The sea mammals are members of either the Cetaceans (Whales, Dolphins and Porpoises) or the Pinnipeds (Seals).
Over 20 species of Cetaceans are known to be present in our waters. These are mammals that have moved back from land to a totally aquatic habitat. Like the land mammals, they are air breathing animals. Cetaceans are extremely well adapted to life in the water. They are able to hold their breath under water, but they have to surface regularly to breathe in fresh air. They have special nostrils on the top of their heads called "blowholes'. Unlike other mammals, they do not have thick coats of fur to keep them warm as these would slow them down when swimming. Instead they have a thick layer of blubber (fat) underneath their skin. They also have flippers and fins (which are better for swimming) than legs and feet. Some whales can hold their breath for two hours or more and can dive to very great depths. Sperm Whales can dive nearly two miles below the surface in search of giant squid in the murky depths. When a whale breathes out, its "blow" or "spout" (water vapour) is clearly visible. The blows vary in size and shape and it is sometimes possible to identify a species of whale just by looking at its blow. Whales sometimes launch themselves out of the water into the air and then crash down into the sea - often many times in a row. No one really knows why they do it, but it is spectacular to watch. It could be a type of communication or signalling, or to remove parasites from their skin, or perhaps it is simply for fun! Some dolphins are particularly well known for their acrobatics at sea. They also like to body surf in the bow waves of fast boats.
The Cetaceans fall into two main groups; the Mysticeti, (the baleen whales) and the Odontoceti (the toothed whales).
The baleen whales have plates made of keratin instead of teeth. These plates hang down from the upper jaw and act like a giant sieve, straining small animals and plants from the seawater. All the bigger whales belong to this group. They eat tiny shrimp like creatures called krill or shoaling fish.
The toothed whales have lots of teeth which they use for grabbing fish and squid. They do not chew their food, but swallow it whole. Dolphins and porpoises belong to this group and are really small whales.
Most baleen whales make very long journeys between their feeding and breeding grounds every year. For example, humpback whales spend the summer months getting fat on the rich feeding grounds of Alaska, but they must move to much warmer areas, like the waters around Hawaii to give birth to their young. The baby whales (calves) would not survive in the cold Arctic waters but there is no food in the warm tropical seas, so the whales have no choice but to migrate between the two every year.
Most whales live in family groups called "pods". Whales and dolphins communicate with one another using sound. The Humpback Whale's song is the longest and most complex in the animal kingdom. It is only the males that sing to woo the females during the mating season. It is possible to hear these songs using underwater microphones called hydrophones. The cetaceans found most commonly in Irish waters include: Humpback whale, Fin Whale, Sei Whale, Blue Whale, Northern Right Whale, Minke Whale, Sperm whale, Pygmy Sperm whale, Beluga, Northern Bottlenose whale, Cuvier's whale, Gurvais whale, Sowerby's Beaked whale, True's Beaked whale, Common dolphin, bottle-nosed dolphin, Striped dolphin, White-sided dolphin, White-beaked dolphin, Harbour porpoise, Killer whale, False Killer whale, Long-finned pilot whale, Risso's dolphin.
For further information on Cetaceans, contact:
Dr Simon Berrow,
Shannon Dolphin and Wildlife Foundation,
Tel/Fax: 065 905 2326,
Mobile: 087 8545450
Of the Order Pinnipedia, (fin-footed) two species are regularly seen, the Grey Seal and the Common Seal.
The Grey Seal
The Grey Seal is the world's first protected species and the only conservation success to date. Early in the last century, their numbers around the British Isles had fallen to about 500 animals. They were declared a protected species and their numbers have risen to approx. 100,000 today.
The male (bull) is much larger and heavier than the female (cow). Bulls can reach a length of 8 feet and a weight of up to 330Kgs. Grey seals only come ashore to rest or to breed. When at sea, they can spend up to 80% of their time submerged and visits to the surface for air are short. Grey Seals congregate on their breeding grounds in the autumn months. The males are the first to arrive. They haul themselves out of the water and clamber over the rocks, often making their way a considerable distance from the sea. A few days later, the first females appear and take their places amongst the assembled males. The females are pregnant from the previous year's mating and must give birth before the breeding cycle begins again.
The Grey Seal pup is born with a white pup coat and usually weigh about 14 Kg. It will suckle soon after birth. They take in about 3 litres of fat rich milk per day. In under 3 weeks they will have trebled their weight, much of it in the form of a thick layer of blubber. The mother seal loses about 4 Kg. in weight every day whilst she is suckling her young. By the time lactation is over, the mother's weight will have reduced by half. Males take no part in the rearing of the pup but do come ashore to mate with the females at the end of lactation. Like the females, they live off fat reserves during the breeding season.
The Common Seal
Male Common Seals can grow up to 2 metres long with an average weight of 100 Kg. The corresponding figures for females are 1.5 metres and 80 Kg. Their favourite haunts are rocky shores and sandy estuaries. They feed on a wide variety of fishes, cockles, muscles, shrimps, squid and even small crabs. Common Seals are usually seen on a sandbank or rocky island which will offer them a quick escape into deep water. Most Common Seal sites are submerged at high tide. The Common Seal spends more time in shallow water and on land than any other seal. Unlike the grey seals, Common Seals are rather quiet animals. Usually you hear the pups calling to their mothers, mostly as a means of keeping in contact while they are swimming over the high tide period. Adults do call occasionally, particularly in early autumn at the time of mating, but they make none of the raucous noise associated with a breeding colony of grey seals. The Common Seal gives birth to its single pup in late June or July; twins are rare. The birth takes place on a tidal rock platform or sandbank at low tide, or occasionally in water. The pup is well developed and relatively independent from birth with adult fur, and as the tide rises and covers the birth site both mother and pup take to the sea and swim until the next low tide. The mother is attentive to her offspring and ushers it to the safety of deep water if danger threatens. Suckling lasts two or three weeks, but after that, pups and adults often remain together. Mating occurs in early autumn and, in contrast to the grey seals, always takes place in the sea.
SEALS and FOLKORE
Seals have often been confused with mermaids and mermen. In Scotland and the Northern Isles there are also many stories of seal folk, or selchies, who were human beings in disguise, capable of resuming human form at certain seasons. The seal skin, according to these tales, is like an outer garment which can be taken off and stored. Matings and marriages between humans and seal folk feature quite frequently in northern folklore, and some highland families claim to trace their descent from such marriages. Because of their near kinship the northern people did not like killing seals. They did so out of need and were usually scared of the outcome.
SEALS and FISHERIES
Both the Grey and Common seals are natural predators of fish. Studies on Grey Seal diet around the coasts of Britain show that 11 fish species make up over 90% of their diet. Of these species, scientists consider that cod is the only one which is also subject to a large commercial catch.
Estimates of total fish consumption by major predators in the North Sea, reveal that fishermen remove 25 times as much fish as seals and that other fish remove 30 times as much fish as seals.
Many fishermen believe that seals are competitors for the limited amount of fish available. They argue that by killing large numbers of seals they will be able to catch the fish that seals would have eaten. However, the marine environment is a complex and dynamic ecosystem. There is no scientific evidence to suggest that fewer seals will mean more fish. In Canada, the culling of seals did not result in the anticipated increase in fish catches and it is now accepted by scientists that culling seals is not an effective strategy for fisheries management. Workshops held under the auspices of the United Nations Environment Marine Mammal Action Plan confirm this view.
For further information on seals, contact, the Irish Seal Sanctuary, Garristown, Co. Dublin.