Lisa Spotowitz

• The Arctic is experiencing warming to a much higher degree compared to other regions on Earth. The annual mean surface temperature between 1971 and 2019 was three times higher than the global average (AMAP 2021). While previous warm episodes like the Early Arctic warming occurring from the 1920s to 1960s are known to be driven by natural processes like changes in the North Atlantic Oscillation (NAO), recent studies provide evidence that the ongoing Arctic warming process differs from the earlier phases and that it is caused by anthropogenically induced large-scale global warming. Consequences for the local marine ecosystem can be, among other things, a shift in species abundance and distribution from a polar towards a more boreal community. In this context, fish species like the Atlantic cod (Gadus morhua) are reported to have risen in abundance in the Arctic region over the last decades. This dissertation focuses on the effects of climate change and climate variability on the population structure and spatial distribution of Atlantic cod in the fjords of Svalbard. A combination of different methods was used and included fishing campaigns in several locations on the coast of Svalbard, genetic studies on the ecotype composition of the catches, otolith studies on subpopulation structure, and year-round in situ camera observations on the occurrence of juvenile cod in the shallow waters of the Kongsfjorden ecosystem. Based on the combined use of observation, otolith shape, and genetic tools, a local ecotype, the “Svalbard coastal cod (SCC)” could be identified as a potential permanent resident in Svalbard fjords. Eggs or larvae of Atlantic cod could not be identified in the catches, nevertheless, eggs of long rough dab have been found during the ichthyoplanktonic surveys indicating spawning in the area. Both, Atlantic cod, and the long rough dab share a similar lifestyle and demonstrate the recent invasion of boreal species into the Arctic marine ecosystem.