barry petersen has the story from the black canyon of the gunnison river. >> reporter: joel evans hasstretch of the gunnison river in western colorado for more than 40 years. like most anglers in these parts, for him, one species of fish is king. the rainbow trout. what's it like to have a rainbow at the end of this line? >> oh, they're fun. they tend to be more active. they jump more. >> reporter: but in the 1990s, that fight shifted to one between rainbow trout and a parasite that invaded colorado rivers. it causes an aquatic plague where young fish are deformed, swim in circles, and die of starvation. what kind of numbers in decrease did you see? >> literally a tenfold decrease. >> reporter: ever since, colorado fish and wildlife manager renzo has been working to keep the rainbow trout alive through various breeding programs, but at great cost and with only limited success. >> the rainbow trout is hugely important to this state. >> how important? is there a dollar number? es $2 billion to thel, it's economy. >> reporter: scientists got a major break when they discovered a small, is