Is Bigger Always Better?

The benefit of studying trout in labs? You get to give them fancy jewelry to track their behaviors.

There’s nothing like field work. Breathing in the fresh mountain air while hiking to a remote population of native trout. Watching the sunset over a stream after a long day’s work.  And, getting back to the office sore and full of new research questions after seeing nature at play.
 
Unfortunately, not every research question I think of in the field, can actually be studied in the field. Nature if far too unpredictable and uncontrollable, and fish far too smart, for scientists to risk putting lots of equipment, time, and money into a field-based study. At least not without some careful pilot studies, often conducted in a laboratory. Before coming to Penn State, I used to dream of having a little indoor stream I could use to test some ideas I picked up along the way about fish behavior. Nothing too fancy- just a couple pools and riffles, and a nice population of brook trout. The possibilities would be endless.
 
Dreams came true within weeks of starting my Ph.D. and finding out that Than Hitt of the USGS Leetown Science Center in West Virginia had…you guessed it…an indoor stream.  Complete with..you guessed it again…pools, riffles, and brook trout. We got to work quickly, setting our eyes on understanding how brook trout use thermal refugia- small areas of groundwater upwelling that, in the summer, have water temperatures that can be much lower than average stream temperature. When we started the research, we knew that studies had shown that trout that occupy areas of thermal refugia may be able to survive periods of thermal stress, which could mean that there might be some hope for trout populations facing future stream temperature rise.
 
But, observing how fish use a thermal refuge in the field had historically proven to be difficult and mostly led to a lot of confusion. For example, previous observations had shown that fish move really far to access a thermal refuge, but then frequently end up leaving the refuge shortly after. This made no sense. If the stream is too hot for the fish, and the thermal refuge is the perfect temperature for the fish, then shouldn’t the fish….you know…stay in the refuge?  Welcome to science.
 
So, why?  Why are fish leaving what seems like their own climate-controlled rooms for what surely seems like a death wish? We had two main thoughts. It could be because competition inside the refuge was so high, that fish that couldn’t hold their own got pushed out. Seems plausible, as brook trout are extremely territorial and aggressive. The second thought was maybe the refuge didn’t have other important resources. It might have thermal habitat, but maybe it doesn’t have food, cover, and good flow. So, fish might occupy the refuge for a while, but eventually they will have to leave to fulfill other requirements.
 
This is where the stream lab proved to be perfect. We could easily manipulate temperatures (thanks to the incredible team of USGS technicians and biologists at Leetown), monitor individual behavior, create some separation between thermal and forage habitats, and start teasing apart why fish were leaving their cushy thermal refugia.  Frequent readers of this blog may have some déjà vu and realize that this isn’t the first time this study has been mentioned, as it’s been a topic that Ben Kline, the lab’s undergraduate research assistant, has been writing about for the last year. After we collected all the data in Leetown, Ben did some heavy lifting to analyze videos of fish aggression and millions of lines of data that documented fish resource use. And, I’m happy to say the data are finally in, and I’m confident to share some conclusions. Like….
 
Big fish really hate hot water. When stream temperatures were cool, big fish ruled the roost.  Again, not surprising because brook trout are aggressive, and big fish are typically the most dominant. But, as stream temperatures increased, big fish stopped defending territories near a feeder in the warm part of the stream and spent most of their time in the thermal refuge. Surprisingly, once in the refuge, they basically stopped fighting. Huh. Now, it’s important to point out that fish don’t do anything “to be nice” to their neighbors. They are mostly selfish pricks. They didn’t stop fighting to let other fish into the refuge, but they probably stopped fighting because they didn’t have the energy to fight. The warm water was really sucking the life out of them.
 
So, the idea about competition influencing fish movement? Wrong. Fish were choosing to leave the refuge.
 
So, let’s consider the resource hypothesis. In our stream lab, the only area that fish could feed was outside of the thermal refuge. What we found was that, yes, fish did spend most of the time in the thermal refuge when stream temperatures were hot. But, all fish did occasionally make forays into hot water to feed. So, it would appear that our hypothesis about fish leaving the refuge in search of resources may hold some weight. It’s also interesting to note that smaller fish tended to leave the refuge more often, as well as stay outside of the refuge for longer, than bigger fish. So, this is another line of evidence to suggest that warm temperatures affected bigger fish more.
 
Why do these results matter?  Well, we typically assume that the presence of thermal refugia alone is good enough to increase population survival when stream temperatures rise. However, what our results may suggest is that the location of refugia relative to other resources in the stream may also be important. If a stream is too fragmented, then fish will need to spend too much time outside of the refuge in search of resources, and so the presence of refugia may do little to conserve fish populations. Alternatively, if resources are nearby, fish can likely make quick trips back and forth among habitat patches, equivalently “charging their batteries” in the refuge before going in search of food. But, also keep in mind, smaller fish may be the most successful at making these jaunts into warm water, so fish size may also be influenced by refuge habitats.
 
Another important finding is that small refugia may have large benefits to populations. Because of reduced competition in the refuge, and the constant movement of fish in and out, a lot of fish may be able to take advantage of the thermal properties of refugia.  So, the population-level benefits of a single refuge habitat may be larger than we currently believe.
 
Now, to take it to the field…..

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