Larval drift dynamics, thermal conditions and the shift in juvenile capelin distribution and recruitment success around Iceland and East Greenland

Global warming is not uniformly distributed, the climate is expected to change most rapidly in the arctic regions. Large scale changes in the arctic biosphere is therefore of particular concern. Capelin (Mallotus villosus) is a keystone species in the many arctic marine ecosystem including the seas around Iceland in Greenland. Its summer/autumn distribution has shifted over thousands of kilometres and production has decreased substantially. Multiple drivers may be involved in this change. Here, we explore the roles of changes in larval drift and thermal conditions around spawning and the larvae. Using model based simulations of the period 1993-2015, we find that vertical behaviour of the larvae, geographical distribution of the spawning and to some extent also timing of the spawning affect the drift trajectories. Our results from the simulations indicate large interannual variation in westwards drift, but without a long-term trend. The shift in distribution of the capelin did consequently not appear to result from a large scale shift in currents. Unlike the current, ambient temperature had a long term trend, and the temperature level changed abruptly and significantly between 2002 and 2003. The longterm shift in distribution might have been driven by the warming through a combination of earlier and more northern spawning and mortality dynamics. However, the timing of the shift in temperature (from 2003) was not in agreement with the timing of the events during the shift (starting in 2002). The first year may, on the other hand, be explained by the strong westwards currents particularly in 2002. Also, we point out a striking match between the abrupt increase in ambient temperature and the observed decrease in recruitment from 2003.