To map all distributions of all seabirds in all sea areas at any given time is, in practice, an impossible task. It would require a system that we do not currently have the technology or resources for. Therefore, we have limited the work both spatially and temporally, while simultaneously choosing species that we believe are important to focus on.

Geographical boundaries

All Norwegian sea areas, as well as adjacent areas where data were available, and which resulted in complete pelagic ecosystems when combined, were included in the analyses. The study area encompasses three more or less demarcated pelagic ecosystems: the North Sea (including Skagerrak and Kattegat), the Norwegian Sea and the Barents Sea. In total, the study area covers more than 1.2 million square kilometres in three seasons. North in the Barents Sea, the study area is limited by the distribution of ice in winter and summer. The study area in the Norwegian Sea and the Barents Sea is additionally limited by the data.

The boundaries of the study areas in different seasons. Map: SEAPOP

Dividing into seasons

The distribution of the seabirds changes throughout the year. It is therefore necessary to analyse different seasons separately. The seasonal division was done by taking into consideration the time of year when the seabirds generally are tied to the breeding area, and when it is assumed that the birds are tied to their overwintering areas. A balance was struck between the desire for the highest resolution possible with consideration to season, while simultaneously maintaining an adequate degree of data coverage for each season.

Three seasons were defined, and all analyses are run separately for each of these seasons. Summer is the period between April 1 and July 31. This season represents the time of year when most species will be tied to the breeding area. Autumn is the period between August 1 and October 31. This represents a period of moulting and migration for a number of species. Winter is the period between November 1 and March 31. This period represents the overwintering period.

Selection of species

In the analyses we have focussed on the most abundant species and species that are typical pelagic species. Purely coastal species are excluded, as these are mapped in other parts of the SEAPOP programme, and as the data collected for these species on “open sea” cruises are limited to those instances when the ships have sailed along the shore. Species were excluded from analyses in seasons and sea areas where they were so few in numbers that the analyses could not be conducted.

It is difficult to distinguish between common guillemots and Brünnich’s guillemots in winter, because the winter plumages of the two species are very similar. This is particularly a problem in the Barents Sea, where these two species readily flock together at this time of year. In the winter datasets from the Barents Sea, 21,855 individuals were identified as Brünnich’s guillemots, 1,489 individuals were identified as common guillemots, and 15,726 individuals were classified as unidentified guillemots. Since 94% of the identified individuals were Brünnich’s guillemots, we assumed that a large portion of the unidentified individuals would also be Brünnich’s guillemots. We therefore combined the unidenified category with the Brünnich’s guillemot category, while the common guillemots were separately analysed. This will lead to a slight underestimation of common guillemots and a slight overestimation of Brünnich’s guillemots in the Barents Sea in wintertime.

In 2010, eight of the twelve analysed species were classified as either near threatened, vulnerable, endangered or critically endangered on the Norwegian Red List for Species. The species in question are the Atlantic puffin, razorbill, Brünnich’s guillemot, common guillemot, northern fulmar, black-legged kittiwake, common gull and glaucous gull.