Aim: To estimate spatiotemporal changes in habitat suitability and inter-specific overlap among three marine predators: Baltic grey seals (Halichoerus grypus grypus), harbour seals (Phoca vitulina), and harbour porpoises (Phocoena phocoena) under contemporary and future conditions.

Location: The southwestern region of the Baltic Sea, including the Danish Straits and the Kattegat, one of the fastest-warming semi-enclosed seas in the world.

Methods: Location data (>200 tagged individuals) were analysed within the maximum entropy (MaxEnt) algorithm to estimate changes in total area size and overlap of species-specific habitat suitability between 1997-2020 and 2091-2100. A total of eleven candidate predictor variables were considered representing anthropogenic activity, environmental, and climate sensitive oceanographic conditions in the area. Sea surface temperature and salinity data were taken from representative concentration pathways [RCPs] scenarios 6.0 and 8.5 to forecast potential climate change effects.

Results: Model output suggests that habitat suitability of Baltic grey seals will decline drastically over space and time, largely driven by changes in sea surface salinity and a loss of currently available haulout sites following sea level rise in the future. A similar though weaker response was observed for harbour seals, while suitability of habitat for harbour porpoises was predicted to remain fairly stable over space and time. Inter-specific overlap in highly suitable habitat was predicted to increase slightly under RCP scenario 6.0 when compared to contemporary conditions but to largely disappear under RCP scenario 8.5.

Main conclusions: Marine predators in the southwestern Baltic Sea and adjacent waters may respond differently to future climatic conditions, leading to divergent shifts in habitat suitability that are likely to decrease inter-specific overlap. We, therefore, conclude that climate change can lead to a marked redistribution of area use by marine predators in the region, which may influence local food-web dynamics and ecosystem functioning.

Location data of Baltic grey seals (Halichoerus grypus grypus), harbour seals (Phoca vitulina), and harbour porpoises (Phocoena phocoena) were collected using GPS and ARGOS tags between 1997-2020 in the southwestern region of the Baltic Sea, including the Danish Straits and the Kattegat. 

A total of eleven candidate predictor variables were generated representing anthropogenic activity, environmental, and climate sensitive oceanographic conditions in the area. Sea surface temperature and salinity data were taken from representative concentration pathways [RCPs] scenarios 6.0 and 8.5 to forecast potential climate change effects.

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