CASEY SANTEE Idaho State Journal | Posted: Tuesday, February 19, 2008 12:00 am
POCATELLO, Idaho - Every day, a Swan Valley man contracted by the state walks to the lower sections of several major tributaries on the South Fork of the Snake River to sort live fish trapped by Idaho Fish and Game.
He releases native Yellowstone cutthroat trout upstream of barriers used to divert the fish into the concrete and mesh metal boxes. He removes invasive rainbow trout and cutthroat-rainbow hybrids, which are then taken to a children's fishing pond in Victor.
The barriers, called weirs, are located on the Pine, Burns, Palisades and Rainy creek tributaries. Some physically divert the fish, while others use electricity to coax the trout into the traps. The South Fork weir program began in 2000.
The weirs are part of a larger Fish and Game effort to protect the genetic purity of the Yellowstone cutthroat. Every spring as rainbow eggs are hatching, high pulses of water are released from Palisades Reservoir upstream to flush away the fry. In addition, Fish and Game has removed limits on the number of rainbow trout anglers are allowed to keep in order to thin the population of the species.
Jim Fredericks, Idaho Fish and Game fishery manager for the Upper Snake River region, said finding a weir that works year around has been a matter of trial and error.
"We've really been trying to find a design that is effective and efficient," Fredericks said. "It's not hard to trap fish in the summer and the fall when water levels are real low. But in the spring when rainbows and cutthroat migrate, trapping can be very difficult."
The newest of the weirs, located on Palisades Creek, diverts fish using a series of four electrodes that span all but a small channel of the waterway leading to the trap. It was built last year after floating and picket type weirs, which sieve the stream, proved ineffective because they are often damaged by debris during high water months. In fact, Fredericks said the picket weir on Pine Creek was shut down last year for that reason.
This year, Fredericks hopes to replace the floating weir on Burns Creek with a velocity barrier, which is a steep concrete slope that diverts trout up a fish ladder and into a trap.
Fredericks said it is still too early to empirically gauge the success of the weir program. He said while the barriers don't prevent rainbows from interbreeding with cutthroats on the main river, it does block them from reaching the spawning areas on the tributaries. That is paramount because most of the pure cutthroats migrate to those tributaries to reproduce.
"The idea is to ensure that rainbows don't continue to invade those tributaries," Fredericks said. "The South Fork cutthroat population is one of the last remaining strongholds of Yellowstone cutthroat in the West. Preserving them is important not only from a fishery standpoint but also from a species conservation standpoint, to maintain the genetic purity of the fish. The South Fork became famous because of the cutthroat trout."