Relationships between estimates of ringed seal (Phoca hispida) and polar bear (Ursus maritimus) populations in the Canadian Arctic

Abstract

Analysis of estimates of population size of ringed s,eals (Phoca hispida) and polar bears (Ursus maritimus) from several areas indicated that estimates of one predicted the range of expected population size of the other in areas where ringed seals constitute the primary prey. In some areas, the closeness of this relationship indicates where estimates of either seals or bears may be inaccurate. The number of seals required to support a population of polar bears of predetermined size was estimated independently using both behavioral and energetic data. Behavioral estimates of the number of seals killed may overestimate energetic requirements and vice versa. Predation and energy matrices indicated that high levels of predation on seals are sustainable only if most animals killed are young-of-the-year. The field metabolic rate of the polar bear appears to be about twice the basal metabolic rate. Densities of seals vary in response to overall productivity of the ecosystem in different areas, and fluctuations in their numbers and reproductive rates between years can be used to monitor changes in productivity of the ecosystem. These changes also cause variation in productivity of bears, which indicates the sensitivity, at the population level, of the relationship between ringed seals and polar bears.

Distribution, abundance and biology of ringed seals (<i>Phoca hispida</i>): an overview

“…They also cite Smith's findings that 40% of 4-year old and 60% of 5-year old female ringed seals were sexually mature in western Baffin Bay (Smith 1973a), while only 20 and 29% of the same age classes, respectively, were mature in the Beaufort Sea (Smith 1987). This wide difference was interpreted as supporting the hypothesis that the Baffin Bay environment is more productive than the Beaufort Sea environment (Stirling and 0ritsland 1995;also see Stirling 1997). It is probably also less temporally variable, judging by the comparative infrequency of seal population crashes.…”

“…Most of the calibration data have been collected during the spring moulting period in areas of "prime" breeding habitat where adult seals predominate. Areas of poor or unsuitable breeding habitat, where sub-adults predominate, have been neglected in spite of the fact that sub-adults comprise the largest component of the total population (Stirling and 0ritsland 1995). Stirling and 0ritsland (1995) reasoned that the correction factors used in previous studies to account for seals in the water during overflights, ranging from about 30% to 90%, were "likely to be conservative when applied universally.…”

“…The "overall robustness" of the relationship between estimates of ringed seal populations (from aerial strip transect surveys) and corresponding estimates of polar bear populations (mainly from mark-recapture analyses) was interpreted by Stirling and 0ritsland (1995) as meaning that "the population' size of one may be used to predict an expected population of the other." There is also a correlation between the reproductive output of ringed seals and the natality and body condition of polar bears in the same region (Stirling and Lunn 1996).…”

“…Some authors have considered the density of ringed seals as a useful index of biological productivity. For example, Stirling and 0ritsland (1995) cite the fact that the lowest seal densities in Kingsley et al's (1985) study area occurred in the most northerly strata, where the water would be expected to be nutrient-poor and where multi-year ice would limit light penetration and thus inhibit photosynthesis. They also cite Smith's findings that 40% of 4-year old and 60% of 5-year old female ringed seals were sexually mature in western Baffin Bay (Smith 1973a), while only 20 and 29% of the same age classes, respectively, were mature in the Beaufort Sea (Smith 1987).…”

“…Among these are the observed segregation of age classes in the fast ice, the strong odour emitted by adult males, the wounds on the bodies of male and immature seals (indicative of intraspecific fighting) and the "disparate sex ratio in the breeding habitat" (Hammill and Smith 1991:133). Stirling and 0ritsland (1995) noted the similarity in densities of ringed seal breathing holes in two areas of prime breeding habitat -the fast ice of western Barrow Strait and Amundsen Gulf (Kingsley 1986). They inferred from this similarity that "the territorial behavior of resident seals" may regulate the under-ice distribution of ringed seals during winter and spring (until break-up).…”