The study area comprises the NE Atlantic from (mainly South) Iceland, via Faroe Islands towards the Norwegian Shelf up to around the Lofoten Islands (Figure 1.1). The subpolar gyre is a large body of water that holds large concentrations of nutrients and zooplankton that are transported northwards with the North Atlantic Current. The East Icelandic current flows from East Iceland towards Faroe Shelf and further onto the Norwegian shelf and therefore affects all three shelves. The results will first be presented by the three shelves, then by trophic level and finally a combined analysis is presented. In every case a comparison with the subpolar gyre and the East Icelandic Current (EIC, or MEIW) is made. The subpolar gyre was largest in the mid 1990s but was small from 1997 to 2013. A peak occurred around 2017, but has decreased afterwards. The East Icelandic Current was strong in the 1990s but was weak from 2003 to 2018 (Figure 1.2). Figure 1.2 is the basis for the results if not stated otherwise.

On the Icelandic shelf, sea surface temperatures have been increasing since the mid-1990s and an increase in the recruitment of haddock was observed (note that temperature is inverted in Figure 1.2). Zooplankton south of Iceland demonstrates peaks 5–10 years apart up to 2002, but past 2002 only one peak has been observed (in 2014). Sandeels were abundant in the 1990s, but began to decline in 2000 with poor period from 2004 to the peak in 2014. The peak in 2014 was mostly sandeels off the Westfjords. In the last four years there has been notable increase mostly due to sandeels off the south and west coast. Capelin recruitment was weak in the period from 2003 to 2019, but has since improved – much the same as the development in EIC (MEIW). Puffin productivity showed periodic changes, with the 2005–2013 period being particularly poor. Cod recruitment was higher prior to 1986, but has remained lower since then.

Figure 1.2 Normalised variables (values minus average and then divided by standard deviation) associated with the three Nordic shelves. SPG: Subpolar gyre with large values showing a large gyre, MEIW: Modified East Icelandic Water, SPG+MEIW: the sum of them, SST: Sea surface temperature (low values indicating high temperatures). Zooplankton: measured at Selvogsbanki in May. Icelandic Sandeels: percentage of cod that were eating sandeels in October from Ingólfshöfði to Hornbjarg in the Westfjords (clockwise). Icelandic Capelin: recruitment of 1–2 year old capelin. Icelandic puffin: production of puffins in town in Vestmannaeyjar. Icelandic cod: recruitment at age 3 shifted to y-2. Icelandic haddock: recruitment at age 2 shifted to y-1. Faroe zooplankton: Calanus finmarchicus and Calanus hyperboreus north of Faroe Islands, Faroe sandeels: percentage in cod that were eating sandeels in March and October, Faroe cod and haddock: recruitment at age 1. Faroe puffin: production in Viðoy or in Mykines, Faroe kittiwakes: breeding success, Norwegian zooplankton: small and large zooplankton at the Svinøy section, Herring recruitment at age 2 shifted to y-2. Norwegian coastal cod north of 67^oN: recruitment at age 3 shifted to y-2. Norwegian puffin: breeding success, Norwegian kittiwakes: breeding success. Each time series is offset to different intervals on the y-axis for better comparison.

On the Faroe shelf, sea surface temperatures have also increased in recent decades (inverted values Figure 1.3). Zooplankton abundances north of the Faroes in the southern Norwegian Sea were low from 2003 to 2018, thus mirroring the development in the MEIW. Puffin production seemed to follow a pattern that was influenced by both the SPG and MEIW. Sandeels were abundant in 2001–2003 and in 2017–2018 but were else infrequent. Puffin production seemed to follow a pattern that was influenced by both the SPG and MEIW. Cod and haddock recruitment fluctuated normally in the same way with peaks prior to 2002 and a poor period from 2003 to 2016.

On the Norwegian shelf, sea surface temperatures were highest between 2003 and 2017, but have decreased afterwards. Zooplankton biomass was high in the 1990s, followed by low values from 2004 to 2015, and somewhat higher afterwards. Cod recruitment was strong in the 1990s followed by low values from 2003 to 2019, after which an increase has been apparent. Puffin production has shown prominent on-off periods that match the year class strength of Norwegian spring-spawning herring).  

Zooplankton abundance values north of Faroe Islands and on the deep stations in the Svinøy section showed the same time development with high values in the 1990s and a poor period from 2003 to 2014 after which there was an increase. This development is similar to the SPG and MEIW. The zooplankton on the shallow Selvogsbanki in Iceland showed a different pattern, but some of the peaks overlapped.

Forage fish seemed to be linked to the oceanographic features, i.e., the SPG and MEIW, especially MEIW and capelin at Iceland. There was probably a signal that first occurred for capelin at Iceland, then in sandeels at Iceland, then sandeels at Faroes and lastly for the Norwegian spring spawning herring recruitment indicating a delayed timing of the effects of the environmental features moving eastwards (Appendix: Figure A1.1).

Demersal fish recruitment was somewhat different for the three shelves. While there was a low period from 2003 to 2014 for the Faroe and Norwegian shelves, the pattern was different for the Icelandic shelf that partly showed a more constant level, although with occasional peaks, after 1993.

Puffin production was quite similar for the Icelandic and Faroese populations with a prominent poor period from 2004 to 2013 that matched the development in SPG and MEIW. The Norwegian population showed a somewhat different pattern that, as already seen, matched the year class strength of Norwegian spring spawning herring (Figure 1.2). The poor period on the Norwegian shelf started a few years later than in Iceland and the Faroes but also lasted a few years longer, with clear signs of improvement only in the last two years.

All the trophic levels are assembled in a single figure in order to facilitate comparisons between the trophic levels as well as between the shelves (Appendix: Figure A1.1). A cyclical pattern is apparent for most of the data series with probably exceptions of zooplankton at Iceland, cod and haddock recruitment at Iceland and puffin productivity at Røst, Northern Norway (Figure 1.3).

A syncronous cyclical pattern was observed for most of the data series where the period from 2003 to 2014 was particularly poor, although an increase was observed afterwards. Comparing the variables with SPG or MEIW showed positive correlations, especially when SPG and MEIW were combined into a single index (Figure 1.3), indicating that nutrient- or zooplankton-rich water may come from both these sources and stimulate the productivity on the three shelves. Interestingly, the recent increase in the MEIW may nearly be counterbalanced by the decrease in SPG. Comparing with SPG and MEIW 1–3 years before indicated that a lag of 1–3 years gave higher correlations (Table 1.1). Combining the three Nordic shelves by taking the average of the productivity in the three shelves gave the strongest correlations (Table 1.1).

Figure 1.3 Average productivity for the Nordic shelves compared with oceanographic features. SPG: Subpolar gyre with large values showing a large gyre, MEIW: Modified East Icelandic Water, SPG+MEIW: the sum of them, SST: Sea surface temperature at the Faroes (low values indicating high temperatures). Each time series is offset to different intervals on the y-axis for better comparison.

Table 1.1 Aid to interpret Figure 1.3: Correlation coefficients when comparing the Subpolar gyre index (SPG), Modified East Icelandic Water (MEIW) and  SPG and MEIW combined (SPG+MEIW) with the average productivity for the Icelandic, Faroese, Norwegian shelves and an average of the three shelves (Nordic). Time lags up to 3 years are shown.