Trout- Goldilocks or Big Bear?

Large floods, like the historic 500-year event that hit Loyalsock Creek in 2011, are surprisingly necessary for long-term trout survival. Photo courtesy of PA State Police.

There’s been a reoccurring theme in my posts- stream flow is really important.  When streams rise too far, trout can wash downstream and die. When there isn’t enough flow, stream temperatures rise, oxygen availability declines, fish become trapped in isolated pools, and, you guessed it, they can die. But, trout aren’t the Goldilocks I may have led you to believe. 

Headwater streams are some of the harshest environments an animal can live in.  They are a complex mosaic of habitats- riffles, runs, pools, glides- all connected by the unpredictable flow of water.  Flood stages and drought conditions can be separated by only a few hours. Stream temperature changes quickly with rain and air temperature. Aquatic macroinvertebrates, a trout’s major food source, follow their own schedule, and fish have to sit and wait for a causality to float downstream (and in doing so expose themselves to predators). The only thing predictable about headwater streams is that they are unpredictable.

Rivers and large streams are less erratic than headwater creeks, but these larger systems are often too warm for trout and don’t have the right food sources (though, some trout can use these habitats…a story for later).  So, for most of the year, trout are basically stuck in these unpredictable headwaters. To survive, trout need to be tough.  But, it also takes a team effort.  In this case, the “team” I’m talking about is the ecosystem. And on this team, unpredictability isn’t just something that organisms have evolved to survive, but is a necessary player. And, one element of unpredictability needed for survival are what we humans have decided to call “natural disasters.”

Take floods, for example. Loyalsock Creek has had its fair share of flooding the last five years.  In 2011 there was a 500-year flood (meaning stream flows are predicted to get that high only once every 500 years).  And, just last week, parts of Loyalsock were hit hard with floods that washed away bridges and homes. These events are devastating for humans because we’ve decided to build structures alongside streams. But, trout depend on floods for long-term health for several reason:  

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Floods clean streams: Trout hate sediment. It scratches their gills, suffocates their eggs, and decreases overall water quality. It can also be a source for diseases and parasites. Even the healthiest streams receive sediment from erosion of riparian land, and the only way to clear it from the stream is with high-velocity stream flows. In a forested watershed, the average rain event will cause little change in stream flows. Streams flows may increase more in an unforested watershed (because there are fewer trees and plants to absorb rain water), but total stream sediment increases because of more riparian erosion. Really high flows and floods are needed to completely clean the system. These high flows cause fine sediment and small gravel to become suspended in the water column and wash downstream. As sediment is passing by, it scrubs algae off rocks and boulders and uncovers a fresh stream bed. What’s left is cleaner trout habitat with less fine particles. 

Floods create habitat. If you fish, you know some of the best trout habitat includes submerged logs and large boulders.  These features create deep pools and are good hiding spots for trout trying to avoid predators.  But, they are extremely heavy.  Luckily, water is very powerful. Heavy rains erode banks and cause riparian trees to fall into the stream.  But, really heavy floods mobilize trees and boulders from much further away. A large pulse of water can cause a downed tree from the hillslope to enter the stream and become trout habitat.  In fact, the lack of fallen trees in watersheds that have been historically logged is a common source of habitat loss in trout populations. Forests less than a century old may appear healthy, but most trees are likely to still be standing and so less wood is available to enter streams during floods. 

To the naked eye, Shanerburg Run looks like a healthy trout stream. But, the lack of submerged logs is a sign of a watershed still recovering from historic logging.

Floods reconnect populations. On average, trout are good at hunkering down and staying put during high flow events.  But, in flood conditions, many fish do get washed downstream. Some likely perish, but survivors find new habitats. This can cause new streams to become colonized or temporarily connect otherwise isolated populations. This is extremely important because connectivity is key to long-term population survival. Populations that are connected have higher genetic diversity, meaning there are more forms of a gene present in the population. This is critical, particularly in highly variable environments like streams, because different genes are better at surviving different forms of stress. Let’s think about a fake example that will bring this point closer to home.  A population with low genetic diversity in humans may have, for example, a high proportion of people with blue eyes. If there is a lethal disease that effects only people with blue eyes, then that population would quickly die. However, a population with high genetic diversity may have an equal number of people with blue, green, and brown eyes.  Now, if the blue-eyed killer hits, the population declines, but the damage is limited to a small subset of individuals and the population survives. Though this is a fictitious example, similar scenarios do happen in natural populations frequently. 

A population with high genetic diversity (represented here as dot colors) is more capable of surviving a catastrophe or disturbance. Here, only the red and blue genes were capable of surviving, but the population did not die. Had the population had less diversity, perhaps only green and orange dots, the population would have died following the catastrophe.

So, in short, trout thrive on human catastrophes.  At this point in my research, I would expect nothing less of my study organism.
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In the week ahead- it’s (maybe) our last full week of tracking!  Hard to believe after six months I can see the finish line.   And, I think we are finally documenting the movement patterns we were hoping to see when we started this study.  Vague?  Yes, but I don’t want to get too excited just yet.