Just Chill

A coldwater seep from Benner Run near Philipsburg, Pa.

Our undergraduate research assistant, Ben Kline, continues to make impressive progress on analyzing fish behavior videos from the laboratory study we did in Leetown, West Virginia last year.  This week, I’m turning the blog back over to him to share a brief introduction on why this study was done. If you missed his first post, be sure to check it out here. Looking forward to getting some results soon!
 
It is no secret that all living organisms require a few basic resources to survive. Often when we think of these necessities we list things like food, water, oxygen, shelter, etc. While it is obvious to us that we need to meet these basic requirements to survive, we often neglect that our aquatic counterparts have needs that vary slightly from our own. My most recent project in the lab is investigating just how important one of these lesser-known resources is.
 
There are many factors that impact the availability of essential resources in any given habitat. At a basic level, aspects of the environment, such as climate and geography, limit resources to a fairly narrow range of possible conditions.  Just consider the resources available in a mountain stream and those in a coastal estuary.  These systems may be fairly close to one another, but the resources are very different because of the physical location of the waterway. 
 
Then there are human practices, such as land and water use, that can further restrict resource availability and/or make habitats more hospitable to particular species. For example, dams have been known to cause considerable changes to species composition, all because of how they alter resource availability, especially flow and water temperature.
 
Taken together, the distribution and abundance of resources have a major impact on the type and number of organisms that can inhabit a certain space. As promised in my previous post, this week’s post will detail a resource that is less understood by us, but critical to the survival of numerous trout species and other coldwater fish: thermal refuge.  

---

Thermal refuge, in its simplest form, describes the availability of a cold water in a body of water that warms. I don’t know about you, but I have taken an unintentional dip or two while fishing in late spring and early summer and I would be hesitant to say that there is any shortage of cold water available for the fish. So what’s the deal?
 
To understand the significance of temperature in trout habitat, we should take a closer look at how water temperature shifts with the seasons. In larger bodies of water, such as lakes and large reservoirs, the issue of cold water access during summer is less pressing because of the depth of the water. In these large bodies of water, thermal stratification creates zones with varying water temperatures. At the surface, water is exposed to high summer air temperatures and heats up. And, while wind can cause it to mix and circulate, warm water stays at the surface because cold water is more dense and sinks to the bottom. Thus, the surface water warms while the deeper layers, which are largely not exposed to air, remain cool, even in the hottest parts of summer. This deep layer provides of a site of refuge for coldwater species to retreat to when the rest of the lake becomes too warm. 

Why can lakes support cold water fish populations. Stratification! Image courtesy of http://www.miseagrant.umich.edu.

Plants and trees along the creek bank can provide some reprieve from the hot summer sun.

While this is great news for our lake dwellers, fish that are native to creeks and streams often find it significantly harder to find cold water during the hot summer months. As I mentioned in my last post, the shallow nature of most creeks and streams means that there is no easy way for these waters to remain cold in the summer. Being close to a cold water source, such as a spring or upwelling, can help to keep the water cool. So can the presence of a dense tree canopy or sufficient riparian vegetation. Ultimately; however, the main body of water can warm to the point that the habitat may be unfit to support trout populations for an extended period of time. 

Contrary to popular belief, the most serious danger in inhabiting warmer waters is not entirely due to the fact that the fish cannot tolerate warmer temperatures. All organisms have an internal set point known as a thermal maxima, which is the highest temperature an individual can tolerate before it perishes. While brook trout prefer cold water, they have actually been observed to have a thermal maxima of around 25C, which is quite warm in terms of water temperature in temperate regions of the US. In fact, it is not uncommon for trout to favor warming waters that are abundant in food supplies over cool, less productive water for a short period of time.
 
The real danger with warm water is a little more subtle and lies in the change to the dissolved oxygen content in the water as it heats up . Trout are very sensitive to drops in oxygen, and, compared to other species of fish, do not fare well when the oxygen concentration is too low. As water heats up, the solubility of oxygen (i.e., the maximum amount of oxygen that can dissolve in water) starts declining. As you can see in the graph below, at around 20°C the solubility of oxygen is less than 55% of what it is at 5°C. In short, this means that warmer water has less oxygen. And, with less oxygen in warmer water, trout start becoming metabolically and physiologically stressed and mortality increases.  

---

As waters warm, the amount of oxygen that can be dissolved in water (the solubility) declines sharply. By 25C, oxygen has a hard time dissolving in water. Image courtesy of www.esf.edu.

But, the plot thickens even more. As mentioned above, in thermally diverse systems, there can be a trade-off between cold, oxygen-rich water and warmer waters where there is more food. Too much of either can be a bad thing, and trout have to constantly make on-going decisions about which habitat type they want to occupy at any given moment.

Often times, though, the decision may not be entirely theirs to make as cold water refuge is a very limiting resource. One of the most common sources of cold water refuge are spring-fed tributaries. These tributaries are much smaller than the main channels and provide less habitat and fewer resources to support larger populations. These zones are therefore a major source of competition among fish. There is significant evidence that shows trout populations actively uses these refuge zones during hot times of the year, but there has really been limited observation of just how individual fish use and compete for these limiting resources. Enter the study I am assisting with, where we seek to understand the subtleties behind how individuals seek out and defend cold water resources. 

​At a basic level, our project involves observing brook trout behavior under a variety of thermal conditions. A population of brook trout, each tagged with a uniquely colored external tag, is placed into mock stream that we have created in the lab. The stream has three pools that we can manipulate to be different temperatures. Our goal is to compare individual interactions as stream temperature changes. So, I sit and watch, for endless hours, videos of fish being fish.  I document how every individual interacts with every other individual in the stream and how much time fish spend in cold water vs. near food that is often in warm water.  Why?  Because we want to know how stable behavior is across temperatures. For example, we want to know how fish rank the importance of food vs. thermal refuge at various temperatures.  We also want to see how fish interact among one another at different temperatures. We generally expect larger fish to rule the streams and dominate all other fish at cooler temperatures. But, when stream temperature heats up, larger fish become more stressed than smaller fish and may no longer be able to successfully compete for limiting resources. In short, big may not be better in warmer temperatures if you're a trout. 
 
While it may be a bit soon to comment on the data we have gathered so far, this project definitely holds promise to shed some new light on how individual behavior may shape the complex populations of brook trout that we know and love. We also hope to demonstrate just how important these limiting resources are for sustaining trout populations in thermally complex environments.
 
Stay tuned!