Prize money being offered to thin South Fork rainbow trout

The Idaho Department of Fish and Game has placed a bounty on rainbow trout in the South Fork of the Snake River in hopes of reducing the population of the invasive species. Prize money ranging from $50 to $1,000 is awarded to any angler who turns in a rainbow trout head containing a coded wire tag.

Fish and Game workers catch the fish and affix the tiny tags within the snout before releasing them in an effort to incentivize anglers to catch and keep the fish. Regional fisheries manager Brett High tells the Idaho State Journal that a few hundred anglers participate in the program each year, and about 4 percent of the heads they turn in have tags.

“Next week, we’re going to start tagging another batch,” High said.

The department started the incentive program in 2010 after the South Fork’s rainbow population jumped.

“Harvest is one spot where we can really improve and do the most good,” High said. “The biggest threat to our cutthroat in the river are the rainbow trout, which can interbreed with and compete with the cutthroat.”

High says rainbow keep rates need to double to make a lasting impact on the fishery. About half the rainbow trout in the river are caught in a given year, but High says only 15 to 20 percent of them are kept by anglers.

The department estimates at least 30 percent of the fishery’s rainbows need to be harvested to put a dent in the population. Anglers who catch more rainbows than they can eat can clean them and donate them to the department, which delivers the fillets to local food banks.

In the 1980s, High said rainbow trout and brown trout were only present at low numbers in the South Fork. Annual monitoring showed rainbow trout and rainbow-cutthroat hybrids made significant gains throughout the 1990s, while pure cutthroats declined. At the low point in 2004, surveys showed the South Fork harbored 771 cutthroats per mile, compared with 2,400 cutthroats per mile in the 1980s.

“We’ve seen some rebound,” High said. “In last year’s survey, there were just over 1,800 cutthroats per mile.”

However, rainbows have fared even better. Last year, the population shot up to 3,073 rainbows per mile, compared to just under 1,300 per mile two years ago.