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Grey Seal |
Classification and names
Marine mammal, order: Carnivora (predators), family: Phocidae
Other and local names:
Engels/English: Grey seal
Frans/Français: le phoque gris
Duits/Deutsch: Kegelrobbe, Grauer Seehund
Description
Seals are sea mammals and belong to the Carnivoria order of mammals. This seal
species exhibits pronounced sexual dimorphism. Male are dark, sometimes with
light patches and have an elongated snout with a wide heavy muzzle; along eastern
Atlantic coasts, at the beginning of the breeding season, the males are 195-250
cm long and weigh 170-350 kg. Females are light-coloured with dark spots, have
a length of 165-210 cm and a weight of 120-220 kg. At birth, pups are 95-105
cm long and weigh 11-20 kg. Pups are born with long-haired white fur, which
they moult two weeks after birth.
Distribution
Grey seals range over the North Atlantic coast. Roughly three stocks can be
identified: one in the West Atlantic, one in the East Atlantic and one in the
Baltic.
The range of the East Atlantic stock extends eastward from Iceland and the British
Isles to the White Sea, and northwards to northern Norway. There are large colonies
in Iceland, the Faeroes, Norway and Russia (Murman coast). Around the British
Isles, largest colonies occur off the NW coast of Scotland; small colonies occur
in France (down to Brittany) and in the Dutch and German Wadden Sea.
The range of the discrete stock in the Baltic includes the Gulfs of Bothnia,
Finland and Riga and extends south to the Gulf of Danzig, the Polish-German
border, and on the Swedish coast as far as Malmö. .
Habitat
Breeding sites of the East Atlantic populations can be exposed rocky coasts,
particularly on uninhabited islands. At several sites, especially on the islands,
seals show a preference for the grassy tops. Rocky shelves, shingle beaches,
and sandy beaches (e.g. Sable Island, the Wadden Sea) are used as well. The
largest colonies are found on sandy beaches, or on grass sites. In the Baltic
Grey seals breed on ice. However, they also use small islands when ice is absent.
In south-west Britain, Ireland and France, most pups are born in caves or on
beaches under steep cliffs. There, a high mortality among pups is observed during
storms. Intertidal flats in estuaries are used for hauling out, but not for
breeding.
Owing to the development of tracking technology, information on grey seal foraging
has been made available and shows a general dispersion from the breeding areas
to adjacent coastal waters. Satellite tracking studies in the U.K. and in the
Baltic revealed that Grey seals make transits between haul-out sites, which
are sometimes hundreds of kilometres apart. Offshore trips could be more than
50 km from shore, and could last several days.
The maximum recorded dive depth was more than 100 m, while the average maximum
dive depth was around 25 m. Most dives seem to reach close to the bottom, suggesting
benthic foraging .
Food
Grey seals feed on a wide variety of fish species, cephalopods and crustaceans.
A survey along the British east coast showed that the most important prey species
were sand eel (family Ammodytidae), Cod (Gadus morhua), Saithe
(Pollachius virens), Haddock (Melogrammus aeglefinus), whiting
(Merlangius merlangus), and several flatfish species, including Flounder
(Platichthys flesus). However, the diet varies significantly between
regions, between seasons and between years at individual sites.
Behaviour & Reproduction
Females become sexually mature at 3 to 5 years of age, males at 6, although
they may not be socially mature until they are 8 years of age or older. In the
Baltic, maturity can be attained at 3 years for both sexes, due to the reduced
population size. Gestation lasts 11 months, including 3 months of delayed implantation.
Females give birth on land or on ice to a single pup. The pup is suckled on
shore for about 17-18 days, increasing in weight by 1.5 to 2 kg per day. Females
do not feed during lactation, and either remain with their pup throughout, or
commute between the pup and the sea. Females come into oestrus towards the end
of the lactation period and are mated several times before they return to the
sea.
During the breeding season Grey seals can get together in large breeding groups
of many hundreds of animals. But in some parts of their range they remain in
small groups with as few as five animals; this phenomenon may depend on the
type of terrain. In the U.K., both sexes show fidelity to pupping sites.
Grey seals are polygynous. Adult males are present near the breeding site, and
come ashore for varying lengths of time. Depending on the terrain, there are
2-10 times as many females than males. Dominant males may prevent other males
from mating or exclude them from the colony.
Pup mortality is lowest in ice-breeding colonies or those on sandy beaches.
Longevity for females is 46 years, and 26 years for males.
The timing of the pupping season differs throughout the range of the species.
The East Atlantic population pups in the autumn, from September to December.
In SW Britain and in the Wadden Sea, this period extends to January. The Baltic
population pups on the ice in February and March.
Predation, competition and threats
There might be some dietary and spatial overlap with the Commmon seal (Phoca
vitulina). Aside from man, the only predators of Grey seals are Killer whales
(Orcinus orca) and large sharks.
Many of the fish species in the grey seals' diet are commercially exploited,
so in a number of areas fishermen have repeatedly proposed culling programs
to control seal numbers in order to secure overfishing.
In Iceland, Faroer, Norway, (and until recently also in Scotland and Denmark)
seals are being killed by fishermen, in order to prevent damage to set nets,
or as a consequence of conflicts over resources, or over the concern that the
Grey seal acts as a vector of the codworm (Pseudoterranova decipiens).
Usually fishermen and owners and employees of marine fish farms are permitted
to shoot seals which may be damaging gear or catches. The government of Iceland
encourages deliberate killing of seals through bounty schemes. It is surprising
that the culling of grey seals in the past has never proven to result in less
damage.
Incidental catch of grey seals is a significant threat in several areas (e.g.
Ireland, Baltic Sea), and can occur throughout their range.
The Baltic Sea is heavily polluted, and fertility rates of seals in the Baltic
are low. Grey seals show a high incidence of reproductive abnormalities, sterility,
and adrenocortical hyperplasia. The abnormalities could be the result of the
effect of either PCB or DDT or of their metabolites, on the adrenal glands of
the seals, as high levels of these organochlorines have been recorded.
All seals are potentially vulnerable to epizootics, such as that which resulted
in the death of large numbers of Common seals in the North Sea in 1988. The
Grey seal has hardly suffered from that virus outbreak.
In the Wadden Sea, seal populations can be easily disturbed by tourism, fishery,
and mining activities, although responses to human presence, underwater noise
and airborne noise are poorly known. The seal populations in the Baltic are
vulnerable to the effects of disturbance by ice-breaker activities, with a possible
impact on breeding success.
Protection
TheIn the light of the increasing numbers of the Atlantic populations, there
is no danger of extinction. Only the small Baltic population is vulnerable.
Grey seals enjoy protection by:
In addition, for the Baltic Sea populations only:
Population size
In 1991 the total size of the British and French population was estimated 95,400,
of which the majority (79,700) occur around the Orkneys, the Hebrides and the
Scottish mainland. The total number in the rest of the Northeast Atlantic was
estimated in around 22,600 (in 1990), including 275 in the Netherlands, 71 in
Germany and 25 in Kattegat-Skagerrak. The Baltic population was estimated at
5,000 in 1993.
Sources & further reading:
P.J.H. Reijnders et al., Status of Pinnipeds relevant to the European Union,
IBN
Scientific Contributions 8, Wageningen 1997.
The Atlas of European Mammals. T & D Poyser Natural History.