Vestmannaeyjar archipelago is located (63°26´N, 20°17´W) on Selvogsbanki 8–46 km off the southwest coast of Iceland. Apart from western (Reykjanesskagi) and eastern (Vestrahorn) extremities Iceland´s south coastline is characterized by glacial riverbeds of black lava sand. Vestmannaeyjar are volcanically active and composed of both lava and tuff. They are the only sizable islands along the whole south coast, characterized by steep grassy slopes and high cliffs. Vestmannaeyjar are more specifically located on the eastern fringe of the shallow (80 m) Selvogsbanki (10,700 km² within the 200 m isodepth) the continental shelf located approximately 20 km to the east. The south coast receives the output of most of Iceland´s the largest rivers, high in silicate (200 micro mol/L) (Stefánsson & Ólafsson 1991). The costal current flows through Selvogsbanki westwards on its clockwise route around Iceland. The Irminger current approaches Iceland in the southeast, and turns southwards along the Reykjanes ridge, with Selvogsbanki in the middle. Selvogsbanki is highly productive and also receives influx of phyto- and especially zooplankton by advection (Gislason et al. 2000). Selvogsbanki is Iceland´s key spawning area of cod, haddock, capelin, sandeel and other important fish species. Sandeel is the neritic ecosystem “wasp-waist” species, being the most abundant 1. level predator and forming the basis for all higher tropic levels ranging from predatory fish to seabirds and cetaceans. Vestmannaeyjar are surrounded to the west by vast areas of sandy bottom, an ideal sandeel burrowing habitat, and only about 20 km from the continental shelf to the east. Vestmannaeyjar are ideally located for foraging on both sandeels, and Euphusiids (mainly Meganyctiphanes norvegicus) in the east, and harbours now the largest puffin colony in the World, some 830,000 breeding pairs, along with kittiwakes (104,000 pairs), fulmars (47,000 pairs), of common guillemots (45,000 pairs), Northern gannets (30,000 pairs), Iceland´s only Leach´s storm-petrel colony (20,000 pairs) and virtually all Icelandic European storm-petrel population (10,000 pairs), and between 7–8000 pairs of razorbills and Manx´s shearwaters (Iceland´s only breeding colony), and other less common seabird species.

For this project the Atlantic Puffin was chosen on the basis of availability if long term data. Puffin is the most abundant breeding bird species in Iceland, and also the most harvested one. 

Puffin productivity has been measured in Vestmannaeyjar from 2007, but puffin pole-net harvest numbers have been compiled for the whole pole-netting record 1878–2008 (Hansen et al. MS). To make the harvest numbers more biologically interpretable, a virtual production index P(H.) was calculated based on average age composition of harvest of birds ringed as chicks. Harvest is 75% composed of immature birds 2–4 years old. The P(H.) calculation sums up each cohort relative size, accounting for survival to age using estimated 87% annual survival in Vestmannaeyjar (Helgason 2012). This index reflects cohort sizes relative to a reference year of maximum cohort size (1882) and is the longest time series on seabirds in existence. It is important to keep in mind that this production index is essentially a product of breeding success (fledglings/pair) and breeding population size in a given year. However, by using fixed age composition, biases small cohorts upwards, and large cohorts downwards in numbers, thus reduces cohort real size variation. To gauge this filtering, another production index P(town) was created based on number of fledglings, either ringed (1971–2002) or rescued by the public annually in the town of Vestmannaeyjar (2003–2023). Each series is standardized by dividing by their respective maximum values (figure 3.1). The number fledglings in town and direct production estimates are highly correlated.

Figure 3.1 Production indices of Atlantic puffins in Vestmannaeyjar, Southern Iceland 1878–2023.

A number of environmental factors have shown substantial changes in Icelandic waters in the last two decades. Most notable are 25% decline in winter Silicate levels since 2001 and simultaneous delay of phytoplankton bloom maximum by a month (Ólafsdóttir 2021; Pétursdóttir et al. 2021). Annual Chlorophyl production has increased substantially since 2010. Migration of mackerel into Icelandic waters increased dramatically since 2005, but the proportion migrating into south Icelandic waters has steeply declined since 2018 and moved to Norwegian waters. 

Strong relationship was found between Puffin production index and SST in Vestmannaeyjar from 1878 until 2005. The effect of temperature can affect puffin production via effects on sandeel egg development time (thus hatching time), winter survival, summer growth, and fertility by direct effect on basal metabolism (van Deurs et al. 2011; Wright et al. 2017; Régnier et al. 2018, 2019). After 2000, sandeels have been observed to be smaller for a given age, with lesser energy content (unpublished data), and 60% of 0-group below a minimum length threshold for winter survival. Warming of the Atlantic in 1995 coincided with a 50% reduction the abundance of northern krill in Icelandic puffins wintering area over the Atlantic ridge (Fayet et al. 2017; Edwards et al. 2021). Northern krill is likely key puffin prey in spring and might affect survival of puffin and many other seabird species wintering in the hotspot (Davies et al. 2021). The relative contributions of summer production or winter survival to the population trajectory are currently not clear.

In 2005 puffin production declined substantially faster than in the beginning of the last warm phase of the Atlantic Multidecadal Oscillation (1930–1965) despite similar SST increase as the current warm phase starting in 1995, suggesting another causative factor/s than SST alone. Furthermore, a large (1.5x SD, or 18 days) delay in Westman puffin breeding phenology (mean fledging time) was observed during this period (after 2005) in comparison to mean (mean fledgling time 1937–2023. This phenological delay is of great interest as it seems indicative of general trophic mismatch in Selvogsbanki, affecting not only production of puffin and their sandeel prey, but also many important commercial fisheries as demonstrated by (Brander et al. 2001).

Maximum cross correlations on the Puffin virtual production P(H.) from Iceland and Faroes together with Röst breeding success are significant, but with different lags, Faroes first, Iceland second and Norway last in the chronology (Table 3.1.).

Table 3.1 Maximum cross correlations (upper panel) and corresponding lags (lower panel) between puffin production in Faroes (Viðoy), Iceland (Westmans) and Norway (Røst). All three correlations are significant.

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