Larval drift dynamics, thermal conditions and the shift in juvenile capelin distribution and recruitment success around Iceland and East Greenland
Introduction
Arctic oceans are changing. Observations and forecasts on sea temperature and ice indicate that the arctic regions are, and will be, changing faster than most other parts of the globe (IPCC, 2019). The warming trend in East Greenland and around Iceland in recent years and the observed changes in the marine ecosystems corresponds to these forecasts. Atlantic mackerel (Scomber scombrus) and Bluefin tuna (Thunnus thynnus) have recently started to migrate into the warming Irminger current in East Greenland (Jansen et al., 2016, Jansen et al., 2020), also deeper living species on the Icelandic and Greenlandic shelfs and shelf slopes like blue whiting (Micromesistius poutassou), haddock (Melanogrammus aeglefinus) and monkfish (Lophius piscatorius) have moved further north (Valdimarsson et al., 2012) following variation in temperature and large scale oceanic features (Post et al., 2020). Among the most noteworthy changes, a radical change in the arctic ecosystem in this area stands out, namely the shift in distribution and production of the keystone fish species capelin (Mallotus villosus) (Carscadden et al., 2013).
Capelin is a small marine pelagic smelt (family Osmeridae) with northern circumpolar distribution, that dominates the mid-trophic tier of many marine arctic ecosystems (Hedeholm, 2010; Vilhjálmsson, 1994). The stock that spawns around Iceland has historically been among the largest and economically most important capelin stocks (Vilhjálmsson, 1994). Spawning of this stock occurs mainly from February to April in shallow (8–90 m, max. densities in 30−50 m), relatively warm Atlantic (5−7 °C) waters off the south and west coasts of Iceland Thors (1981). Spawning off the north coast is typically sparse (Vilhjálmsson, 1994), however, during a warm period in the 1920s and 1930s spawning on the south coast was abandoned in the favor of the north coast (Sæmundsson, 1934). Most adults spawn at the age of 3 years and die after spawning. The eggs stick to the bottom substrate and hatch after approximately 3 weeks (Fridgeirsson, 1976). Main hatching time was found to be 4 April to 6 May based on back-calculations from aged larvae sampled around west Iceland in April and May (Pálsson et al., 2012). The larvae ascend to the uppermost part of the water column and drift towards the nursery areas (Fortier and Leggett, 1983). Generally, the nursery areas before 2002 have been the Icelandic East, Northeast, North, and Northwest shelf, as well as the Greenlandic shelf area just west of the Denmark Strait (Carscadden et al., 2013) (Fig. 1a). When maturing, the vast majority migrate clockwise around Iceland. In January, they follow the East Iceland Current from North to East of Iceland (East of 16 °W, North of 64.5 °N) at depths of around 30–225 m. The route follows a relatively narrow thermal funnel of near constant temperatures (approximately 2.5 °C), which is situated along the shelf break in waters where the bottom depth typically range between 150 and 500 m (ICES, 2019; Olafsdottir and Rose, 2012; Vilhjálmsson, 1994). Around the end of January or beginning of February, they reach the Iceland-Faroe front, and enter the substantially warmer and more saline North Atlantic Current. Here they turn west and move onto the shelf (30–500 m bottom depth). The capelin are typically found between 30 and 150 m depth in this final part of the migration to the spawning areas that take around 3 weeks for the majority of the capelin (ICES, 2019; Olafsdottir and Rose, 2012; Vilhjálmsson, 1994).
Carscadden et al. (2013) reviewed fisheries and survey data on larvae, juvenile and adult capelin and concluded that radical shifts in distribution and production of capelin had occurred in the early 2000’s. The changes started with the 2001 and 2002 year classes. Both cohorts were large, i.e. similar to the level of the cohorts in the 1990s. The 2001 year class was found in the normal areas when surveyed as 0-group in August 2001, but this changed in 2002. The 2002 year class was nearly absent from these areas as 0-group in August, and in November both cohorts were absent from the eastern areas. Later surveys indicated that they were further west on the Greenlandic shelf. However, when maturing, they returned to spawn around Iceland where they were measured by acoustic surveys and caught in the winter fishery. Later cohorts from 2003 onwards (with the possible exception of the 2005 cohort) were also absent in the normal nursery areas, and (when surveyed) found further west primarily on the Greenlandic shelf. This shift in spatial life cycle pattern was summarized in Fig. 1b. Furthermore, recruitment to the fishery decreased radically starting with the 2003 year class (Fig. 1c).
Carscadden et al. (2013) suggested that changes in larval drift due to currents, timing and location of spawning could have been driving the change in distribution of the juveniles, and that the decrease in recruitment could have been related to increased temperature and salinity. However, they did not provide data to support these hypotheses. We continue this line of thought about oceanography and capelin by analysing the currents (larval advection), and temperatures experienced by the capelin from 1993 to 2015.
In this study, we simulate the larval drift from hatching in spring on the Icelandic continental shelf to the end of the larval phase in early autumn, and analyse the spatial patterns and thermal conditions during the drift. Furthermore, we document the thermal conditions during the prespawning migration and the spawning. Finally, the results are discussed in relation to the observed changes in distribution and recruitment.
Section snippets
Materials and methods
The study area covers the marine territory around Iceland, the Denmark Strait between Iceland and the east coast of Greenland, more precisely the area between 12 and 45 °W and between 60 and 70 °N (Fig. 2).
Results
Larval drift from the Iceland - East Greenland capelin stock was simulated for the first time.
Substantial interannual variation in larval drift was indicated by the accumulated density distributions resulting from simulations using the ‘base run’ settings (Fig. 4 and Table 1). Three recent years were selected as representative for this variation. 2010, 2013 and 2015 had medium, low and high western drift, respectively (Fig. 4). These years were used in the sensitivity runs to test the influence
Discussion and conclusion
Simulation of capelin larvae drift, done for the first time for this stock, indicated that vertical behaviour of the larvae, geographical distribution of the spawning and to some extent also timing of the spawning affected the drift trajectories in the period 1993 to 2015. We found a large interannual variation in westwards drift, but without a long-term decadal scale trend. Also, we found an absence of westwards drift in 1999–2001, followed by substantial westwards drift in 2002–2003. Finally,
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
We wish to thank Elsevier for granting permission (License Number: 4905861346474) to reproduce Fig. 6, Fig. 9 from Carscadden et al. (2013).
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