Changes in Icelandic groundfish community structure based on life- history strategies

Ólafur Ármann Sigurðsson

Auðlindadeild Viðskipta- og raunvísindasvið Háskólinn á Akureyri 2021

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Changes in Icelandic groundfish community structure based on life- history strategies

Ólafur Ármann Sigurðsson

90 eininga lokaverkefni sem er hluti af Magister Scientiae-prófi í Auðlindafræði

Leiðbeinandi Steingrímur Jónsson

Meðleiðbeinandi Jón Sólmundsson

Auðlindadeild Viðskipta- og raunvísindasvið Háskólinn á Akureyri Akureyri, maí 2021

Titill: Changes in Icelandic groundfish community structure based on life-history strategies Stuttur titill: Icelandic groundfish and life-history strategies 90 eininga meistaraprófsverkefni sem er hluti af Magister Scientiae- prófi í viðskiptafræðum/raunvísindum Höfundarréttur © 2021 Ólafur Ármann Sigurðsson Öll réttindi áskilin

Auðlindadeild Viðskipta- og raunvísindasvið Háskólinn á Akureyri Sólborg, Norðurslóð 2 600 Akureyri

Sími: 460 8000

Skráningarupplýsingar: Ólafur Ármann Sigurðsson, 2021, meistaraprófsverkefni, Auðlindadeild, viðskipta- og raunvísindasvið, Háskólinn á Akureyri, 81 bls.

Akureyri, maí, 2021

Ágrip

Margvíslegar breytingar hafa átt sér stað á útbreiðslu og stofnstærð botnfiskategunda á landgrunninu og út á landgrunnshlíðina við Ísland á síðustu tveim áratugum, sem hafa einkennst af tiltölulega háum sjávarhita. Meginmarkmið rannsóknarinnar er að kanna breytingar á botnfiskasamfélögum við Ísland á síðustu áratugum með flokkun tegunda í hópa sem byggja á lífsferilseinkennum þeirra. Gögn úr árlegum stofnmælingaleiðöngrum Hafrannsóknastofnunar frá tímabilinu 1987-2020 voru notuð til að skipa íslenskum botnfiskategundum í þrjá megin lífssöguhópa: periodic, opportunistic og equilibrium. Í heildina litið bregðast hóparnir misjafnt við breytingum í umhverfinu, en periodic hópurinn er algengastur við Ísland og hefur haldist nokkuð stöðugur yfir árin. Skilyrði síðustu áratuga virðast hafa verið hagstæð fyrir opportunistic hópinn, en einstaklingum innan hans hefur fjölgað nokkuð þétt yfir tímabilið. Á sama tíma hefur einstaklingum innan equilibrium hópsins fækkað þegar á heildina er litið, þó svo að flestar tegundir innan hópsins séu á uppleið. Breytingar eru þannig ekki einsleitar milli tegunda og breytt skilyrði virðast vera hagstæð fyrir sumar tegundir en óhagstæð fyrir aðrar innan sama hóps.

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Abstract

Various changes have come about in the distribution and abundance of many groundfish species in the waters around Iceland in the past two decades, which have been characterized by relatively high temperatures. The main objective of the thesis is to examine changes in groundfish assemblage structure based on life-history strategies. Using annual bottom trawl survey data from 1987- 2020, Icelandic groundfish species were categorized into three main groups based on previously defined life-history strategies: periodic, opportunistic, and equilibrium. Overall, the groups respond differently to environmental changes, but the periodic group is the most abundant in Icelandic waters and has remained quite stable over the study period. Conditions of the last two decades seem to have been favorable for the opportunistic group, which has steadily increased in abundance over the period. At the same time, it seems that these conditions have been unfavorable for the equilibrium group, which has decreased in overall abundance, even though most of the species within the group display an upwards abundance trend. Species within each group therefore respond differently to the environmental changes, which seem to have favoured some species over others.

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Foreword and acknowledgements

This 90 ECTS thesis is part of a master‘s degree study in Natural Resource Sciences at the University of Akureyri and was supported by the University of Akureyri Research Fund. I would like to thank my supervisor Steingrímur Jónsson and my co-supervisor Jón Sólmundsson for their invaluable guidance throughout the process.

In addition, special thanks to Pamela J. Woods at the Marine and Freshwater Research Institute for advice regarding statistical analysis and Klara Jakobsdóttir and Hreiðar Þór Valtýsson for useful discussions.

I would also like to express my gratitude to the Marine and Freshwater Research Institute for providing me with extensive survey and hydrographic data.

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Table of contents

1 INTRODUCTION ...... 1 2 ACADEMIC OVERVIEW AND LITERATURE REVIEW ...... 3

2.1 THE SEAS AROUND ICELAND AND THEIR TOPOGRAPHY ...... 3 Atlantic inflow...... 6 Inflow from the Arctic Ocean ...... 7 Currents and hydrographic properties around Iceland ...... 8 Long-term changes ...... 18 2.2 FISH SPECIES IN ICELANDIC WATERS ...... 20 Groundfish ...... 20 Groundfish in Icelandic waters ...... 21 3 MATERIALS AND METHODS ...... 25

3.1 SPRING GROUNDFISH SURVEY (SMB) ...... 25 3.2 SURVEY DATA ...... 26 Station data ...... 26 Species data ...... 27 Abundance and diversity ...... 28 3.3 LIFE-HISTORY STRATEGIES ...... 29 Ternary graphs ...... 30 LHS-group abundance ...... 31 4 RESULTS ...... 32

4.1 BOTTOM TEMPERATURE ...... 32 4.2 SPECIES RICHNESS ...... 36 4.3 LIFE-HISTORY STRATEGIES ...... 38 Ternary graphs ...... 42 LHS-group abundance ...... 44 Species abundance within LHS-groups ...... 48 Equilibrium species ...... 48 Periodic species ...... 49 Opportunistic species ...... 50 Mixed species...... 51 5 DISCUSSION ...... 53 6 CONCLUDING REMARKS ...... 57 REFERENCES ...... 59

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Table of Figures

Figure 1. Bottom topography of the waters around Iceland ...... 4 Figure 2. Nordic seas and their connections to the North Atlantic and the Arctic Ocean ...... 5 Figure 3. Surface ocean currents around Iceland...... 9 Figure 4. Hydrographic stations (dots) around Iceland ...... 11 Figure 5. Hydrographic conditions (temperature, °C) in Icelandic waters at 50 m depth in 2016...... 12 Figure 6. Hydrographic conditions (salinity, ppt) in Icelandic waters at 50 m depth in 2016 ...... 13 Figure 7. Potential temperature for Faxaflói hydrographic station in February and August in 2016 ...... 15 Figure 8. Potential temperature in Siglunes hydrographic stations in 2016 .. 16 Figure 9. Near-bottom temperatures at hydrographic stations Faxaflói (FX3, FX8) and Siglunes (SI1, SI7) ...... 17 Figure 10. Annual, quarterly mean of Atlantic water transport to the northern shelf area ...... 18 Figure 11. Index for Atlantic Multidecadal Oscillation north of the equator and mean temperature from stations 2-5 at the Siglunes section from 0-200 m depths ...... 19 Figure 12. Stations in 2020. Deep (>250 m) stations are blue. Shallow (<250 m) are red...... 27 Figure 13. Mean of bottom temperatures registered in the SMB, arranged by depth range and area ...... 33 Figure 14. Bottom temperature (°C) at stations by years...... 35 Figure 15. Yearly abundance (above) and number of relevant species (below) observed in the groundfish survey ...... 36

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Figure 16. The number of observed, species at different depths and areas by years...... 37 Figure 17. Weighted mean of species depth and temperature ...... 41 Figure 18. Relative life-history strategy values of species and their respective order ...... 42 Figure 19. Proportional representation of each life-history strategy (0-100) based on annual abundance ...... 43 Figure 20. Annual abundance of LHS-groups...... 45 Figure 21. Proportional abundance values for each LHS group with five-year intervals from 1990 ...... 46 Figure 22. Correlation of abundance (Y-axis) and temperature (°C) by LHS groups ...... 47 Figure 23. The annual abundance of equilibrium species...... 49 Figure 24. Annual abundance of the most abundant periodic species...... 50 Figure 25. Annual abundance of the most abundant opportunistic species .. 51 Figure 26. Annual abundance of the most abundant mixed species ...... 52

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List of tables

Table 1. Mean bottom temperatures (and standard deviation) before and after 2000, separated by areas (North and South) and depth intervals (Deep (>250m) and Shallow (<250m))...... 34 Table 2. Notable species from each LHS group and their life-history scores. A complete list of species can be found in Appendix...... 38 Table 3. Species and weighted averages of various environmental factors in which they have been observed in the SMB. Species are arranged by abundance, in descending order. A complete list can be found in Appendix along with calculated abundances for each species...... 39

List of abbreviations AMO Atlantic Multidecadal Oscillation

EEZ Exclusive Economic Zone

EGC East Greenland Current

EIC East Icelandic Current

IC Irminger Current

LHS Life-History Strategy

MAR Mid-Atlantic Ridge

MFRI Marine and Freshwater Research Institute

NIIC North Icelandic Irminger Current

SST Sea Surface Temperature

SMB Spring bottom trawl survey

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1 Introduction

In the recent two decades, the waters around Iceland have been relatively warm (Ólafsdóttir et al., 2020), and many changes have been observed in the marine environment which have been proposed to be induced by increasing temperatures, mainly through increased inflow of warmer Atlantic water to the region (Jónsson & Valdimarsson, 2012; Tsubouchi et al., 2021). Both pelagic and demersal species have responded to these changes. For pelagic species, the Atlantic mackerel (Scomber scombrus) has extended its feeding grounds from the Norwegian Sea into Icelandic and Greenlandic waters, and at the same time, capelin (Mallotus villosus) has been observed to shift its distribution west towards East-Greenland (Astthorsson et al., 2012; Óskarsson et al., 2016). Increasing temperatures in the bottom layers of Icelandic waters have led to various developments in the distribution and stock sizes of many groundfish species. E.g. species such as monkfish (Lophius piscatorius) and haddock (Melanogrammus aeglefinus) saw a spike in abundance in the early 2000s and were observed to extend their distribution considerably (Sólmundsson et al., 2010, 2021). Many southern commercial species have extended farther north while northern species have been observed to retreat, and many new species have been recorded in Icelandic waters during this period (Valdimarsson et al., 2012).

The effects of climate-driven changes on northern fish communities have been studied in various ways, e.g. by categorizing species into groups based on biogeographical factors and species life-history traits (Frainer et al., 2017; Pecuchet et al., 2017). Frainer et al. (2017) reported that Arctic fish

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communities in the Barents Sea, which were mostly composed of small-sized, bottom-dwelling benthivores, were being rapidly replaced by traits of incoming boreal species, which were mostly larger, longer-lived, and more piscivorous species. Pecuchet et al. (2017) analyzed fish community compositions across European seas and reported an increase in opportunistic and equilibrium strategies in recent years, which they believe to be due to increased temperatures and a decrease in fishing effort. As environmental changes in marine habitats affect all species within them in some way, community wide approaches might prove useful in predicting the overall effects on a given fish assemblage, instead of focusing only on specific species.

The Marine and Freshwater Research Institute (MFRI) of Iceland has conducted an annual spring bottom trawl survey (SMB) on groundfish habitats since 1985, gathering substantial data on marine species and environmental factors which are used as the basis for Icelandic stock assessments and fishing advice. Furthermore, the survey provides data on distributional changes of individual fish species over the study period, both those commercially and non- commercially exploited (Sólmundsson et al., 2020)

The goal of this study is to examine abundance and distribution changes in Icelandic groundfish communities. Analyzing data from the SMB with a community-wide approach, species were categorized into groups of life- history strategies (LHS) based on their life-history traits; equilibrium, periodic, and opportunistic.

The study is confined to data from the SMB, which only provides information for a limited period (March), annually. This study further only analyzes demersal fish species, although with a few exceptions. Furthermore, all analysis based on life-history strategies is limited to the species which had previously been given a life-history score by Pecuchet et al. (2017).

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2 Academic overview and literature review

2.1 The seas around Iceland and their topography Iceland is an island in the North Atlantic Ocean, with a surface area of about 103.000 km2 (FAO, 2021a) (Figure 1). Iceland is the largest part of the Mid- Atlantic Ridge (MAR) that reaches above sea level. From the southwest of Iceland, the Reykjanes Ridge extends far south into the North Atlantic and separates the Irminger Sea to the west and the Iceland Basin to the east. The extension of the MAR north of Iceland is the Kolbeinsey Ridge that stretches from the central north coast of Iceland towards the Jan Mayen Fracture Zone. The continental shelf area around Iceland is around 111.000 km2, and since 1975, the area of Iceland’s Exclusive Economic Zone (EEZ) has been 758.000 km2 (FAO, 2021a). Iceland’s shoreline is characteristically jagged, excluding the southern shore from Hornarfjörður in the southeast to Reykjanes in the southwest. This area is also where the continental shelf around Iceland is the narrowest. West of Iceland is where the continental shelf is the widest, reaching around 100 km west from Faxaflói and Breiðafjörður.

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Figure 1. Bottom topography of the waters around Iceland. Map from the Icelandic Coast Guard. Red line encircles Iceland‘s EEZ

Iceland is situated at the southern border of the Nordic Seas, which can roughly be defined as the body of water north of the Greenland-Scotland Ridge and south of the Fram Strait. The Nordic Seas include three main “seas”- the Greenland-, Norwegian-, and Iceland Sea (Figure 2). The Nordic Seas area covers about 2.5×106 km2 or 0.75% of the world's oceans (Drange et al., 2005). Despite its relatively small size, the area is quite dynamic and diverse, with Atlantic and Polar currents with contrasting water properties converging, along with a diverse seabed topography of basins, ridges, plateaus, and fracture zones. With its geographical position, the Nordic Seas serve as the

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main connection between the North Atlantic and The Arctic Ocean (Blindheim & Østerhus, 2005).

Figure 2. Nordic seas and their connections to the North Atlantic and the Arctic Ocean. Abbreviations in alphabetical order are as follows: BSO = Barents Sea Opening, FN = Faroe North, FS = Fram Strait, FSC = Faroe-Shetland Channel, SNW = Svinøy Northwest. (Lien et al., 2016).

The Nordic Seas area is a tale of two main surface layer currents, wherein, warm and saline North Atlantic water enters the area from the south over the Greenland-Scotland Ridge, and cold, Polar water with low salinity enters

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through the Fram Strait in the north. Figure 2 shows the flow of these two opposite currents through the area. Topographic features play a major role in dividing the area into different seas and they influence the flow of water through them. The ridges direct the water masses in certain directions and amplify or restrict flowrate through certain areas. For example, The Greenland-Scotland Ridge is a transverse ridge area that extends from the East coast of Greenland through the Denmark Strait eastward towards the Faroe and Shetland Islands. This ridge area plays a major role in the inflow of Atlantic water into the Nordic Seas as it mainly occurs in three locations; through the Denmark Strait, across the Iceland-Faroe Ridge, and through the Faroe- Shetland Channel. Another example is how the MAR extends north of Iceland to Jan Mayen as the Kolbeinsey Ridge, and from there to Spitsbergen as the Mohn Ridge and later the Knipovich Ridge.

As has been discussed, two major water masses enter the Nordic Seas area on its north-south axis. Cold, low salinity Polar water enters the area from the Arctic Ocean through Fram Strait with the East Greenland Current (EGC). The Polar water has temperatures around 0°C or lower and is relatively low in salinity (S<34.4) due to heavy freshwater inflow, largely from Siberian and Canadian streams (Fogelqvist et al., 2003). Relatively warm, Atlantic water enters the area from the south through gaps in the Greenland-Scotland Ridge. This water mass is much warmer and more saline (T>5°C, S>35.0) than the Polar water (Fogelqvist et al., 2003; Hansen & Østerhus, 2000). It is worth noting that these properties correspond to upper ocean layers rather than deep water.

Atlantic inflow The inflow from the North Atlantic Ocean enters the Nordic Seas through three main pathways – the Irminger Current (IC) west of Iceland, the Faroe Current

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(through the Iceland-Faroe Ridge), and through the Faroe-Shetland Channel. Most of the Atlantic water mass enters the Nordic Seas on the eastern side, mainly through the Faroe-Shetland Channel and over the Iceland-Faroe Ridge (Østerhus et al., 2019). Downstream of its bifurcation south of Iceland, a smaller portion of the North Atlantic Current flows along the south and west coast of Iceland, forming the IC and later the North Icelandic Irminger Current (NIIC) when it flows into the North Icelandic shelf area through the Denmark Strait. The NIIC flows along the west coast to the north of Iceland, mixing with coastal water close to shore as well as Polar water from the EGC and Arctic water from the EIC. This mixing of warm Atlantic water and Arctic water masses along with the coastal Icelandic shelf water creates the basis for different water masses around Iceland.

Inflow from the Arctic Ocean Located at 79°N, the Fram Strait is the passage between Greenland to the west and Spitsbergen to the east which is thought of as the northern boundary of the Nordic Seas. Through it, west of Spitsbergen the Atlantic water from the south enters the Arctic Ocean with the West Spitsbergen Current. In the western part of the strait, cold, low salinity Polar water from the Arctic Ocean enters the Nordic Seas, forming the EGC. Flowing along the east coast of Greenland, the EGC carries cold and fresh Polar surface water along with intermediate and deep waters from the Arctic Ocean. During its southward journey, the waters of the current gradually mix with other water masses. The current starts mixing with the waters of the Greenland basin and topographic constraints such as the Greenland Fracture Zone leads some of the EGC into the Boreas Basin (Quadfasel & Meincke, 1987). The current makes its way further south and reaches the Jan Mayen Fracture Zone, where a large portion of its waters enter the Jan Mayen Current, flowing eastwards and entering the

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Greenland Sea Gyre on its southern limb (Bourke et al., 1992). Further south, a portion of the EGC enters the East Icelandic Current (EIC), which flows eastward into the Iceland Sea and the Iceland Sea Gyre. The remaining part of the EGC flows out of the Nordic Seas through the Denmark Strait and along the west coast of Greenland into the Labrador Sea. The EIC flows eastward into the Iceland Sea, where it carries an admixture of Arctic Water and Polar Water southwards onto the North-Eastern Icelandic shelf, causing the waters there to be more Arctic than Atlantic (Casanova‐Masjoan et al., 2020; Logemann et al., 2013). The EIC meets the Atlantic water off southeast Iceland and side by side they flow eastwards over the Iceland-Faroe Ridge.

Currents and hydrographic properties around Iceland Iceland's geographical location and topography of the surrounding ocean make it a very interesting region, hydrographically. Located at the junction of the MAR and the Greenland-Scotland Ridge with two very different surface water masses flowing from their respective meridional direction and converging around the island, make the waters around Iceland spatially diverse. Originating from around 50°N, the warm and saline water of the North Atlantic Current meets the southern shores of Iceland more than ten latitudinal degrees further north. East of the Reykjanes Ridge the current is part of the sluggish North Atlantic Drift, but west of the ridge, the current is more dynamic, flowing towards the west coast of Iceland as the IC (Logemann et al., 2013). Most of the IC recirculates westward to Greenland’s continental shelf and subsequently flows towards the south over the East Greenland continental shelf and slope parallel to the EGC that flows southward over the continental shelf. A small portion of the IC continues further north, along the western- and northern coast of Iceland where it becomes the NIIC (Jónsson & Valdimarsson, 2012) (Figure 3).

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Figure 3. Surface ocean currents around Iceland (Astthorsson et al., 2007).

From the north, the EGC carries low salinity cold water from the Arctic Ocean. A portion of this current branches off into the Iceland Sea through the Spar fracture zone and eventually into the north-eastern Icelandic shelf, causing the water there to be characteristically more Arctic than Atlantic despite the presence of the NIIC (Logemann et al., 2013). The EIC continues its journey, carrying Arctic water to the east and reaching the northern flank of the Iceland- Faroe Ridge where it meets Atlantic inflow from the Faroe Current and the two

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water masses flow towards the east and form a strong front, sometimes called the Iceland-Faroe Front (Blindheim & Østerhus, 2005), or the Arctic Front, which is characterized by sharp temperature gradients (Hansen & Meincke, 1979; Logemann et al., 2013; Orvik et al., 2001).

Logemann et al. (2013) propose that four major types of surface water masses interact in the waters around Iceland. First, and most important is the Atlantic water mass that enters Icelandic waters through the IC and NIIC. The second is the so-called Arctic Intermediate Water (Arctic waters) which is carried by the EIC but formed through the circulation along the Arctic front, which stretches eastward along the Iceland-Faroe Ridge and partly turns north towards Jan Mayen, where a fraction flows back into the Iceland Sea (Blindheim & Østerhus, 2005). The third water mass is Polar water from the Arctic Ocean, which was discussed earlier. This water mass has low salinity due to freshwater discharge from Canadian and Siberian rivers and is very cold (T<0°C), mostly due to atmospheric cooling in the Arctic region. A part of this water mass leaves the Arctic Ocean through the EGC, where it is carried southward along the East Greenland shelf. The cold Polar Water contributes greatly to the forming of the so-called Polar Front, which forms in the Denmark Strait at the interface of Arctic and Atlantic water masses. Some parts of the Polar Water can enter the NIIC through fresh and cold eddies forming and separating from the Polar Front (Våge et al., 2013). This Polar water can become dominant in Icelandic waters given certain conditions, such as when strong winds cause the Polar Water to drift eastwards into the North Icelandic shelf (Logemann & Harms, 2006). Lastly, the fourth water mass is the coastal water formed by mixing of the freshwater discharge around Iceland's coast with the Atlantic water masses in the IC and NIIC. This water mass is thought to be vital to the marine ecosystem, injecting nutrient-rich water into the

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coastal waters as well as having an effect on stratification, facilitating the spring algal bloom, and causing a clockwise flow which is considered a crucial part of the recruitment process of many fish species in Icelandic waters (Logemann et al., 2013; Marteinsdóttir & Astthorsson, 2005; Thordardottir, 1986). However, Logemann et al. (2013) further suggested that a distinct Icelandic coastal water mass only exists to the south-west of Iceland and that the NIIC dominates the near-shore circulation along the north-west and north coast of Iceland, eroding most of the coastal freshwater signature, with the coastal water mass still appearing sporadically in shielded bays and fjords. Seasonal hydrographic observations have been conducted in Icelandic waters for decades at fixed stations around Iceland (Figure 4).

Figure 4. Hydrographic stations (dots) around Iceland and their corresponding section (names). Also shown are the main ocean currents around Iceland. Red arrows represent Atlantic water and blue arrows represent Arctic, and mixed water (Ólafsdóttir et al., 2020).

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Hydrographic measurements are usually conducted at regular intervals (quarterly) over the year, making it possible to observe seasonal variability in hydrographic conditions. The sections generally extend over the shelf and into deep waters over the slope.

Figure 5. Hydrographic conditions (temperature, °C) in Icelandic waters at 50 m depth in 2016. Measurements from February (above) and August (below) (sjora.hafro.is).

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Figure 6. Hydrographic conditions (salinity, ppt) in Icelandic waters at 50 m depth in 2016. Measurements from February (above) and August (below) (sjora.hafro.is)

A clear difference can be observed in the waters around Iceland, both spatially and seasonally as is shown in Figure 5 and Figure 6 for February and August 2016, respectively. The presence of warm Atlantic water causes the

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waters to the south and west to be relatively warmer than in other regions around Iceland. North of the Látrabjarg section in the northwest, the IC starts to mix with cold water flowing from the North with the EGC, creating the NIIC as it flows northeastward and then along Iceland's northern coast. In the northern, and northeastern regions of Iceland, the influence of the EIC starts to become noticeable with the region being decisively colder than the southern regions. The formation of the Arctic Front can be seen, both in temperature and salinity, off the eastern coast of Iceland, where cold water from the EIC meets the Atlantic inflow at the Iceland-Faroe Ridge (Logemann et al., 2013).

Furthermore, cross-sectional plots based on the same CTD/Sonde data reveal the potential temperature signature of the water columns at a given hydrographic section. Cross-sectional plots show the interplay between the different currents well (Figure 7 and Figure 8). The potential temperature in the Faxaflói section reveals a rather homogenous water column, with the water being mostly of Atlantic origin. The seasonal difference in this section is due to the upper layers being warmer in the summertime because of atmospheric warming of the surface layers and increased solar radiation. However, a cross-section of the Siglunes section depicts a more dynamic water column. In this section, the NIIC and EIC converge and create a more stratified water column, where the colder water from the EIC characterizes the bottom layers, while the warmer water from the NIIC defines the upper layers, due to a density differential between the water masses. The middle layer is most likely Arctic intermediate water, carried by the EIC.

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Figure 7. Potential temperature (°C) for Faxaflói hydrographic station in February (above) and August (below) in 2016. Plots are made with CTD/Sonde data from 2016 surveys. Retrieved from MFRI oceanographic homepage (sjora.hafro.is).

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Figure 8. Potential temperature (°C) in Siglunes hydrographic stations in 2016. Plots are made with CTD/Sonde data from 2016 surveys, from February (above) and August (below). Retrieved from MFRI oceanographic homepage (sjora.hafro.is).

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Along with spatial and seasonal changes in the waters around Iceland, the temporal difference between years can also be quite variable (Figure 9). Near- bottom temperatures registered at Faxaflói and Siglunes transect sections are examples of this variability, indicating fluctuating temperatures, but an overall upwards trend from 1990 until around 2010, where some discrepancy has started to form, mainly due to decreasing near-bottom temperatures at Faxaflói station FX8 (~400 m) as Atlantic water temperatures have decreased in recent years (Ólafsdóttir et al., 2020).

Figure 9. Near-bottom temperatures (°C) at hydrographic stations Faxaflói (FX3, FX8) and Siglunes (SI1, SI7). Stations FX3 and SI1 represent temperatures at ~70 m while stations FX8 and SI7 represent temperatures at ~400 m. Seasons shown are winter (January-March), spring (April-June), and autumn (October-December). Y-axes are separate, line is smoothed and error shading increases with variability. Graph reproduced from (Campana et al., 2020).

Another thing to consider is that the inflow of Atlantic water over Iceland‘s continental shelf varies, both annually and seasonally. For example, as

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measured from the Hornbanki section, the inflow of Atlantic water to the northern shelf area can be quite variable, with mean annual transports ranging from around 0.68 to 1.16 Sv (106 m3/s) over the study period (1994-2015) (Jónsson & Valdimarsson, 2012) (Figure 10). Increased transport, especially during periods where the Atlantic water is relatively warm, is likely to have affected the water column and ecosystem in the area which it flows into.

Figure 10. Annual, quarterly mean of Atlantic water transport to the northern shelf area. Measurements are from the Hornbanki hydrographic section. Graph modified from Jónsson & Valdimarsson (2012).

Long-term changes Further back, and on a larger scale, the environmental conditions in Iceland during the 20th century have been observed to fluctuate between warm and cold periods, roughly corresponding to phases of the Atlantic Multidecadal Oscillation (AMO) (Schlesinger & Ramankutty, 1994; Valtýsson & Jónsson,

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2018).

Figure 11. Index for Atlantic Multidecadal Oscillation north of the equator (above) (Enfield et al., 2001) and mean temperature from stations 2-5 at the Siglunes section from 0-200 m depths (below) (data made available by MFRI). Lowess smoothing has been applied with error shading included.

Hanna et al. (2006) analyzed the variability of the Icelandic climate using long-term sea surface temperature (SST) measurements, revealing that long- term variations and trends in coastal areas are broadly similar to Icelandic air temperature records, i.e. with cold conditions at the start of the 20th century and then strong warming in the 1920s which ended abruptly in the mid-1960s with subsequent cold conditions dominating the climate until the 1990s (Jónsson, 2007). These oscillating conditions have an impact on the ecosystem, e.g. fluctuations in phytoplankton productivity, where temperature and salinity are strong indicators of productivity and seasonal bloom timing, i.e. higher temperatures and increased Atlantic inflow have a positive effect on

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the phytoplankton spring bloom. These variations can be very pronounced in the northeastern region due to the variable strength of the main currents around Iceland and their effect on Atlantic inflow to the region, which has been linked with increased phytoplankton productivity (Gudmundsson, 1998; Zhai et al., 2012).

2.2 Fish species in Icelandic waters Observed fish species around Iceland are estimated to be well over 300 (Jónsson & Pálsson, 2013), but commercially exploited fish species are around 30 in total (MFRI, 2020e; Statistics Iceland, 2020). Both demersal and pelagic species are commercially exploited, with pelagic fish landings having been larger than demersal ones since the early 1990s (ICES, 2019). Notable demersal species include cod (Gadus morhua), haddock (Melanogrammus aeglefinus), saithe (Pollachius virens), redfish (Sebastes norvegicus), and Greenland halibut (Reinhardtius hippoglossoides) to name some of the most important ones, while the main pelagic species are capelin (Mallotus villosus), herring (Clupea harengus), blue whiting (Micromesistius poutassou) and mackerel (Scomber scombrus).

Groundfish Groundfish, also known as demersal fishes, are fish species that live on, or near the bottom of a body of water and show morphological and behavioral adaptions to life on or near the seabed (Bergstad, 2009). Species from this group occur at all depths and are found in most near-bottom habitats of the ocean. While pelagic fishes are in most cases similarly adapted to their environment, with slender and often torpedo-shaped bodies adapted for sustained fast swimming, the demersal fishes are adapted to life in the near- bottom zone in a plethora of ways (Bergstad, 2009). Adaptations of groundfish

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can be highly variable, from eel-shaped species to laterally compressed flatfishes and dorsoventrally compressed rays or even the more classical fish shape of the gadoids (Bergstad, 2009). Groundfish are generally carnivores and are comprised of both benthic and benthopelagic species, with the latter usually performing vertical migrations to feed.

Groundfish in Icelandic waters Many of Iceland's most commercially important fish species are groundfish, such as cod, haddock, redfish, and Greenland halibut (Statistics Iceland, 2020). Most (but not all) of these groundfish species generally spawn in the warmer Atlantic waters off the south and southwest coast of Iceland.

Demersal species are widespread around Iceland and occur at a wide range of depths and temperatures. During recent decades of increased temperatures, many changes have been observed in the distribution of demersal species, e.g. species such as saithe, plaice (Pleuronectes platessa), lemon sole (Microstomus kitt), whiting (Merlangius merlangus), and deepwater redfish (Sebastes mentella) have been observed moving from a mostly southern shelf assemblage structure to a more widespread one (Stefánsdóttir et al., 2010). Changes in recent decades have not only been observed for exploited fish but also less common species and previously recorded vagrant species. Additionally, many new species have been recorded in Icelandic waters (Valdimarsson et al., 2012). A recent study by Campana et al. (2020) revealed that most geographic shifts of species in Icelandic waters have been to the northwest, but also indicated that there was no overall tendency to move to deeper waters.

Predation by cod on northern shrimp and the subsequent decline of shrimp stocks, especially inshore ones, has been linked to increased

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temperatures (Jónsdóttir et al., 2012). The biomass index of cod (observed in the Icelandic groundfish survey) rose steadily from 2007-2017, peaking in 2015-2017 where the spawning stock was in one of the best conditions it had been in since the 1960s (Valtýsson & Jónsson, 2018), however, the biomass index has been declining since 2017 (Sólmundsson et al., 2021).

Around the turn of the century, the monkfish population in the south of Iceland saw a surge in abundance and distribution, with large cohorts being recruited annually and the stock extending its range to the whole south and west coast, even onto the northern shelf (Sólmundsson et al., 2010), but since then the surge has subsided and the stock decreased substantially (Sólmundsson et al., 2021). The haddock population also saw a spike in abundance around 2001-2005 due to a few large cohorts but has since reverted and remained relatively stable (Sólmundsson et al., 2021).

While increasing temperatures seem to have induced many initial changes in the marine environment around Iceland, many of them have been reverting despite persistent warm conditions (Valtýsson & Jónsson, 2018). While marked changes have been observed for demersal species since 2000, there still remains much uncertainty, with further research needed. More community- wide approaches might reveal some new insights into these changes, such as categorizing species into groups based on biogeographical factors and information about their life-histories (Frainer et al., 2017; Pecuchet et al., 2017). A study by Frainer et al. (2017) reported that Arctic fish communities in the Barents Sea, which were mostly composed of small-sized, bottom-dwelling benthivores, were being rapidly replaced by traits of incoming boreal species, which were mostly larger, longer-lived, and more piscivorous. A study by Pecuchet et al. (2017) analyzed fish community compositions across European seas and provided a theoretical framework outlining three life-history

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strategies of fish; opportunistic, periodic, and equilibrium. Their analysis revealed an increase in opportunistic and equilibrium strategies in recent years, which they believe to be due to increased temperatures and a decrease in fishing effort.

In this study, the framework introduced by Pecuchet et al. (2017) was used to analyze temporal changes in fish community distribution and abundance in Icelandic waters, with special consideration of environmental changes.

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3 Materials and methods

All analyses were performed using R Statistical Software (version 3.5.3) (R Core Team, 2021).

3.1 Spring groundfish survey (SMB) Data on fish distribution and species composition were obtained from MFRI‘s annual spring groundfish survey, which started in 1985 and has been conducted annually since then. The research area is Iceland's continental shelf, down to depths of around 500 m. Data is collected at fixed sampling/towing stations (hereinafter „stations“), where various information about fish stocks and the marine environment is collected (Sólmundsson et al., 2020).

SMB data is collected annually in a systematic and statistically designed manner (Sólmundsson et al., 2020). The MFRI currently operates two research vessels, Árni Friðriksson and Bjarni Sæmundsson, and has contracted commercial fishing vessels each year to assist in the survey. Samples are collected using standardized bottom trawls and information about each tow is registered, such as the date, time, location, fixed station number, the vertical and horizontal opening of the trawl, and length of the tow to name a few. Environmental variables such as wind speed, atmospheric temperature, sea surface temperature, and bottom temperature are registered at each station. All fish species encountered are registered, measured, and counted, with some variability in the criteria for each species. Selected species are also further sampled, e.g. they can be weighed, their otoliths removed (for age

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analysis), their sex and degree of maturity determined, and their stomach contents analyzed.

3.2 Survey data A few adjustments were made to the SMB dataset before any analysis took place. This study only includes data from 1987-2020, as data for some fish species and temperature for part of the study area from 1985-1986 were deemed to be incomplete and unreliable.

Station data The Iceland-Faroe ridge area was excluded, as well as stations that had been sampled <26 times in the 34 years study period. The research area was divided into two areas; North and South, with the dividing lines between the areas being 64°12‘N and 22°00‘W. This roughly correlates to the hydrographic variability previously discussed (Figure 5, Figure 6, Figure 7 and Figure 8), i.e. the South area represents Atlantic dominant waters and the North area represents the colder waters where mixing of Atlantic and Polar water has taken place. Furthermore, the stations were categorized as either deep (>250 m) or shallow (<250 m) (Figure 12). The bottom temperatures tend to differ between the two depth zones, especially north and east of Iceland (Jónsson & Valdimarsson, 2012; Sólmundsson et al., 2021).

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Figure 12. Stations in 2020. Deep (>250 m) stations are blue. Shallow (<250 m) are red.

Out of all the samples in the refined dataset (n = 17800), around 46% are in the North and 54% in the South. There is a bigger discrepancy between deep and shallow, with around 81% of samples being from shallow bottom depths and only 19% from deep bottom depths. In this dataset, there are on average 524 usable stations annually; 240 stations in the North area (194, or 81% of which are shallow), 283 in the South area (231, or 82% of which are shallow).

Species data Invertebrate species (e.g. mollusks and arthropods) were omitted as well as fish species that are not classified as groundfish (e.g. capelin) or had some sampling uncertainty. However, some species which are typically pelagic, such as herring and blue whiting, were included in the study due to their prevailing presence in these groundfish habitats.

Various eelpouts (Lycodes reticulatus, Lycodes esmarkii, Lycodes seminudus, Lycodes eudipleurostictus, Lycodes pallidus, Lycodes squamiventer) were grouped together as coldwater eelpouts due to inconsistent species identifications in the first years of the survey.

Arctozenus risso, Paralepis coregonoides, and Magnisudis atlantica were grouped together as Barracudinas (Paralepididae).

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Ammodytes tobianus, Ammodytes marinus, and Hyperoplus lanceolatus were grouped together as sandeels (Ammodytes).

For further information regarding scientific names of species used in the final dataset, refer to Appendix.

Abundance and diversity An abundance component was calculated to estimate the relative representation of a particular species or LHS at a given station or set of stations over a defined period. Abundance was defined as the number of individuals per nautical mile and transformed using natural logarithms to offset the skewing caused by abnormally large samples. Also, before applying a logarithm, the value of 1 was added to the number of individuals per nautical mile to account for values that would otherwise be negative.

푛푢푚푏푒푟⁡표푓⁡푖푛푑푖푣푖푑푢푎푙푠 퐴푏푢푛푑푎푛푐푒 = ln⁡(( ) + 1) 푛푎푢푡푖푐푎푙⁡푚푖푙푒푠⁡푡표푤푒푑

It is important to note that the abundance variable does not take into account species biomass. For example, this method does not differentiate between a single Norway pout and a single Greenland shark, they weigh as much in the abundance calculations of their respective strategies or species.

Non-transformed abundance was used when determining biodiversity, which was done by using species richness and Shannon-Wiener index of diversity (H‘). The Shannon-Wiener index is a measure of diversity that combines species richness (number of species in a given area) and their relative abundances to determine the level of diversity in a particular area, where s is the number of species collected in a sample and pi is the proportion of species i.

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푠 ′ 퐻 = ⁡ − ∑ 푝푖⁡ln⁡(푝푖) 푖=1 Weighted means were used to determine the average environmental conditions in which species were found (i.e. depth, temperature, and latitude), where transformed abundances were used as weights. A complete list of species can be found in Appendix, along with their calculated abundance and environmental factors.

3.3 Life-history strategies Species observed in the SMB data were assigned pre-defined life-history strategies scores from 0-1 (where applicable) based on their life-history traits, a method introduced by Pecuchet et al. (2017) using archetypal analysis (AA). This method is based on assigning proportional values (0-1) to species based on six traits; maximum length, life span, trophic level, fecundity, offspring size, and parental care. These values were fitted to the species abundance data, and groundfish species in Icelandic waters were analyzed based on their values for life-history strategies. The three main strategies, or archetypes, are as follows: equilibrium, periodic, and opportunistic. The equilibrium strategy is characterized by species with high trophic levels, long lifespan as well as length, low fecundity but large offspring size and parental care, e.g. skates and sharks such as thorny skate (Amblyraja radiata) and dogfish (Squalus acanthias). The periodic strategy is characterized by species with medium-to- long life span and length as well as high trophic levels and fecundity but low parental care and offspring size, such as haddock and cod. The opportunistic strategy is characterized by species with low trophic levels, small size, and short life spans but relatively high fecundity, e.g. Norway pout, dab (Limanda limanda), and herring.

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In this study, species were further categorized into groups based on their life-history strategies. The criteria for categorization was as follows: if a species had a score for one life-history strategy which was equal or greater than the sum of the other two (≥0.5), then it was categorized into a life-history group (LHS-group) corresponding to its dominating life-history strategy. A fourth LHS- group, “mixed“, was created for species that had no dominating life-history strategy. Furthermore, this study uses total abundances of life-history strategies at each station to analyze temporal changes, while the study by Pecuchet et al. (2017) used strategies proportions (based on relative abundances) in defined grid cells.

Ternary graphs Ternary graphs were used to examine how species and their taxonomic order are distributed in relation to life-history scores. For clarification, Scorpaeniformes, like redfish and Norway haddock (Sebastes viviparus), were excluded due to these species being quite mixed in regards to life-history scores, i.e. not having one predominating life-history strategy. Ternary graphs were further used to examine temporal differences in the proportional abundance of the three life-history strategies. Standard years were selected at 5-year intervals, starting from 1990. Selected years were also included: 1989, 1993, 2003, 2013, and 2019. The coldest and warmest mean temperatures were registered in 1989 and 2013, respectively. The year 2003 was selected due to large Atlantic water transport to the Northern shelf (Jónsson & Valdimarsson, 2012), 2019 was selected due to recency of data and 1993 was randomly selected to better represent the period before 2000.

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LHS-group abundance Temporal changes in spatial distribution and abundance of LHS-groups were examined using scatterplots with a geographical function. For comparison, registered bottom temperatures from standard years along with selected years were plotted using the same geographical function. The relationship between abundance and temperature for all four groups was also examined visually using scatterplots and smoothing lines. Finally, the annual abundance of the ten most abundant species from each group was examined to see the composition and annual variance within each group.

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4 Results 4.1 Bottom temperature Bottom temperature varied considerably between years but has mainly been rising since the 1990s (Figure 13). The mean bottom temperature for deep stations in the South area is usually warmer than for shallow stations in the area for this time of the year i.e. March. This is likely because in the South area both the shallow and the deep stations are in the Atlantic water mass which is warm and saline and during the winter the bottom temperature of the shallow stations are affected by the winter cooling that does not affect the deep stations as much. The water column in the South is much more homogeneous than in the North area (Figure 7, Figure 8), and atmospheric cooling in winter causes the shallower bottom areas to be colder than the deeper ones, where the effects of atmospheric cooling are not as prominent.

In the North area, both shallow and deep, temperature is much lower than in the South and in the North the deep stations are 1-2°C colder than the shallow stations, contrary to what was observed in the South. The reason for this is that the deep near-bottom areas the North can be affected by deep water masses of Polar origin (Figure 8). It was shown by Jónsson and Valdimarsson (2012) that the Atlantic water over the north Icelandic shelf usually does not extend deeper than 200 m. Thus the situation in the North area is different from that in the South and this was one of the reasons for the separation of the stations into different areas and depth intervals. The mean temperatures in shallow and deep areas in the North are further apart than in the South, due to a more pronounced stratification of the water column. The most rapid

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increase in temperature occurs during the first one and a half-decade of the study period, but after that the curves level off and there is even a decrease at the shallow stations in the South after 2013 (Figure 13).

Figure 13. Mean of bottom temperatures registered in the SMB, arranged by depth range and area. Smoothed lines represent Lowess smoothing with error shading included.

Increasing temperatures can be observed at deep and shallow stations in both the North and South areas (Table 1). The mean temperature in the North before 2000 with the shallow and deep areas combined was 1.13°C (SD = 0.89) and increased to 1.97°C (SD = 0.99) after 2000, a 0.84°C increase in mean temperature between the two periods. A similar trend was observed for the South area as the registered mean temperature before 2000 was 4.73°C (SD = 0.76) and 5.48°C (SD = 0.48) after, i.e. a 0.75°C difference between the two periods.

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Table 1. Mean bottom temperatures (and standard deviation) before and after 2000 (before = 1987-1999, after = 2000-2020), separated by areas (North and South) and depth intervals (Deep (>250 m) and Shallow (<250 m)).

Area and period Deep (mean °C) Shallow (mean °C) Deep (SD) Shallow (SD)

North (< 2000) 0.52 1.74 0.54 0.75

South (< 2000) 5.10 4.36 0.69 0.66

North (> 2000) 1.10 2.84 0.37 0.55

South (> 2000) 5.69 5.27 0.41 0.46

Although temperatures increased in both deep and shallow stations, there was a more noticeable increase in bottom temperatures registered at shallow stations. The temperature increase at deep stations was around 0.58°C for the North area and 0.59°C for South area while temperatures at shallow stations increased by 1.10°C in the North area and 0.91°C in the South area between the two periods. As the overall trend, the standard deviation was lower in the period after 2000 for both deep and shallow stations.

A trend towards increasing bottom temperatures can be seen all across Iceland (areas covered by the SMB), where the warmer Atlantic water dominates to the south, while relatively colder water masses dominate the areas to the north and east (Figure 14). The years before 2000 are noticeably characterized by lower temperatures compared to the years after 2000. A clear difference can be seen between 1989, the lowest annual mean temperature in the dataset, and 2013, the highest annual mean temperature in the dataset. Also, in year 2003 which was characterized by large Atlantic water transport to the Northern shelf (Jónsson & Valdimarsson, 2012), the northern and eastern areas have noticeably high mean bottom temperatures (Figure 14).

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Figure 14. Bottom temperature (°C) at stations by years.

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4.2 Species richness The adapted dataset for groundfish on Iceland‘s continental shelf included a total of 115 species. Although almost all of these species are considered groundfish, they can vary considerably in their preferred environment, i.e. some species are on average found in quite cold, and deep waters while others prefer shallower and warmer waters.

Figure 15. Yearly abundance (above) and number of relevant species (below) observed in the groundfish survey. Smoothed line represents Lowess smoothing.

There was a gradual upwards trend for the total number of species encountered each year, with the annual number of species rising from about 60 in the 1990s to more than 70 in the 2010s (Figure 15). Furthermore, there seems to be a temporal increase in the number of species observed both in the North and South of Iceland as well as at shallow and deep areas (Figure

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16). This development is in line with the overall trend in rising temperatures (Figure 13). However, while the average number of species per station has increased in the South since 1996, the same variable has rather declined at deep bottom areas in the North (Figure 16). A sudden decrease in the annual number of species observed at shallow stations in the South seems to coincide with decreasing temperature (Figure 13).

Figure 16. The number of observed, species at different depths and areas by years. Red lines (primary Y-axis) represent the number of species observed annually. Black lines (secondary Y- axis) represent the average number of species per station.

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4.3 Life-history strategies As described in chapter 3, species were grouped into three main categories based on their life-history strategies, a classification introduced by (Pecuchet et al., 2017). However, due to the heterogeneous nature of some species' life- history strategy, a fourth category was introduced for „mixed“ species, where no life-history score was greater than the sum of the other two (Table 2).

Table 2. Notable species from each LHS group and their life-history scores. A complete list of species can be found in Appendix. Life-history strategies Species names score Equilib- Oppor- Peri- Icelandic English Scientific name LHS group rium tunistic odic Sebastes Gullkarfi Redfish 0.43 0.1 0.46 Mixed norvegicus Melanogrammus Ýsa Haddock 0 0.22 0.78 Periodic aeglefinus Norway Trisopterus Spærlingur 0 0.68 0.32 Opportunistic pout esmarkii Thorny Tindaskata Amblyraja radiata 0.83 0.08 0.09 Equilibrium skate The abundance of species in the survey was highly variable, with species like redfish, haddock, Norway pout, and cod being especially abundant (Appendix). The environmental conditions in which species were on average found can also vary considerably, e.g. concerning average temperature, depth, and latitudinal distribution (Figure 17). E.g. haddock was on average found at depth of 165 m and temperature of 4.2°C while Norway pout was found, on average, at 207 m and 6.0°C (Table 3). This variability is also present within LHS groups, e.g. compared to Norway pout, dab, another opportunistic species, was on average found at 86 m and 5.0°C (Appendix).

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Table 3. Species and weighted averages of various environmental factors in which they have been observed in the SMB. Species are arranged by abundance, in descending order. A complete list can be found in Appendix along with calculated abundances for each species.

Species names Temper- Depth Latitude Icelandic English Scientific name LHS group ature (m) (°N) (°C) Sebastes Gullkarfi Redfish Mixed 193.4 65.51 4.0 norvegicus Melanogrammus Ýsa Haddock Periodic 164.5 65.40 4.2 aeglefinus Norway Trisopterus Spærlingur Opportunistic 207.2 64.41 6.0 pout esmarkii

Þorskur Cod Gadus morhua Periodic 192.1 65.83 3.1

Long Hippoglossoides Skrápflúra Periodic 189.1 65.64 3.4 rough dab platessoides Norway Litli karfi Sebastes viviparus Mixed 200.8 64.59 5.5 haddock Blue Micromesistius Kolmunni Periodic 309.8 63.87 5.8 whiting poutassou Atlantic Steinbítur Anarhichas lupus Mixed 159.1 65.84 3.4 wolffish

Sandkoli Dab Limanda limanda Opportunistic 86.2 64.42 5.0

Greater Gulllax Argentina silus Mixed 292.9 64.16 6.1 argentine Merlangius Lýsa Whiting Periodic 161.9 64.42 5.4 merlangus Thorny Tindaskata Amblyraja radiata Equilibrium 194.8 65.83 2.9 skate

Ufsi Saithe Pollachius virens Periodic 196.7 64.94 5.0

Vahls Litli mjóri Lycodes vahlii Mixed 214.2 66.26 2.1 eelpout

Síld Herring Clupea harengus Opportunistic 174.3 64.96 4.8

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The opportunistic strategy was observed in a wide range of conditions, i.e. at deep and shallow bottom depths and in cold and warm waters (Figure 17). Mixed species are also quite scattered in their average temperatures and depths, possibly taking advantage of various conditions. The periodic species are relatively abundant and most of them are found in similar conditions, from around 100-200 m depths and between 2.5 and 6°C. However, some periodic species are prominent outliers, such as the Greenland halibut, which prefers deeper and colder conditions. The equilibrium species are few and scattered, but the group's most abundant species, the thorny skate, is very similar to cod in its average observed temperature and depth. The groups vary in their species count; equilibrium (9), mixed (19), opportunistic (38), and periodic (25), with some species having relatively few observed individuals (Appendix).

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al or greater than the no. of sampling years were excluded. were sampling years of than the greater no. al or

. Weighted mean of species depth and temperature. Color represents LHS groups. Size represents abundance. Species whose abundance. Size groups. represents represents LHS Color species temperature. depth of and Weighted mean .

17

Figure not individualequ count was

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Ternary graphs Ternary graphs were used to examine how species and their taxonomic order were distributed concerning life-history strategies scores, with Scorpaeniformes, like redfish and Norway haddock being excluded due to them not having one predominating life-history strategy (Figure 18).

Figure 18. Relative life-history strategy values of species and their respective order. Species within an order are represented with the same symbol and encircled with a corresponding order color.

Members of the Squaliformes order are distinctively equilibrium, which is not surprising given the typical life-history of squaliform sharks. Rajiformes skates are also characterized by the equilibrium strategy. Clupeiformes, Gadiformes, and Pleuronectiformes species lean towards either periodic or

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opportunistic life-history strategies. There is at least some context between species order and life-history strategies, with species within a particular order being similar in their adherence to a certain life-history strategy.

Life-history strategies scores were examined temporally to observe any potential changes in proportional abundance between years (Figure 19).

Figure 19. Proportional representation of each life-history strategy (0-100) based on annual abundance. Years ≤ 2000 are light-blue, years after 2000 are orange. Axes have been cropped.

While differences between years are not particularly pronounced, some differences were observed, e.g. years before 2000, which were relatively cold, seem to have a slightly higher proportion of equilibrium influenced

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abundance. The equilibrium strategy has historically been the least represented strategy, with years before 2000 having an average of 19% representation, compared to 17% after 2000. The difference in average representation seems to have been made up with an increase in abundance of opportunistic species, increasing from 28% on average before 2000, to 30% after 2000. The average representation of the periodic strategy remains the same before and after 2000, at 53%, but the range over the study period was 51.5 to 55%.

LHS-group abundance The abundance of species by life-history strategy groups reveals that opportunistic species abundance has been rising since the mid-1990s, with equilibrium abundance rather decreasing (Figure 20). This visualization also reveals how the equilibrium group is relatively less abundant than the other three. Opportunistic abundance varies considerably between years, while mixed and periodic species seem to be more steady, especially the mixed group. While mostly stable, the periodic group shows a sharp rise and fall in abundance around 2005.

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Figure 20. Annual abundance of LHS-groups. The periodic LHS-group had a strong representation all around Iceland and seems to be able to thrive over the northern shelf, but the group is also prevalent around the southwestern coast. The sharp increase in abundance observed in 2005 (Figure 20) can be seen all around Iceland, but is especially noticeable in the North area. The rise in opportunistic abundance after 2000 occurred in the southwest, even reaching further north to Northwest Iceland. The equilibrium group is scattered around Iceland but seems to be somewhat losing its foothold in the north, cropping up in various areas but not exhibiting a steady trend. The mixed species are prominent off the southwest and west coast, and appear to be increasing in abundance in those areas. However, their presence off the northeast and east coast seems to be declining. The mixed group also has a noticeable presence off the southeastern coast (Figure 21).

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Figure 21. Proportional abundance values for each LHS group with five-year intervals from 1990. Color of points represents relative abundance according to abundance gradients for each group. Size of points also represents relative abundance within each LHS group.

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Abundance of different LHS-groups seems to have some relation to temperature (Figure 22). E.g. equilibrium abundance is more noticeable in colder temperatures, around -1 – 1°C, but decreases with temperatures higher than that. Opportunistic abundance is lowest around 1-3°C but increases after that, becoming very noticeable in temperatures over 6°C. The periodic group has a relatively steady abundance from 0°C but no clear peak in abundance. The mixed group seems to increase in abundance similar to the opportunistic group, albeit with less variability, rising around 3°C, with peak abundance just under 6°C.

Figure 22. Correlation of abundance (Y-axis) and temperature (°C) by LHS groups. Lowess smoothing applied.

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Species abundance within LHS-groups While LHS-groups have a certain overall trend in abundance, the development of species within the groups is not entirely homogenous (Figure 23, Figure 24, Figure 25, Figure 26).

Equilibrium species The equilibrium group is the scarcest of the four groups regarding abundance, richness (9 species), and diversity (Shannon-Wiener, H‘ = 0.40). The most notable species within this group is the thorny skate, which has been rather steady in abundance over the years, but has been declining since 2000, although not as noticeably as the abundance fall observed for dogfish. While not necessarily a coldwater species, the thorny skate does have the second- lowest observed mean temperature (2.9°C) of the group. Most other equilibrium species have mostly been increasing in abundance, e.g. rabbit fish and velvet belly (Figure 23). The Arctic skate has been observed on average at noticeably lower temperatures (0.3°C) than the other equilibrium species and has mostly been decreasing in observed abundance since 2010.

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Figure 23. The annual abundance of equilibrium species. Abundance values are transformed. Color represents the weighted average of each species mean temperature (Appendix).

Periodic species The periodic group consists of 25 species and is the most diverse of the four groups (H‘ = 1.51). The group contains many abundant species, such as cod, haddock, and long rough dab, and while most prominent species are relatively stable in their annual abundance, with occasional peaks and drops, other fairly abundant species are highly variable in their annual abundance, e.g. blue whiting, whiting, and saithe (Figure 24). Greenland halibut abundance has declined noticeably in recent years. The Greenland halibut also has the lowest observed mean temperature of the group (0.5°C) and is the only species that can truly be considered a coldwater species. The periodic abundance „boom" observed around 2005 (Figure 20 and Figure 21) can be seen in various species, most notably in haddock, whiting, saithe, and cusk (Brosme brosme). Other

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species such as cod and long rough dab did not seem to contribute to this sudden increase, and a sharp decline in Greenland halibut abundance can also be observed around that time.

Figure 24. Annual abundance of the most abundant periodic species. Abundance values are transformed. Color represents the weighted average of each species mean temperature (Appendix).

Opportunistic species The opportunistic group has the highest species richness, with 38 species in total but has a relatively low diversity score (H‘ = 0.82). The most abundant species in the group, the Norway pout, has seen a significant increase since 2000 (Figure 25). Other species, such as the moustache sculpin (Triglops murrayi) and sandeels have also seen an overall upwards trend in abundance, and the silvery pout (Gadiculus argenteus) had a very sharp spike in abundance between 2010-2015 but has since declined. Species such as dab and Atlantic

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poacher (Leptagonus decagonus) have rather seen a decline since the 2000s, while other species have remained relatively stable, e.g. witch (Glyptocephalus cynoglossus), Atlantic hookear sculpin (Artediellus atlanticus), and sea tadpole (Careproctus reinhardti).

Figure 25. Annual abundance of the most abundant opportunistic species. Abundance values are transformed. Color represents the weighted average of each species mean temperature (Appendix).

Mixed species The mixed species group, defined by not having a predominating score for any single life-history strategy, consists of 19 species in total and is relatively diverse (H‘ = 1.09). Most of the species within this group have either increased or decreased in abundance during periods of warming since the 2000s (Figure 26). Redfish is noticeably the most abundant species in this group and has increased in abundance. Norway haddock has seen a steady increase in

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abundance since around 1996. Greater argentine has also been increasing in abundance, but with more annual variance than Norway haddock. Atlantic wolffish, Vahl's eelpout, coldwater eelpouts, snakeblenny (Lumpenus lampretaeformis), and spotted wolffish (Anarhichas minor) have all seen a distinctive decline in abundance since the turn of the century. Interestingly, mixed species with relatively low observed mean temperatures such as Vahl's eelpout (2.1°C), coldwater eelpouts (0.6°C), snakeblenny (2.54°C), and spotted wolffish (2.3°C) have been decreasing in abundance since the 2000s while species that have been increasing in abundance such as Norway haddock (5.5°C), greater argentine (6.1°C), and redfish (4.0°C) have relatively higher observed mean temperatures (Figure 26; Appendix).

Figure 26. Annual abundance of the most abundant mixed species. Abundance values are transformed. Color represents the weighted average of each species mean temperature (Appendix).

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5 Discussion

This study analyzes data from the Icelandic spring groundfish survey from 1987-2020, where the abundance of species, as well as the number of species observed annually, has been mostly rising since the 2000s. The last two decades have been marked by an increase in the overall heat transport into the Nordic Seas (Tsubouchi et al., 2021), which has been mirrored by increased temperatures in Icelandic waters (Ólafsdóttir et al., 2020). In this period, various biological changes have been observed in Icelandic marine habitats, which are consistent with the findings in this study, such as geographical shifts in fish species distribution as well as changes in abundance and species richness (Astthorsson et al., 2007; Campana et al., 2020; Jónsdóttir et al., 2019; Stefánsdóttir et al., 2010; Valdimarsson et al., 2012). However, while increased species richness has been linked to increasing temperatures in many habitats, they have also been reported to have an adverse effect on some deep-water communities, e.g. in East Greenland where richness and total abundance have been observed to decrease concomitantly with increased bottom temperatures (Emblemsvåg et al., 2020).

A sharp abundance increase of both opportunistic and periodic species in 2002-2006 seems to coincide with increased transport of Atlantic water to the North Icelandic shelf, being a maximum in 2002-2003 (Jónsson & Valdimarsson, 2012), accompanied by increased heat transport to the area (Tsubouchi et al. 2021). The number of species observed has also been increasing, which seems to correlate with increasing temperatures. Furthermore, there was a concurrent increase in richness and number of

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species per station in the North and South areas as well as in shallow and deep water, except for the deep waters in the North, where the average number of species per station has rather decreased while the annual species count in this area has increased. These changes could indicate a spatial shift in species distribution similar to those observed by Campana et al. (2020). It is worth noting that the deep waters in the North are of Arctic origin, while the deep waters in the South are characterized by Atlantic water.

In this study, the periodic life-history strategy was the most represented each year. The equilibrium strategy was the scarcest, but relatively more represented in the years before 2000. The average representation of the equilibrium strategy in the years between 1989 - 2000 accounted for around 19% of the total abundance but decreased to 17% after 2000. On the other hand, the the average representation of the periodic strategy increased from 28% before 2000 to 30% after 2000. In the study by Pecuchet et al. (2017), the overall trend for European seas indicated a relative increase in opportunistic strategies similar to what was observed in this study, but contrary to this study, Pecuchet et al. (2017) also observed an overall relative increase in equilibrium strategies. In this study, the changes in strategy representation are not very pronounced, but they do coincide with a noticeable increase in bottom temperatures observed in the data, which is around 1°C higher on average in the period after 2000. Another thing to consider is that species within LHS groups have in most cases either been increasing or decreasing in abundance, so the overall balance is likely due to the interplay of increasing and decreasing abundances.

Unlike Pecuchet et. al. (2017), which examined relative strategy representation, species in this study were also arranged into LHS-groups to examine abundance changes for each group as well as the species within them.

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Periodic abundance has been fairly stable over the study period, and a sharp increase around 2005 is due to a few species but primarily haddock, which has historically been more noticeable in the warmer waters to the west and south of Iceland but has been increasing its presence in the North in the last two decades (MFRI, 2020f). The abundance of opportunistic species increased considerably after 2000, and almost exclusively in the southern region. This is primarily due to the emergence of Norway pout, a mostly pelagic zooplankton feeder with spawning grounds in Iceland (FAO, 2021b). Contrary to the overall downwards trend of equilibrium abundance, most species within the group exhibit increased abundances since 2000, but the most abundant species, thorny skate, has seen a gradual decline and dogfish abundance has also plummeted since the 2000s. Although mixed species have no dominant life- history adherence, notable species within this group mostly display a clear downwards or upwards abundance trends. Increased temperatures seem to have been advantageous for species like redfish, Norway haddock, and greater argentine, which are mostly found in warmer waters in the south and west of Iceland (MFRI, 2020d, 2020b, 2020c), but unfavorable for species like Atlantic wolffish and spotted wolffish, which are mostly found further north, northwest of Iceland (MFRI, 2020a, 2020d), along with Vahl‘s eelpout, coldwater eelpouts, and snakeblenny, all of whom were observed to prefer considerably colder waters than the redfishes and greater argentine (Appendix).

In conclusion, the periodic strategy is usually the prevailing strategy in the groundfish communities living at the Icelandic continental shelf and has displayed a relatively stable abundance trend over the study period. Opportunistic abundance has been rising with increased temperatures while equilibrium abundance has been decreasing. However, when looking at the underlying species composition for the equilibrium group, most species exhibit

55

increasing abundance trends but a dramatic decrease in dogfish abundance and a gradual decrease in thorny skate abundance cause an overall decline in the groups' abundance. It seems that the life-history strategies do not uniformly predict species response to changing temperatures as species within each group respond differently to increased bottom temperatures.

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6 Concluding remarks

This study set out to explore changes in distribution and abundance of groundfish communities at the Icelandic continental shelf, based on species‘ life-history strategies (Pecuchet et al., 2017), with consideration of increasing temperatures in recent decades. The prevailing strategy in Icelandic waters is the periodic one, and like the results reported by Pecuchet et al. (2017) for European waters, the opportunistic strategy seems to be responding well to increasing temperatures, with the overall abundance of the group steadily increasing since the late 1990s. The increase in abundance of opportunistic species is mostly confined to areas in the south and west, where Atlantic water reigns supreme. Equilibrium strategies in Icelandic waters have rather been decreasing in overall abundance, contrary to the aforementioned findings by Pecuchet et al. (2017) for European seas. Furthermore, it is important to note that in this study the equilibrium strategy was the most scarce group regarding species richness and diversity, with only 9 species in total, and only one species that could be considered notable in overall abundance (thorny skate). Most of the equilibrium species were observed to be increasing in abundance, but a gradual decrease in the abundance of thorny skate and the sharp decline in dogfish abundance has led to a decrease in the overall abundance of equilibrium species.

Species within each LHS-group do not respond uniformly to environmental changes, with increasing temperatures seemingly being advantageous to some species within a particular group, but unfavorable to other species. While a life-history strategies approach could prove useful in

57

studying changes in Icelandic groundfish communities, further research and statistical analysis is needed to determine the validity and applicability of this method, taking into account the variance within each group and possibly adding a more refined temperature preference component to the classification process.

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Appendix

List of species by abundance (see Methods for definition) and their life-history strategies scores and corresponding group. Abundance-weighted averages of environmental factors for each species are also included.

Equilib- Oppor- Peri- Abun- Depth Latitude Temper- Icelandic English Scientific name LHS group rium tunistic odic dance (m) (°N) ature (°C) Gullkarfi Redfish Sebastes norvegicus 0.43 0.10 0.46 14.27 Mixed 193.4 65.51 4.0 Melanogrammus Ýsa Haddock 0.00 0.22 0.78 14.07 Periodic 164.5 65.40 4.2 aeglefinus Spærlingur Norway pout Trisopterus esmarkii 0.00 0.68 0.32 13.77 Opportunistic 207.2 64.41 6.0 Þorskur Cod Gadus morhua 0.00 0.19 0.81 13.34 Periodic 192.1 65.83 3.1 Hippoglossoides Skrápflúra Long rough dab 0.01 0.34 0.65 13.28 Periodic 189.1 65.64 3.4 platessoides Litli karfi Norway haddock Sebastes viviparus 0.43 0.24 0.33 12.85 Mixed 200.8 64.59 5.5 Micromesistius Kolmunni Blue whiting 0.00 0.49 0.51 11.79 Periodic 309.8 63.87 5.8 poutassou Steinbítur Atlantic wolffish Anarhichas lupus 0.36 0.30 0.34 11.76 Mixed 159.1 65.84 3.4 Sandkoli Dab Limanda limanda 0.00 0.62 0.38 11.71 Opportunistic 86.2 64.42 5.0 Gulllax Greater argentine Argentina silus 0.12 0.42 0.46 11.58 Mixed 292.9 64.16 6.1 Merlangius Lýsa Whiting 0.00 0.37 0.63 11.35 Periodic 161.9 64.42 5.4 merlangus

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Equilib- Oppor- Peri- Abun- Depth Latitude Temper- Icelandic English Scientific name LHS group rium tunistic odic dance (m) (°N) ature (°C) Tindaskata Thorny skate Amblyraja radiata 0.83 0.08 0.09 11.12 Equilibrium 194.8 65.83 2.9 Ufsi Saithe Pollachius virens 0.00 0.21 0.79 10.82 Periodic 196.7 64.94 5.0 Litli mjóri Vahls eelpout Lycodes vahlii 0.32 0.45 0.23 10.72 Mixed 214.2 66.26 2.1 Síld Herring Clupea harengus 0.04 0.61 0.35 10.47 Opportunistic 174.3 64.96 4.8 Þykkvalúra Lemon sole Microstomus kitt 0.00 0.49 0.51 10.42 Periodic 139.9 64.71 5.3 Pleuronectes Skarkoli Plaice 0.00 0.38 0.62 10.40 Periodic 107.0 65.54 3.7 platessa Kaldsjávarmjórar Coldwater eelpouts Lycodes 0.32 0.45 0.23 10.02 Mixed 336.4 66.56 0.6 Glyptocephalus Langlúra Witch 0.00 0.55 0.45 9.95 Opportunistic 192.0 64.83 4.8 cynoglossus Þrömmungur Moustache sculpin Triglops murrayi 0.17 0.59 0.23 9.84 Opportunistic 176.4 66.04 3.1 Atlantic hookear Krækill Artediellus atlanticus 0.29 0.64 0.07 9.73 Opportunistic 289.3 66.16 0.9 sculpin Lumpenus Stóri mjóni Snakeblenny 0.29 0.49 0.22 9.44 Mixed 176.1 66.00 2.5 lampretaeformis Hlýri Spotted wolffish Anarhichas minor 0.31 0.23 0.46 9.22 Mixed 234.1 66.24 2.3 Hrognkelsi Lumpfish Cyclopterus lumpus 0.25 0.30 0.45 9.08 Mixed 153.4 66.13 2.8 Keila Cusk Brosme brosme 0.00 0.26 0.74 9.03 Periodic 198.6 65.00 4.2 Silfurkóð Silvery pout Gadiculus argenteus 0.00 0.66 0.34 8.82 Opportunistic 297.3 63.31 7.0 Leptagonus Áttstrendingur Atlantic poacher 0.18 0.69 0.12 8.79 Opportunistic 298.6 66.72 0.5 decagonus Reinhardtius Grálúða Greenland halibut 0.16 0.13 0.71 8.70 Periodic 360.2 66.12 0.5 hippoglossoides Djúpkarfi Deepwater redfish Sebastes mentella 0.42 0.34 0.24 8.69 Mixed 319.9 64.91 4.1 Lepidorhombus Stórkjafta Megrim 0.00 0.31 0.69 8.37 Periodic 238.7 63.50 6.5 whiffiagonis

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Equilib- Oppor- Peri- Abun- Depth Latitude Temper- Icelandic English Scientific name LHS group rium tunistic odic dance (m) (°N) ature (°C) Fourbearded Enchelyopus Blákjafta 0.00 0.50 0.50 8.35 Mixed 183.3 64.96 4.6 rockling cimbrius Langa Ling Molva molva 0.00 0.21 0.79 8.25 Periodic 209.6 64.28 5.9 Urrari Grey gurnard Eutrigla gurnardus 0.00 0.44 0.56 8.08 Periodic 164.6 63.62 6.9 Helicolenus Svartgóma Blackbelly rosefish 0.32 0.41 0.26 7.89 Mixed 241.7 63.50 6.9 dactylopterus Geirnyt Rabbit fish Chimaera monstrosa 0.62 0.09 0.29 7.84 Equilibrium 340.7 63.56 6.4 Blálanga Blue Ling Molva dypterygia 0.00 0.14 0.86 7.83 Periodic 297.8 63.91 6.0 Háfur Dogfish Squalus acanthias 0.94 0.00 0.06 7.81 Equilibrium 153.2 63.57 6.3 Careproctus Hveljusogfiskur Sea tadpole 0.30 0.57 0.12 7.70 Opportunistic 317.0 66.51 0.5 reinhardti Síli Sandeels Ammodytidae 0.01 0.84 0.15 7.51 Opportunistic 90.7 65.13 4.4 Hippoglossus Lúða Halibut 0.04 0.00 0.96 7.49 Periodic 143.7 65.63 4.0 hippoglossus Gaidropsarus Rauða sævesla Arctic rockling 0.00 0.57 0.43 7.28 Opportunistic 290.3 65.81 1.7 argentatus Skötuselur Monkfish Lophius piscatorius 0.02 0.11 0.87 7.13 Periodic 172.2 64.52 5.6 Litla brosma Greater forkbeard Phycis blennoides 0.00 0.35 0.65 7.02 Periodic 279.6 63.60 6.6 Marhnýtill Polar sculpin Cottunculus microps 0.29 0.50 0.21 6.96 Mixed 267.6 65.95 1.8 Myoxocephalus Marhnútur Bullrout 0.26 0.49 0.24 6.70 Mixed 77.7 65.70 1.5 scorpius Loðháfur Velvet belly Etmopterus spinax 0.79 0.21 0.00 6.59 Equilibrium 352.4 63.22 6.8 Ískóð Polar cod Boreogadus saida 0.02 0.61 0.37 6.37 Opportunistic 275.0 66.82 0.9 Spotted snake Leptoclinus Flekkjamjóni 0.08 0.73 0.19 5.81 Opportunistic 161.2 65.91 3.2 blenny maculatus Fuðriskill Twohorn sculpin Icelus bicornis 0.21 0.69 0.11 5.58 Opportunistic 181.9 65.89 3.4

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Equilib- Oppor- Peri- Abun- Depth Latitude Temper- Icelandic English Scientific name LHS group rium tunistic odic dance (m) (°N) ature (°C) Anarhichas Blágóma Arctic wolffish 0.38 0.14 0.48 5.48 Mixed 279.4 65.74 2.7 denticulatus Skata Skate Dipturus batis 0.66 0.00 0.34 5.26 Equilibrium 169.8 63.90 5.7 Pólskata Round skate Rajella fyllae 0.52 0.41 0.07 5.02 Equilibrium 322.5 64.82 5.8 Hollowsnout Coelorinchus Trjónuhali 0.07 0.47 0.45 5.01 Mixed 368.7 63.23 6.8 grenadier caelorhincus Amblyraja Skjótta skata Arctic skate 0.83 0.08 0.09 4.80 Equilibrium 390.9 66.50 0.3 hyperborea Gymnelus Guli brandáll Aurora pout 0.26 0.74 0.00 4.80 Opportunistic 255.0 66.18 1.2 retrodorsalis Agonus Sexstrendingur Hooknose 0.07 0.71 0.22 4.53 Opportunistic 74.9 64.02 5.5 cataphractus Ciliata Ljóskjafta Northern rockling 0.00 0.74 0.26 4.13 Opportunistic 114.2 65.00 4.9 septentrionalis Threebearded Gaidropsarus Bletta 0.00 0.61 0.39 3.83 Opportunistic 194.0 64.00 6.1 rockling vulgaris Lycenchelys Blettaálbrosma Checkered wolf eel 0.38 0.62 0.00 3.71 Opportunistic 378.5 65.74 0.4 muraena Dökki sogfiskur Gelatinous snailfish Liparis fabricii 0.30 0.57 0.12 3.61 Opportunistic 230.0 66.58 1.2 Callionymus Flekkjaglitnir Spotted dragonet 0.00 0.82 0.18 3.60 Opportunistic 148.0 63.61 7.3 maculatus Phrynorhombus Litli flóki Norwegian topknot 0.00 0.31 0.69 3.56 Periodic 123.8 65.19 5.0 norvegicus Makríll Atlantic mackerel Scomber scombrus 0.00 0.48 0.52 3.55 Periodic 227.7 63.91 5.9 Hvítaskata Sailray Rajella lintea 0.52 0.41 0.07 3.40 Equilibrium 254.2 65.86 3.7 Stóri sogfiskur Seasnail Liparis liparis liparis 0.15 0.79 0.06 3.10 Opportunistic 121.3 65.52 3.5 Geirsíli Barracudinas Paralepididae 0.00 0.67 0.33 2.85 Opportunistic 378.6 65.98 1.0 Álbrosma Moray wolf eel Lycenchelys kolthoffi 0.38 0.62 0.00 2.68 Opportunistic 251.9 65.53 1.6

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Equilib- Oppor- Peri- Abun- Depth Latitude Temper- Icelandic English Scientific name LHS group rium tunistic odic dance (m) (°N) ature (°C) Sogfiskur Montagu´s seasnail Careproctus sp. 0.30 0.57 0.12 2.58 Opportunistic 252.3 66.26 1.4 Roundnose Coryphaenoides Slétthali 0.09 0.33 0.58 2.46 Periodic 318.6 63.28 7.0 grenadier rupestris Bluntsnout Xenodermichthys Bersnati 0.30 0.56 0.14 2.35 Opportunistic 281.3 63.56 5.3 smoothhead copei Skrautglitnir Dragonet Callionymus lyra 0.00 0.71 0.29 2.21 Opportunistic 146.1 63.61 7.4 Four spotted Lepidorhombus Dílakjafta 0.00 0.50 0.50 1.95 Mixed 340.8 63.30 6.9 megrim boscii Litli sogfiskur Montagu´s seasnail Liparis montagui 0.10 0.88 0.02 1.69 Opportunistic 131.7 66.18 1.6 Roughhead Snarphali Macrourus berglax 0.17 0.04 0.79 1.47 Periodic 347.3 64.57 3.2 grenadier Stinglax Black scabbard fish Aphanopus carbo 0.00 0.27 0.73 1.45 Periodic 207.9 63.41 6.3 Somniosus Hákarl Greenland shark 0.89 0.00 0.11 1.45 Equilibrium 229.8 66.91 3.4 microcephalus Sarsálbrosma Sars´ wolf eel Lycenchelys sarsii 0.38 0.62 0.00 1.39 Opportunistic 241.6 65.79 3.1 Flundra Flounder Platichthys flesus 0.00 0.52 0.48 1.10 Opportunistic 47.0 64.22 4.0 Ísalaxsíld Glacier lanternfish Benthosema glaciale 0.00 0.89 0.11 0.69 Opportunistic 241.6 66.86 1.5 Brynstirtla Horse mackerel Trachurus trachurus 0.00 0.41 0.59 0.56 Periodic 293.0 63.24 6.8 Litli langhali Smooth grenadier Nezumia aequalis 0.03 0.59 0.37 0.56 Opportunistic 370.0 64.02 6.8 Stóra sænál Snake pipefish Entelurus aequoreus 0.47 0.49 0.04 0.56 Mixed 195.3 65.33 4.7 Reticulated Callionymus Rákaglitnir 0.00 0.88 0.12 0.45 Opportunistic 125.3 64.48 6.6 dragonet reticulatus Keilubróðir Fivebearded rockling Ciliata mustela 0.00 0.74 0.26 0.44 Opportunistic 29.5 65.70 1.6 Alepocephalus Gjölnir Baird´s smoothhead 0.30 0.25 0.46 0.41 Mixed 157.3 63.49 7.1 bairdii Atlantic spiny Eumicrotremus Gaddahrognkelsi 0.31 0.69 0.00 0.22 Opportunistic 217.0 66.87 4.5 lumpsucker spinosus

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Equilib- Oppor- Peri- Abun- Depth Latitude Temper- Icelandic English Scientific name LHS group rium tunistic odic dance (m) (°N) ature (°C) Ísþorskur Arctic cod Arctogadus glacialis 0.11 0.56 0.33 0.22 Opportunistic 278.0 64.82 0.1 Jeffreys kýtlingur Jeffrey´s goby Buenia jeffreysii 0.16 0.84 0.00 0.22 Opportunistic 107.0 63.67 7.9 Lýr Pollack Pollachius pollachius 0.00 0.28 0.72 0.22 Periodic 68.0 63.35 6.8 Scophthalmus Sandhverfa Turbot 0.00 0.34 0.66 0.22 Periodic 49.5 63.41 6.9 maximus Suðræni Short silver Argyropelecus 0.00 1.00 0.00 0.22 Opportunistic 175.0 64.65 5.6 silfurfiskur hatchetfish hemigymnus

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