Climate-driven redistribution of fish and reconfiguration of coastal food webs in Northern Norway

Abstract

Rapid climate change at northern latitudes induces poleward redistributions of fish leading to reconfigurations in biodiversity and food webs. This thesis aims to broaden our understanding of the climate-driven biogeographical alterations in fish and the subsequent reorganization of food webs along the coast of Northern Norway. Documenting and understanding the ongoing, rapid ecological change in coastal waters is crucial to inform climate adaptation of conservation and ecosystem-based management. This thesis examines the climate-driven reorganization of coastal communities over the period 1995-2019, comparing three study areas in Northern Norway (> 67), using trawling samples from the Norwegian Institute of Marine Research together with metaweb data. The objectives are to i. assess the climate-driven, temporal change in fish species diversity, prevalence, and co-occurrence frequencies; ii. investigate changes in the topological food web structure due to incoming species; and iii. characterize the changes in frequency of interactions between fish due to changes in co-occurrences over time. The implications of fish redistribution for species richness, prevalence, and co-occurrence were studied using occurrence data from the coastal trawling survey. The changes in food web topology due to incoming species in the three study areas used available metaweb data for the region, with selection and aggregation of trophospecies tailored to the scope and purpose of the study. The changes in interaction frequencies within the fish food webs were visualized using co-occurrence data, providing insights into the dynamic relationships among species in the studied areas. The findings reveal a notable increase in species richness and prevalence across all study areas from the earlier, cooler period (1995-2000) to the later, warmer period (2014-2019). The increase is consistent with expectations that increasing ocean temperatures along the north Norwegian coast facilitate boreal species' expansions into northern latitudes, altering local biodiversity and community structures. The increased prevalence of incoming boreal species leads to higher co-occurrence frequencies among fish, suggesting a reconfiguration of northern coastal food webs. The food web implications of fish redistributions included a distinct increase in the number of links, and a decrease in connectance and clustering indicating a shift towards less densely connected but more structurally complex food webs.