A new study led by Montana Fish, Wildlife & Parks (FWP) fisheries biologist Sam Bourret is giving researchers a better understanding of where bull trout in the Swan River basin originate and which streams are most important to the species’ future.
The study, Estimates of Spawning Stream Contributions to an Imperilled Bull Trout Metapopulation Utilizing Otolith Geochemistry and Redd Counts, uses decades of bull trout spawning data and new research methods to see which streams produce the most bull trout in Swan Lake.
Bull trout depend on cold, connected waterways to survive. Populations have declined across much of their historical range due to habitat loss, warming water temperatures, barriers that limit migration, and competition from nonnative fish. FWP has worked diligently to protect bull trout in Swan Lake, which is a historically important waterbody for the threatened native trout. Along with the monitoring research, FWP operates a lake trout suppression program to limit the nonnative species in Swan Lake.
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“We continue working hard to ensure bull trout persevere in the Swan Lake system,” said FWP Region 1 Supervisor Amber Steed. “Sam’s research is an important piece of that effort.”
Bull trout spend much of their lives in Swan Lake, but they hatch in mountain streams throughout the watershed before moving to the lake for adulthood. For years, biologists have counted bull trout nests, called redds, to estimate how different streams contribute to the overall population. This new study builds on that work by using small calcium carbonate structures located in a fish’s inner ear, called otoliths, to trace individual fish back to the streams where they hatched.
Much like the rings of a tree, otoliths grow throughout a fish’s life. As they grow, they record the unique mix of naturally occurring minerals found in the water where the fish first hatched. By examining those chemical fingerprints, researchers can identify a fish’s home stream years after it has moved into Swan Lake.
Using otoliths collected from bull trout that were unintentionally caught during ongoing lake trout suppression efforts in the lake, researchers discovered that some streams produce more surviving bull trout than redd counts alone would suggest, while others contribute fewer fish despite similar spawning activity.
The findings give fisheries biologists a clearer picture of which streams are making the biggest contributions to the Swan Lake bull trout population. That information will help guide future habitat restoration, monitoring efforts, and conservation work where it can have the greatest benefit.
“Protecting the right tributaries today can help to ensure that future generations continue to see bull trout in the Swan River system,” says Bourret.
