Think of the hatchery rainbows of our time the same as commercial strains of corn, or tomatoes, or, especially, apples and chickens.
Stocking rainbows, for the most part, is like stocking Purdue or holly hills farms chickens in the woods to supplement hunting opportunities.
There’s a temperature manipulation that is commonly used on very young rainbows that results in them all being female. (Sex determination in fish, and amphibians, is governed by XY chromosomes, but several other factors as well. Fish and amphibians are very weird, compared to us.)
Trout genetics are also odd, with many situations where chromosomes exist in sets of more than two (triploid or more, as opposed to our typical diploid).
The short answer is that intermittent stocking of put and grow female fingerling is very unlikely to result in a wild population. But it’s not genetically impossible. There are documented cases of fish changing from male to female, or female to male. There’s good reason to believe that rockfish (striped bass) start as male but become female after getting to a certain size. And smallmouth bass in the Potomac are believed to have experienced non XY sexual determination, but this has been theorized as resulting from birth control sex hormones entering the river via wastewater treatment plants.
So there’s a theoretical pathway for male and female rainbows of spawning age existing in the wild as a result of fingerling stocking.
But there’s another important reason why it’s unlikely: habitat.
Wild rainbows survive in temperatures as high as the mid to upper 70s, but they thrive in conditions similar to those where brook trout survive: under 65F, high oxygen. Consider the places where we have wild rainbows (limestone, and a few pristine cascades) and it makes sense. Rainbows are from the Rocky Mountains, so they do best in places where conditions are like the Rocky Mountains.
Enter the brown trout. They thrive in temperatures up to about 68, but cannot tolerate temperature spikes as well as rainbows. That said, temperature spikes in the mid to upper 70s are survivable. Also, they coexisted with European human civilization for millennia, and, for whatever reasons, are able to coexist with agriculture and low levels of urbanization.
Much of the modern mid Atlantic is a lot more like Western Europe than like the forested precolumbian land of the wild brook trout. Brown trout can live near us, brook and rainbow trout, not so much.
As for the stocking of fingerling rainbows, I ok with it. Antietam creek in Maryland has a lot of ok trout water, but its nursery waters and spawning places are heavily impacted by human activities. Maryland manages it a a put and grow fishery.
It’s very close to one of the state hatcheries, so at the end of the season when eggs for the coming year are hatching, the fingerling pens are cleared out and the extra fish end up in Antietam.
No doubt birds, mink, otters, and, yes, a fair number of big brown trout, have a feast. And, of course the meat hole crowd feels no shame about killing a limit of tiny trout.
But I have caught some nice rainbow trout in the creek as well.
Mainly, though, the attraction of Antietam is the possibility that the next cast could bring a big brown out of the depths.
