White Fish (C. nelsonii) with Saprolegnia Fungus

Observation: Pus found on white fish (Coregonus nelsonii) from Chakchak, Alaska. More than 75% of white fish caught with under ice net had pus on them. We checked total of 4 nets and we caught 52 total white fish. Only 12 fish had no pus on them. Other people were seeing the same pus on the fish. I've never seen these kind of pus since I started setting nets about 8 years ago. I don't think people are eating the fish. Some white fish had pus all over their body. I never seen pus on the other fish we catch like the burbot, shee fish, and pike.

LEO says: Seeing an occasional fish appearing like this is probably not unusual, although certainly worth posting as a LEO observation. If however, you observe several fish with the same condition, we would begin to question if there is an environmental or other cause. In this instance with so many fish showing evidence of illness, environmental conditions are a possible factor that can contribute to this condition.

NOTE: LEO is interested in the observations of members to help better understand the scale of this event; especially in the Yukon-Kuskokwim region. If you are seeing similar conditions in the fish where you live, you can post your observations to the LEO Network.

Alaska Department of Fish and Game Consult: (2016-12-02) Ted Meyers, Fish Pathologist writes, "The whitefish appears to have a fungal infection of the skin, probably caused by Saprolegnia, the common water mold that is naturally present in fresh and slightly brackish water. It is important to note that water mold is found worldwide and all fish are susceptible to infection. Generally, Saprolegnia is a secondary invader meaning the skin or invaded tissue must first become traumatized or damaged. The damage may have resulted from a mechanical injury by a predator or in this case more likely from previous encounters with netting gear under the ice. Another possibility is from an external bacterial infection from trauma or stress that produces tissue death that then becomes infected with fungus. If these fish were caught under the ice, dissolved oxygen often is limited causing environmental stress. The stress and cold water temperatures reduce the immunity and resistance of affected fish predisposing them to slower tissue healing of any mechanical injury that is more likely to become infected with bacteria and fungus. Whatever the cause, the lesions will become infected with fungus if the fish survives long enough. More detail is provided in the ADF&G field guide on common diseases of fishes located on the ADF&G website- pg 34 for Saprolegnia." (Meyers et al., 2008)

Ted continues, "The prevalence for the fungal infection is high but if due to poor or otherwise stressful environmental conditions it is likely that many of the fish in the population will be affected in the same way. Other apparently normal appearing species may not be as susceptible or as exposed to whatever the sub-optimal conditions may be. Also, no information was given on how local or widespread this event is which would be indicated by how far apart the nets were set or whether fishers in other locations are observing similar diseased fish. To be sure there are some fish parasites that can infect humans and other warm-blooded animals. However, coldwater bacterial and fungal infections in fish generally are not harmful if fish are consumed. That said, I do not recommend eating any of these fish to error on the side of caution and because any fish with a disease is simply not a wholesome food product for humans or their domestic animals." Source:Fish Pathology Laboratories, ADF&G