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.