Our results support the hypothesis, that biomass of upper trophic levels on the Faroe shelf is driven by both on-shelf production and off-shelf food abundance. Using a simplistic straightforward approach, that considers the volume of SAW as the sole off-shelf driver and the 0-group length index as the on-shelf production indicator, we can explain three phenomena since the late 1970s: Faroese cod stock recruitment, cod stock biomass development and guillemot attendance at the Faroese seabird cliffs.
The previous proposed linkage between eastward transports from the Icelandic tongue and the volume of SAW at both the northern Faroe slope (Section N) and along the Norwegian slope (Svinøy Section) (Hátún et al., 2022b; Kristiansen et al., 2019; Skagseth et al., 2022) is supported. Our SAW volume indicator therefore likely represents a fundamental aspect of the marine environment in the entire southern Norwegian Sea. Our findings demonstrate a marked shift to subarctic conditions in the 2020s, similar to a shift that took place during the early 1990s (Fig. 2.3). The marine environment was also highly subarctic during the late 1970s, before shifting to an Atlantic state in the late 1980s, with a similar abrupt shift to an Atlantic state in 2003. The rapid ‘subarctification’ in the 2020s resulted in a dramatic increase in subarctic zooplankton abundance north of the Faroes in May, which strongly supports the previous hypothesis (Kristiansen et al., 2019; Skagseth et al., 2022) that a variable influx from the Icelandic tongue regulates the zooplankton community in the southern Norwegian Sea. This large zooplankton increase is, however, not reflected in broad-scale zooplankton biomass estimates in the Norwegian Sea (ICES, 2024). The discrepancy between localized abundance and regional biomass measurements highlights the complexity of marine ecological assessment. Determining the ecological importance of species-specific zooplankton indicator records compared to coarse biomass indicators warrants a more in-depth discussion.
Our new hypothesis can explain why seemingly favorable conditions - some peaks with strong on-shelf primary production, and numerous and large juvenile fish in the summer, do not translate to high adult cod stock numbers. The weaker than expected cod recruitment peaks (1987–1988, 2008–2009 and 2016–2017) coincide with Atlantic conditions, characterized by small SAW volume and, by inference, limited ocean-to-shelf food supply throughout the year. While abundant food supply shortly after hatching is certainly a prerequisite for a good year-class, young fish need continuous food supply throughout the year, and for several years, which the relatively short spring/summer bloom cannot sustain. Ocean-to-shelf food transport can benefit many marine species (e.g. greater sandeel – Hyperoplus lanceolatus) providing larger cod with a richer and more persistent diet. Guillemots are known for their ability to dive deep (up to 150–200 meters) to catch prey, making them efficient marine hunters, but their prey size is generally limited to smaller fish species (Piatt & Nettleship, 1985). This seabird species feed both on and off the Faroe shelf (Olsen et al., submitted), and thus also over region directly influences by the SAW. However, elaborating of this bottom-up trophic cascade is beyond the scope of the present work.
Strong recruitment peaks consistently lead to a marked increase in adult cod stock biomass within 1–3 years. The low SAW volumes and low recruitment peaks quickly resulted in major declines in the cod stock biomass in the late 1980s and in the 2000s. Although the SAW volume indicator and cod biomass are not strongly correlated, they share important characteristics. Excluding the 2020s peaks in the SAW volume, both series clearly show a negative trend. It is evident that subarctic periods have led to higher biomasses, while Atlantic periods result in reduced biomasses. Excessive fishing pressure has certainly contributed to these declines, but, the explanation power of our new hypothesis suggests that consideration of both on-shelf production and off-shelf marine climate and food abundance could complement management of this stock.
The cod stock has, however, not reacted to the 2020s increase in SAW volume and increased abundance of oceanic subarctic copepods, as our hypothesis would predict. Given the cod recruitment estimate is uncertain for the recent 2–3 years (Beverton & Holt, 1993), it is possible that the Faroe cod recruitment and biomass are somewhat higher than indicated by the presented assessment records.
A strong correlation between cod biomass and guillemot attendance in the Faroes enables using the guillemot record as an assessment-independent indicator for the cod stock. The low guillemot numbers in recent years further support evidence of a very low cod biomass, validating the cod assessment. Guillemot attendance numbers have declined dramatically since the 1970s, with significant decreases in the late 1980s and early 2000s (Olsen et al., submitted). Rounded guillemot peaks, which precede the cod biomass peaks by 1–3 years, concur with peaks in cod recruitment.
The major decline in Faroe guillemot populations initiated in the 1960s, and continuing until present remains a puzzle to seabird scientists and hunters alike. This negative trend was linked by Olsen et al. (submitted) to changes in the marine environment, but the here demonstrated close link to SAW volume provides a new perspective. Hunting has, as with the cod, certainly put additional pressure on the guillemot population, but this cannot account for the population growth during the 1990s and the peak in 2008–2009. And, like with cod, there is a stark contrast between the 2020s SAW increase and the low guillemot attendance counts.
However, the 2020s discrepancy may not invalidate our hypothesis. The cod stock on the adjacent Faroe Bank started to rise in the late 2010s, and increased sharply in 2022, exceeding the previous high levels of the 1990s and early 2000s. Relationships between the 0-group length index, haddock recruitment, and SAW volume are similar to the presented cod linkages, and haddock recruitment and biomass increased during the 2020s (Fig. A2.1), supporting our hypothesis. Furthermore, some seabird species have experienced improved conditions in recent years (e.g. Faroe puffins and kittiwakes, Olsen et al., submitted).
Given the recent high SAW volume, large abundance of subarctic oceanic zooplankton, a moderate lower-level on-shelf production peak in 2023–2024 (Olsen et al, submitted) and the above-mentioned improvement for other species, the persistently low cod/guillemot populations appear counterintuitive. This may be because intense fishery has reduced the cod stock to levels too low for recovery, consequently impacting the guillemot population more severely than other seabird species (puffins and kittiwakes). Additionally other environmental drivers beyond SAW volume, such as the long-term decline in pre-bloom silicate concentrations in the Atlantic water passing the Faroe plateau (Hátún et al., 2022a, 2017) may have impacted the cod-guillemot trophic pathway more severely than other trophic paths. Although marine scientists often associate environmental drivers with seawater temperature (Edwards et al., 2021), we do not consider this the sole driver of the discussed trends. While the unproductive Atlantic period (2004–2014) was associated with high temperatures, the highly productive period of the late 1950s to the early 1960s experienced equally high temperatures (www.hav.fo). Correlations between the presented biological time series and temperature are, furthermore, not significant.
A third possibility is that according to our hypothesis, an anticipated cod-guillemot increase has indeed taken place around 2023–2024, but remains not evident in data on the populations sizes of adults – yet. Under this scenario, we should expect a marked increase in cod biomass and guillemot attendance during the coming year(s). Similar conditions potentially occurred in the early 1990s; SAW volume reached a high level in 1993, followed by high on-shelf production (proxied by the 0-group length index) in 1994. Cod recruitment increased dramatically in 1994, and cod biomass rose from 1991 levels, which were comparable to current very low estimates, to very high values in 1995 and 1996 (Fig. 2.4b). Notably, due to poor fishery in the late 1980s to the early 1990s, the Faroe Islands went bankrupt and the fishing fleet was largely dismantled in the early 1990s, resulting in very low fishing pressure (ICES, 2023).