Cutthroat Trout: Actually Four Different Species?

Dec 4, 2018

An angler displays his catch before releasing a wild Yellowstone Cutthroat Trout.
Credit Jon McFarland

Here on the campus of Utah State University, I’ve got this book sitting on my desk. It’s green, hardcovered, and wider than it is long. It has some of the best and most detailed paintings of fish in it that I’ve ever seen- and as a fisheries grad student, I’ve seen a lot of books on fish. 

But this particular book is special, not just because it has some sweet pictures in it, but because of who wrote it. This particular book is written by none other than Bob Behnke himself… Dr. Trout!

Now I know you’re probably saying, “who is Bob Behnke?” Don’t worry, we’ll get back to Bob. First I want to tell you about lumpers and splitters. 


Splitter (Noun) a biologist who divides groups into smaller categories.

Lumper (Noun) a biologist who attaches more importance to similarities than to differences when classifying species.

Ok. So let’s say we have a group of fish in a river and that a geologic event, like an earthquake, creates a new barrier on this river so the fish are separated and can no longer breed.  Now remember back to high school biology - species are always evolving. Since those two groups can no longer interbreed, they will then evolve differently. And if those two groups are separated long enough, they evolve to look so different that scientists “split” them into two different species. However, since they share a common ancestor, they still might be “lumped” into a broader category like genus.

The cover art for Dr. Roper's new book.
Credit American Fisheries Society

"How people decide what’s a species is often discipline specific. There’s long-term arguments of how many species of grass there are just because there’s lots of splitters in the field of botany and that makes sense. You know because plants don’t tend to run very far. But with fish you have people that tend to lump things because most of these systems are connected by rivers," said Dr. Brett Roper, the U.S. Forest Service’s National Aquatic Monitoring Program Leader and one of my professors at Utah State University. He’s also one of the editors of a recent book about Cutthroat Trout evolution in which Bob Behnke took center stage.

Remember Bob, the author of that big green book on my desk with all the nice drawings of fish?

"It’s pretty neat. What [Bob] Behnke did by just sorta driving around and looking at individuals is supported by the much more highly technical genetic data. And so, it does show you that being a naturalist pays off- that being out there, taking your time, thinking about how things work gets at these large-scale patterns really well," said Roper.

Bob Behnke was a Korean War veteran and long-time professor at Colorado State University who is probably most famous for researching how Cutthroat Trout (Oncorhynchus clarkii) evolved. And before new genetic tools had become widespread, he was able to simply drive around the West and examine Cutthroat anatomy to determine how Cutthroat had evolved into different subspecies.

According to Roper, Behnke would look at features like a fish's, "…gill rakers, how many scales along the lateral line, the spotting patterns. Certainly, he was pretty amazing. And, he would figure out how that nested within the evolution of the landscape. So he put those things together in a time when that was really difficult to do. I find it pretty amazing. Even the list of subspecies falls out under the same groups that he had proposed. One of the big things is, depending on how you count them, there are 13 or 14 of these sorta subgroups under Behnke where, when you look at the genetic data, there’s support for more, around the mid-20s. "

So while Behnke’s work got a lot of the Cutthroat story right, new genetic data is rewriting some parts. In the book Roper helped edit, some contributing scientists wondered whether the major groups of Cutthroat Trout that Behnke had called subspecies, should actually be considered full species in their own right. But other contributors to the book say there’s just not enough data yet to say if Cutthroat Trout are actually more than one species.

According to Dr. Dennis Shiozawa, a fisheries professor at Brigham Young University, "we are still at a point where I don’t know that we can fully draw the lines as to what is what on these subspecies and try to elevate some to species." 

I called up Dr. Shiozawa who explained that figuring out how species separate from distant ancestors and evolve isn’t very straightforward.

"You have this separation taking place continually over a period of at least 10 million years," he said. "Some might have separated out at 10 million, others may have separated out at 5 million years ago. Some may have separated out a 100,000 years ago. So the question is, where along that continuum do you draw the line to say this is where I’m gonna call this a separate species or I’m gonna recognize this as a separate subspecies?"

A Cutthroat x Rainbow Trout hybrid recovers after being reeled in by a fly fisherman.
Credit Jon McFarland

Shiozawa predicts that we’re anywhere from three years to a decade away from being able to determine for sure whether Cutthroat Trout make up more than one species. But this kind of work has real-world consequences for which subspecies or groups of Cutthroat are listed as threatened or endangered.

"Obviously going from 13 or 14 subspecies to 24 subspecies can put at play the Endangered Species Act," Dr. Roper said. "It could go both ways. It could increase the possibilities of listing because you’ve got more subspecies. But it could lead, eventually, to the delisting because the genetic information doesn’t suggest some of these subspecies were distinct.

"Ya know, I think that’s one of the products of science. Science is a process. We get better and better understandings. It’s amazing that someone like Behnke, again just driving through, was able to do as well as he has. No one should expect that’s going to be perfect when you can get down to the genetic information of the individuals. This is better scientific information on how things should be looked at but it doesn’t make the decisions on what should and shouldn’t be protected."

Roper also says that this work has led to legal debates.

"You look at Greenback Cutthroat Trout, some of the issues with misidentification or reidentification. I think that’s always gonna be in play when you have decisions like Endangered Species Act listings that have economic and ecological impacts," he said. 

But according to Roper, the major groups of Cutthroat Trout that may be considered separate species in the future like Westslope or Coastal Cutthroat Trout, have already been through the process of determining whether they are threatened or endangered.

"It won’t affect those," he said. "I think it will affect some of these smaller groups that probably warrant more protection because their small, isolated and really do need the help of the Endangered Species Act. I do think there’s lots of implications of this. It’s just gonna take time to play out."

Dr. Roper’s new book now sits beside that old green one on my desk. The new one is purple if you were wondering- a really nice color complement there.  And while the body of work represented by these two volumes will continue to be consequential for the foreseeable future, I’ll be here looking at fish drawings and reporting for Utah Public Radio. 

Cutthroat Trout: Evolutionary Biology & Taxonomy is a publication of the American Fisheries Society- the world's oldest and largest organization of fisheries scientists. To learn more about Cutthroat Trout, visit the web pages of Brett Roper and Dennis Shiozawa.