Glaucous gull (Larus hyperboreus)

Closeup of a glaucous gull sitting on an ice formation.
Photo: Stein Ø. Nilsen / Norwegian Polar Institute

The glaucous gull's position uppermost in the food chain makes it susceptible to pollutants, and the species is listed as Near Threatened in the 2010 Norwegian Red List. The glaucous gull has a circumpolar, high-Arctic distribution and breeds throughout Svalbard. It is a generalist that exploits many different kinds of prey such as fish, molluscs, sea urchins, crustaceans, eggs, chicks and adults of other seabirds, as well as insects, carcasses and rubbish.

What is being monitored?

Breeding population on Bjørnøya and in Kongsfjorden

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The figure shows the development of the breeding population of glaucous gulls in the study areas on Bjørnøya and in Kongsfjorden, Spitsbergen. The numbers have greatly declined on Bjørnøya since monitoring started in 1987. Reduced prey availability, high levels of pollutants, increasing predation from arctic foxes and competition from a rising population of great skuas are all possible explanations for the observed decline.

Status and trend

The breeding population of glaucous gulls on Bjørnøya has greatly declined in the past 30 years.

There were estimated to be approximately 2000 pairs in 1980, whereas in 1986, in connection with a total census of the seabird populations on the island, the estimate was about 2350 individuals – approximately half of the estimate 6 years previously. A new total census in 2006 gave about 700 breeding pairs, a 65% reduction over the three decades prior to 2005.

The species was monitored on Bjørnøya prior to 2005, when it began to be monitored in Kongsfjorden, the number of glaucous gulls breeding there has increased substantially in the decade after 2005.

The status of the species elsewhere in Svalbard is poorly known. The glaucous gull is known to be declining in other parts of the Arctic, including Iceland and Canada.

Causal factors

Several factors are probably affecting the glaucous gull population on Bjørnøya, contributing to its decline. The most important ones are probably 

  • Pollutants
  • Shortage of food
  • Predation by a growing arctic fox population
  • Increasing competition with great skuas is also a potential factor

Dead and dying glaucous gulls have been found on Bjørnøya in the breeding season since the 1970s. Autopsies and analyses of tissue samples from these birds have revealed very high levels of various pollutants in their brain and liver. Even though the cause of death cannot be determined with certainty, high pollutant levels are likely to be responsible, either directly (physiologically) or indirectly through, for example, impaired fitness.

A series of studies from 1997 onwards has documented effects of pollutants on several reproductive parameters, including

  • survival of chicks
  • brooding behaviour, and
  • survival of adult birds

The collapse of the common guillemot population in 1986–87 removed a large part of the food base for glaucous gulls which live on eggs, chicks and adults of this species. This may have been a contributory cause of the population decline in the 1980s and 1990s. The common guillemot population has recovered since.

For many years, the arctic fox population on Bjørnøya was very small, due to intensive hunting. It has, however, been rising since 2000, and foxes are now breeding in several places on the island. Arctic foxes take glaucous gull eggs and chicks, and may contribute to the population decline by reducing the breeding success. This may be a particularly important factor for glaucous gulls that are debilitated due to high levels of pollutants.

The great skua began breeding on Bjørnøya in 1971. The population has grown greatly since then and now amounts to at least 500 pairs. The great skua is a top predator like the glaucous gull, and to a great degree the two species have an overlapping choice of food. Competition from the growing great skua population may also be a factor contributing to the population decline of the glaucous gull. However, studies to substantiate this are lacking.

Consequences

The glaucous gull, along with the great skua, is a top predator in Svalbard, and the species plays an important role in the ecosystem as a predator and scavenger.

About the monitoring

The glaucous gull populations are monitored because such high levels of pollutants are present that survival and/or reproduction may be jeopardised.

In addition, GLS logging (Global Location Sensing) is being used to study activity patterns, migratory routes and winter ecology, but this is not part of the ordinary monitoring.

Monitoring the number of breeding pairs is the best, and the only internationally accepted, means of revealing short-term and long-term population changes in the glaucous gull. Annual counts and recording the number of nests in colonies or sample plots offer an opportunity to reveal changes in the population provided any changes discovered in the sample plot or colony are representative for the entire population.

Breeding success gives a measure of the production of chicks in the year in question and thereby a measure of the environmental conditions. The production of chicks forms the basis for recruitment to the population and a failure in the chick production will lead to a decline in the population if the recruitment does not compensate for the adult mortality.

Places and areas

Bjørnøya is the largest breeding site in Svalbard and the rest of the Barents Sea. This population is affected by pollutants. Kongsfjorden was chosen because an adequate number of pairs breed there and other seabird monitoring is taking place in the same area.

Relations to other monitoring

Monitoring programme
International environmental agreements
Voluntary international cooperation
Related monitoring