© Ian Stirling
1/10/2017 1:29:13 AM
What About the Ringed Seals as the Arctic Climate Warms?
By Dr. Ian Stirling, Research Scientist Emeritus, Environment Canada and Adjunct Professor, Department of Biological Sciences, University of Alberta
It is not surprising that most people first think of polar bears when contemplating the rapidly disappearing sea ice in the Arctic due to human-caused climate warming. The reasoning is both straightforward and logical. Even children in elementary school know that polar bears find it difficult to catch seals in the open water; they depend upon the presence of sea ice as a platform from which to more easily hunt them.
Consequently, as the sea ice breakup comes earlier and earlier, while freeze-up gets progressively later, the duration of the open water period through which the bears have to survive on their stored fat reserves, without being able to catch more seals, gets longer and longer. In the Barents Sea around Svalbard (the Norwegian Arctic) for example, the number of ice-free days per year is now increasing at the staggering rate of 65 days per decade (Parkinson 2014). The negative effects of sea ice loss because of climate warming on some populations of polar bears, such as those in Western Hudson Bay and the Southern Beaufort Sea, are clear and have been well documented. But, what about the ringed seals—the polar bear’s most important food source—in a steadily warming Arctic?
Many people assume that as long as sea ice is present during part of the year, there will be lots of seals and, if the polar bears have a platform to hunt from, everything will be fine. But, could it really be that simple? Our knowledge of the status of most seal populations in the Arctic, but especially the all-important ringed seal, is rudimentary at best. However, we do know enough about the breeding ecology of ringed seals, the primary prey species of polar bears, to clearly identify some huge concerns. In fact, it is abundantly clear that the negative effects of climate warming on ringed seals in much of the Arctic are already dramatic. Ultimately, what happens to ringed seals is the most important single factor in how climate and the loss of sea ice impacts the polar bear.
Throughout the winter and spring, ringed seals live beneath the solidly frozen ice along the coastlines and in the bays throughout the Arctic. They self-maintain their breathing holes by scratching the re-freezing sea ice with the claws on their fore flippers (Smith and Stirling 1975). In fact, each seal maintains several holes so that, if a polar bear discovers one, the occupant may be able to escape and breathe safely at a different one.
In about early April in most areas, pregnant female ringed seals give birth to their pups in small birth lairs; they dig these in snowdrifts that form over one of their breathing holes during winter (e.g. Figure 1).
Figure 1: Example of a perfect snowdrift for concealing a ringed seal birth lair and protecting the occupant. (© Ian Stirling)
In prime ringed seal breeding habitat, under normal conditions, the hard wind-blown snow over the birth lair provides a thick protective lair over the pup inside a birth lair (Figure 2).
Figure 2: Digging out an abandoned ringed seal birth lair illustrates the protection provided by a layer of hard wind-blown snow. (© Ian Stirling)
(To understand how incredibly important that protective snow cover is to a birth lair, compare Figure 2 with Figures 4 and 5!) Such a lair hides the seals from hunting bears and provides protection from the cold to the tiny pup while it begins to accumulate a protective layer of fat from its mother’s rich milk. At birth, the pups weigh only 4-5 kg (10+ lb) and have little body fat. However, by the time they are weaned at the age of about six weeks, they may weigh more than 20 kg (40-45 lb), of which 50% or more is fat.
Because of that fat, the most important time of the whole year for polar bears to feed is spring, from the time the pups first start to fatten up inside their birth lairs, through the period of melting and disappearance of the lairs, to the eventual breakup of the ice in early summer. During that period, the abundance of newly weaned ringed seal pups represents the biggest accessible source of fat for most hunting bears at any time in the entire year. Also, because the young seals are less experienced at avoiding predators than older ones, they are more vulnerable to predation. Thus, during that critical feeding period from late spring through early summer, most bears may accumulate up to two-thirds or so of the energy they will require throughout the entire year.
The critical importance of the annual crop of ringed seal pups to the reproductive success of polar bears was dramatically confirmed in the southern Beaufort Sea in the mid-1970s and again in the 1980s. In both decades, following an almost complete reproductive failure of ringed seals from natural causes, there was an immediate large-scale reduction in both the production and survival of polar bear cubs (Stirling 2002). The most important conclusion was that the large supply of fat from ringed seal pups in spring and early summer is the most important single factor in maintaining a large viable population of polar bears.
But the ringed seals and the polar bears that depend on them now face a severe and increasing new threat, directly resulting from climate warming. The rapidly warming weather in spring throughout much of the Arctic in recent years makes the protective snow roof of a birth lair more likely to collapse and melt prematurely. When this happens, the newborn pups are exposed to predators on the sea ice before they have had time to accumulate much fat or are mature enough to swim with their mothers to another breathing hole and escape (Figure 3).
Figure 3: A fat female ringed seal and her newborn pup lie exposed on the sea ice in Svalbard after the roof of her birth lair collapsed. (© Mats Forsberg)
In such cases, a bear can easily see the large dark-colored adult female from a distance and, if it does, a lethal attack will quickly follow. The adult female ringed seal will desert her pup and flee if a predator approaches, but the oblivious pup will be killed, even though it is of little nutritive value at such a young ages. Sometimes such young pups are not even eaten because they don’t have enough fat to make it worthwhile to the bear (Figure 4).
Figure 4: Newborn whitecoat ringed seal pup killed by a polar bear but not eaten because it has not yet accumulated any fat. (© Ian Stirling)
Similarly, unseasonably early rain, instead of snow, may prevail for long enough to simply wash away a birth lair completely, leaving the pups completely without protection (Figure 5), as a colleague and I witnessed on SE Baffin Island in Canada in 1979 (Stirling and Smith 2004). All the birth lairs in the two bays where the rainfall was focused were washed away, leaving the pups exposed on the ice (Figure 6) where most were vulnerable to heavy predation by arctic foxes and polar bears. Sadly, although the bears killed many seal pups, they were of little energetic value because they had not yet lived long enough to accumulate a significant amount of fat from their mothers’ rich milk.
Figure 5: A dead newborn ringed seal pup lies on the sea ice beside where its birth lair was washed away by unseasonably early rain. (© Ian Stirling)
Figure 6. A newborn ringed seal is vulnerable to predators on the sea ice after its birth lair was washed away by rain. (© Ian Stirling)
At this point, we do not know how extensively or frequently the loss of protective birth lairs for ringed seals is occurring throughout the circumpolar Arctic but some worrisome events have been observed already. For example, in the last two springs, many of the fiords in Svalbard have not frozen early enough to accumulate sufficient snow cover to facilitate construction of safe birth lairs. When that happens, the pups may be born unprotected on the open ice and suffer high mortality rates. Such large-scale problems with ringed seal productivity, and the survival of their pups, are likely already far more widespread than we are able to quantitatively document. Even so, it is already clear that as the climate continues to warm throughout the Arctic, the potential for large scale, and progressively more widely spread, negative effects of warm weather on ringed seal pupping habitat are increasing.
Clearly, simply having some ice to walk on and hunt from is not enough on its own to ensure the health of polar bear populations.
Parkinson, C. L. 2014. Spatially mapped reductions in the length of the Arctic sea ice season, Geophysical
Research Letters 41:4316–4322.
Smith, T.G. and Stirling, I. 1975. The breeding habitat of the ringed seal (Phoca hispida): The birth lair and associated structures. Canadian Journal of Zoology 53:1297-1305.
Stirling, I. 2002. Polar Bears and Seals in the Eastern Beaufort Sea and Amundsen Gulf: A Synthesis of Population Trends and Ecological Relationships over Three Decades. Arctic 55, Supplement 1:59-76.
Stirling, I. and Smith, T.G. 2004. Implications of Warm Temperatures and an Unusual Rain Event on the survival of Ringed Seals on the Coast of Southeastern Baffin Island. Arctic 57:59-67.