Research Themes -
Impacts of climate change on marine mammals
Climate change is an ongoing phenomenon in the Arctic, an area expected to exhibit the most accelerated changes associated with global warming. Sea ice represents a key component in Arctic ecosystems: it regulates heat, water and gas exchanges between the ocean and atmosphere. Sea ice is also a selected habitat by some marine mammal species which use it as a platform to haul out on (e.g. ringed seals, walrus), while it restrains access to some areas for other marine mammal species (e.g. beluga whales). However, over the last four decades the extent, duration and thickness of sea ice has been declining, and climate change scenarios predict that the current trend will continue.
The rate and intensity of climate change are predicted to be greatest at the southern and northern distribution limits of ice-obligate and seasonal migrant species, respectively. Studies aiming to anticipate the consequences of climate change on marine mammals and Arctic ecosystems have focused on a region where both of these conditions prevail, Hudson Bay. Impacts of climate change have also been studied in a sub-Arctic species in Northwest Atlantic and the Gulf of St. Lawrence, at the southern limit of pack-ice formation in North America.
Hudson Bay is a large (≈ 1.2 million km2), seasonally ice-covered inland sea inhabited by a variety of Arctic or ice-obligate resident marine mammal species, as well as seasonally ice-associated migrant species. Top predators in Hudson Bay are currently Inuit hunters and polar bears. Hudson Bay represents the southern limit of polar bears' range. This species uses ice from freeze-up to breakup to forage, primarily on ringed seals and then fasts throughout the summer on land. Climate change is associated with decreasing feeding opportunities for polar bears through two mechanisms. First, since the ice-covered period is crucial for polar bears to acquire the energy needed to survive during the summer, a decrease in the duration of sea ice shortens their critical foraging period. Second, polar bears' primary prey abundance will be negatively affected by climate change. Indeed, ringed seals depend on fast-ice for their reproduction: they give birth and nurse their pups in subnivean caves throughout a ≈6 week suckling period. Reductions in fast-ice cover and duration will likely result in increased predation on young and due to a reduced lactation period, will result in an increase in pup mortality, ultimately leading to a decline in ringed seal abundance.
Although considered as a northern temperate specie, small numbers of harbour seals are found in Hudson Bay. This species prefers ice-free waters and gives birth and nurse their young on land. The reduction in ice coverage and duration in Hudson Bay has led to the hypothesis that harbour seal abundance could increase in this area, eventually replacing ringed seals as a primary food source for polar bears. However, a study demonstrated that harbour seals prefer nearshore, shallow water areas. This habitat preference would likely limit their ability to increase to insufficient numbers to replace ringed seals in the diet of polar bears. Moreover, ringed seal pups remain relatively passive in their caves throughout lactation, and adults return to breathing holes in the ice, making them accessible prey for polar bears. In contrast, harbour seals pups are highly aquatic from birth to weaning, and the use of ice-free waters makes this species less predictable and therefore possibly less vulnerable to predation by polar bears.
Unlike beluga and bowhead whales, killer whales have high profile dorsal fins which reduces their capacities for movement through heavy ice. There are some killer whale sightings from as early as 1900 in Hudson Strait, but this species was historically not present in Hudson Bay. However, since the 1940s, the number of killer whale sightings has increased exponentially since then. Killer whales are now regularly seen in Hudson Bay. Evidence suggest that sea ice in Hudson Strait acted as a choke point restricting killer whale access to Hudson Bay, and that the decline in sea ice has now enabled the species distribution to expand. Killer whales in Hudson Bay were observed to feed on whales and seals, with primary preys suggested to be narwhals and belugas in the spring and fall, and bowhead whales in the summer. As temperate waters shift to higher latitudes as a result of climate change, killer whales are expected to expand their range farther into Canadian Arctic waters. This may result in an ecological shift, with killer whales replacing polar bears as the top marine predator in some areas. While polar bears mostly feed on seals, killer whales primarily feed on whales. In addition, Inuit harvest polar bears for food and offset their expenses through the sale of the hide. The replacement of polar bears by killer whales as top predators are thus expected to result in complex ecosystem and social perturbations.
Gulf of St. Lawrence and Northwest Atlantic
The most abundant marine mammal in the North Atlantic is the harp seal, which plays a keystone role in and the ecosystem. Harp seals are also harvested both for commerce and subsistence in the Northwest Atlantic, therefore representing social and economic interests. This species depends on pack-ice to give birth, nurse their young and moult. The nursing period lasts on average 12 d, after which the weaned animals continue to remain on the ice for several weeks. For harp seals to successfully reproduce, the presence of pack-ice pans of sufficient size and thickness are thus required from whelping (mid-February) until young of the year have developed their swimming abilities and undertake a northward migration to the Arctic.
Following the general trend towards deteriorating ice conditions in northern Canada, pack-ice extant and thickness have been decreasing in the North Atlantic and the Gulf of St. Lawrence over the last 40 years. Climatic simulations predict a 70% decrease in ice duration in the Gulf of St. Lawrence by 2050. In 2009-2010 and 2010-2011, ice conditions were the lowest in the Northwest Atlantic since the Canadian Ice Service began compiling daily ice charts in 1969. The way in which harp seals responded to these extreme ice conditions provides insights into how this species may respond to future changes in ice-conditions.
In March/April -2010, ice was absent from the Gulf of St. Lawrence at the time females usually give birth. No significant whelping was observed to occur in the southern gulf in this year. Instead, females moved to more suitable ice, outside of the gulf, to an area referred to as the Front, off the southeast part of Labrador. In 2010-2011, ice formed late in the Gulf and Northwest Atlantic, but the ice was thin. Pupping occurred in the normal areas, but broke up rapidly after most of the females had given birth. Mortality in March/April 2011 was considered to be very high. Observations from these two years suggest that if there is suitable ice at the beginning of the pupping season, females will give birth in the traditional breeding areas. If this ice is destroyed, then mortality could be high. However, if ice is not present, then females will attempt to move elsewhere in search of suitable ice.
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