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
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
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’m back! And, boy was my absence untimely. While I enjoyed soaking up the rays attending the annual meeting of the American Fisheries Society in Florida, I unfortunately missed the Pennsylvania Wild Trout Summit. The PA Fish and Boat Commission was quick to post presentations online, so I’ve been able to catch a few talks (including the one below by my advisor, Ty). But, I’ve also been reading some feedback from a few attendees and my takeaway is that the best talk wasn’t by a platform presenter- it was among members in the audience. One of the reasons I love studying trout is the passionate anglers and citizen scientists that are invested and devoted to wild trout conservation and restoration. There is no other angler base that is as informative and fun to interact with as you all, and I was sad to miss the opportunity.
My other observation is that there was some disappointment in what wasn’t discussed. Most notably, it seems a lot of people in attendance wanted to discuss the state’s trout stocking plans. I’m not surprised. Stocking is controversial and there will probably never be a stocking plan that makes everyone happy. But, I’m also encouraged. The public is trying to voice their opinions on this really complex problem, and, from what I’ve seen, seem to largely understand the delicate balance between the science of native fish conservation and the social dynamics of recreational fishing. It’s not an easy line to walk.
I’m also encouraged because it means there is interest in our current research beyond the scientific community. Our manuscript on native and hatchery fish interbreeding is nearing completion, and the results are getting closer to being released. Until then, I’ve been spending most of my days pouring over manuscripts published over the last 20+ years from other studies of hatchery-wild interbreeding and trying to summarize their findings. From this, I’ve already summarized the pros and cons to hatchery stocking, but I’ve left you in limbo the last two weeks. Overall, do hatcheries have more of a positive or negative effect on wild trout populations?
Before I answer that question, there are two caveats. First, I’m only discussing recreational stocking- or stocking done to temporarily increase population sizes to allow for increased angling opportunities. The potential pros and cons to conservation stocking are a bit different. Second, I am only focusing on the hard science. I’m not going to attempt to compare the social benefits of stocking with the impacts to native fish diversity. But, you should. Everyone should weigh the pros and cons and make their own informed decisions about stocking. It’s not my place to make the decision for you, but it is my job to present the science so that you can be informed. We know that stocking increases recreational opportunities and can be an economically profitable business, both of which valuable. Taking that into consideration, I have drawn a line in my mind where I think stocking is worthwhile and where it’s not. You need to find that line without someone telling you where they think you should put it.
So, after 20+ years of study, what do we know about the effect of hatchery stocking on wild trout populations?
So, where does that leave us? With a lot of uncertainty. Hatcheries can have negative effects on wild populations. But, not always. And, hatchery interbreeding can be high in stocked populations. But, not always. And, we know that there are long-term negative consequences of interbreeding. But, yet again, not always. We just don’t know.
Perhaps a more important question- where does that leave you in your thoughts on stocking?
I wrote last week of the two types of grad student vacations, conferences and field work. But, there’s another holiday that’s even rarer (at least for me) and merits even more celebration. I’m talking about your advisor’s vacation week, otherwise known as Grad Student Independence Week.
Truth be told, my advisor’s whereabouts don’t really influence my work ethic. For the time being, I’m working at my own self-defined pace (cross my fingers I can keep it that way). But, the closer we get to the beginning of the semester, the more sparse the office gets. With no one to pester during the day, why bother going in?
So, I didn’t. I slept in a little later (which for me is 6am), enjoyed coffee on my patio, and had one main goal: start working on the hatchery-wild hybridization manuscript. Data analysis is still on going, but at this point I know what the results are going to say. There’s no need to wait for the final numbers to crunch to start the long process of preparing the work for publication.
When I was an undergrad, I always thought that scientific publications were the works of brilliant scientists who wrote the equivalent of Shakespearian prose. I never thought I’d be smart enough to accomplish a similar feat. I actually still think that, except I’ve somehow been let into that elite crowd of published scientists seven times now. It still hard to believe I’ve reached the point in my career where I am the authority on a topic- someone out there is reading my manuscript and thinking I am the brilliant scientist. Crazy.
One thing I have learned along the way is that regardless of how smart you are, how great your research is, or how well you write, all manuscripts start in the same place. With a blank Word document that just stares at you. For me, it’s probably the single most intimidating and frustrating part of the publication process. Literally anything I put down “on paper” would represent an improvement over the blank page, but I just sit there for hours- staring, erasing, and getting more frustrated.
There’s all sorts of advice out there about how to be the best, most efficient writer- outline your ideas, write 30 minutes every day, discuss your paper beforehand, etc.- and I defy every single recommendation. That long, frustrating, fight with the blank page is just part of my process, and I need to work through before I can write something worth saving. And, the fight needs to be long and uninterrupted. Not a great task for tackling at the office where distractions are imminent, but a perfect job for celebrating my Grad Student Independence Week at home.
I actually only got one full day at home, but it was enough to win the battle and get a solid start on the manuscript. Time to save it, back it up, and not look at it for at least a few days. In the meantime, I go back to square one- read published manuscripts that I know are important for my study and that I will cite in my own publication to support why our study was needed and to add credibility to the results we found.
As I’ve said before, there aren’t a lot of studies on hatchery-wild interbreeding in brook trout. But, I did find one by Andrew Harbicht and colleagues (see below for a link to the manuscript) that looked at how the probability that hatchery trout will breed with wild trout changes depending on the environment. I’m still not releasing the result of our analysis, but studies like this are important regardless of what we find. Whether we find a high degree of interbreeding or not much at all, we need to know WHY we are getting that result. And, it makes sense that environmental conditions influence how much hatchery trout breed with their wild counterparts.
The study was conducted on several lakes in Algonquin Provincial Park in Ontario, Canada, of which some were never stocked with hatchery brook trout, and others had historic stocking that had been stopped 10+ years prior to their study. Immediately, you’ll notice there are some differences between their study and ours: we work on streams, and in areas that are currently being stocked with high densities of fish. Nevertheless, their results are important to keep in mind as we move forward. Most importantly, they found:
So, why is this study important for us? For starters, streams often support lower populations of brook trout than lakes, making us nervous that interbreeding may be more prevalent in streams than lakes- particularly, again, because stocking in our systems is frequent and on going. Our streams also have a wide range accessibility, pH, and other environmental variables (e.g., gradient and temperature) that influence population sizes and competition. Big picture, this study just shows us that introgression isn’t an all or nothing phenomena. Location matters a whole lot, and our results can’t be taken as the definitive response of trout to stocking.
But, all of this presumes that we are finding interbreeding. Which I’m not saying we are. I’m also not saying we aren’t. You’ll just have to stay tuned.
*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.
For states fortunate enough to have cold water flowing through their hydrologic veins, native trout conservation tops the list of management goals for many state and federal fisheries biologists. Often times, we take a “if we build it, they will come (and stay)” approach to conservation. In other words, more habitat equals more fish. Every year, state and federal agencies, non-profit organizations, and local citizen groups spend millions of dollars on stream restoration and habitat additions. This includes everything from riparian plantings to decrease water temperature and sediment transport, instream structures to create pools and slow down stream flow, and even reconstruction of the stream channel.
Does it work? When done properly, yes. Stream restoration activities are great at increasing (sometimes for decades) local trout abundance and survival. But, habitat restoration does not discriminate between species. Good faith efforts to increase one trout species (like native brook trout on the east coast), will also increase populations of nonnative trout- in this case brown and rainbow trout.
If fish shared habitat peacefully, this wouldn’t be a problem. But, nothing in nature is ever that easy. Trout species share habitat like two toddlers in a toy box. Competitions for the best spawning and feeding spots are common, and champion fighters get a major advantage- their first pick of home territories; places that have the most food, the best hiding spots from predators, and not too much flow (otherwise the fish has to use too much energy to swim around). These spots are generally won by nonnative species, who’s faster growth rates and tolerance to warmer temperatures make them gold medal fighters. Worse yet, native species don’t just lose the fight, they are usually kicked entirely out of the playground.
Competition between brook and brown trout is not a new topic. We already know brown trout typically outcompete brook trout because brook trout grow slower and shift their habitat use when brown trout are present. However, figuring out exactly how the two species interact and divvy up space is more of a challenge. Streams are very complex environments with limited controllability. It’s hard to figure out how fish compete for small-scale habitat features (like the features we would typically add to a stream during restoration) when habitat quality changes so fast. We can develop very complex maps that accurately predict the best place in the stream for a fish, and then observe fish interact for those spots. But, one storm can completely change habitat availability and desirability. Likewise, one fish moving in to, or out of, a pool can shake up the competitive dynamics and turn winners into losers, and vice versa. It’s very difficult to make very small scale observations in natural systems.
Enter the experimental stream lab at the USGS Leetown Science Center in West Virginia. Than Hitt recently lead a study that looks at how brook and brown trout compete for different habitat requirements with rising stream temperature. The setup was fairly straightforward- four streams, each with three pools and two riffles. Stream temperature was gradually increased form 57°F to 73°F, all while the last pool was held at a constant 57°F to mimic cold water upwelling areas common in mountain streams. There was also a feeder that continually released food, but it was located at the top of the stream, far from the cold water upwelling. Two streams were stocked with 10 brook trout, and two streams were stocked with 5 brook and 5 brown trout.
The idea behind this design was to supply two areas of required habitat – food and cold water- and see how fish compete for each as temperature increased. When temperatures were cooler, food should be the most desirable resource, and competitions near the feeder should be fierce. But, as temperatures increased, competitions should shift away from food and towards spots in cold water. Brown trout added a layer of complexity, and the expectation was that brook trout should be the best fighters at cold temperatures and win access to food, but at warmer temperatures they would start losing competitions to brown trout.
The result? As expected, the desirability of the food patch declined with temperature. In the brook trout-only stream, fish slowly shifted from spending their time near the food, to spending the majority of their time in the cold water. Not a surprise. Fish can survive several days without food, but they can only survive a few hours in stressful temperatures.
But, when brown trout were present, brook trout couldn’t get near the food. Not at cold temperatures, and not at warm temperatures. Brown trout excluded brook trout from habitat patches were food was most abundant and, overall, brown trout influenced brook trout habitat selection more than temperature.
What this study shows us is that just because habitat is available, doesn’t mean that your target species is able to use it. Instead, removing competing species may do more to increase habitat availability than physically increasing the amount of habitat in a stream. In fact, because nonnative species can exclude native species from desirable habitats, increasing habitat availability could increase nonnative species abundance without doing much to increase population size of native species.
In this study, brook trout were excluded from foraging locations and restricted to habitat that was still thermally suitable. What if they had been kicked out of cold water and into warm water? In this case, brown trout would be pushing brook trout into lethal habitats. This is likely to be the reality moving forward with stream temperature rise. There are a growing number of streams that get seasonally too warm for trout, yet they still maintain populations because trout move into areas of cold water refuge during temperature spikes. For fish that are thermally stressed, these refugia are their last lifeline, and fish are willing to spend their last bit of energy vying for even a few minutes in cold water. Inevitably, competition for such a limiting resource reduces populations sizes as not all fish can occupy the refuge and many are forced into lethal habitats. But, when two species start competing, it will likely result in extirpation of the less successful competitor. And, if history repeats itself, we already know that brook trout are likely to lose.
*Note: Content in this post is my own and may not reflect the opinion of the manuscript's authors or the agencies they represent. I encourage you to read the manuscript so you can contribute to the discussion.
In the next few weeks, millions of trout (nearly 4.6 million in Pennsylvania, alone) will be stocked nationwide to improve and encourage angling opportunities. We all have opinions on fish stocking. And, if you’re like me, those opinions may not be so cut and dry. Native vs. nonnative. Stream location. Source population. Angling pressure. It all weighs into what you believe is the “right” choice for a particular stream.
But, if you’re also like me, your opinions about stocking probably focus mostly on the fish. Will the stocked fish compete with the native fish? What happens if the stocked fish start reproducing in areas they are released? Sometimes we might extend our thinking to other organism, such as how adding more fish to a stream could impact the abundance of macroinvertebrates (i.e., food), which could ultimately decrease fish growth.
It’s not too often we think about how fish stocking could affect an entire ecosystem. And, when we do, it usually still revolves around organism-level effects (such as declines in bats, birds, and spiders with nonnative fish stocking…a story for another day). But, what about the effects of stocking on, say, nutrients? Thinking about nutrients can be a little difficult because you can’t usually see them, or their effects, directly. But, flowing through the water are microscopic minerals and particles that are the building blocks for life. You’re probably more familiar with this concept than you realize- when you fertilize your garden you are adding, among other things, the nutrients phosphate and nitrogen to speed up growth. In streams, these nutrients are floating in the water, and organisms, particularly algae and other plants, absorb them to grow.
The amount of nutrients available can limit the number of plants, insects, fish, even terrestrial animals that an ecosystem can support. Natural ecosystem have evolved a tight network that maximizes nutrient use to support the most number of critters possible. And, generally speaking, we don’t notice nutrient problems until something gets out of balance. For example, algal blooms are the result of excess nutrients entering streams, often from runoff from urbanized areas. Too many nutrients leads to too much plant growth, which ultimately can lead to loss of oxygen in the water and death of aquatic life.
Now, let’s turn our attention back to the fish. You can think of fish (and any living organism, for that matter), as a super concentrated packet of nutrients. Body tissues are loaded with nutrients, and at any given moment fish are eating, absorbing, digesting, and secreting even more nutrients.
So, if fish are concentrated packets of nutrients, what effect does stocking have on the balanced ecosystem? This question was addressed by a group of researchers from Cornell University who evaluated the effects of stocked, nonnative brown trout on stream nutrient levels. They specifically focused on forms of nitrogen and phosphorous- two of the most prevalent nutrients that can quickly become too abundant and decrease overall water quality.
For starters, fish stocking results in immediate increases to ecosystem nutrients. If you add thousands of new fish bodies to an ecosystem, then you are also increasing the amount of nutrients, often by orders of magnitude. This is a pretty obvious conclusion, yet I had never thought about stocking in that way before.
Once fish are in the streams, they start excreting waste, and waste is full of nitrogen. Stocked fish excreted up to 85% of the total ecosystem nitrogen demand when, in comparison, native fish only excrete 0.5% of ecosystem demand. That difference is huge! And, with more nitrogen floating around streams, there is greater potential for nutrient imbalances that can harm fish. Interestingly, stocked fish didn’t excrete much phosphorous, and so there was no effect on that nutrient.
From there, things get interesting. In this study, angler harvest, predation, and natural mortality resulted in quick removal of most stocked fish from the systems. So, the effects of stocking to nutrient loads were not long-lasting. However, what if those fish had survived? What if fish were stocked in a catch-and-release system? In this case, stocked fish have the potential to not only have long-term impacts to nitrogen, but also increase other nutrients through reproduction (fish eggs are very high in nutrients) and in death (fish carcasses are even higher in nutrients). Mortality is particularly important when considering that many trout are stocked in streams that get too warm in late spring and summer, meaning that stocked fish are predicted to all die around the same time of year. If those carcasses are all decomposing in spring, at roughly the same time as many plants are starting to come out of dormancy, then excess nutrients could increase growth of aquatic plants and cause declines in overall ecosystem health.
The effects of stocking are somewhat limited in small streams where nutrient levels are already high, and so ecosystem processes are unaffected by the addition of even more nutrients. However, mid-reach rivers and lakes are often nutrient poor. Initially it might seem like stocking in nutrient poor areas is a great thing. But, remember, ecosystems have evolved to operate under their own natural nutrient levels, even when they are low. So, adding nutrients will open the door for growth of plankton and algae and, ultimately, loss of water quality (you may be sensing a theme, here).
It may sound like I’m trying to convince you that fish stocking is bad. It’s not. At least not always. It’s just a very complex issue, and the complexities are a lot deeper than many, including myself, sometimes realize. So, hopefully this post just makes you think a little harder next time you see the trout stocking schedule for your favorite steam.
I’m not even going to try to hide it. Very little got accomplished this week in the field. We had a lot of struggles early with equipment malfunctions, and no sooner than we started making progress it started raining. That’s field work, and we’ll try again next week.
When I started this blog I intended it to be both a source of updates for my studies, but also a place where interesting research by other trout ecologists could be made more accessible. While I tackle a few big-picture questions in the Previous Research tab, hundreds of research articles are published every day. Unfortunately, these articles are often hard to access without a university affiliation, and sometimes even harder to understand without a dictionary and a lot of patience. The knowledge used to manage our natural resources shouldn’t be held captive in the hands of a select few. Science has to do better.
The unexpected lull in field work left me a little time to catch-up on the growing stack of articles I’ve saved over the past few months. So, there’s no better time to start realizing the next phase of this website and discuss some interesting research by my fellow ecologists. In this inaugural research blog I discuss a paper that addresses the question:
How High Can a Trout Jump?
First off, why do we care? In addition to the increasingly large number of man-made barriers to trout movement (e.g., bridges), mountain streams have many waterfalls of various shapes and sizes. Knowing whether a fish can swim upstream of a barrier has significant implications for management. If a barrier is passable, then the population is said to be “connected,” which is important because connected populations are more resistant to extirpation (a concept I explain more detail here). However, if a barrier is not passable, a population will become separated into two subpopulations, each with a higher risk of collapsing and being extirpated when there is a disturbance.
One way to determine whether a population is connected is to measure genetic relatedness within and among populations. Now, bear with me. I know a lot of people hear “genetics” and tune out thinking the words to follow are going to be impossibly complicated and uninteresting (I once, and sometimes still do, fall into that category). But, the concept is quite easy, and it’s fascinating that we can do this study in fish. When we check for genetic relatedness, we are interested to see how similar the genetic composition is of fish within and among populations. If the genetics are similar, then we know the population is connected.
Put another way, checking for genetic connectivity is basically the equivalent of determining how genetically related everyone is at a family reunion (within-population) and then comparing that family to families across the United States (among-population). You would expect that within-population genetic relatedness would be high because you’re measuring parents, siblings, and cousins that share similar genes. You would also expect that families that live near one another would be genetically different, but still somewhat similar because individuals can easily cross over (marry) into other families. However, we wouldn’t expect a family from Pennsylvania to be too closely related to one from California because not too many people move that far away.
The question is, how many states away do we start seeing families becoming distinctly different? To bring this back to trout, how tall does a barrier have to be before trout can no longer swim past it and there are two genetically distinct populations?
This is the question that Anne Timm and her colleagues addressed in a recent paper published a few months ago in Environmental Biology of Fishes. They measured genetic relatedness of populations upstream and downstream of waterfalls ranging in height from 5-200 feet. They found that 13-foot barriers were large enough to significantly reduce genetic relatedness between two populations, but population separated by smaller barriers still had some degree of connection. This means that brook trout are at least sometimes capable of moving upstream of barriers that are less than 13-feet tall. That’s a really tall jump!
That statement is qualified by “sometimes” because movement is highly dependent on stream flow, availability of pool habitat near the falls for resting, and slope of the fall. So, in reality, 13-feet is an extreme maximum, and it’s unlikely that trout are regularly swimming upstream of barriers that tall. In fact, other biologists have found maximum jumping height to be less than 5 feet for brook trout, which is probably a more realistic height for trout to regularly jump.
More connected populations also have more genetic diversity, meaning that there are more genes in the entire gene pool. Thinking back to humans, a more connected population might have genes for all the possible colors of hair, but disconnected populations might lose the genes for red hair. As predicted, Timm and others also found that genetic diversity of upstream populations decreases when falls exceed 13 feet tall. This is problematic because more genetically diverse populations are usually more likely to survive environmental change, so this suggests that populations upstream of large falls may be more prone to extirpation.
What can we do with this information? Of course we aren’t going to remove waterfalls or artificially connect population near falls. But, it does give a quick threshold for identifying impassable barriers, which could help locate disconnected populations that may be at a higher risk of extirpation. Once identified, populations can receive special consideration for receiving additional habitat protection or possibly stocking to supplement genetic diversity. But, stocking has its own problems, a topic we’ll have to save for another time.