Seals and Sea Lions
Seals and sea lions
Members of the suborder Pinnipedia, seals and sea lions are characterized by paddle-like flippers on a pair of limbs. Most pinnipeds are found in boreal or polar regions and are the most important predators in many high latitude areas, feeding primarily on fish and squid. Seals and sea lions catch their prey during extended dives of up to 25 minutes at depths of 2,625 ft (800 m) or more. Pinniped biologists are interested in the physiological changes these animals undergo during their dives. Known as mammalian dive response, a combination of reduced heart rate and a lowered core body temperature enables these warm-blooded animals to complete such dives.
Seals and sea lions must return to beaches and ice floes each year to give birth. Here they raise their pups in large congregations known as colonies. In some species the larger, more aggressive males will form polygamous mating groups, or harems, and claim and defend territories in these limited breeding areas. Males that do not get to mate form "bachelor male" groups in areas less suitable for weaning pups. Males of most species of pinnipeds do not mate before age four but may acquire harems when they attain large size and fighting experience. Some individual seals have been known to live as long as 46 years.
Because of the commercial value of their pelts, many species of seals and sea lions are threatened by hunting . One of the largest pinnipeds, the Stellar sea lion (Eumetopias jubatus ) has been steadily decreasing in numbers and is now listed as endangered by the IUCN. Within 15 years, the new pup count at the Marmot Island Rookery (the largest known sea lion rookery in the world) decreased from an average of 6,700 to only 804. The northern or Pribilof fur seal (Callorhinus ursinus ), which had been heavily exploited for its fur and in steady decline since the mid-1950s, is now listed as vulnerable. In California, the Guadeloupe fur seal (Arctocephalus townsendi ) is also listed as vulnerable. The Monk seal (Monachus monachus ) in the Mediterranean is listed as critically endangered.
These animals are in danger from natural and maninduced pressures, and the consequences of their demise are unknown. Other than hunting, perceived competition with fishermen, pollution , and habitat destruction are threatening pinnipeds. Despite the sanctions afforded by the Marine Mammals Protection Act (1972), fishers continue to kill seals that are "interfering" with fishing operations. Recent revisions to the act require some vessels to include observers who can monitor compliance with the law. Some environmentalists contend that it is the fisher who "interferes" with the seals. The California Department of Fish and Game estimates that 2,000 seals die each year from accidental entanglement in fishing nets.
In the summer of 1988, 18,000 harbor seals washed up on European shores. They were discovered to be suffering from an acute viral infection, possibly due to an immune-system breakdown. Chemical pollutants, mainly polychlorinated biphenyl (PCB), have been implicated in reduced immune function in marine mammals, and levels of PCBs have been constantly increasing in many coastal waters. A connection has also been made between mass seal die-offs and increasing temperature. Four of the six recorded mass mortalities have occurred in the past 12 years, a period that includes some of the warmest weather in the twentieth century. Other pollutants, such as heavy metals , may also affect seal health or reproductive ability. Oil spills also poison seal food sources and reduce the insulation ability of the seals' fur.
Finally, the reduction of habitat for feeding and reproduction has not been investigated thoroughly. Reputedly caused by reductions in prey-fish stocks due to overfishing and by development of coastal shorelines, this reduction may negatively affect pinniped populations.
[William G. Ambrose Jr. and Paul E. Renaud ]
Sumick, J. L. An Introduction to the Biology of Marine Life. 5th ed. Dubuque, IA: W. C. Brown, 1992.
Gentry, R. "Seals and Their Kin." National Geographic 171 (1987): 474–501.
Hersh, S. "Saga of North Sea Seals (Phocine Distemper Virus)." Sea Frontiers 36 (1990): 55.
Rosenberg, S. "Sea Lions of Monterrey." Sea Frontiers 35 (1989): 97–103.
Stirrup, M. "A Sea Lion Mystery." Sea Frontiers 36 (1990): 46–53.
Stolzenberg, W. "Seals Under Siege: A Heated Warning." Science News 138 (1990): 84.