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Fauchald P, Anker-Nilssen T, Barrett RT, Bustnes JO, Bårdsen B-J, Christensen-Dalsgaard S, Descamps S, Engen S, Erikstad KE, Hanssen SA, Lorentsen S-H, Moe B, Reiertsen TK, Strøm H, Systad GH (2015) The status and trends of seabirds breeding in Norway and Svalbard – NINA Report 1151. 84pp. This report presents the updated sizes, trends and spatial distributions of the breeding populations of 17 seabird species breeding in Norway and Svalbard. The analyses are based on available census and monitoring data from SEAPOP; the Norwegian monitoring and mapping program for seabirds. In addition, the report presents results from a species-specific literature review of the most important prey items and drivers of population change. The report documents large-scale decadal changes in the seabird communities along the coast of Norway and Svalbard. A division of the populations into five geographical regions (North Sea & Skagerrak; Norwegian Sea; Barents Sea; Bjørnøya; and Spitsbergen) was used as a basis for the analyses of population dynamics from 1980 to present. 13 of the 35 regional seabird populations assessed have declined by more than 50% the last 25 years. 5 regional populations increased by more than 100% in the same period, while 8 populations showed large decadal fluctuations. Several populations were not assessed due to the lack of census and/or monitoring data. In order to improve the dataset, it is recommended that a census of breeding seabirds from Vesterålen to the Swedish border is completed. Declining populations were found in all regions and included all major ecological groups (i.e.; Pelagic surface-feeding (Ps), Pelagic diving (Pd), Coastal surface-feeding (Cs), Coastal benthic-feeding (Cb) and Coastal diving (Cd) seabirds). Populations with more than a 50% decline the last 25 years were: Common Gull (Cs), Lesser Black-backed Gull (Ps) and Atlantic Puffin (Pd) in the North Sea & Skagerrak; Great Cormorant (Cd), Common Eider (Cb), Black-legged Kittiwake (Ps) and Common Guillemot (Pd) in the Norwegian Sea; Herring Gull (Cs), Great Black-backed Gull (Cs), Black-legged Kittiwake (Ps) and Brünnich’s Guillemot (Pd) in the Barents Sea; Northern Fulmar (Ps) and Glaucous Gull (Ps) on Bjørnøya; and Brünnich’s Guillemot (Pd) on Spitsbergen. The populations of European Shag and Great Cormorant have shown large fluctuations with a notable increase in the population of Phalacrocorax carbo sinensis in North Sea & Skagerrak. Common guillemot has been increasing in the Barents Sea since the collapse in the population in the 1980s, however the population in the Norwegian Sea has been steadily declining since the early 1980s. Atlantic Puffin is declining in the North Sea and Norwegian Sea, but the population in the Barents Sea is stable or is increasing slightly. The datasets were too small to assess several of the large gull species in the Norwegian Sea. However, extensive monitoring in the North Sea & Skagerrak and recent censuses in the Barents Sea suggest declines by more than 50% in several of the gull populations in these areas. Black-legged Kittiwake has declined in all regions except for Bjørnøya. The large colonies of Brünnich’s Guillemot on Spitsbergen have declined from 1.15 million pairs in 1988 to 522 000 pairs in 2013. The colony on Bjørnøya (about 100 000 pairs) has in the same period been stable or declined slightly, while the small populations on the Norwegian mainland have almost disappeared. Northern Gannet has been increasing in Norway since the establishment of this species on Runde in the 1940s. The species has expanded northward and has recently established a small colony as far north as Bjørnøya. The review of diet studies highlighted the importance of the young age-classes of cod fish, the importance of pelagic forage fish species and in particular the importance of sandeel. However, the differences in diet among ecological groups combined with the fact that declining seabird populations were found in all regions and included all major ecological groups suggest that the recent changes in Norwegian seabird communities cannot be explained by changes in the abundance of a single group of resources alone. On the contrary, this might suggest a combined effect of simultaneous changes in several prey items, possibly involving entire trophic levels. Alternatively, it might suggest that bottom-up regulation through food is less important, and that top-down mechanisms such as anthropogenic stressors and predation are more involved in the present changes. A large number of studies have been conducted to investigate how different anthropogenic and environmental factors affect seabird populations. Factors such as fisheries by-catch, harvest and intentional killing, pollution and disturbance are all anthropogenic stressors with a welldocumented negative impact. Although most of these stressors have been reduced in Norwegian waters due to the implementation of regulatory mechanisms and protection measures, they might still have impact on local populations. For example, the decline in the population of Glaucous Gull on Bjørnøya has been related to high levels of persistent organic pollutants. Several case studies suggest that predation from avian and small mammalian predators in the seabird colonies might be important, and we cannot exclude this driver as an important mechanism behind the observed declines. The large spatial and the relatively long temporal scale of the population changes observed in the present report, might suggest that fluctuations in the marine ecosystems, possibly partly due to climate change and past and present fishing pressures, might be important. This is corroborated by numerous studies documenting a direct impact from food deprivation and an indirect impact from climatic factors on seabird population dynamics. Such factors often involve complex indirect trophic links which make it difficult to point out the ultimate cause of the observed change. We conclude that the two most likely candidates to explain the recent declines in Norwegian seabird populations are 1) increased predation in the seabird colonies from avian and mammalian predators and 2) ecosystem changes affecting the availability of prey. The impact from these drivers might be difficult to document and even more challenging to control. In contrast, more easily managed direct anthropogenic stressors such as fisheries by-catch, pollution, hunting and disturbance have either been constant or have shown a decreasing trend. Although these drivers cannot explain the recent population declines, they still contribute to the cumulative impact on seabird populations and these stressors are therefore especially important to control and minimize in rapidly declining and threatened populations. © Norsk institutt for naturforskning. Publikasjonen kan siteres fritt med kildeangivelse.