The reason is rooted in a primary trait that not only inhibits biomass gain, but makes browns more difficult to catch. Most places, they're already at carrying capacity. In a recent snorkel study of 10 sites in the Coller Wildlife Area on the Rio Grande River near Creede, DOW crews found that 80 percent of the browns resided in close proximity to a boulder 2 feet in diameter or larger. Browns require those underwater condos. Rainbows, on the other hand, thrive in condensed numbers in an open-water environment, a trait that not only makes them more accessible to anglers, but adaptable to hatchery life.
Rainbows are cosmopolitan.
You can cram many more rainbows into smaller volumes of water than browns. Nehring said that, in the absence of angler harvest, browns rather rapidly reach a maximum carrying capacity, both in terms of cover and food supply. Typically, this results in an oversupply of stunted fish, particularly in special-regulation waters with restrictive creel limits. Because it isn't likely that natural rainbow reproduction will rebound soon in those rivers impacted with whirling disease, the only real relief rests with a state hatchery system fighting its own battle to produce rainbows free of the malady often fatal to young fish in their formative phase. Thing is, producing hatchery rainbows cost lots of money.