By Kristin Cook
Fish & Wildlife Biologist, Ennis National Fish Hatchery and Graduate Student, Montana State University – Ecology Department
The western pearlshell is the only native freshwater mussel found in western Montana’s trout streams. Unfortunately, it’s rare now, and we need to know why.
The decline of western pearlshells is particularly alarming because despite their lackluster personalities they serve important functions; they filter impurities and pathogens out of water, are important parts of the food web, and are great monitors of stream health. For example, an adult mussel can filter up to 20 gallons of water a day, removing viruses, bacteria, and even pharmaceuticals from the water. Freshwater mussels also filter algae and debris from the water column and excrete them as large, nutrient-dense masses for consumption by other organisms; i.e., aquatic insects (trout food) eat their poop! Because of this, mussels are associated with high insect diversity and densities and healthy trout streams.
Western pearlshell restoration (naturally in streams or artificial propagation in hatcheries) requires basic information on their reproduction and biology that is currently lacking. Pearlshells release their larvae into river water where they attach to fish gills to hitch a ride to good habitat and develop to the juvenile stage; the fish are not harmed. Not just any fish will do; mussels require specific host fish. It’s unknown when western pearlshells produce and release their larvae in Montana streams and which fish species host the mussel larvae.
My collaborators and I are investigating these unknowns regarding the basic biology of western pearlshells in Montana. We need to capture salmonids in several streams, on multiple occasions, to determine which salmonid species host pearlshell larvae and for how long. We electroshock to capture fish, then look at their gills to visually determine if they are carrying mussel larvae.
With Trout Unlimited’s help, I was able to capture over 900 salmonids this summer to check for the presence of pearlshell larvae.
The Westslope Chapter provided 8 volunteers to contribute ~80 hours of work in the Big Hole and Flint-Rock watersheds and several members of the Missoula Fly Gals of Missoula came out to help. Laurie Lane, WSCTU board member, and her brother documented the project in photos and video for sharing with local grade schools and social media. Board members of the George Grant Chapter also came out and helped me electroshock, pack gear in and out of sites, and collect samples from mussels in the Big Hole watershed. I would not have been able to accomplish my research without the help of Trout Unlimited volunteers.