Intertwined Lives: Migratory Fish and Freshwater Mussels

Migratory fish and freshwater mussels have more in common than you might think. Each have unique life cycles and depend on clean, free flowing water to thrive.

Migratory fish spend time both in freshwater streams and in the ocean. Some species, like herring, live in the ocean and reproduce in freshwater. Other species, like American eel, live in freshwater and reproduce in the ocean. Either way, these migratory species need undammed access to riverine habitat to complete their life cycle. At American Rivers we are always thinking about connecting rivers and as we think about World Fish Migration Day, we consider how important migrating fish are to our freshwater mussels.

Freshwater mussels need fish, sometimes migratory fish, to complete their life cycle. Why? Well, adult mussels combine egg and sperm to create small freshwater mussel larvae (called glochidia). These larvae look like Pac Man and must attach to the gills of a fish for a period of time in order to transform into an adult freshwater mussel. Some species create a lure made of these larvae that mimic fish, crayfish, or other fish food to ensure the larvae make it on the gills of their host. Pretty weird, huh? The fish are not hurt in this process and the mussel gets to hitch a ride to a new place in the stream. You can see many more examples of freshwater mussel lures at the Unio Gallery.

Historically, our great rivers were paved with mussels. I’m talking billions of mussels filtering the water and hitching rides on fish. People used these great populations of mussels to make buttons and exported them for pearl culturing, too. Yep, freshwater pearls come from freshwater mussels!

These great populations are unfortunately a thing of the past. Freshwater mussels are one of the most imperiled groups of organisms threatened by habitat loss and fragmentation from dams, and pollution from sedimentation, chemicals, and fertilizers. In fact, 76% of all freshwater mussels are imperiled and 10% are already extinct.

People noticed the decline of migratory fish sooner than freshwater mussels due in part to the long lifespan of the mussels. Some species of freshwater mussel can live to be 100 years old. Luckily, now that we understand the unique life cycle of freshwater mussels, we can work to protect migratory fish and freshwater mussels together by working to conserve, protect, and restore our streams.