Risso's Dolphin
Grampus
griseus
 
Key
Facts
Gaelic name:
Deilf-risso
Length: Max.
4 metres (males slightly longer than females)
Weight: Up
to 500 kg
Range: Temperate
and tropical seas worldwide
Threats:
Hunting, bycatch and pollution
Physical Description
The Risso's
dolphin is a large dolphin with a robust, stocky body that becomes more
slender behind the dorsal fin. The head is bulbous. Striking
is the tall dorsal fin, which is at the mid-point of the body.
Large animals
are typical for being silver-grey or cream-white, although the dorsal
fin, flukes and distal half of the flippers generally remain darker.
The body is covered with scratches and scars, which increase with age.
Life Span
Risso's dolphins
are believed to have a life span of at least 20 years. They are
probably preyed upon by killer whales, sharks and possibly false killer
whales (although no incidences of predation on Risso's dolphins have
actually been observed).
Distribution
Risso's dolphins
have a world-wide distribution in tropical and temperate seas, but do
not generally penetrate far into high latitudes.
Behaviour
Risso's dolphins
tend to form small to medium-sized pods of 2 to 45 animals, although
they may be seen singly or in large groups of several hundred or even
thousands.
Risso's dolphins
social behaviour haas been described by researchers as 'rough and obviously
physical'; behaviours included slapping, splashing and sometimes striking
one another. The high degree of scarrng on the skin of these dolphins
is though to be attributable to this intraspecific behaviour.
Risso's dolphins
are often seen travelling and surfacing slowly, although they may be
highly active, occasionally bow-riding and often breaching clear of
the water and slapping their heads, tails or sides on the surface.
Feeding
The diet
of Risso's dolphins consists of mainly cephalopods (especially squid),
crustaceans and occasionally small fish.
Current Situation
Risso's dolphins
are hunted in small numbers world-wide, including off Southern Europe,
off Sri Lanka, the Philippines, the Lesser Antilles, South America,
the US, Japan, Indonesia, the South Pacific and China. They are
also widely caught incidentally in fishing nets, and have been killed
deliberately in Japan to try to reduce competition with fisheries.
Crude estimates of population size and catches in Sri Lanka indicate
that the exploitation rate there is probably unsustainable.
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