CLIMATE CHANGE AND RINGED SEAL (PHOCA HISPIDA) RECRUITMENT IN WESTERN HUDSON BAY

MARINE MAMMAL SCIENCE, 21(1):121 135 ( January 2005) Ó 2005 by the Society for Marine Mammalogy CLIMATE CHANGE AND RINGED SEAL (PHOCA HISPIDA) RECRUITMENT IN WESTERN HUDSON BAY STEVEN H. FERGUSON Fisheries and Oceans, 501 University Crescent, Winnipeg, Manitoba R3T 2N6, Canada E-mail: fergusonsh@dfo-mpo.gc.ca IAN STIRLING Canadian Wildlife Service, 5320 122 Street, Edmonton, Alberta T6H 3S5, Canada PHILIP MCLOUGHLIN Department of Biology, University of Saskatchewan, 112 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada ABSTRACT Climate warming is predicted to reduce the extent of ice cover in the Arctic and, within the Hudson Bay region, the annual ice may be significantly decreased or entirely lost in the foreseeable future. The ringed seal (Phoca hispida), a key species that depends on sea ice, will likely be among the first marine mammals to show the negative effects of climatic warming. We used 639 ringed seals killed by Inuit hunters from western Hudson Bay (1991 1992, 1999 2001) to assess trends in recruitment relative to snow depth, snowfall, rainfall, temperature in April and May, North Atlantic Oscillation (NAO) from the previous winter, and timing of spring break-up. Snowfall and ringed seal recruitment varied from lower than average in the 1970s, to higher in 1980s and lower in 1990s. Prior to 1990, seal recruitment appeared to be related to timing of spring ice break-up which was correlated with the NAO. However, recent 1990 2001 environmental data indicate less snowfall, lower snow depth, and warmer temperatures in April and May when pups are born and nursed. Decreased snow depth, particularly below 32 cm, corresponded with a significant decrease in ringed seal recruitment as indicated by pups born and surviving to adults that were later harvested. Earlier spring break-up of sea ice together with snow trends suggest continued low pup survival in western Hudson Bay. Key words: Arviat, Inuit, North Atlantic Oscillation, Nunavut, Phoca hispida, pup survival, rainfall, sea ice break-up, snowfall, temperature. 121

122 MARINE MAMMAL SCIENCE, VOL. 21, NO. 1, 2005 Warming of the earth s climate is forecast to be greatest at the poles and, within the Arctic, western Hudson Bay is one of the areas in which the rate and degree of climatic warming is predicted to be greatest (Prinsenberg 1986, Parkinson 2000, Krajick 2001, Gough and Wolfe 2001, Comiso 2003). The challenge for resident species to accommodate such change is increased in the Arctic because of its large scale, the rapid rate at which the warming is predicted to occur, large interannual variation in climate, and the accelerated pace of human development (IPCC 2001). As a result, climate change in the Arctic is expected to have large effects (Hughes 2000, Levitus et al. 2001). Higher ocean temperatures and lower salinities, contraction of seasonal sea ice extent, rising sea levels, and a host of other effects (Munk 2003) are certain to have significant impacts on marine species. For marine mammals adapted to life with sea ice, the effects of reductions in sea ice are likely to be reflected initially by shifts in range and abundance (Tynan and DeMaster 1997). Demographic changes associated with shifts in geographic range will likely be observed as decreased recruitment in areas of reduced sea ice. Northern seal and walrus (Odobenus rosmarus) populations may represent useful indicators of ecological change in northern ecosystems because of their dependence upon annual sea ice (Tynan and DeMaster 1997) which has already undergone significant changes in annual cycles of distribution and abundance and significant further reduction is predicted (Comiso 2003). Although ringed seals (Phoca hispida) are the most abundant seal in the Arctic and are considered a keystone species in the marine ecosystem (Smith et al. 1991), little research has focused on the effect of climate warming on this species (Harwood et al. 2000, Stirling and Smith 2004). The evolutionary adaptations of ringed seals to exploit sea ice habitat for reproduction and overwinter survival makes this species particularly suitable for examining effects of climate warming (Lowry et al. 1980, Ryg et al. 1990, Smith et al. 1991, Moulton et al. 2002). Ringed seal pups are born from mid-march to mid-april, suckled for about six weeks, and weaned prior to spring break-up in June (McLaren 1958, 1993, Hammill and Smith 1991). During lactation pups spend about half their time in subnivean dens on top of the ice, and half underwater diving (Lydersen and Hammill 1993a, Furgal et al. 1996), during which time they and adults are hunted by polar bears (Stirling and Archibald 1977, Smith 1980). Pups in subnivean birth or haul-out lairs with thin snow roofs are more vulnerable to predators than those in lairs with thick ones (Smith and Stirling 1975, Hammill and Smith 1991, Furgal et al. 1996). Although there is considerable information on ringed seals in some other parts of the Arctic, information on their distribution, abundance, breeding habitat, and reproductive rates in western Hudson Bay is relatively limited (Smith 1975, Lunn et al. 1997, Holst et al. 1999). Largely because the sea ice in western Hudson Bay is both relatively unstable and inaccessible from shore-based locations by surface travel in April and May, there are no data on the distribution of birth lairs, habitat preferences, or vulnerability of pups to predation. In the absence of data, it has generally been assumed there are enough ringed seals to satisfy the needs of Inuit hunters and bears and that seal population sizes are large and stable. In a preliminary study however, Holst et al. (1999) reported that the pregnancy rate of ringed seals and the proportion of young-of-the-year in an open water sample from Arviat (Fig. 1) in 1991 1992 were unexpectedly low. Even though there are no direct data on ecological aspects of the birth lair habitat or productivity on the sea ice of western Hudson Bay, we are able to define and

FERGUSON ET AL.: CLIMATE CHANGE AND SEAL RECRUITMENT 123 Figure 1. Study area in western Hudson Bay. Communities from which seals were harvested and from which weather stations provided environmental data for this study are indicated. Box denotes area used to determine sea ice break-up dates and dotted line represents approximate area hunted. evaluate several environmental factors that may affect the quality of natal dens in relation to recruitment of seals into the population. The amount of snowfall in April and May is likely important, and may correlate with factors such as the predation rate (negative) on seal pups by polar bears (Hammill and Smith 1991), depth of drifts with subnivean birth lairs (positive), or rainfall (negative) (Stirling and Smith 2004). In addition to predation risk, survival of pups in April and May may be influenced by the negative effects of exposure, including hypothermia (Smith et al. 1991, Kelly 2001), in the event of den collapse. Our goal was to determine whether annual variation in the survivorship curve was correlated with these environmental factors. METHODS Study Area Our study area encompassed the western Hudson Bay region (Fig. 1). Hudson Bay is a relatively shallow salt-water body with counter clockwise currents (Prinsenberg 1986, Saucier et al. 2004). Hudson Bay is covered by annual ice from mid-november to May and in some years can be almost free of ice by late June (Collin and Dunbar 1964). Although shore leads and polynyas are present throughout the winter, spring break-up generally begins in April with openings observed in the Roes Welcome Sound polynya to the southwest of Southampton Island (Stirling 1997). Ice decays rapidly to the south and southeast because of

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