The Hunchback of the Bighorn River

“The Hunchback of Bighorn River”


We are all familiar with the classic fable the “Hunchback of Notre Dame” but what does it mean when fables surface within prime wild trout waters?

For years anglers have questioned the cause of the “humpback appearance” found on brown trout in the Bighorn River fisheries below Yellowtail Dam.  While not prominent in all Bighorn brown trout, a certain percentage of Salmo trutta do contain a prominent hump on their dorsal anatomies, linking a likeness to the “Hunchback of Notre Dame.”

Yet- what causes this morphological difference?  Is it a recessive trait resurfacing from prehistoric ancestors, a reflection of increased genetic diversity, a parasitic infection, or rather an indicator of environmental damage?

Figure 1.  X-ray image of morphologically normal (top) and deformed (bottom) Bighorn brown trout.

Figure 1. X-ray image of morphologically normal (top) and deformed (bottom) Bighorn brown trout.

First off, it is important to note that it is not unusual to observe a few individuals with obvious morphological deformities in large samples from natural population of fishes.  In fact, both brown trout andrainbow trout have been observed with “humps” on many waterways throughout the nation.  Yet, in contrast to most populations, the population of brown trout that inhabit the upper reaches of the Bighorn River, MT contain an “appreciable percentage” of morphologically deformed, or “humpy” individuals¹.  These individuals contain a variable number of fused and compressed vertebrate in the “hump” region of the fish- that is linked to the result of abnormal growth processes caused by genetic or environmental conditions (Figure 1).

In 1984 samples of 24 “humpy” brown trout were collected from the Bighorn River during spring electrofishing population estimates conducted by Montana Fish Wildlife and Parks.  The 24 individuals collected at random all contained a “humpy” appearance.  Dissection of these samples revealed a 50/50 sex ratio between male and females, making theories of sex linked genetic disorders doubtful. Furthermore, larger brown trout sampled showed evidence of previous years spawning- meaning a “humpy” appearance did not limit the reproductive success of said individuals.  Further dissections of humpy trout showed no parasites, pathogens or tumors and found the hump to consist of healthy, normal muscle tissue. Therefore, a “humpy” appearance does not limit the survival or reproductive success of hunchback brown trout- which is good news,  but do “humpy” brown trout differ genetically from normal Bighorn brown trout?

Picture2To determine this, 25 humpy brown trout and 25 normal brown trout were collected from the Bighorn River in the fall of 1985 and compared.  Findings revealed that average genetic diversity  in both normal and “humpy” Bighorn brown trout was “high compared to natural populations of brown trout from Sweden and the British Isles” but that the heterozygosity (genetic diversity) of the “humpy” Bighorn brown trout was the highest value that had ever been observed in Brown Trout¹ !  This means that “humpy” Bighorn brown trout are unmatched in genetic variability compared to other fisheries which is great news for the population, but still inconclusive in linking the humpback characteristic to genetic conditions.

Over the years, anglers have voiced speculation linking “humpback” trout to electrofishing, a standard sampling technique used to assess trout population in many rivers.  The extent of damage and mortality from electrofishing has been a concern of fishery workers for years and was actually studied in 1985. In the study 250 trout (125 rainbow trout, 125 brook trout) were held in experimental water bays and electrofished using high voltages and then x-rayed to detect injured vertebrate and then compared to x-rays of control or non-electrofished trout. Normal appearing “shocked” trout in this study had a 1% level of injured vertebrate with all fish from all different voltage studies showing fused vertebrate “probably not caused by electrofishing[2].”  The low level of vertebrate damage in this study suggests that the high observances of “humpback” brown trout on the Bighorn River is not due to environmental damage caused through current electrofishing sampling, which occur at much lower voltages than in the controlled study.

The above findings remain inconclusive in determining whether the “humpback” growth found in some Bighorn brown trout is of genetic or environmental origin.  However, it is likely that experimental matings could help determine if the abnormal growth of brown trout is attributed to genetic variation.   A study that- in looking towards the future- the BHRA may be able to answer in the classroom with local Bighorn kids.  Stay tuned!

Anne Marie Emery,  executive director


[1] Leary, Robb F, Fred Allendorf and Kathy L. Knudsen.  Electrophoretic Variation and Levels of Fluctuating Asymmetry in Morphologically Normal and Deformed Brown Trout from the Bighorn River, Montana.  Population Genetics Laboratory Report.  University of Montana.

[2] Hudy, Mark. Rainbow and Brook Trout Mortality from High Voltage AC Electrofishing in a Controlled Environment.  North American Journal of Fisheries Management 5:475-479. 1985


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