The central coast of Norway between Stad (62°11’N) and Andfjorden (69°N) in Vesterålen forms the eastern limit of the Norwegian Sea. This area is not only characterised by a rugged landscape with deep fjords sheltered by thousands of islands on the outer coast but also has a large shelf continental that on the widest extends more than 200 km offshore.

As discussed in other parts of this report, the Norwegian Coastal Current (NCC) runs along the entire Norwegian coast, from the Skagerrak and the eastern North Sea in the south to the southern parts of the Barents Sea in the northeast. It transports a wide range of zooplankton and spawning products of fish that forms an important food base for a variety of marine life, including many species of seabirds. A branch of the East Atlantic Current (EAC) runs offshore of and parallel to the NCC, and the interaction between these currents, the bottom topography and wind-induced turbulence are likely to affect both the advection of nutrients and zooplankton onto the shelf, and the growth and retention time of fish larvae and forage fish (e.g. Sætre et al. 2022a).

Immediately upstream of the widest shelf area, the extremely offshore archipelago Røst is located 100 km west of the mainland coast (Bodø), 50 km SW of the Lofoten Islands and about 100 km east of the shelf edge (Eggakanten). This strategic position makes it an excellent breeding place for pelagic seabirds. The long distance to the mainland coast, where local fjord systems may provide alternative sources of food when the food availability offshore is poor, also makes Røst the ideal place to study how physical and lower trophic level processes affect the performance of long-ranging pelagic seabird populations. In the southwestern part of Røst, five steep bird cliffs surrounded by many lower islands used to hold the world’s largest breeding population of Atlantic puffins Fratercula arctica (hereafter puffin), substantial numbers of black-legged kittiwakes Rissa tridactyla (hereafter kittiwake), common guillemots Uria aalge and razorbills Alca torda, as well as a wide range of more coastal seabirds.

In the context of this project, we chose to focus on breeding puffins and kittiwakes, which in Røst are documented by GPS tracking to feed far offshore in their search for prey to sustain their offspring (Anker-Nilssen & Lorentsen 1990; Anker-Nilssen & Aarvak 2009; Fayet et al. 2021; Fayet et al. in prep.). Whereas the puffin is a medium-sized auk that dives for prey down to > 50 m depth (e.g. Piatt & Nettleship 1985), the kittiwake is a small surface-feeding gull that only can plunge-dive down to < 1 m depth and thus depends on finding prey in the top surface layer. The puffin usually feed their chick bill-loads of small fish, whereas the food the kittiwakes regurgitate to their chicks at times also contains a significant proportion of zooplankton, most often krill (Euphausiidae). In contrast to puffins, the kittiwakes sometimes also search food in near-shore waters, especially when local abundance of age 0 saithe there is high (T. Anker-Nilssen, pers. obs.). These two seabird species thus reflect two different foraging niches in the pelagic ecosystem offshore, and the foraging ranges of both species in the breeding season cover the entire extent of the shelf area around Røst.

Productivity has proven to be the most important demographic factor determining the population growth rate for puffins in Røst (e.g. Layton-Matthews et al. 2023). Although both puffins and kittiwakes are highly philopatric, they have delayed maturity and don’t start breeding until age 3–5 (kittiwakes) or 5–7 years (puffins). Changes in breeding numbers therefore reflect mainly lagged effects of immature survival and recruitment and are consequently not a very precise indicator of the conditions the birds experience in the breeding areas. This is further substantiated by both species travelling far away to spend the non-breeding season in very different parts of the North Atlantic (Frederiksen et al. 2012; Fayet et al. 2017). From the same line of arguments, survival rates are even less accurate indicators of breeding conditions. Seabird mortality usually peaks in winter, and populations which breed far apart but overlap in wintering areas, exhibit temporal synchrony in survival rates (Reiertsen et al. 2021). Although carry-over effects from environmental conditions experienced on the wintering grounds to some extent may also affect the birds breeding performance (Bogdanova et al. 2017; Keogan et al. 2022; Charrier et al. 2024), productivity is still by far the best parameter linking seabird demography with trophic interactions in the breeding area.

The productivity of puffins and kittiwakes in Røst has been monitored annually since 1964 and 1979, respectively (Fig. 2.1), and are as such among the longest data series for seabird performance in the world (Cury et al. 2011; Sydeman et al. 2021). Besides long-term reproductive problems driven by poor food supply, the breeding success of kittiwakes in the largest natural cliff (25,000 pairs in 1979) has more recently also been severely affected by disturbance and predation from an increasing number of immature white-tailed eagles Haliaeetus albicilla (Anker-Nilssen et al. 2023) that spend the summer in Røst, and the colony went extinct in 2020. The resurrection of the white-tailed eagle population in Norway is the result of targeted management actions on land and not directly related to changes in the marine ecosystem. To avoid this source of bias, we therefore chose to use the almost equally long data set from a colony breeding on buildings in the Røst harbour area, where human presence has kept the eagles on a distance and breeding success is unaffected by eagle numbers (Anker-Nilssen et al. 2023). For puffins, the eagles are not a huge problem as they breed sheltered in earth burrows and rock crevices that eagles are unable to enter. The data series used is the proportion of puffin chicks that survived the nestling period (fledging success) in each year, as registered for a selection of breeding burrows on the island of Hernyken. This island holds 8.5% of the total population of puffins in Røst, which counted approximately 1.4 million breeding pairs in 1979 (Anker-Nilssen & Øyan 1995). Based on the census and monitoring method developed by Anker-Nilssen & Røstad (1993), the population size of puffins in Røst 2023 was estimated at 208,500 pairs (SE=10,988), corresponding to only 15% of the initial population estimate (T. Anker-Nilssen, unpubl. data).

Figure 2.1 Productivity of Atlantic puffins (chick fledging success) and black-legged kittiwakes (chicks fledged per pair) in Røst, northern Norway 1964–2023. Estimates for years before 1980 are based on semi-quantitative assessments published by Lid (1980).

The trophic link between the availability of age 0 herring produced by the Norwegian spring-spawning (NSS) stock and the productivity of puffins in Røst, is a classic example of the high importance of forage fish as food for breeding seabirds (e.g. Anker-Nilssen 1992; Durant et al. 2003; Durant et al. 2005). Both the breeding success of the Røst puffins and the size of herring in their chick diet, have proven good indicators of the year-class strength of NSS herring (Sætre et al. 2002b, Durant et al. 2003), the longer-term validity of which is currently being re-examined (Walnum et al. in prep.). The threshold of this relationship also fits a global pattern, showing that breeding success of seabirds is severely reduced when the abundance of their key prey drops below a third of its historical maximum (Cury et al. 2011). As seabirds only consume a small fraction of their prey population (Saraux et al. 2021), this threshold most likely reflects the abundance needed for the birds to find enough prey to sufficiently sustain their offspring without spending too much time and energy that could jeopardize their own survival. This life-history balance is typical of long-lived seabirds, and likely depends not only on the abundance of prey but also on the abundance of seabirds competing for the same prey (Fayet et al. 2021).

The northward drift of herring and cod larvae with the NCC explains the distribution of the largest bird cliffs along the Norwegian coast, with most colonies found where these larvae appear most abundant and predictable between years in the breeding season for seabirds (Sandvik et al. 2016). This illustrates the importance of these fish as food for seabirds, as well as the value of the retention areas for young herring on the vast shelf areas surrounding Røst. Further north, age 0 cod is shown to be an important diet component of adult common guillemots breeding in the southwestern Barents Sea. It also affects the breeding success of this species there, even if the adults only carry one fish at a time and therefore need to raise the chick on larger fish prey of other species, such as sandeel and capelin (Myksvoll et al. 2013).

Compared to interactions with forage fish, the direct or indirect value of crustacean zooplankton for seabirds is less well studied on the Nordic shelves. As for the puffins, the productivity of the kittiwakes in Røst is linked to the quality of herring larvae (Anker-Nilssen et al. 2023), yet diet samples for this population are fewer and still being worked up for further analyses. However, in periods when schooling-size age 0 herring (> c. 45 mm long) are plentiful, this is also clearly the most abundant prey for the kittiwakes, even if they load small prey more easily than puffins and therefore can keep their offspring alive for longer periods by feeding them krill when fish prey is scarce. This is easily detected in the colony because the half-digested crustaceans stain the birds’ plumage orange when the adults regurgitate the food to their chicks. In line with this, the breeding success of kittiwakes is highly and positively correlated (r²=0.66) with the ICES/PGNAPES abundance indices for zooplankton in the Norwegian Sea basin in May, while there is no such relationship for the puffins (r²=0.06). Kittiwake breeding success also correlates to some extent with the SPG index in the previous year (r²=0.22) and in the last winter (r²=0.24), but similar relationships are not evident for the puffin (both r²<0.01).

The pooh of adult puffins is sometimes reddish in colour, indicating they may sometimes feed themselves on crustaceans that are too small to carry efficiently in the bill to support the chick. This is most often seen at the start of breeding and during incubation. Interestingly, puffin breeding success correlates better with zooplankton indices measured NE of Iceland (r²=0.18) than with those in the other areas of the Norwegian Sea (r²<0.09).

There are also occasional events on lower trophic level that can affect seabird productivity. Good examples are the occurrences of unusual prey species that turn up in the seabirds’ chick diet. In the early 2000s, poor quality snake pipefish Entelurus aequoreus constituted a significant part of the diet for pelagic seabirds in many NE Atlantic colonies (Harris et al. 2007; Harris et al. 2008), including also puffins and kittiwakes in Røst (Anker-Nilssen & Aarvak 2006). In 2023, age 0 mackerel was the most frequent prey fed to puffin chicks in Røst in late summer, an event that had not been seen since 1974. Although this contributed to explain a moderate breeding success, ongoing analyses indicate that the size of age 0 herring in the diet of puffin chicks in Røst is negatively correlated (r²=0.53) with the proportion of age 0 mackerel within the Norwegian Exclusive Economic Zone (EEZ NO) in the North Sea in the previous winter (range 5.8–17.4%, ICES/IBTS data 1998–2020). This could suggest that this part of the mackerel year classes follows the NCC northwards where they prey upon herring larvae (and their crustacean prey), and/or that parts of the spawning stock also can follow this route and add to that top-down effect. A significant amount of age 0 mackerel was also found in the diet of puffins breeding on Runde, immediately north of the North Sea border in western Norway (Noever et al. unpubl. data). Still, diet studies of mackerel in the eastern Norwegian Sea in spring and summer are scarce. One study made along a large part of the Norwegian coast in July 2018 showed low proportions of Teleostei prey in both immature and adult mackerel at most stations sampled (Bjørdal et al. 2022). The proportion of age 0 mackerel in the EEZ NO in the preceding winter (9.9%) was however rather low, suggesting mackerel was less abundant on the Norwegian shelf that year. 

Although the above correlations are interesting and warrants further research, it is still not known in much detail how the breeding performance of puffins and kittiwakes in Røst is linked to the large-scale oceanographic mechanisms in the Northeast Atlantic. Local physical properties of the seascape off central Norway may also act to mask the larger-scale processes. This vast shelf area has a varied bottom topography with trenches and shallow bank areas that affect the northward flow of NCC water, creating rings, eddies and jets that increase the retention time of fish larvae and other planktonic organisms (Sætre et al. 2002a). In concert with the parallel EAC and onshore/offshore winds, this is also likely to affect local advection and upwelling of zooplankton and mesopelagic fish onto the shelf. How these physical mechanisms act to affect food availability for breeding seabirds is poorly understood. Their importance is however substantiated by the fact that sea temperature and salinity within the NCC off the Lofoten Islands in March, i.e. two months prior to egg laying, explained more of the variation in puffin breeding success on Røst than the abundance or quality (size) of their main prey, age 0 herring (Durant et al. 2003; Durant et al. 2005; Durant et al. 2006; Walnum et al. in prep.).

Effects of large- and mesoscale oceanographic processes on the Norwegian shelf ecosystem. As detailed in other chapters of this report, large-scale oceanographic variability is an important driver of the productivity of Nordic shelf ecosystems. This includes evident effect of the Subpolar Gyre (SPG) on seabird productivity in the Faroes (Hátún et al. 2017; Olsen et al. in prep.). Both the SPG and Modified East Icelandic Water (MEIW) transported by the East Icelandic Current (EIC) bring nutrient-rich water into the Northeast Atlantic. The effects of these physical processes on the productivity of keystone species of plankton, fish and top predators on the Norwegian shelf are however less well understood and warrants further research.

Top-down effects of mackerel as a predator on seabird prey and lower trophic levels need to be studied in more detail. The northward expansion of Northeast Atlantic (NEA) mackerel in summer has apparently had several effects on seabirds. The occurrence of age 0 mackerel in the diet of puffins is one effect. The establishment of the world’s northernmost colony of northern gannets Sula bassanus on Bjørnøya is probably another. Gannets plunge-dive to several meters depth and are among the few seabirds that can feed efficiently on older mackerel than age 0.

Spatial and temporal dynamics of seabird foraging habitat use. Along the Norwegian coast, twenty large marine areas have recently been identified as especially attractive for establishing offshore wind power development, provided impacts on the environment are assessed to be acceptable (https://www.miljodirektoratet.no/aktuelt/nyheter/2023/april-2023/20-mulige-havvind-omrader-langs-hele-kysten/). The largest of them, Nordvest A, is a 11,300 km² area situated in the middle of the eastern Norwegian Sea shelf, approximately 40 km off the mainland coast and 100 km southwest of Røst. On this background alone, it is imperative to understand how different pelagic seabird species (including kittiwakes, puffins, and several other species) utilise different types of foraging habitats on this shelf in relation to variation in food availability, as reflected by what they bring to their chicks. For instance, it may seem that kittiwakes breeding in Røst travel far offshore (off the shelf edge) when their diet is dominated by krill, whereas they spread over large parts of the shelf when feeding on herring larvae or feed in near-shore waters (within Røst or along nearby Lofoten Islands) when the availability of age 0 saithe is high. Some of these patterns are currently being explored by the SEAPOP programme (www.seaopp.no/en), including the foraging behaviour of chick-feeding kittiwakes tracked with GPS from Røst each summer since 2017 (Fayet et al. in prep.).

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