Seal skins and oil were among the first of New Zealand's natural resources to appear in world markets. The early enterprises of sealers, here as elsewhere, were conducted in secrecy; consequently the records are few. But it seems probable that commercial sealers first hunted in New Zealand waters about 1792. The industry flourished briefly; in less than 30 years the seals were hunted almost to extinction both in New Zealand and in the sub-Antarctic islands. Only in recent years have they been allowed, under legal protection, to increase in numbers and approach their former population strength.
By modern systematists, seals are grouped in the order Pinnipedia, although some biologists would still prefer to regard them as aquatic members of the order Carnivora. Superficially, they are remarkably doglike, and the pups of many species bark as though to claim affinities with the dogs. Seals are, however, considerably modified both internally and externally for life in the sea. Their limbs are paddles, their shape is streamlined, and their blood system, muscles, and skin show many adaptations which help them to dive and swim efficiently. Under water the heart slows to a rate of three to six beats per minute. Some seals, possibly all, have a muscular collar on the diaphragm which constricts their blood system, so that circulation is maintained only in the vessels of the head during a dive; the muscles of viscera and limbs work under anaerobic conditions, and thus all available oxygen is reserved for the brain. Seals have more blood than do land animals of similar size, and so take down considerable stores of oxygen when they dive.
Most seals live in cold water, where food is plentiful; New Zealand is toward the northern edge of the range where seals would be expected, although tropical and subtropical species are known. Living in waters colder than their own bodies, they require insulation to prevent excessive heat loss; hence the thick blubber, hide, and fur which attracted the sealing gangs of the eighteenth and nineteenth centuries. Fur is found only on sub-Antarctic animals, together with comparatively thin blubber; in climates where icing might become a problem seals have a much thinner covering of hair, without the dense under-pelt which furriers value, and insulation is maintained by a much thicker layer of blubber. On the sub-Antarctic islands of New Zealand both fur-yielding and oil-yielding species were found; in New Zealand itself only fur seals were of any commercial importance, although it is possible that sea lions (which yield oil and hides) may also have been taken at one time from the west coast of the South Island.
Biologically, seals are divided into three families of which one (Odobenidae, the walruses) is found only in the Northern Hemisphere. The remaining families, Otariidae, the eared seals (including fur seals and sea lions), and Phocidae, the so-called “true seals”, have a world-wide distribution, with a tendency to congregate in the colder waters of high-latitude seas and polar currents. Three species of seals breed at present in the New Zealand region (including the sub-Antarctic islands): two are eared seals, and the third is the largest of the true or Phocid seals, the sea elephant. Only one species, the New Zealand fur seal Arctocephalus forsteri, breeds north of the forty-eighth parallel in the South Island; the other two are entirely sub-Antarctic in range and are seldom seen even as stragglers on the main islands of New Zealand.