Cutthroat Trout in the Weber River

By Matt McKell, Utah Division of Wildlife Resources

Tributary culvert that dozens of PIT - tagged fluvial cutthroat trout have ascended during spring spawning.

Tributary culvert that dozens of PIT

tagged
fluvial cutthroat trout have ascended during
spring spawning.

So there we were…” It was July 2009, in the heat of a typical summer day. We were sampling the lower Weber River for bluehead sucker near the town of Mountain Green. This was pretty much your ordinary, average day of raft electrofishing, including the typical, frantic netting on the front of  he raft. Tons of brown trout (for which the Weber is best known), mountain whitefish, some rainbow trout, pods of large Utah suckers, and an occasional bluehead sucker. Then, unexpectedly, our sampling moved beyond the typical. At the confluence with one of the small tributaries was an unusually large number of cutthroat trout. Totally unexpected, and super cool! But also a little perplexing. The Weber River, as broken up and hammered by dams, diversions, altered habitats, non-native fishes, and all the potentially negative impacts it had been subjected to for many years, still harbored a population of cutthroat trout.

Amazing! But lots of questions…How did they hang on? Did the cutthroat we sampled represent a remnant of the native stock? What were they doing at that tributary? With few answers right then but wanting to explore a little, we decided on the spot to collect some fin clips to look at genetic (…not too surprising, there appeared to be genetic evidence of past stocking of non-native cutthroat trout).

Fast forward a couple years. In 2011, our office began to collaborate with Dr. Phaedra Budy and the Fish Ecology Lab at Utah State University to look at cutthroat trout population dynamics in the lower Weber mainstem and a handful of tributaries. The study included assessments of genetics, movement, spawning, barriers, and fish passage.

Fast forward a few more years to the present. We now have a boatload of data from three field seasons, including genetics results for the tributaries, PIT tag and antenna data for assessing movement of individual fish, information on passage, mainstem abundance estimates, survival estimates, water chemistry for each stream/section and otolith microchemistry for a handful of individuals for assessment of natal origins, and more.

It turns out there is much more to this cutthroat population than we anticipated. It’s larger than we originally thought, we’ve PIT-tagged nearly 2,000 cutthroat trout and still get fairly low recapture rates. There is more movement and tributary use than expected, and we’ve identified significant spawning tributaries, which are likely the primary reason the fluvial population has persisted. The gene pool in the mainstem may not be as muddled with non-native DNA as previously thought…there are native genes in the Weber River that are very similar to non-native ones, and there is a possibility that some of the alleged non-native genes are actually native.

In addition, four of the five tributaries have populations that are genetically pure, even in fluvial spawning areas. The genetics in the mainstem and tributaries look so good that we’re managing these fish as a sportfish/conservation population. We even changed fishing regulations in this portion of the Weber River to catch-and-release for cutthroat.

Large diversion at the mouth of Weber Canyon, now fish passable since 2011.

Large diversion at the mouth of Weber Canyon, now fish passable since 2011.

There are also fewer complete barriers than we thought, and these cutthroat have exhibited an unbelievable ability to surmount obstacles that we, the smart humans we think we are, identified as full barriers. In fact, one cutthroat trout pit-tagged in 2012 traveled 12 river km upstream from the point of tagging, passing three mainstem “barriers” and a few large beaver dams in a tributary before being detected by the antenna in that tributary this past spring. The data we’ve collected have already spawned projects aimed at improving passage, some of them recently completed or soon-to-be (two fully funded and in the hopper), plus more in the works. One impassable barrier, a diversion structure at the mouth of Weber Canyon (see photo), was modified with a bypass channel in 2011 and passage of cutthroat trout and lots of other native fishes has been confirmed via weir trap. This is the first passage of fish through this diversion in nearly a century!

Cutthroat trout are amazing! What an extraordinarily persistent fish, and what a great story they’re telling!

——–

Part of the July 2014 (PDF) issue of Lateral Lines.

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