By Michael Howell
The Sula Clubhouse was the place to be last week for anyone interested in bull trout. It was the gathering place this year for the Salvelinus confluentus Curiosity Society. The use of the Latin scientific classificatory appellation for bull trout in the club’s name is a dead giveaway that the group is composed of lot of scientists. The three-day event included plenty of data laden scientific reports as well as some of the latest in scientific modeling that may be used in the implementation of the federal Bull Trout Recovery Plan. But one day was spent in the field looking for bull trout and looking at some of the core area of bull trout habitat in the Bitterroot.
Interest in bull trout seems to be growing in inverse proportion to the rate at which the species is declining. It is the decline in populations across most of their native habitat that has scientists and fishermen especially concerned. Protected under the Endangered Species Act, it was placed on the list of Threatened Species in 1999.
Bull trout are cold-water-loving fish and require colder water temperatures than other salmonids and they require the cleanest stream substrates for spawning and rearing. They need complex habitats, including streams with riffles and deep pools, undercut banks and lots of large logs. They also rely on river, lake and ocean habitats that connect to headwater streams for annual spawning and feeding migrations.
Because they thrive in pristine environments and seem to be the first to suffer and decline as water quality declines and water temperatures increase, bull trout are considered by USFWS scientists to be an “indicator species” for water quality.
According to these experts, “bull trout are primarily threatened by habitat degradation and fragmentation, blockage of migratory corridors, poor water quality, the effects of climate change and past fisheries management practices, including the introduction of non-native species such as brown, lake and brook trout.” (www.fws.gov/pacific/bulltrout)
Montana Department of Fish Wildlife and Parks Fisheries Biologists Chris Clancy and Leslie Nyce and Bitterroot National Forest Fisheries Biologist Mike Jakober gave presentations about the status of bull trout in the Bitterroot where they have been doing some intensive study in core area bull trout habitat for years.
Clancy said that, according to historic reports, bull trout were far more plentiful in the late 19th and early 20th century than they are today in the Bitterroot drainage.
“Newspaper accounts and other popular literature report that large, fluvial fish were common in the Bitterroot River,” said Clancy. “Today few fluvial fish are found in the mainstem of the Bitterroot River.” Causes for the decline, he said, are most likely human related impacts, including dewatering of streams, dam construction, introduction of competing species, and land management practices.
Monitoring studies, which began in 1989 on Sleeping Child Creek, have validated the overall decline in the bull trout populations in one of the core areas in the Bitterroot. After increasing in population over the first few years of study, the population began to decline, even before the catastrophic fires of 2000 which sent the numbers down to their lowest point. Although the population slowly recovered, it has not approached the previous high numbers of the early ‘90s.
Some of the decline is certainly due to rising water temperatures. Following the introduction of digital temperature measuring devices in the ‘90s, FWP and the US Forest Service did a lot of temperature monitoring across the Bitterroot Basin in dozens of streams. The average temperatures in Sleeping Child Creek from 1993 to 2014 have increased by 30% over that time period.
Bull trout, according to Clancy, start looking for another place to live when the water temperature starts knocking on 60 degrees Fahrenheit, unlike cutthroat trout which can tolerate up to 65 degrees and brown trout which can tolerate temperatures up into the low 70s.
Clancy said that evidence suggests here and elsewhere that the decline in bull trout populations has more to do with habitat changes than invading species like the brown trout. The bull trout in Sleeping Child Creek were showing signs of trouble long before the first brown trout showed up in 2003. A few more browns moved in in 2006 and a few more over the next couple of years. But it was not until the year after the big fires of 2000 that the population of browns began to boom…and boom.
“They are not just moving in,” said Clancy, “they are coming in like a freight train.” He said at a checkpoint about four miles above Sleeping Child Hot Springs, the last count was 152 Browns to 4 Bull Trout.
“That’s a 40 to 1 ratio,” said Clancy.
BNF biologist Mike Jakober gave a presentation about the fish populations following the fires of 2000. The East Fork of the Bitterroot River drainage was particularly hard hit, according to Jakober, with about half of its drainage area (124,481 acres) burned, and about 25% of the watershed burned at high severity. Eighty-seven miles of Westslope Cutthroat Trout and/or Bull Trout habitat were burned at high severity, major fish kills were observed in eight tributaries to the East Fork of the Bitterroot River, and widespread population declines occurred across the watershed. To add insult to injury, he said, numerous debris flows occurred the following summer.
After 16 years, mark-recapture estimates in reaches with pre-fire data indicate that Westslope Cutthroat Trout numbers are now higher than they were pre-fire, while Bull Trout numbers have declined in most reaches. But this decline in Bull Trout numbers mirrors that which has also been occurring in unburned streams. Brook Trout numbers are also still down in most burned reaches, while Brown Trout appear to be invading many of the reaches.
Fisheries biologist Leslie Nyce has also added to our knowledge base of the Bull Trout in the Bitterroot by performing some genetic studies, taking non-lethal fin samples from 17 sites, nine on East Fork tributaries, the mainstem East Fork and seven other tributaries across the drainage.
One thing she turned up was the fact that the East Fork samples formed a distinct cluster compared to other tributaries sampled. Her results showed that most individuals assigned to their tributary of capture with over 90% probability, suggesting the tributaries contain genetically divergent populations. The East Fork sample tended to form its own group, but some fish collected from it also assigned to tributaries. According to Nyce, these data suggest the East Fork may contain a mixture of individuals produced from spawning in the upper mainstem and migrants from different tributaries.
“The mainstem East Fork appears to be an integral component for maintaining the migratory form of Bull Trout in the drainage and serves as a vehicle for potential genetic exchange among tributary populations,” said Nyce. She said conservation and management efforts in the drainage need to simultaneously focus on the tributaries and the mainstem of the East Fork.
The U.S. Forest Service, Rocky Mountain Research Station, National Genomics Center for Wildlife and Fish Conservation, in Missoula, gave a presentation about a new model, called the Climate Shield model, which makes accurate (78-85%) and spatially explicit predictions about natal habitat for Bull Trout and Cutthroat Trout and identifies likely future habitats The model identified over 5,000 potential natal habitats for Bull Trout, but thousands have rarely or never been sampled.
To address this information gap, they developed an environmental DNA (eDNA) sampling method that provides precise, robust information about the presence of bull trout (and other species) that can be collected quickly and at low cost across a species’ range.
They claim the method has a better detection efficiency than electrofishing, costs less, and takes less time. Initial studies were aimed at precisely delineating the distribution of Bull Trout within select watersheds, as well as confirming their absence from potential habitats and discovering previously unknown populations. Using eDNA surveys, they are now inventorying occupied Bull Trout habitat throughout river basins across the interior Columbia River basin.