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.