Whether you love them or hate them, once a nonnative fish species invades a stream or river, it is often impossible to get them out. Sometimes these colonization events are entirely accidental, like the Eurasian-native round goby that is thought to have been released in ship ballast waters sometime in the 1980s. Other times, nonnative introductions are deliberate, like the stocking of nonnative trout throughout the United States.
While present-day stocking efforts reflect the desire to have fish in certain locations, the distribution of nonnative trout largely represents the ghosts of managements past. Today, it is unlikely (I hope) that we would stock nonnative brook trout in the Rocky Mountains where they readily outcompete native cutthroats. Likewise, we might think twice about the extent of brown trout stocking on the east coast if we knew how readily they displace native brook trout populations. However, at the turn of the 20th century, we didn’t know better. Nonnative trout stocking became the status quo and, to a large degree, it set the precedence for modern-day fish management.
But, as science evolves, and as managers, anglers, and conservationists seek to find a balance between native fish conservation and recreational fishing, we’re often find ourselves in a position of regret. We now know the threat nonnative trout can have on native species; however, there is very little we can do about it. Manual removal of nonnative trout is often ineffective because it is labor intensive (read: expensive), and requires managers to electrofish large stretches of stream and pick out natives from nonnatives. It’s also possible to chemically remove undesired fish, but this method of removal also kills native fish and can have other ecosystem-wide impacts. And, in the end, neither manual or chemical removal prevents a nonnative trout from outside the study reach moving into a managed stream and undoing all the efforts. With our hands seemingly tied, a lot of people now argue for management and conservation of nonnatives, quoting that “nonnatives are the future of the fishery”, that “something is better than nothing,” or “let’s do the best with what we’ve got.”
But, what if we could rewind the clock? What if we could actually eradicate nonnatives? Manual and chemical removal are unlikely to be effective eradication measures but, ironically, stocking might just be a saving grace for some native trout populations.
The trick? Stock “supermales”- male fish that are only capable of producing male offspring, In theory, over several generations of reproduction with supermales in the population, the sex ratio of a population would become so far skewed towards males that the population would not be self-sustaining and would collapse.
So, how do supermales work? Recall from basic biology that all females have two X chromosomes, and all males have one X and one Y chromosome. During reproduction, females contribute an egg with an X chromosome, and then offspring sex determination is decided by whether the egg is fertilized with sperm that has an X or a Y chromosome. In supermales, all sperm have Y chromosomes, and so all offspring from supermales are males. So, stocking supermales is basically an effort to remove X chromosomes (and thus females) from a population.
It sounds difficult, but the production of supermales is actually relatively easy and has been used in aquaculture for decades. Producing a supermale requires feeding normal fish estrogen-infused food, which causes males to produce eggs rather than sperm. When hormone-treated males (with eggs) mate with untreated males (with sperm), about 1/3 of the offspring will only have Y chromosomes (the other 2/3 will have at least one X chromosome). If you mate the Y-only feminized males with Y-only supermales together, 100% of the offspring with be supermales that only have Y-chromosomes and, when stocked, will only produce male offspring. And, because supermales themselves were never exposed to hormones, there is no concern about consumption of stocked fish or introduction of chemicals into the environment.
Seems like a win, right? Now that hatchery production methods for supermale trout have been ironed out, it seems like the possibilities could be endless. However, now there is another problem. Survival and reproduction of stocked trout is often very poor compared to wild counterparts. And, for supermale stocking to result in complete eradication of a nonnative trout population, supermales have to comprise a relatively large proportion of the spawning population.
Fisheries managers in Idaho are now in the process of evaluating survival and reproduction of supermales, and the potential efficacy of supermale stocking for nonnative species control. Simulating possible scenarios, they determined that stocking juvenile supermales could result in complete eradication of nonnative brook trout populations in less then ten years, with faster eradication rates occurring when stocking is combined with manual removal of wild fish. This was a promising result; however, another study of actual fish populations showed that stocked adult supermales had low survival and reproduction compared to wild counterparts. So, while the supermales did reproduce (which is encouraging), only about 4% of wild offspring had a supermale father.
For supermale stocking to be an effective method of population eradication, managers will have to find a way to increase reproduction of stocked supermales. How to achieve this goal remains a little uncertain. It can’t be achieved by simply stocking a higher density of supermales. Higher stocking densities are known to increase mortality of stocked fish, and mortality of stocked supermales has already been shown to be high. However, it may be that a combination of stocking and manual removal could increase survival and reproduction of supermales, which could increase the probability of eventual eradication. Or, survival and reproduction may be higher if juveniles are stocked rather than adults, or vice versa. All of these hypotheses are currently being tested to improve supermale reproduction in Idaho streams.
I think it’s also important to note that so far the end goal of supermale stocking has been complete eradication. However, even if eradication is not possible, supermales may still be effective for suppression of nonnative populations. This could help preserve native fish populations while still allowing for nonnative fish persistence. However, regardless of whether the goal becomes eradication or suppression of nonnatives, the success of supermale stocking is also going to depend on management of adjacent tributaries. If stocking continues in nearby tributaries, then movement of nonnative fish back into managed waterways will ultimately make supermale stocking efforts futile.
So, are supermales too good to be true? For now, only time will tell.
*Note: Content in this post is my own and may not reflect the opinion of the manuscripts' authors or the agencies they represent. I encourage you to read the manuscript, found here and here, so you can contribute to the discussion
Time flies when you’re working on a dissertation. I turn around and it’s suddenly the end of the semester and I haven’t posted any updates in over a month. Oops.
Truth be told, there hasn’t been a lot to update on recently. Preparing for outreach events had me spending more time photo shopping in Microsoft Paint than running data analysis in R. But, Ben and I put together some impressive posters, so I consider that time well spent. And, we showed them off this past week at an Earth Day event at the local elementary school.
I’ve also taken the show on the road, and have been visiting several chapters of Trout Unlimited to present the hatchery-wild brook trout interbreeding (or introgression) results that I discussed in a previous blog. At first I felt a little guilty about making introgression the main topic of my talk because I thought most everyone in attendance had already read my blog post. But, as science turns, after we submitted the manuscript for publication- and after we thought for sure we had covered all of our basis with analyses- reviewers unanimously wanted to see another analysis. Of course.
Truth be told, we were actually happy to do the requested analysis. Simply put- they wanted us to quantify how introgression varies across different habitats. Is introgression higher in sites with higher temperatures? Lower pH? What about introgression rates in small vs. big streams? Does distance to a stocking location influence introgression? These are all great questions because they can help us understand if there are site-level characteristics that make a site more likely to have higher rates of introgression. And, if fish are more likely to introgression in certain habitats, then we can use that information to potentially adjust our stocking protocols.
Unfortunately, while there was no doubt this analysis was worthwhile, we knew before we started crunching numbers that none of the results would be statistically significant. If you remember, less than 6% of all fish we tested showed signs of being introgressed. And, most fish at any of our 30 sample sites assigned to pure wild origin. Without the presence of introgressed fish in our study, we weren’t going to be able to find strong relationships between habitat and introgression. It’s like trying to quantify elephant habitat in Pennsylvania- if the elephants aren’t here, we can’t really quantify their habitat.
But, because there are so few studies of introgression on stream trout, we knew that we could still use the results of the analysis to start building hypotheses about habitat variables that could matter. So, that’s what we did. We analyzed whether the probability of an individual being introgressed was related to eight “site-level” and three “watershed-level” habitat measures. The site-level variables includes measures of water quality and physical habitat that you would see if you were standing in the stream like pH, dissolved oxygen, and stream width (and we got lucky here, because all of that data was collected by Susquehanna University and the analysis would not have been possible without their willingness to share data). The watershed-level variables were those I measured back at the computer and were things like watershed area and landuse.
Like we expected, none of our models implicated any of our habitat variables as a smoking gun that increases introgression- all models showed a statistically insignificant relationship between introgression and the 11 habitat variables. But, a bit to our surprise, we did have a few variables that approached significance. Again, this was surprising because we had so little introgression in our data, and so even a variable that trends towards significance is worth a second look.
Probably not ironically, the variables that trended towards significant were also variables that have been suggested in other studies of lake-dwelling brook trout and stream brown trout as being modulators of introgression. It seems there is a consensus among studies that introgression rates are lower in larger streams, and at sites with low pH and higher adult brook trout densities. This makes some sense as higher wild trout densities increase competition (which likely decreases reproductive success of stocked fish) and larger streams have more stable flows that can help promote a large, healthy wild population. It’s suggested that pH might also be an important predictor of healthy wild trout populations, as macroinvertebrates densities are often higher in streams with higher pH.
So, at this point, these results can’t yet be used to really direct stocking efforts. They really just point to a need for more directed studies of introgression in stream trout. But, it does make you think. Most of the time we only stock streams without brook trout, or in streams with “marginal” wild populations. Those “marginal” populations, which tend to be smaller in size but existing in decent habitat, may be the populations that are most vulnerable to introgression. So, by trying to avoid wild-hatchery interactions and stocking in marginal populations, could we actually be increasing the probability of introgression? Again, we just need more data to tell.
He's back. While I work to meet a few deadlines, Ben is filling in again and providing some more perspective on his experience as an undergraduate. Hopefully this is just a start, and he'll be providing more details in future posts as he navigates many "firsts" in his career as a fisheries biologist. Notably absent from his description is any mention of full his calendar became after he started working in the lab, and how much sass he receives from me daily. I'm giving him the true fisheries experience.
Getting involved in research as an undergrad can certainly feel like a daunting task. When you first arrive at college, no matter what school or major, it is undoubtedly going to be a lot different than anything you have ever experienced before. In the midst of trying to make friends, adjusting to college classes, and trying your best to get involved, thinking about your future is something that often gets thrown on the backburner. Nonetheless, when the time comes to take on a new challenge, it can be difficult to know where to start. Luckily, I am here to share my struggles and triumphs in finding an undergrad research position so all of you can live vicariously through me.
When I first decided that I wanted to get involved in undergrad research, the first person I went to was my academic advisor. At a regular advising meeting one day we were just discussing my courses for the upcoming semester and I brought up the possibility of starting to get some volunteer hours in a biology lab. Her recommendation to me was to go online to faculty webpages and start looking for professors doing research that might interest me. I took her advice and began searching for what I hoped would be my future home. While this was pretty interesting at the beginning, freshman me had no clue about half the research I was reading about. And with riveting topics such as “atypical heterotrimeric G protein Y sub-unit and guard cell K channel regulation in morphological development” and ”chronic unpredictable stress causes long-term anxiety”, I think I was starting to develop some long-term anxiety of my own. Not to mention the nightmare that is trying to meet with faculty that have schedules that are equally or more busy than your own class schedule.
There is nothing more awkward than walking into the office of a professor you have never met before and trying to simultaneously impress them while also trying to pretend like you know more about a research topic than you actually do. After my fair share of uncomfortable meetings with professors that were studying nothing close to what I was interested in, I decided to talk to an instructor that was teaching a class that I was taking in my major about research opportunities. Meeting with her was a great way to become exposed to researchers that were doing work that was more relevant to what we were studying in class. While I was disappointed that she was not able to offer me a research position in a lab of her own, I left her office a bit more interested in continuing my search. I sent out another round of emails to some of the people that she suggested to me and eventually heard back from two of them. This time when I met with the each of the professors I did my best to be straightforward about what my skills were and what I was hoping to get out of my experience.
Within the next few weeks I had heard back from both of them with two very different offers. The first of which was the potential for a full-time position, 40 hours a week, for the entire summer sampling in local state parks. The second offer was to work part time in the summer on a brook trout project. I think by now it is obvious which choice I made, but there were a few other considerations that I had to make when choosing a position. I knew that there was no way that I would be able to afford to spend the summer at school without another part time job. I did what I thought at the time was “biting the bullet” and declined what seemed like it would be a really amazing opportunity and opted for the part time position so that I could work part time at the university advising center in order to save a little bit of money. I could not have been happier about my choice.
Finding an opportunity to work in research as an undergrad forced me to make a lot of difficult decisions and really reflect more seriously about what I wanted to do in the future. It is extremely difficult to make choices for reasons based on things other than simply academics, but as is the nature of life that we often have to choose between what we think is best for us and what is actually feasible. In this case, everything ended up working out quite well for me. After I took some time to get oriented to the undergrad research life, I was able to find a fair amount of success with the help of my mentor and other lab-mates. Through undergrad research I got to experience my first taste of field research. It was really engaging to see that skills that are so often talked about in the classroom coalesce into a real-world application used by scientists every day. As you heard in my last post, my research experiences have been invaluable when it comes to networking and developing effective science communication skills. I am also so grateful for all of the opportunities that have been made available to individuals like me through funding offered by a number of locations on campus that support undergraduates in research. It is thanks to these generous contributions that I am able to continue to perform research and better define my interests as I learn more and more about the world of fisheries science. I hope that one day in the future I can reflect again on my undergraduate research experience and how it has shaped me into the person that I am, but for now I will just sit back and hope for the best.
Ben Kline, undergrad extraordinaire, graciously offered to pick up my slack and do a few blog posts over the next few weeks. Ben has been working in the lab for almost a year now, and taken leadership on a project looking at individual variation to heat stress. Results of the project are still pending, but we’re introducing him to a whole new side of science- professional conferences. In this post, Ben talks about his first impressions of an American Fisheries Society (AFS) meeting, which he presented at in February. He did so well that he's packing his bags for the National AFS meeting in August in Atlantic City.
A couple of weeks ago I was lucky enough to give my first oral presentation at a professional conference on fisheries science. It wasn’t my first ever presentation, as last fall I attended the Susquehanna River Symposium at Bucknell University. If you follow The Troutlook on Twitter, then there is a good chance you saw my first crack at a research poster that I attempted to pull together for the conference (or perhaps you were able to pick out the glaring typo instead). The event was quite memorable, and I was pleased with how everything turned out. The poster presentation was a great way to get my feet wet in the fisheries scene, and I thoroughly enjoyed meeting and talking with other people that were interested in the same realm of science as me.
Making the poster and hearing feedback was something that also helped me to reflect on my current and potential future research experiences. As people began to talk to me about my project and ask questions, it became evident that, although I had already spent months working on my project, I still had significant knowledge gaps about all of the moving parts behind our experiment. This experience was somewhat disappointing, as I felt that I had not learned very much in my time working on the project, but also simultaneously quite motivating in a sense that there was a bit of urgency for me to get back to the lab and keep working on the project.
I left the symposium with some thoughtful critique and some questions in need of answering. Upon returning to the lab after some time for the winter holiday, another opportunity presented itself to me. A few weeks prior I had submitted my first abstract to give an oral presentation at a local conference. When I logged into my email I had discovered that my abstract was accepted and I would get a chance to speak at the annual meeting of the Pennsylvania Chapter of the American Fisheries Society. Upon sharing the news with Shannon, we immediately got to work. I had never before given a professional talk about my research and I knew that there would be a lot for me to learn. I prepped my talk and meticulously edited my PowerPoint until I was pleased. When it finally came time to give my practice talk to our lab I knew that there would be plenty of comments, but I was eager to learn and improve in any way that I could. To my surprise, my talk was received warmly by the other members of the lab. With the meeting coming up in just a few short days, I continued to refine my work until the story came out exactly how I wanted it to.
The morning of the conference was a big groggy. Getting up at 4:30am and then taking a 2 hour drive in subfreezing temperatures is not the most glamorous way to start the day, but I was excited nonetheless. We arrived at the conference center and got things set up. I had the chance to chat with a few of my classmates from Penn State that were in attendance while we waited for the show to begin. There were about 100 people in attendance, a healthy mix of managers and academics with few students- and even fewer undergrads. I was a bit nervous to give my talk, and being the first person to go wasn’t very inspirational. But a curt nod from Shannon was enough to motivate me towards the podium and the rest was downhill from there. The talk went quite well, and only one person was asleep, so I would consider it a success. With a major weight lifted from my shoulders I had the opportunity to sit back and enjoy the rest of the conference for what it truly was. I had plenty of opportunities to listen to other talks about research that is going on in the fisheries world. As a future graduate student, it was really helpful to hear what kind of projects other people are working on so that I could better define my interests. I was also pleased to have the chance to meet a number of other scientists and managers and hear about their experiences firsthand.
Through it all I was certainly able to learn a number of valuable lessons. From my first attempt at a research poster to my second attempt with the oral presentation, I was able to identify that I maybe didn’t fully understand my project to the best of my ability, and that was okay for the time being. Having a hard time putting your work into words was a great way of identifying what parts of my research I should spend some time becoming more familiar with. After all, it is one thing to try to understand something yourself. However, being able to not only explain, but also really convey the essence of your research to someone else is something that is truly difficult to do, which brings me to my second lesson. Being a good presenter is about being a good story-teller. The more clear and vivid of a picture you paint for your audience, the better your talk will be as a whole. I spent countless hours thinking about the story that I was trying to tell with my presentation and as a result, the final product was much different than the original. It isn’t about having perfect transitions or the most articulate vocabulary in every instance, but instead about finding common ground that is relevant and meaningful to your audience and using that as a way to leverage the most important parts of your research into what you are trying to say.
I’m happy to report that after months of studying (and neglect of this blog), I successfully passed my comprehensive exams this week. Over the course of a 40-hour written and 3-hour oral exam, I delved into everything ranging from statistical models, population genetics, adaptive management, and the history of ecology. My brain is mush.
But, in the middle of my exam, I took one day to attend the Pennsylvania Chapter of the American Fisheries Society to present the results of our study on hatchery and wild brook trout interbreeding. I know I’ve been holding those results hostage for a few months, but now that I’ve started spreading the word and our manuscript is nearly published, I think it’s about time I let you all in our secret. Drum roll please…..
In Loyalsock Creek, hatchery brook trout don’t seem to breed much with wild brook trout.
Are we sure? About as sure as we can be.
How can this be? We don’t know exactly, but we have guesses.
Let’s step back a little and digest this study. Throughout their native range, hatchery brook trout are commonly stocked in streams frequented by anglers to increase the probability of catching a keeper trout. Stocking programs are largely successful in accomplishing that goal, but yet often remain controversial because of the possibility that hatchery trout might breed with wild trout. The process of wild and hatchery interbreeding is more formally known as introgression.
Why do we care about introgression? In short, introgression can decrease wild population reproductive success and increase the likelihood that a wild population will be extirpated. The mechanisms behind this response are a bit complicated, and have been covered in another blog post. But, it largely centers on the fact that fish used in the hatchery system are so genetically different than wild fish (especially when the focus of the hatchery is in recreational stocking…more on that in other blogs). The last time a wild fish was brought into many hatcheries was 50-100 years ago. In that time, humans have bred hatchery fish to grow really fast, really large, and produce a lot of offspring. While these traits produce a great hatchery fish, it often makes hatchery fish very unsuccessful at life in the wild. So, when a wild and hatchery trout introgress, their offspring often have low survival and low reproduction. To put another way- think of offspring as the average of their parents. The average of two wild fish is usually going to be far superior than the average of a wild and hatchery mating.
It only takes a few generations of introgression for there to be a noticeable decline in population size, and for the risk of extirpation to rapidly increase. However, much of our understanding about introgression in brook trout is borrowed from studies of salmon, and in situations where there was a large-scale accidental release from a captive facility and/or where a stocking program was terminated 10-20 years ago. There is very little information about introgression in a ‘typical’ stocking reach- where stocking is still ongoing, and has been occurring for 10-20 years. So, prior to this study, we didn’t really have a great idea on how active stocking might influence introgression in wild brook trout streams.
And, we are just as surprised as you when our results showed very low rates of introgression. We surveyed 30 sites across the Loyalsock Creek watershed and analyzed genetics for 1742 fish. Only 97 of those fish (<6%) had evidence of introgression. And, it’s worth noting, our methods were very sensitive, so if anything we overestimated the number of introgressed fish.
Now, this isn’t a call to go out and stock freely and openly. One troubling find was that we did find introgression at sites several miles away from the closest stocking location. Often times, stocking is justified because it doesn’t occur directly on top of a wild brook trout stream. However, our results suggest that the impact of stocking can extend far beyond the spatial scale of direct stocking efforts. How stocking could influence wild trout populations needs to consider not only the status of the stocked stream, but also streams nearby.
Second, it’s important to keep in mind that we did not account for how stocking could influence wild trout reproductive success. Does competition for food and space with hatchery trout decrease wild trout reproduction? Do wild trout readily attempt to breed with hatchery trout, but offspring die before we are able to sample them? We’re not sure, but both of those scenarios are plausible non-genetic effects of stocking that our study cannot address, and that would still result in declines in wild population survival.
Why introgression seems to be substantially lower in our study relative to other studies of trout and salmon is a bit of a mystery. It’s possible that stocked brook trout die (either through harvest or natural mortality) quickly after stocking. We know there is some truth in this, as stream surveys rarely find stocked fish even a few months after stocking. It’s also plausible that there is an environmental gradient over which stocking is more or less possible. Because we did not find much introgression to begin with, we can’t really test hypothesis about how the environment may influence introgression. However, we did find some patterns where introgression did seem marginally higher at sites with higher densities of wild adult brook trout, and where pH was higher and temperature low. In short, it seems (though very preliminary and without high certainty) that robust wild trout populations may be a little more resistant to introgression.
Overall, our study suggests that until a better understanding of the factors that influence hatchery and wild trout introgression are known, a cautious prescription of hatchery stocking may still be warranted.
I have, and will continue to be, sparse with my updates. I’m in one of the most dreaded times of a Ph.D. student’s tenure- the preparation for, and eventual taking of, my comprehensive exams (colloquially known as comps).
For those unfamiliar- comps are arguably the biggest hurdle that stand between a Ph.D. student and graduation. No two people have the same comps, but for me it will entail a 40-hour written test taken over the course of four days and a three-hour oral exam. But, it’s not the length that’s daunting. It’s the topic. Science. Know anything and everything about science (and statistics, because good ecologists need to know what to do with their data after it’s collected). Just about the only way to prepare is to read as many textbooks, articles, and online guides as possible. So, read I must. Every day, all day, for the next month. Panic hasn’t quite set it, but I can tell it’s getting close.
While comps definitely add a bit of pressure, I can’t say the experience has been entirely miserable so far. It’s easy to get hyper-focused on one project and lose focus of how your work fits into broader ecological contexts. So, it’s been fun stepping back and thinking more broadly outside of hatchery introgression, which has been the object of all of my attention lately (but which we did finally submit for publication). And, I’ll write about all of that after comps. Promise.
For now, in all my readings I stumbled across one very short article that seemed perfectly fit for this blog. No long explanation required, but a research finding I suspect many of you will be interested in. The topic is one of my favorites: nonnative fish invasion on native populations. So often I hear people argue something to the effect of ‘nonnative fish are not causing declines in native populations, but simply taking over as native populations are dying off due to climate change, habitat loss, etc.’ To put another one, people often think that nonnatives are REPLACING, not DISPLACING native fish.
If this were true, if nonnative fish were simply replacing natives, then there wouldn’t be a lot we could (or probably should) do about it. When a population of a species starts declining, it opens space in the ecological niche that usually needs to be filled in order to maintain ecosystem functioning. It’s like a job opening- if Debbie was going to quit, it’s better that someone fills the position rather than just leaving it vacant and hoping Debbie returns. Replacing Debbie would be like filling the empty ecological niche.
But, what if Debbie had no intentions of leaving and is being forced out? In this case, Debbie would be displaced. The intruder will likely fill some of the same roles Debbie left behind, but may also neglect certain tasks. As we all know, two employees with the same job description rarely have comparable work effort and quality. The intruder is going to leave some of the niche open.
Bringing this back to fish, it’s really difficult to determine if a nonnative species are replacing or displacing a native species. If we were to track the number of fish of each species over time, we’d likely see that one species (usually the native) was increasing and one (usually the nonnative) was decreasing. But, that is not evidence for either replacement or displacement. If the nonnative species was absent, the native species may still decline due to habitat loss, genetic collapse, or overharvest. Or, it may thrive despite all the aforementioned stressors. The only way to know for sure would be to do several very controlled experiments where we artificially added or removed fish from streams and then monitored their populations for several generations. But there are time constraints, and largescale changes to species communities are generally frowned upon in conservation.
So…enter statistics, where we can model the relationship between the abundance of each species, time, and environmental variables to determine how each species effects one another. If that sounds vague, it is. But, the details aren’t worth describing here. It’s just worth noting that a researcher from Japan recently used these models to investigate how invasion of nonnative brown and rainbow trout influenced the abundance of native white-spotted char (a close cousin to brook trout) using data collected over 15 years.
And his findings? Nonnative trout clearly DISPLACE native trout. Moreover, rainbow trout also displace brown trout, so not all nonnatives are created equal. If you think about Pennsylvania, right now brown trout are outcompeting (whether it is replacement of displacement, we don’t know) brook trout. In the future, could we see displacement of brown trout by rainbow trout? Certainty possible.
Perhaps more noteworthy, there was a significant time lag (8-13 years) between the initial invasion of nonnative trout and displacement of white-spotted charr. But, once displacement started, it was achieved rapidly (just a couple years). This suggests that monitoring efforts following invasion may have to extend for several decades before the effect of invasion are realized. It’s not enough to make conclusions about invasion based on only a few years of data, and certainly not enough to make inferences on the cause (be in replacement or displacement) with such limited information.
*Note: Content in this post is my own and may not reflect the opinion of the manuscripts' authors or the agencies they represent. I encourage you to read the manuscript, found here, so you can contribute to the discussion
Take a second and think about a healthy brook trout population. Your mind may wander to a stream rambling through a backwoods valley on a crisp autumn day, where there’s a deceptively calm mirror glaze reflecting the forest canopy. It’s deceptive, of course, because you know underneath the water’s surface trout are swirling around and preparing to spawn. Now, habitat aside, what qualities really constitute a “healthy” population, and what do biologists use as a measure of health?
I’m willing to bet most you said something about population size. And, why wouldn’t you? It’s one of the most common metrics biologists use to gauge population status, and everything you’ve probably ever been told about trout suggests more fish is better. You’re probably feeling pretty confident that large populations are healthy populations.
Science is a cruel beast. As a biologist, I’ve come to learn that the only thing you can be confident in is that nothing is ever ALWAYS true. Even things that seem so completely obvious and undeniable….like the correlation between population size and health. Yes, there are many benefits to large population sizes such as higher genetic diversity, higher adaptive potential, and more resiliency to disturbance. Many conservation objectives aim to increase population sizes in order to preserve exactly these qualities.
But, bigger is not always better. There’s a limit to how big populations can get before high abundance actually becomes a negative thing. How can this be? An obvious answer is food availability- more fish in a stream means less food for each individual. Have you ever gone fishing in a pond and caught what seems like hundreds of small bluegill? You could be catching the same individual, or just fishing at the wrong time of year. But, more likely you were fishing a stunted population- a common phenomenon in sunfish were there are too many fish and not enough resources for any of those fish to grow very large.
Food is not the only resource fish compete for, and may not even be the most important limiting resource. As population sizes increase, competition for habitat, specifically spawning habitat, also increases. As competition for spawning habitat increases, the average reproductive success of individuals starts declining. So much so that some fish may not get to reproduce during their entire lifetime. In other words, if there is a lot of competition for spawning habitat, population size may be large, but the number of breeders can actually remain quite small. In brook trout, it’s not uncommon for a population of 1,000 adults to only have 100 breeders, or only 10% of the population contributing towards reproduction.
In this case, population size is a poor indicator of overall population health. If I surveyed 1,000 adult trout in a stream, I would be amazed at how healthy that population is. But, if only 10% of those fish are contributing towards reproduction, then that population may be rapidly losing genetic diversity. You have to remember that to increase adaptive potential and maintain genetic diversity and population stability, the goal is to increase the number of fish that are reproducing- not necessarily the number of fish in the stream. So, a population that has fewer individuals but has, say, 40% of the individuals contributing towards reproduction may actually be much healthier than a large population where only 10% of adults are breeding.
More problematic is that we often have no idea the relationship between population size and the number of breeders. It’s pretty easy for us to go out and sample fish and determine the population size. To determine the number of breeders, we have to sample juveniles, and then run genetic analyses to determine how related the juvenile fish are. If the fish are mostly siblings and cousins, then we know the number of breeders must be fairly small and there are only a few families in the population.
What does this mean for those of you that may be actively trying to plan and execute various projects to improve the health of your local trout stream. Yes, it’s difficult to determine the number of breeders in your stream. But, that doesn’t mean you should just throw your hands up and hope for the best. The number of breeders is often directly related to habitat availability. So, to increase the number of breeders, we should often focus restoration efforts on increasing habitat, not stocking to increase abundance. Stocking fish in streams with limiting spawning habitat could actually decrease the number of breeders, thus decrease the overall adaptive potential of the population.
Of course, this isn’t to say that small populations are healthy populations. The ratio of breeding adults to total population size isn’t the only metric of population health. It’s just one of the many things to consider as you think about protecting the health of your local waterway.
I’ll start by acknowledging that this is a bit of a bold idea, but bear with me.
Researchers recently determined that egg incubation temperature decreases the social learning ability of adult lizards. They conducted an experiment where they incubated some eggs at about 80°F and others at 86°F. Once those lizards reached adulthood, they tested their ability to socially learning a new behavior. Specifically, lizards were allowed to watch videos of other lizards opening a sliding door (a behavior that lizards don’t usually know how to do). After watching the videos, the researchers tested whether lizards hard learned to open the sliding door for themselves. Lizards that were incubated at warmer temperatures were significantly less capable of socially learning the behavior, and thus were less successful at opening the door compared to lizards incubated at the cooler temperatures.
So what? Why does a trout biologist care about lizard egg incubation, social learning, and sliding doors? Okay, maybe I don’t care about sliding doors. But, social learning, or simply learning by way of watching or imitation, is something that humans take for granted. We watch a friend solve a puzzle a certain way and we instantly know how to solve the puzzle the same way. Or, if we see a group people heading for a different register at the store, we’re inclined to follow thinking they found a faster way to check-out. Humans use social learning all the time, both consciously (like solving a puzzle) and unconsciously (like finding a new cash register). But, what about fish?
My first few research projects as an undergraduate all focused on understanding how trout acquire important information about their environment. The underlying assumption is that every individual learns information the hard way via trial and error where each fish has to learn everything on it’s own. While some information is acquired this way, it’s not a very good learning strategy for most things. It can take a long time to develop a new behavior or learn about a threat, and a fish only gets one chance to learn about a new predator before it’s eaten. So, one alternate strategy is to pay close attention to the behavior of other fish, and pick up new information via social learning.
Social learning speeds the learning process, and is particularly helpful in situations where information changes quickly and individuals need to be ready to adapt to their surroundings. For example, and completely hypothetical, streams are subject to rapid changes in flow conditions, which can also change where the best location is for a trout to sit. A fish can try to roam around and actively determine where to go as flow changes. But, the individual is unlikely to gather all the information before flow changes again, plus moving around exposes them to a lot of predation. Alternatively, a fish can use social learning to watch to see what all the other fish are doing around them, and use that information to update their map of the stream without really moving.
As it turns out, trout are incredible social learners. One of my first research projects focused on determining how brook trout gather information about changing food resources in a stream. As many anglers know, prey availability changes frequently in small streams, and trout have to constantly make decisions about whether something floating past them is food, a stick, or potentially something lethal. So, they typically develop a search image for a few insects, and let everything else float past (this is why it’s important to “match the hatch” and pick flies that resemble bugs currently in the stream). When food resources for which a fish has developed a search image to run out, trout have to learn a new search image and target a type of insect. It turns out, it can take over two weeks for a fish to develop a new search image on its own. That’s two weeks a fish could go with very limited food as it tries to learn what to eat. But, if a fish is watching another fish eat a new type of insect, it will develop a search image almost instantly. Here, social learning leads to more calories that can be used for growth, reproduction, and even survival.
Another project I did showed that brook trout also use social learning to avoid interacting with other fish they know will outcompete them. Brook trout readily fight with one another for spots in the stream that have the best access to food, concealment from predators, and where flow is not too fast or slow. Naturally, the most aggressive fish (which is usually the biggest) has access to the best spot, number two has the second best spot, and so on down the chain. There’s a benefit to occupying spots of highest quality, but it’s dangerous for a fish to pick a battle with another fish that it is going to lose to.
So, how does a trout decide who to fight? As before, it could be a trial and error process wherein a fish fights with most of the other fish around it to determine its rank. But, fighting is energetically costly and can be lethal, so trout try to avoid interactions when possible. To do that, trout watch other fish compete, and from those observations learn which individuals are more and less dominate. It’s like watching a series of playground fights to identify the bully you never want to mess with. By watching other fish compete, a fish can socially learn the competitive ability of many individuals in a pool without ever having to interact with them directly.
So, brook trout use social learning to find new food resources (which increases energy that can be used for growth and reproduction) and to limit competitive interactions with rivals (which decreases energetic output and the chances of injury). If increased stream temperature during incubation decreases social learning as it did with the lizards, what effect will that have on trout? It’s hard to say. We know that trout use social learning, and we can speculate on the energetic benefits to using social learning. But, we don’t know exactly what happens if trout suddenly lose the ability to learn socially. Could we see reduced growth, reproduction, competitive ability, and extirpation? Yes. But, it would be hard to say that those outcomes happened because of a loss in learning rather than the effects of some other stressor such as stream temperature rise or competition with nonnative species. It’s difficult, perhaps impossible, to isolate all of those stressors from one another.
But, what this highlights is that climate change is more pervasive that we probably think about on a daily basis. Yes, stream temperature rise can make certain streams too hot for trout to occupy. But, there are negative consequence of stream temperature rise that occur before population extirpation and that may affect more subtle aspects of fisheries ecology and behavior.
Everything’s fine until the invasives move in.
I’ve preached this before. Invasion by nonnative trout results in declines in native trout abundance. On the east coast, I’m talking specifically about invasion of nonnative brown and rainbow trout causing declines to native brook trout. But, what is the mechanism of decline?
Is it competition? Sure. Nonnative trout can outcompete native trout for food, habitat, and sometimes even mates (enter tiger trout).
Is it habitat preference? Yep, that too. Brown and rainbow trout tend to have higher thermal tolerances, and so they can live in a wider range of habitats. They can also occupy streams with altered flow regimes, higher sedimentation, and lower water quality.
What about growth? We have a trifecta- nonnative trout tend to grow faster than natives. This makes nonnatives better competitors, but bigger fish also tend to produce more offspring. So, populations of nonnative trout tend to grow fast and can quickly outnumber native trout (this usually isn’t the case of rainbows in Pennsylvania, but down south rainbow trout populations are taking off and outnumbering brook trout).
But, you know what else it could be? Maybe nonnative trout act as a strong selection pressure. This could cause native trout to become maladapted to their local environments because interactions with nonnative fish are acting as a stronger, more acute selection pressure than the environment. Huh?
Let’s break this idea down a little. We often think about the environment as the strongest selection pressure that shapes the genetics of populations. And, that’s not wrong. Through hundreds of years of natural selection and adaptation, trout populations have accumulated the genes and outward characteristics that make them best at surviving in coldwater stream habitats. At this point in the evolutionary time scale, the amount of variation in those characteristics is really quite small. Yes, brook trout show a lot of variability, but you can still identify a brook trout from, say, a bass that has spent millions of years evolving for life in a different type of habitat. Almost every brook trout is now well-equipped for life in the typical stream environment.
So, now we’re at the stage of fine-tuning the genes in populations. There’s a lot of genes that are good for life in a stream, but only a subset of those are also good for surviving a catastrophic flood. And, only another subset for devastating droughts, or unseasonably hot summers. So, natural selection is still at work. But, it has to wait for these very rare events to occur before there is large shift in the genes in a population. Until then, populations just maintain the characteristics that make them good at life in their streams.
But, then life in the stream changes. A nonnative fish invades, and starts imposing a new selection pressure. Suddenly brook trout, which are often the top predator in a small stream, need to compete with another species for food and habitat. And, because presence of the nonnative species is a constant pressure that can act on native species every day and in multiple ways, it starts acting as a stronger selection pressure than rare environmental events.
Think of the red line as the genetics in trout populations. Historically, back when fish were new to the animal kingdom, trout and bass probably looked very similar to one another. As evolution occurred, trout genes started becoming more adapted to stream life until there was very little variation in the genes of trout populations (relatively speaking). That was, until the nonnatives moved in....
It may sound a bit far-fetched, but a team of researchers recently completed a study to see if invasive trout could be acting as a selection pressure that overrides selection from the environment. Their work was conducted in Sweden, so in this case the invasive fish was our beloved brook trout, and the native was brown trout. What they found was that, in the presence of nonnative brook trout, brown trout developed stouter bodies, had a smaller home range, and even shifted their diets to consume more terrestrial prey. When brown trout weren’t in the presence of brook trout, they had short daily movements, high metabolic rates, and high activity.
How did brook trout cause this change? It seems to be related to a change in how brown trout live their daily life. When the only top predator, native brown trout can afford to live a high risk, high reward lifestyle. They are free to swim around, eat a lot of the best food (which are often bugs living on the stream bottom), live in the best environments, and defend quality territories from subordinate individuals. To sustain this lifestyle, fish need to have high metabolisms (to keep up with energy needs for swimming and fighting) and body shapes that are more slender, which are better for sustained swimming and foraging.
Now, add nonnative brook trout to the mix and brown trout are no longer standing at the top alone. There’s less freedom to move around and find insects on the stream bottom, and so trout switch to a “sit and wait” feeding strategy. Instead of actively foraging, they become drift feeders and wait for terrestrial insects to fall into the stream near them. The addition of brook trout also means there’s generally less food available for each individual, and so slower metabolisms (which require less food to sustain basic biological function) are favored over faster metabolisms. But, slow metabolisms are associated with reduced growth, reproduction, and movement, and so body shape changes and fish develop smaller home ranges.
So, the addition of a nonnative trout species results in more than just competition. It can also induce evolutionary change and alter the native species’ behavior, morphology, and physiology. Do these changes then make native species maladapted for everyday stream life? Or, could it reduce survival when there are catastrophic events? How does the presence of a nonnative change the adaptive potential of a native species? I think we need more study to really answer those questions.
*Note: Content in this post is my own and may not reflect the opinion of the manuscripts' authors or the agencies they represent. I encourage you to read the manuscript, found here, so you can contribute to the discussion
I know you’ve done it. You’ve gone fishing in two different streams, or maybe even just looked at photos of brook trout from different areas, and thought “why are these fish so different from one another?” And, I’m not just talking about length. You may have noticed that brook trout can show a very large range in color, patterning, body depth, fin size, etc. Even fish within the same stream can sometimes look completely different. What gives?
It’s a great question. And the answers potentially have big implications for our understanding of how trout respond to fragmentation. I’ve already told you that fragmentation results in population isolation, which can lead to loss of genetic diversity and eventually lead to local extirpation. But, there are other changes that can occur before an isolated population collapses- the population size might decline, the ratio of males: females might become skewed, fish may start behaving differently, and, that’s right, individuals may start taking on different physical appearances.
The question is whether fragmentation leads to predictable changes in morphology across populations. If so, it indicates that fragmentation could be a factor that influences the evolutionary trajectory of populations in a predictable way. For example, if fragmented populations show a tendency to have reduced body coloration, then we would know that fragmentation somehow operates as a selection pressure, and that bright coloration is somehow not advantageous in fragmented populations (this scenario is purely hypothetical, by the way).
So, scientists being scientists, someone went out and tried to determine if fragmentation is an evolutionary selection pressure that acts on brook trout morphology. In a recent paper, researchers from Canada sampled individuals from 14 brook trout populations in Newfoundland, Canada. These populations are all genetically distinct, and upstream of barrier waterfalls (begs the question as to how the brook trout got there. Let’s leave that story for another day, but it was all natural, promise). At each site, the team took measures of body size, weight, and sex. They also took very detailed pictures so that they could later digitally measure things like body color, number of spots, body depth, fin length, hump size, jaw length, etc.
Is fragmentation a significant selection pressure on brook trout morphology? Yes and no. The researchers definitely found that populations had very different morphologies. Isolation has prevented gene flow, which has put each population on it’s own evolutionary trajectory. But, population size and standing genetic diversity, which act as a proxy for the strength of the effect of fragmentation, didn’t predict morphology that well. This indicates that fragmentation, itself, isn’t a factor that influences morphological change. Rather, current habitat conditions seemed to have a stronger influence of morphology. Trout in warm, slow streams tended to grow larger and develop a larger hump on their backs (reminder: warm is relative, these populations are in Canada), fish from faster streams tended to have longer pectoral and pelvic fins, and sites with more acidic water had fish with redder color tones.
Interesting, the environmental associations tended to be stronger in females than in males, and females also had more morphological traits that were correlated to habitat. For example, females developed redder tones in deep, fast, warm water, but the association was weaker for males. Why this is the case isn’t entirely clear. But, it likely has something to do with the fact that sexual selection acts much more strongly on males. Sexual selection is a form of natural selection that is specific to traits that increase reproductive success. Think about brook trout spawning behavior, and how sexual selection may act differently on each sex. Females build redds and then wait for eligible bachelors to arrive. Males have to compete for access to females, and subordinate males aren’t going to produce many offspring. This means that natural selection (via sexual selection) is going to strongly favor males that have traits associated with fighting ability during spawning, even if perhaps they are a little less adapted for the environment outside of spawning. Sexual selection acts less strongly on females, and so natural selection is going to favor females that are generally adapted to their local environment. As a result, female morphology is more strongly correlated to habitat than male morphology.
So, why do we care about these results as fish managers and conservationists? While many organizations are making strides to increase movement corridors and reconnect populations, streams are still becoming fragmented by loss of thermal habitat, road crossing, dams, etc. The results of this study suggest that fragmentation, itself, doesn’t seem to pose a strong selection pressure. But, the habitat that the fish become isolated in does. By building a road crossing, we could be effectively deciding the morphological fate for brook trout populations. How this could influence population survival remains unclear, but changes in morphology don’t seem, at least right now, to result in rapid loss of population survivability.
The more you know…
*Note: Content in this post is my own and may not reflect the opinion of the manuscripts' authors or the agencies they represent. I encourage you to read the manuscript, found here, so you can contribute to the discussion.