Harbor porpoise displacement by a solitary bottlenose dolphin in the Baltic Sea

Top predators affect ecosystems by controlling prey populations both directly through consumption and indirectly through fear. Even a small number of novel top predators can cause significant shifts in trophic chains. In 2019, a male bottlenose dolphin relocated from Scotland and settled in southern Funen, Denmark. Bottlenose dolphins are known to harass and kill harbour porpoises. Harbour porpoises are the only native cetaceans in the Baltic Sea, so the local porpoise population does not normally experience dolphin aggression. Even though these species are known to co-exist in many places, dolphins’ colonization of areas with naïve porpoises can have far-reaching ecological implications. Here we report the results of passive acoustic monitoring of dolphin and porpoise presence in southern Funen. Our findings indicate a substantial reduction in porpoise acoustic detections in areas where the dolphin was frequently observed, in comparison to nearby dolphin-free regions during the same timeframe. We suggest two potential consequences of bottlenose dolphin colonization of new areas inhabited by porpoises. First, lethal bottlenose dolphin attacks may directly decrease the abundance of harbour porpoises. Second, these interactions could potentially create a “landscape of fear” leading to the reduction in fitness as a result of exclusion from suitable habitat.