Smallmouth bass are prized by anglers but, when introduced to waters outside of their natural range, can outcompete native species.
Despite years of eradication efforts in the Adirondacks, researchers at Cornell University find small mouth bass populations have not decreased significantly and have, in fact, evolved.
WAMC's Lucas Willard spoke with evolutionary biologist Dr. Liam Zarri, now a Smithsonian postdoc at the National Zoo in Washington D.C.
Among anglers, smallmouth are widely recognized as being this amazing sport fish. They fight really hard. They're an amazing fish to fish for, they're really fun. But they also have sort of this infamous ecological background, which is they are spreading north right now. They're native to North America, but they're spreading north as a result of our warming, warming climate. But they've also been introduced to a lot of waters as sport fish because they're so desired by anglers to fish for. They're pound for pound, one of the hardest fighting fish out there. And the work that we did is set up in the Adirondacks, which they're not native to, but they're introduced all across those waters in the 1800s and 1900s, for anglers, essentially to come in and fish for. But when smallmouth are introduced to a water body that's new to them, they very commonly, after a couple of generations, they can completely take over the lake or stream ecosystem, because there are these voracious predators, and they just chow through most of the of the native food web.
What are some of the native species that small mouth bass are consuming in upstate New York and the Adirondacks that has become a problem?
Yeah. So, a lot of the native minnows, for example, that form the base of the food web for other predatory fishes, like brook trout, once they get larger, and lake trout, once they get larger, all of these fish are feeding on our native minnow assemblage, of which we have many, many minnows in upstate New York and the Adirondacks. But smallmouth also are voracious feeders on crayfish as well. And the lakes that we work on, we see that they're not only eating the native minnow population, but they also eat a lot of crayfish, as well. So, for all the anglers out there, crayfish are, if you're not already on to them, those bottom jigs are great for catching smallies.
So, there's been efforts for years to eliminate problem fish like small mouth bass and the Adirondacks. And not only are they popular with sportsmen and anglers, like you had mentioned, but they're also seen as a nuisance. Your study actually looked at how smallmouth bass have changed over time in response to efforts to remove them from waters where they are a danger to native species. Now, how exactly did smallmouth bass populations change over time? And how did your team go about discovering?
Sure, yeah, so, so in the lakes that we were working on in the Adirondacks, this was a suppression effort, and I was started in the years 2000 and 2003 by the principal investigator of our lab at the time, Dr Cliff Kraft. And there are a couple different methods to remove invasive fishes. There's sort of the nuclear option, which is a chemical application called rotenone. But for a lot of water bodies, that's not really an option, because there are these native species that and that chemical kills everything in the lake, if you can treat the entire water body, which is difficult to do in lakes that are really deep, for example, or have really extensive wetlands. So, another option that's a more targeted approach, is called electro-fishing, which is essentially introducing electricity to the water, whether that's from a backpack carried battery unit, or whether that's a generator on a boat and the lab that I was working in went for that ladder option, the generator on a boat. Essentially the way the removal works is moving along the edges of the lake, shocking the water, and we stun all the fishes in the vicinity. We scooped them up, and the invasive smallmouth bass go in the cooler, and all the native fishes were able to get some basic information about those, such as their lengths, and those then go back over the side. So, it's a way to do a targeted species removal.
Over the course [of[ 25, 22 to 25 years, over the last couple of decades, what were some of the changes that your team noticed in the bass themselves?
Sure, well, so this, this work. I came in. I started my PhD at Cornell in 2018 so I was working. Working with this, these data and samples that Cliff Kraft and then his successor, Dr Pete McIntyre, decided to keep an archive. And this work really wouldn't have been possible without the preservation of that data and those samples. But the surprising life history phenomenon that they documented was in the first few years of this really intensive eradication effort, I mean, removing thousands and thousands of fish from this lake every year there, in the first year or two, it looked like populations were starting to decrease, but then they exploded, which was completely counter to, of course, what they wanted. They wanted to eradicate the smallmouth bass from this lake. They were able to get rid of a lot of the large old individuals but then what they found is that every single year there was this huge bumper crop of small young of year bass that had just been born in that year. And that's extremely odd for smallmouth bass populations. Normally, bass have a good recruitment year, which is survival of the babies to their first year. That only happens every once in a while, every, every three, four, five years. But what they're observing is every single year was a great year for these small juvenile bass. And really, what, you know, there are a couple things going on. It's complicated, because you're completely changing the food web in this in this ecosystem, and so what we think is going on is that all these bass were getting removed. We would remove as about 25% of the population every year, we estimate. But for all the bass that are left, this is a great lake to be a smallmouth bass, because there's far less competition for their food resources. And so those young-of-year bass, they could grow much more during their first year. They could make it through that first winter. And the first winter is really the, the major, first major mortality even once the bass have left their dad's nest. And so, there was sort of this initial, what we call a demographic effect, where, where our removal removed a lot of individuals, but left all of these, for all the bass that were left, they could do really well. And then through time, what we documented is that the bass who were steadily investing more and more and more into early maturation. So, the bass…the ones who were the earliest to mature, which is ages three and four for males and females respectively, they're investing more and more and more into early reproduction. And so, what we did is we examined their genome between the years 2000 and 2019, and we found that this early reproductive investment, this rapid growth rate, is actually an evolutionary change. So, before the removal got going, there was likely a mix of life history strategies all the way from live-fast-die-young to grow-slowly-and-reproduce-when-you're-older. And what we think we've done with our removal, and by essentially doubling the annual mortality of this population every year is we just selected for and allowed that live-fast-die-young life history strategy to take over the lake, and in doing so, this population has evolved and is thus becoming more and more resilient to our removal efforts every year that passes by.
So, the fish have actually shrunk over time. Is that what you're saying? That the ones that have survived, that are the smaller fish, and that are able to reproduce when they're younger animals and don't grow to a larger size, those are the ones that are taking over the smallmouth bass populations in these lakes. So, these are kind of like incredible shrinking fish over time. Is that a good way of describing it?
Yeah, it's interesting. Because if you just look at the population itself, without taking into account the age structure of the lake, it would look like, yes, this population of bass, every individual is now smaller. But that's underlain essentially by it's a younger population of bass. The individuals that we see now are actually, and this is somewhat counterintuitive, but age one smallmouth bass, for example, they're. growing faster than age one smallmouth bass did before the removal got underway. Same with age two, age three, age four. So, our individuals are actually growing faster, and that's an evolutionarily mediated trait. We found that there's been genomic change that actually underlies those rapid growth rates. And why we think that is, is because smallmouth bass, like many other fish, need to hit a certain size in order to reproduce. And so, what we think is going on is that we've just grabbed a subset of the population that grow really quickly to get to this threshold size to mature as young as possible, because they're probably not going to make it to the next year.
So, what does this research tell you about how, one, an invasive species can be managed, but also, two, how native animals can be protected under the threat of invasive species like, in the Adirondacks, smallmouth bass?
Yeah, absolutely. I mean the best, the best way to manage invasive species, and we've learned this time and time again in all sorts of different ecosystems and with all sorts of different species, the best way to deal with invasive species is to just not let them in in the first place, because once they're in and once they're established, it is so hard to get rid of them. And in other contexts, there's been, sort of, more broadly for managing invasive species an evolutionary adaptation to the measures we take to eradicate them. That's been documented in response to things like herbicides, trying to get rid of invasive weeds, Those weeds can evolve a physiological resistance to those herbicides. Or, in terms of pesticides, you can think of insects that that plague our agricultural systems, and they can also evolve a physiological resistance to the pesticides that we apply. What we found is that even in the absence of, kind of, a clear physiological pathway invasive species can involve, or can undergo life history evolution. So, the main driver, we think that that is pushing this evolutionary pressure along is just the doubling of annual mortality, and that alone can push populations to mature, do whatever they have to do, to mature earlier. And there are a whole other now that we've sort of, you know, sorted out this life history evolution. There are all sorts of other follow ups that we want to do to see, you know, is there behavioral evolution? Are they eating differently than their ancestors did, but that's all work that we're hoping to look at in the next coming years.
