It’s Not Always Climate Change

We tried, but Mother Nature just wasn’t having it this week. Ice-capped streams and high flows followed by heavy rains and 50 mph wind gusts made the decision to delay sampling easy. Unfortunately, the next ice age is about to descend on Pennsylvania, so shop is closed for the foreseeable future.
 
But, we did manage to add 43 fish to the collection on Monday. And, with water temperatures peaking around 33°F, I think I also managed to lose some nerve endings in my hands. 

The Colorado River Cutthroat Trout is a relative of rainbow trout (both in the genus Oncorhynchus). Image courtesy of Wikipedia.

Thawing back in the office, I’ve been thinking through the genetics dataset a little more.  With the preliminary analyses done, we’re starting to think about other, less common, analyses that may give interesting results. Sure, the descriptive statistics are great, but we have a suspicion there’s more interesting conclusions yet to be uncovered.
 
Scientific detective work isn’t that glamourous- it’s mostly trying different search terms in Google and reading literature. And, let me tell you, those genetics papers are not exactly page turners. After amassing a large collection of manuscripts that seemed relevant (thank goodness for electronic copies) and running out of excuses as to why I hadn’t started reading, I poured myself a big cup of coffee and…..couldn’t click on a title.  I guess I’ve gotten a little burnt out on the topic.
 
But, in the long list of genetics papers there was one title that stuck out like a sore thumb.  It had nothing to do with genetics, which of course made it an obvious choice to read first. 
 
The paper was titled “Nonnative trout invasions combined with climate change threaten persistence of isolated cutthroat trout populations in the southern Rocky Mountains.” That’s right, the manuscript (which can be found by click on the hyperlink in the previous sentence) isn’t about east coast streams, and the title doesn’t even directly mention my precious brook trout (but, knowing the system, I knew the “nonnative” trout they were referencing was brook trout).  
 
But, this paper intrigued me for a few reasons.  For starters, the second author, Kurt Fausch, had sent it to me a few weeks ago. I’ve academically ‘grown up’ reading Kurt’s work, and his studies have always inspired me to be a better fisheries ecologist. When you’re deep in the weeds of your project, a little inspiration never hurts. 

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What I wouldn't give to go back to my undergrad research in Colorado, 2009. Picture right to left, Josh (still one of my best friends), Kurt, Chas. (my undergrad advisor), and his son Charles. And, yes, I'm short...but Charles and Kurt are also tall.

Second, though the main actors are different, the story line could have easily been written for east coast trout populations. The manuscript models the effect of climate change and nonnative fish invasion on the persistence of 309 native trout populations.  In this case, the native trout were cutthroat trout, and the nonnative are brook trout (recall: brook trout are only native on the east coast, and are an invasive nuisance out west).
 
You may be able to guess the main result- climate change and nonnative species cause a decline in cutthroat trout populations.  But, which factor has the strongest effect? And, how fast do we expect cutthroat populations to decline once invaded?
 
This is where I found the results to be a bit surprising. In the absence of nonnative species, the effects of climate change on cutthroat trout populations are almost negligible- the authors only predicted one of the 309 populations would go extirpated by 2080. But, add in brook trout invasion, and that number goes to 122 extirpated cutthroat populations with another 113 at risk of extirpation.
 
Shocking still, extirpation happens really fast. On average, the authors estimated that brook trout move upstream about 50 feet a day (mostly during summer), which results in about 10 miles of invasion per decade. Looking at historic trends and doing a little more math, they estimated that it takes as little as 7 years for cutthroat trout populations to become extirpated once brook trout move in.
 
To summarize another way, for the cutthroat trout populations studied, invasive species matter a whole lot more than climate change. And, the timeline from invasion to extirpation is fast, giving managers very little time to implement management decisions to prevent extirpation.
 
Bringing things back to the east coast, many of you may be thinking I’m getting ready to climb atop my soapbox and talk about the destruction of native brook trout populations by nonnative brown and rainbow trout.  But, I’m not.  At least not entirely. Cutthroat trout populations are more sensitive to invasion than most brook trout populations. So, I do not want to imply that the results of this manuscript are directly transferable to native brook trout streams. As many of you know, brook trout and brown trout can persist together far longer than 7 years. However, nonnative trout do threaten brook trout, and this paper does a great job of summarizing the mechanisms behind those invasions and species turnovers (there’s the soapbox).
 

Still dominated by snowmelt in July, Colorado streams aren't entirely comparable to those on the east cost ecologically, but they are just as pretty. Photo courtesy of Josh Harris.

What I do want to call attention to is that the authors found that climate change alone had very little impact of cutthroat trout populations. This is important. Climate change is definitely a major threat to trout populations, and certainly more of a threat on the east coast than the Rockies.  But, we have become so fixated on climate change as THE threat that we may have lost focus on other, potentially remediable, threats to brook trout populations. Yes, stream temperature rise is going to extirpate brook trout, likely having more severe threats than climate change on cutthroat trout.  But, studies have shown that many mountain headwater streams are buffered from the effects of climate change and may not see the big increases in temperature that we project for larger waterways. For these systems, nonnative fish invasion may matter a lot more for brook trout persistence than climate.  
 
And, at some level, we can fix nonnative fish invasions. At the conclusion of the article, the authors discuss how their findings can be used in cutthroat trout management.  In particular, they suggest refocusing management efforts away from “lost cause” populations (my words, not theirs) that are already heavily invaded and towards cutthroat trout populations that are still uninvaded by brook trout. They even go so far to suggest building barriers to prevent upstream migration of nonnative trout.
 
Of course, movement barriers will also isolate populations causing potential negative consequences to genetic diversity.  And now I’ve circled back around to genetics. I just can’t escape.  

*Note: Content in this post is my own and may not reflect the opinion of the manuscripts' authors or the agencies they represent.  I encourage you to read the manuscript so you can contribute to the discussion. 

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