Fish Kills in a Future Fueled by Disease and Rapid Environmental Change

By Jack Williams

When disease combines with stressful stream conditions, small problems that might typically fall under the radar screen become transformed into headline events. The Yellowstone River is a case in point. 

This large-scale mountain whitefish die-off, caused by Proliferative Kidney Disease (PKD), prompted the closure of more than 183 miles of the iconic Yellowstone River and its tributaries downstream of Yellowstone National Park to fishing and all water-based recreation. State officials noted that drought and warm water were contributing factors to the outbreak. 

Staff from Montana Fish, Wildlife & Parks documented more than 2,000 dead whitefish but estimated that the kill may be much larger. Estimating numbers in fish kills are tricky as fish can be hard to spot and scavengers take their toll. Also, juvenile fish are at highest risk from PKD and smaller fish are less noticeable. Regardless, the numbers appear to be large.

PKD results from an internal parasite that attacks the kidney and spleen causing fish to become lethargic, anemic, and ultimately to die.  Studies suggest that mortality rates from PKD are rising and mortality can be as high as 90 percent in infected fish. Salmonids, including trout and whitefish, are usually the victims. 

In 2011, researchers from the Natural History Museum in London and the University of Bern in Switzerland reported that the incidence of PKD was on the rise in Europe and North America, both in trout raised in aquaculture facilities and in wild populations. In Switzerland, brown trout crashed with PKS being the primary cause. In that instance, researchers noted that environmental change, particularly higher stream temperatures, was likely to cause PKD outbreaks because higher temperatures made the trout more susceptible to the disease by suppressing their immune system and increasing the number of parasites that cause the disease in the first place. 

Drought, high temperatures, and degraded habitat degradation all have been tied to PKD outbreaks. Laboratory studies document that rainbow trout infected with the PKD parasites show fully swollen kidneys in fish held at 59 to 66 degrees Fahrenheit. But the disease moderates and fish show little kidney swelling at water temperatures of 48 to 54 degrees. 

As climate change warms our streams and rivers, we should expect further disease and parasite outbreaks to become more severe and largescale fish die-offs to become more common.

On August 19^th, the Yurok Tribe in California announced that “ich” or “white spot disease” which is caused by external parasites, had surfaced on salmon in the Klamath River. In 2002, the disease caused a severe salmon kill on the river. Fisheries biologists for the Yurok indicated that lower temperatures and increased flows were needed to reduce the impact of the disease. Again, high temps and low flows are driving factors in the intensity and rapid spread of this disease.

What to do about the increasing threat posed by parasites and diseases? 

In the near-term, the shorter day length of fall days coupled with cooler nights and perhaps some rain will lessen the immediate problems in most streams.   

But in the long run, we must focus on restoration efforts aimed at restoring stream flows and better management of riparian areas to provide cooler water and conditions that may lessen the severity of these outbreaks. For example, in the Yellowstone Basin, TU is working with farmers to upgrade irrigation systems in a way to produce more reliable, colder flows in the river. 

Additionally, better monitoring is needed to fully understand disease outbreaks and their impacts.  Our knowledge of mountain whitefish populations for instance, is sorely lacking in many western streams.

Anglers can help by inspecting their wading shoes and other fishing gear to make certain they are clean and dry between trips so that we they do not contribute to the spread of diseases.

Ultimately, however, it is our warming world that is the game changer resulting in more and larger fish kills. Adaptation efforts aimed at stream and flow restoration are needed and can help to moderate outbreaks and disease severity. But solutions also must come from our social and political systems.

To make a lasting difference we must take climate change seriously and make significant reductions in carbon emissions. Our society and our politicians can be slow in recognizing longer-term environmental threats and even slower in acting upon them. If these past few years are a sign of things to come, I hope we don’t wait much longer.

 

Jack Williams is TU’s Senior Scientist