Seabird and Mammal Ecology in Polar Marine Ecosystems and Climatic Changes
Joiris Claude R
Published on: 2021-07-15
This article aims at summarising the changes in abundance or in geographical distribution of top predators-seabirds and marine mammals – detected during our long-term study of the at-sea distribution, mainly in polar seas. They concern increasing large whale stocks in the northern Atlantic Ocean, probably from the higher stocks in the Pacific Ocean, the decrease in ivory and Ross’s gulls in the Greenland Sea. Such evolutions do not necessarily concern the status of the whole species but might reflect changes in geographical distribution. No decrease of polar bear population was detected.
KeywordsSeabirds; Marine mammals; At-sea distribution; Polar seas
Climatic changes, i.e. the increase of temperature probably due to human activities, are strongly affecting polar ecosystems, especially in the Arctic. Pack ice extent decreased in summer (from 14 to a minimum of 7 million km2 in July 2012), much less in winter (from 14 to 12.7 million km2 in May 2021) . Major ecological consequences are thus to be expected in a significant part of the Arctic, changing from multiyear to one-year ice ecosystems. They are strongly affecting the populations of the higher trophic levels - seabirds and marine mammals. In the frame of our long-term study of their at-sea distribution in polar areas, we aim at detecting such changes in population density, both in abundance as in geographical distribution.
A major consequence concerns the distribution of the large Mysticeti cetaceans. The North Atlantic stocks strongly declined due to the historical over-whaling and used to be separated from the higher stocks of the Pacific Ocean by ice. The major decrease in ice coverage from 2005 and 2007 on caused the opening of the North-East and North-West Passages, allowing an important influx of cetaceans to the Atlantic. Most striking was the increase of the rare bowhead whale Balaena mysticetus, but also affected blue Balaenoptera musculus, humpback Megaptera novaeangliae and fin whales Balaenoptera physalus (Figure 1) .
Figure 1: bowhead, humpback and blue whales; evolution of numbers of observations for bowhead (blue diamonds) and blue whale (red dots) Joiris 2016.
(Figure 1) Bowhead, humpback and blue whales Temporal evolution of numbers of observations for bowhead (blue diamonds) and blue whale (red dots) in the Greenland Sea and Fram Strait; mean number per 30 min count .
An important decrease of ivory gull Pagophila eburnea breeding populations in the northern Canada, the northern Greenland and Wandel seas might be due to a decreasing ice coverage, but also might correspond to moving populations toward North Siberia more than an actual decrease of the species. The Ross’s gull Rhodostethia rosea on the contrary used to regularly appear in the area in low numbers, but apparently stopped moving west from its eastern breeding grounds and left the area from 2015 on [3, 4] (Figure 2).
Figure 2: Ivory gull; temporal evolution of numbers of observations for ivory and Ross’s gulls in the Greenland Sea and Fram Strait; mean number per 30min count.
The example of polar bear Ursus maritimus as threatened by the decrease of ice cover is often cited. One must however take into account that the species is not distributed on the whole pack ice but limited to the outer marginal ice zone (OMIZ). This is probably why we did not notice any significant decease in polar bear populations. On the contrary the presence of many cubs – up to three in one case - can be considered a sign of a healthy population. Moreover, a very high summer concentration of more than 3500 individuals was tallied on Wrangel Island, North Siberia . The Hudson Bay marginal population constitutes a possible exception due to increasing distances between their summer and ice distributions (e.g. in Churchill) . Such data, however, do not necessarily mean that the whole species is threatened (Figure 3).
Figure 3: Polar bear with cub.
Discussion and Conclusion
Two complementary approaches can be applied in order to detect possible changes in seabird and marine mammal populations. On the one hand, one can follow their evolution by regular counts at their breeding grounds, most seabirds and pinnipeds breeding in highly concentrated large colonies. On the other hand, we try to quantify seabird and marine mammal abundance and density at sea. The low reproducibility of this method, however, might not allow to detect changes by less than one order of magnitude, especially when hotspots cause an important heterogeneity in distribution, and thus a lack of reproducibility . In both cases, tallied changes can be due both to changes in abundance, to geographical changes (i.e. cetacean populations), in breeding distribution (ivorygull) or in migration pattern (Ross’s gull) and thus do not necessarily reflect evolutions at the species level.
- National Snow and Ice Data Center, University of Colorado, Boulder, USA.
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