Common guillemot (Uria aalge)

Semi-closeup of three guillemots on the surface of the ocean.
Photo: Stein Ø. Nilsen / Norwegian Polar Institute

The common guillemot is a food specialist which, in the breeding season, chiefly lives on pelagic fish such as capelin and herring. This makes it a good indicator of changes in fish stocks. The common guillemot population has declined dramatically in the Barents Sea and the northern part of the Norwegian Sea, and the species is listed as "Critically Endangered" in mainland Norway and "Vulnerable" on Svalbard in the Norwegian Red List. The common guillemot is now being monitored on Bjørnøya, where by far the largest breeding colony in Norway and the Barents Sea is found, and on Jan Mayen.

What is being monitored?

Breeding population on Bjørnøya and Jan Mayen

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The population is shown as the percentage of the average number in the colony in the entire monitoring period. The population on Bjørnøya collapsed in the winter of 1986–1987 due to the collapse of the capelin population. The common guillemot population has been growing since then. The collapse in the capelin population seems to have been triggered by increasing numbers of juvenile herring in the Barents Sea in the previous years. The time series is too short to say anything relevant about the population trend on Jan Mayen.

Status and trend

In 1986, the breeding population on Bjørnøya was estimated to be 245,000 nesting pairs. The Bjørnøya population collapsed in the winter of 1986–87 due to lack of food, and only 10–15 % (ca 37,000 pairs) returned to the nesting ledges in 1987.

The population has been growing since 1987. A rapid recovery was recorded in the first two  years, indicating that some of the breeding population survived the winter of 1986–87, but refrained from nesting in 1987. The population was comparatively stable from 1989 to 1995, but began growing again after that. Viewed as a whole, the population has substantially recovered since 1986–87.

A total census carried out in 2006 gave a minimum of about 75,000 pairs. There is reason to believe that the number of common guillemots in the selected sampling areas shows an artificially high growth, since the species seems to be recolonizing the steep cliffs before the open, flat areas where it was very concentrated prior to 1987. The population growth since 2000 has been  7 % per year.

The growth in the population of common guillemots on Bjørnøya must be viewed in the context of the collapse in the population in the winter of 1986–87. The growth has remained comparatively smooth and stable since 2000. However, the species is still vulnerable and the population has not returned to the same level as before the collapse in 1986–87.

There is reason to believe that the most important impact factors that led to the decline in the Norwegian common guillemot population in the last 40 years (lack of food, by-catching and oil pollution) are still affecting the population or may potentially do so in the future.

In mainland Norway, predation by white-tailed eagles has recently become a problem in colonies that have been greatly reduced in numbers. The collective defence provided by many individuals nesting together ceases to have effect and the remaining nesting birds become predation victims for white-tailed eagles (or great black-backed gulls).

Causal factors

The cause of the collapse of the Barents Sea common guillemot population in the winter of 1986–87 was lack of food, chiefly capelin, in combination with climatic factors (Mesquita et al. 2015). This resulted from a corresponding collapse in the capelin stock, probably due to the immigration of large numbers of young herring into the Barents Sea. Simultaneously, no alternative prey was available.

Subsequent studies of common guillemot movements through the year have shown that common guillemots nesting on Bjørnøya overwinter in the southwestern Barents Sea, inside a triangle linking Bjørnøya to Lofoten and the Kola Peninsula.

The capelin stock has suffered several collapses since 1986–87 (e.g. 1995 and 2005), but these have not had a corresponding effect on the common guillemot population, probably because alternative prey has been available.

Several of the most important colonies in mainland Norway have not recovered since the collapse in 1986–87. This especially applies to those located west of North Cape, on Hjelmsøya in Finnmark and Røst in Nordland, for example.

The population decline in mainland Norway probably began as early as the 1960s in response to several factors, including by-catching in fishing gear, lack of food and oil contamination.

Consequences

The population of common guillemots on Bjørnøya is growing, but is still vulnerable and has probably still not reached the level prior to its collapse in 1986–87.

The Bjørnøya colony is the largest in Norway and the rest of the Barents Sea. The annual growth seen on Bjørnøya since 2000 gives grounds for believing that the colony also helps to recruit birds to colonies in Russia and on the Norwegian mainland, which have not recovered since the collapse in 1986–87. Bjørnøya may therefore be a source population that is valuable for recruitment to other colonies in the region, too.

About the monitoring

The common guillemot is a food specialist which, in the breeding season, chiefly lives on pelagic fish such as capelin and herring. This makes it a good indicator of changes in pelagic fish stocks and is one reason why it is monitored. It is also one of the most vulnerable species for by-catching in fishing gear and for oil pollution.

The common guillemot population has declined dramatically in the Barents Sea and the northern part of the Norwegian Sea, and the species is listed as "Critically Endangered" in mainland Norway and "Vulnerable" on Svalbard in the Norwegian Red List.

It is now being monitored on Bjørnøya, where by far the largest breeding colony in Norway and the Barents Sea is found.

Monitoring the number of individuals on nesting fledges is the best and the only internationally accepted method to reveal short-term and long-term changes in its population size. Annual counts in plots provide opportunities to reveal changes in the population, provided changes in plots are representative for the entire population. Several other parameters are monitored annually to explain and predict those changes. The most important are

  • reproduction (timing of breeding, chick growth, chick condition when leaving the colony, breeding success)
  • survival (adult survival)
  • prey (food choice)

In addition, TDR-loggers (time, depth and temperature recorders) and GLS loggers (Global Location Sensing) are being used to study activity patterns, migratory routes and winter ecology.

Places and areas

Relations to other monitoring

Monitoring programme
International environmental agreements
Voluntary international cooperation
Related monitoring