| Summary: | Blue mussels (Mytilus spp.) are declining in coastal areas around the world. In Norway, they thrive on floating structures, while on rocky shores they have widely disappeared. Proposed and elsewhere reliable drivers such as climate change, pollution, disease, parasites, hybridization, and failed recruitment would not discriminate between floating structures and rocks. Therefore, we hypothesize that crawling predators, unable to reach floating structures, drive the Norwegian decline. A known ferocious crawling predator without pelagic stage is the dogwhelk Nucella lapillus. The antifouling tributyltin (TBT) made this snail sterile but is now banned and populations are recovering rapidly. We surveyed trees hanging into seawater and floating docks together with nearby rocky shores for blue mussels and dogwhelks, and conducted a predator exclusion experiment with caged blue mussels (40-80 mm). Blue mussels were present on all floating docks (65% cover), but only on 18% of rocky shores (≤5% cover). Similarly, they were found on 83% of trees without bottom contact, but only on 1% touching the seafloor. In cages, mortality due to other factors than dogwhelks was extremely low (1%) and confirms that blue mussels continue to thrive when out of reach from predators. Additionally, we conducted pilot experiments to assess dogwhelks’ feeding and crawling potential to drive a blue mussel decline of the observed magnitude and pattern. Dogwhelks drilled and fed effectively on blue mussels of any size (up to 131 mm). They more often managed to reach blue mussels on top of branches and concrete bricks than on top of ropes and chains that usually hold floating docks in place. Muddy bays are another refugium for blue mussels in Norway, and dogwhelks needed markedly more time to reach blue mussels on mud than on sand. This tentatively suggests that Norwegian blue mussel refugia, such as floating docks and muddy bays, are out of reach from dogwhelks. Shifts in community structure towards a new ecosystem state with few blue mussels ...
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