Infected Browns on the Mo

We have been seeing a lot of browns that look like this fish... George from FWP has helped clear up some of the confusion and here is what he has to say about it.

"Please let me try to explain in some detail what we think is happening to the fish that you are observing that are in bad shape or dead, which should be primarily brown trout. The fungi that you are seeing on the brown trout is likely Saprolegnia spp, or a closely related genera. Saprolegnia parasitica and S. diclina are the principal species which cause the disease in salmonids. These species occur worldwide and appear as whitish cottony-like growths on eggs or fish. They are most commonly a secondary infection or invader to bacterial or viral infections, malnutrition, physical injury and/or physical stress and are opportunistic in nature."

As John Wilson's email suggested, our fish health biologist went out and collected some live fish with Saprolegnia fungus present in conjunction with scheduled redd counts after speaking with Hal Harper. He examined the live fish, and most all had fungus on and near the ventral (bottom) portion of the body on and near the tail.

Internal clinical signs of infection were not present, which indicates a normal condition and no additional disease present. As you would expect from spawning fish that were focused on spawning and not feeding, internal fat was absent and all except one had their gall bladders swollen and dark, which indicates they had not been feeding. If the fish is not feeding, that is an additional stressor on the fish. One female's gall bladder was not completely full and had eggs in her stomach. She would have likely survived the infection.

We are aware of the substantial numbers of dead browns that can be observed in pools of tributaries or at the mouth. For example, 15 could be observed in a single pool in Sheep Creek today.

When infected individuals within a spawning stream are present, you can assume all individuals are exposed to fungal spores' since other fish are not infected, the Saprolegnia are opportunistic parasites. The infection is promoted by primary traumatic damage inflicted on females during redd construction and on males from abrasion and wounds during redd defense. The occurrence is also favored by sexual maturity and mature males are more susceptible than mature females. Mature males that spawn possess fewer mucous secreting cells in their epidermis, which results in a reduced mucous slime coating. The mucous is thought to inhibit colonization of fungus. The mucous inhibits fungus by physically removing attached spores, it includes a morphogen that inhibits mycelium growth, and it contains lymphocytes and neutrophils that attack the mycelium. The concentration of the mucus producing cells decrease posteriorly along the fish, which is also the areas more prone to mechanical damage during spawning.

Males produce higher levels of androgenic steroids during spawning and as a result, have lower concentrations of mucous secreting cells, and are more susceptible. Larger males are thought to produce higher peak levels of androgens or for a longer period of time. This along with larger fishes tend to be older and occupy higher positions within the dominance hierarchy in spawning areas, they would then be challenged frequently by younger, sub-dominant males, use up their energy reserves, have a great chance for mechanical injury, as well as having less resistance to a secondary infection. So, older larger fish will tend to be at much greater risk, especially if they are males. However, any fish could be at some risk, especially if they are exposed to a primary infection or injury.

One thing that is quite important that we have not yet analyzed is water temperature. We will compare water temperature data this fall and to other years to determine if the Indian summer and potential warmer water temperatures could have helped to accentuate the more widespread incidence. However, it could be a result of a brown trout population with a higher proportion of larger, older fish.

Back in late September we did a health inspection of wild brown trout on the Missouri in the Craig area. We have received the majority of the results back from that and to date, no major pathogens have been identified. We did observe some clinical signs of Mycobacteria, which is a form of tuberculosis that we have found in mountain whitefish and rainbow trout in the Mo below Holter in the past, but those test results have not been received.

Those clinical signs would not be consistent or be related with what is being observed on the dead or dying brown trout. We are not aware of any threat to human heath by this species of Mycobacteria. The health inspection provides us confidence that we do not have a major disease problem developing on the Missouri at this time. We expect to see the Saprolegnia infections fade as brown trout spawning decreases in early December. About 25 years ago we saw a similar outbreak in the Missouri River near Beaver Creek below Hauser Dam. Please let me know if you have any questions.