Skagit Bull Trout Primer Ch 4: A Mystery - Dolly Varden or Bull Trout?

For decades the native char of the Skagit basin was thought to be Dolly Varden. The fish had access to the salt and some of the population spend time there foraging much like the local sea-run cutthroat. That thinking was challenged after T. M. Cavender in 1978 published his paper “Taxonomy and distribution of the bull trout Salvelinus confluentus from the American northwest. He looked in detail at museum specimens focusing on the skulls of fish thought to be bull trout. He found that skull and its bones had a unique shape(s). In the simplest terms he found that bull trout have a larger and broader and flatter head and a more ventrally flattened body. As we look at Skagit native pink spotted char it was clear that a portion of them (maybe 15 to 20%) fit that description of bull trout. Does what our local species are or what we call them make a difference? For the angler it seems unlikely. For the scientist a precise answer is the goal. For the fishery manager considering the needs for species specific management, the questions of whether they were dealing with one or two species and whether each species would require a different approach were of interest. This state of uncertainty or confusion continued for more than a decade with those on the wet side of the state continuing to call our char "Dollies" while those on the dry side opted for "Bulls".


Photo by @Greg Armstrong

In the fall of 1991 Gordon Hass published his paper “Systematics and distribution of Dolly Varden (Salvelinus malma) and bull trout (Salvelinus confluentus) in North America”. Like Cavender he looked at morphological and meristic characteristics from various collections and field locations. He found that there were two distinct morphotypes and that the two species occurred in strict sympatry in several areas with no evidence of interbreeding. In the lab he was able to artificially breed the two species and found that hybrid was intermediate between the two parent species. In his sampling no natural hybrids were conclusively found and there was no evidence of introgression. He found that Dolly Varden were distributed largely along the coast and ranged further to the north. Bull trout were mostly in the interior and ranged further to the South. But for this discussion as part of his work he developed a linear discriminant function (a formula if you will) based on counts of three features; the number of branchiostegal membranes or rays (those soft plates below the gill plate that cover the gills), the number of anal fin rays, and the length of the upper jaw. Enter those values into that formula and depending on whether the answer was above or below a certain value you would have the answer as to whether the fish sampled was a bull or Dolly. There was now a tool easily used in the field that would at last solve this mystery!

That winter a few days after reading that article I headed to the river with the formula copied in a little field notebook, pencil, and pocket calculator tucked into my shirt pocket to see what we had. Using barbless flies I would slide the fish into the shallows keeping the fish in couple inches of water, take the 3 counts, release the fish and run those numbers through the formula to see what I had. The first was a Dolly but the third was a bull so we were still left with the question. If there were two species, how are they maintaining their uniqueness as separate species?


Photo by @DanielOcean

To gain insight into that question on one of the North Sound rivers WDFW bull trout spawning index streams in the weeks prior to spawning dozens of staging fish were captured. Then each the 3 key meristic values were collected and ran through the formula. Based on that evaluation the bull trout and Dolly Varden received different colored floy tags. The hope was as spawning commenced where and when those 2 groups of fish spawned would provide insight to the Dolly/bull trout questions. Once spawning began it was clear that both groups were spawning at the same time and in the same area. In several cases fish with different colored tags were seen spawning with each other. It was clear that regardless of what those fish were called there was either a single hybrid population or a single species population. While the whole effort was interesting it did not definitively answer the species question. As so often was the case in the world of our native char, the more I learned the more questions I had!

Fortunately, it was not long before science once again provided a tool to potentially answer the question. Reliable genetic testing for both species became available. To identify the species of a specific fish all that was needed was a small piece of fin tissue preserved in its own vial of alcohol. Testing of several hundred individual char from the Skagit basin below the Skagit Dams or in the Snohomish showed that they were all bull trout! It was not long before a tagged fish that was also genetically shown to be a bull trout was documented to be anadromous. The Skagit fish are bull trout that include anadromous fish. In fact, that anadromous life history is now the dominant life history.

However just to keep folks on their toes headwater resident Dolly Varden have been found in a couple of small Skagit tributaries above Ross Reservoir and in a couple Nooksack headwater streams.


Photo by @kerrys