You’re pretty likely to find Pacific Blue Mussels all over the shorelines of the North Shore, especially when the tide is out. Mussels like to attach themselves to rocks and and other hard surfaces like docks using their strong foot. It is not uncommon to find Pacific Blue Mussels in groups of hundreds or even thousands! Being attached to rocks make it easy for Mussels to filter feed on plankton and detritus as they float by. Mussels are a popular food for predators like seagulls and other sea birds, crabs and fish. Because of their many predators and their role in filter feeding, Pacific Blue Mussels play an important role in the shoreline ecosystem.
Information from E-Fauna BC: https://linnet.geog.ubc.ca/efauna/Atlas/Atlas.aspx?sciname=Mytilus%20edulis&ilifeform=186 & https://www.pugetsound.edu/academics/academic-resources/slater-museum/exhibits/marine-panel/pacific-blue-mussel/