Do Antarctic seals feel El Nino?

Living in the near-frozen waters around Antarctica, Weddell seals rank among the world's most southern animals. But even in their home waters around the icy continent, Weddell seals apparently feel the effect of climate disruptions in the tropics, more than 6,000 kilometers distant.

For the last 12 years, James W. Testa of the University of Alaska in Fairbanks has monitored changes in population in a seal community in the Antarctic, just offshore from Ross Island where the National Science Foundation supports a research center. Testa has recorded an average of 400 seal pups born every year near his study site. But this was a bad year for baby seals: Only 317 have been born so far, and few more are expected this late in the season.

Several years ago, Testa noticed that the number of births declines every four to six years, apparently coincident with a climate phenomenon known as the E1 Nino/Southern Oscillation (ENSO). An ENSO occurs when a tropical pool of warm water shifts from the western Pacific eastward and alters weather in much of the tropical and temperate latitudes. Since an ENSO developed last year and gripped the Pacific through the middle of this year, the current dip in birth rates supports Testa's theory that tropical weather can disturb seal populations. He suggests that the seal declines may result from changes in the fish population, caused possibly by shifts in ocean currents.

The lowered birth rate for seals represents the most southern biological effect ever recorded for an ENSO, Testa says. Because biologists have not tracked other Antarctic animals over such a long period, it remains unclear whether ENSOs also affect neighboring creatures.

Work at Britain's Signy Station on the Antarctic peninsula has revealed a similar pattern of population swings among Weddell seals there. But the seals have not shown the same variations at Australia's Davis Station.