HERMAN GULCH — If Colorado’s imperiled state fish can still survive anywhere in its native South Platte River Basin, government wildlife biologists say, it would be here: isolated tundra headwaters 4 miles above traffic racing toward Eisenhower Tunnel on Interstate 70. The biologists have purged this gulch of all other fish competitors. But the first pure greenback cutthroat trout dropped into chilly streams Monday morning simply quivered at edges of eddies. These captive-bred 1-year-olds — 960 of them — are thought to be hardier than the 4,000 hatchlings that Colorado Parks and Wildlife biologists put in Herman Gulch last year.