Saving River Mussels: Choosing a Zoo Conservation Plan

Mussels of the Clearwater River

1Riverbend Zoo's conservation program works with farmers and the state wildlife agency to protect the Clearwater River. Freshwater mussels live half-buried in the gravel of the riverbed. A mussel pulls river water in, filters out tiny algae and bits of mud to eat, and pushes the water back out. A healthy mussel bed can help keep river water clear for fish, other wildlife and the towns downstream that take their drinking water from the river. Old surveys found 20 kinds of mussels in the Clearwater; recent surveys find 9, and one of them is endangered.

2Mussels have an unusual life cycle. A female releases tiny larvae that clamp onto the gills of a certain kind of fish for a few weeks. Then the young mussels drop off wherever the fish has swum and settle into the gravel. A young mussel is about the size of a grain of sand, and fine mud washing off fields can bury it and clog the spaces in the gravel where it lives. An old mill dam, no longer in use, blocks fish from swimming upstream past it.

3Zoo biologist Paloma Ferreira ran two tests. First, she filled two 100 L tanks with the same cloudy river water and kept them at the same temperature and light. One tank held 20 adult mussels and the other held none. She measured the cloudiness of each tank in NTU (a higher number means cloudier water) at the start, after 6 hours and after 24 hours (Table 1). Next, the zoo raised young mussels on host fish in its lab and released 200 at each of two river sites. Site A is below a farm field with bare banks, where mud coats the gravel. Site B is beside a farm that planted a 30 m wide strip of trees and grasses along the bank years ago, and its gravel is clean. A year later, divers counted the survivors (Table 2).