this area described as an
exposed zone to potentially toxic elements and characterized by
an intensive maritime traffic.