In Defense of Nonnatives
We stock. We stock nonnative brown and rainbow trout in native brook trout streams, but we also stock brook trout in native brook trout streams. You may feel uneasy about nonnative stocking in native streams- there is clear evidence that brown and rainbow trout cause declines to native brook trout populations. But, what about stocking brook trout in brook trout streams? That seems pretty harmless, right?
Or is it?
You may have already heard about the negative genetic consequences that can occur if native brook trout spawn with stocked brook trout (otherwise known as introgression). In short, stocked fish are not genetically compatible with local stream conditions. If a native trout spawns with a hatchery fish, the offspring generally have lower survival, reduced growth, and go on to make fewer, less successful offspring. It’s a negative feedback and ultimately results in overall reduced population health that can take many generations to repair.
But, hatchery and wild brook trout don’t always reproduce with one another, and even some evidence that there are very adults that are the product of hatchery and wild interbreeding (whether this is a choice or a failure for eggs to survive remains unclear). So, genetic consequences of stocking are sometimes quite minimal. In that case, the effects of stocking may depend more on how hatchery and wild brook trout interact with one another on a daily basis, otherwise known as conspecific (meaning same species) interactions.
Following my post last week on the influence of stocking on ecosystem nutrients, I was sent a paper that discussed whether native or nonnative trout stocking is a bigger disruption to native ecosystems. The authors reviewed hundreds of research papers that documented the effects of stocking on everything from individuals, to populations, and on up to entire ecosystems. They found that native fish stocking can actually be WORSE on wild, native populations than nonnative stocking.
How can it be so? How can more of a good thing (native fish) be a bad thing? It all makes sense in light of ecological theory which states that the more similar two individuals are, the more they will compete for resources. So, two toddlers are going to compete over the same toy more fiercely than a child and adult might.
There is no doubt that brown and brook trout compete with one another- so much so that it causes declines in one species (usually brook trout). But, individual brook and brown trout compete less with one another than two brook trout compete with each other. Or, to put it another way, intraspecific (same species) competition is always higher than interspecific (between species) competition.
And, that’s where we have a problem. Competition between hatchery and wild trout of the same species can cause a shift in individual-level properties. Things like stress, physiology, growth, reproduction, movement, behavior…basically everything…are influenced by competition. And, because hatchery fish are often artificially selected to have higher growth, the competitive edge is given to them. Once hatchery fish outcompete wild fish, only the non-adaptive hatchery genes are left to sustain the population, which could speed up population collapse.
Of course, I’m not going to let nonnative stocked fish off the hook entirely. The authors showed that nonnative stocked fish have a significant negative effect on aquatic communities (which entails all living organisms in the stream, including frogs and other terrestrial species that only occasionally visit the water) and the ecosystem (which includes not only interactions with other organisms, but also nutrient processing and energy flow). Unfortunately, there aren’t many studies on how native stocked fish influence community and ecosystem-level processes, so we can’t be sure whether native or nonnative stocked fish have a greater impact at these levels of organization.
So, once again, I warn you. Stocking has significant positive influences on angling opportunities, and is probably responsible for getting people off couches and into streams. But, the long-term consequences of stocking may be both negative and long-lasting, and in ways we still don’t entirely appreciate or understand.
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 so you can contribute to the discussion.
What Else Are You Stocking?
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.
It’s Not Always Climate Change
We tried, but Mother Nature just wasn’t having it this week. Ice-capped streams and high flows followed by heavy rains and 50 mph wind gusts made the decision to delay sampling easy. Unfortunately, the next ice age is about to descend on Pennsylvania, so shop is closed for the foreseeable future.
But, we did manage to add 43 fish to the collection on Monday. And, with water temperatures peaking around 33°F, I think I also managed to lose some nerve endings in my hands.
Thawing back in the office, I’ve been thinking through the genetics dataset a little more. With the preliminary analyses done, we’re starting to think about other, less common, analyses that may give interesting results. Sure, the descriptive statistics are great, but we have a suspicion there’s more interesting conclusions yet to be uncovered.
Scientific detective work isn’t that glamourous- it’s mostly trying different search terms in Google and reading literature. And, let me tell you, those genetics papers are not exactly page turners. After amassing a large collection of manuscripts that seemed relevant (thank goodness for electronic copies) and running out of excuses as to why I hadn’t started reading, I poured myself a big cup of coffee and…..couldn’t click on a title. I guess I’ve gotten a little burnt out on the topic.
But, in the long list of genetics papers there was one title that stuck out like a sore thumb. It had nothing to do with genetics, which of course made it an obvious choice to read first.
The paper was titled “Nonnative trout invasions combined with climate change threaten persistence of isolated cutthroat trout populations in the southern Rocky Mountains.” That’s right, the manuscript (which can be found by click on the hyperlink in the previous sentence) isn’t about east coast streams, and the title doesn’t even directly mention my precious brook trout (but, knowing the system, I knew the “nonnative” trout they were referencing was brook trout).
But, this paper intrigued me for a few reasons. For starters, the second author, Kurt Fausch, had sent it to me a few weeks ago. I’ve academically ‘grown up’ reading Kurt’s work, and his studies have always inspired me to be a better fisheries ecologist. When you’re deep in the weeds of your project, a little inspiration never hurts.
Second, though the main actors are different, the story line could have easily been written for east coast trout populations. The manuscript models the effect of climate change and nonnative fish invasion on the persistence of 309 native trout populations. In this case, the native trout were cutthroat trout, and the nonnative are brook trout (recall: brook trout are only native on the east coast, and are an invasive nuisance out west).
You may be able to guess the main result- climate change and nonnative species cause a decline in cutthroat trout populations. But, which factor has the strongest effect? And, how fast do we expect cutthroat populations to decline once invaded?
This is where I found the results to be a bit surprising. In the absence of nonnative species, the effects of climate change on cutthroat trout populations are almost negligible- the authors only predicted one of the 309 populations would go extirpated by 2080. But, add in brook trout invasion, and that number goes to 122 extirpated cutthroat populations with another 113 at risk of extirpation.
Shocking still, extirpation happens really fast. On average, the authors estimated that brook trout move upstream about 50 feet a day (mostly during summer), which results in about 10 miles of invasion per decade. Looking at historic trends and doing a little more math, they estimated that it takes as little as 7 years for cutthroat trout populations to become extirpated once brook trout move in.
To summarize another way, for the cutthroat trout populations studied, invasive species matter a whole lot more than climate change. And, the timeline from invasion to extirpation is fast, giving managers very little time to implement management decisions to prevent extirpation.
Bringing things back to the east coast, many of you may be thinking I’m getting ready to climb atop my soapbox and talk about the destruction of native brook trout populations by nonnative brown and rainbow trout. But, I’m not. At least not entirely. Cutthroat trout populations are more sensitive to invasion than most brook trout populations. So, I do not want to imply that the results of this manuscript are directly transferable to native brook trout streams. As many of you know, brook trout and brown trout can persist together far longer than 7 years. However, nonnative trout do threaten brook trout, and this paper does a great job of summarizing the mechanisms behind those invasions and species turnovers (there’s the soapbox).
What I do want to call attention to is that the authors found that climate change alone had very little impact of cutthroat trout populations. This is important. Climate change is definitely a major threat to trout populations, and certainly more of a threat on the east coast than the Rockies. But, we have become so fixated on climate change as THE threat that we may have lost focus on other, potentially remediable, threats to brook trout populations. Yes, stream temperature rise is going to extirpate brook trout, likely having more severe threats than climate change on cutthroat trout. But, studies have shown that many mountain headwater streams are buffered from the effects of climate change and may not see the big increases in temperature that we project for larger waterways. For these systems, nonnative fish invasion may matter a lot more for brook trout persistence than climate.
And, at some level, we can fix nonnative fish invasions. At the conclusion of the article, the authors discuss how their findings can be used in cutthroat trout management. In particular, they suggest refocusing management efforts away from “lost cause” populations (my words, not theirs) that are already heavily invaded and towards cutthroat trout populations that are still uninvaded by brook trout. They even go so far to suggest building barriers to prevent upstream migration of nonnative trout.
Of course, movement barriers will also isolate populations causing potential negative consequences to genetic diversity. And now I’ve circled back around to genetics. I just can’t escape.
*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 so you can contribute to the discussion.
After a short three-month hiatus, the Loyalsock trout crew is loading the trucks and preparing to hit the streams for “spring” sampling. The term spring is really being thrown around loosely here…there is snow on the ground, it was 13°F this morning, and trees and flowers still appear dead (some may call this death-stage dormancy, and while science would call them right, I say you can’t trust a plant until it comes back green).
The goal of this sampling is easy- go out, find fish, collect gill samples, and ship gills to West Virginia so they can analyze for gene expression. If all goes as planned, at the end of next week we will have collected gill samples every 1-3 months for a full year. It will also up our total sample size to near 600.
That’s a lot of vials of fish gills. More vials of fish gills than most other studies. And, our gills are collected on wild trout in natural systems. My background on gene expression studies is severely lacking, but I’ve learned that we are sailing in largely uncharted waters with this study. Specifically, our sample sizes are large, we are not working in a controlled laboratory environment, and we have multiple samples on the same individual fish over time.
That’s not to discount the significance of any other study- all of this stuff is pretty cutting edge, and valuable data is being gathered from everyone working in this field. In fact, lab studies have a lot of advantages that wild streams don’t. I have to wait for stream temperature to rise and, because Loyalsock is two hours away, hope I pick a critical time for collecting tissue samples. But, with the flip of a switch, we can change temperature in artificial stream labs and immediately expose fish to temperatures that we are interested in measuring heat shock expression in (recall: heat shock proteins are produced in response to heat stress to prevent cell death).
But, streams don’t really work like labs. Stream temperatures rise and fall much faster than we can readily mimic in a laboratory environment. And, gene expression is highly sensitive to this variation. This leaves us at a bit of a catch-22. We can control temperature in labs but, because tight control leaves little variation, the data may not reflect patterns of gene expression in wild trout populations. Streams, on the other hand, have all the variability we need, but we don’t know what aspect of the variation actually matters to trout. Simply put, we don’t actually know what triggers heat shock protein expression. Yes, it’s heat. But is it average stream temperature over the last three days? Week? Month? Or is it difference in maximum and minimum stream temperature? Or, time at a certain temperature? Even if we did know what aspect of variation triggered protein expression, it would be really hard to find the sampling window of most interest without camping streamside and recording temperature every 30 minutes.
We’ve hit this wall with our data so far. Some of our lowest heat shock protein expression is from July, when water temperature were near their highest. No, this doesn’t mean our data are wrong. It likely means that conditions between May-July triggered heat shock protein expression, and once the proteins were floating around in the fish’s’ system, they stopped expressing the gene. So, we missed the critical window.
And, that’s okay. It’s still providing us great information into how trout are responding, molecularly, to stream temperatures. It also gives us a better idea of when we should sample if we want to catch peak gene expression. Like, maybe March. No, stream temperature are not high in March, but they will be starting to rise from winter lows. And, we know expression in May is not high. So, could it be this increase in temperature in March that triggers expression? Maybe. And we’ll soon know