One of the first things you’d learn about, if you ever yearned to become a fish ecologist, is the danger posed by non-native species. And the most simple formulation of the prevailing theory on this runs something like this: Don’t introduce a new fish (we’ll stick with fish here for simplicity) to an ecosystem in which it did not evolve. The problems that have resulted from these rampant introductions are one focus of my own book, The Dying Fish.

Almost none of U.S. flowages, lakes or ponds are what they were ecologically in the pre-European era. This is how pervasive introductions have been since our arrival here but especially beginning around 1850 when men such as Seth Green and Robert Roosevelt began to experiment with using the nation’s developing rail lines to shuffle the distribution of North American fishes. By 1850 many eastern fisheries were already nearing depletion. Couldn’t these waters be re-invigorated with infusions of rainbow trout from California, smallmouth bass from the Great Lakes or brown trout from Europe?

And so today, it’s in fact hard to imagine what original species composition might have looked like in our favorite local ponds and streams. Here in Pennsylvania some of the most pervasive fishes are rainbow trout, brown trout and carp and none of these are native to the Keystone State. Non-natives cause real problems for the original fishes as well, competing heavily for limited forage and even eating the natives at times. And furthermore, for most introduced fishes, most of the time, it’s impossible to eradicate them once they’re established.

So, for most of my life I’ve been kind of a zealot on this issue. As a kid, I wouldn’t release a brown or rainbow trout alive. I designed a study when I was fifteen to look at the effects of stocked trout on nutrient enrichment of a small lake. I always stood up for the native but unglamorous fishes, whether they were perch, burbot or bullheads. I longed to see the Great Lakes free of Pacific salmons and emerald shiners.

But I’m writing today to tell you that my views have softened and that’s quite an admission from someone who’d always like to be known as a radical. I started to see that the habitat itself, in so much of the east, has been radically changed since pre-colonial times. Many of the streams that might have been solely the domain of the cold-loving brook trout no longer have the shade to keep them cold, as one example. Brown trout, rainbow trout and smallmouth bass now would be a better fit. Too many streams have lost their meanders and have been impounded, leaving them far better habitat for warmwater species.

Then too, in most eastern American waters, we’re simply too far gone to even hope to regain original species distributions. Both state agencies and everyday Joes with milk pails have been meddling with Mother Nature’s plan for hundreds of years and so today, would it really cause an ecological emergency for, say, a new species of minnow to arrive in the upper Allegheny River watershed? The accepted view is that it would but Mother

Nature has ways of dealing with these things and has been integrating our human designs into her plan for quite a while.

With most introduced fishes, or especially predatory fishes, they cause some chaos when first introduced, sometimes more, sometimes less and sometimes in ways that are not at all obvious to us. But the typical pattern is for a new predator to devastate the population of existing small fishes. They never saw this new threat coming and don’t know how to cope with it. But after the small fishes are greatly reduced, the population of the new predator also collapses when there’s little food left. Subsequently, the little fishes start to regain their numbers while the predatory fish also slowly recover and in time they reach a new equilibrium, almost like they’d always been there. But it’s not perfect, and yes, sometimes old species are simply wiped out by the new.

Viewpoints on this subject are widely varied and often impassioned. I would still prefer the native species in almost all situations. And that’s coming from more of a philosophical standpoint rather than an ecological one. I want to just find the best in what Mother Nature’s set before me and instead of being a malcontent when I find only chubs and bullheads, I can just keep on moving to see what lives in the limitless miles of water beyond. What do you think?