As we are all pretty much aware, the Yellowstone was shut down last week indefinitely by FWP because of Proliferative Kidney Disease killing thousands of whitefish and trout. It's a parasitic disease transmitted by spores hosted in freshwater Bryozoan which are aquatic invertebrates more commonly known as moss animals. I'm definitely not a biologist and quite honestly, slept my way through most of my biology courses in high school and college so I'm not going to pretend to know any more of this than I do but what seems to happen, based on the research I've been doing, is the parasite finds a host such as these invertebrates and when conditions are right, spores are released and infect fish in the waters the parasite inhabits.
My understanding is that the spores can survive in the infected fish for a while without the fish showing symptoms. When certain environmental conditions are met that create more stress on the fish such as warmer water temperatures, the fish succumbs to the disease, showing signs of a bloated abdomen as the kidney's swell and the fish dies. I think what's important to note here is that the water temps that were recorded as the most crucial to the morbidity rates that were studied in fish farms were around 54 to 59 degrees Fahrenheit, which to me, doesn't seem that warm.
From what I read, the disease was most concerning for fish farms where close to 100% of the trout in those farms would die from the disease. Hearing that can sound off the alarms if we generalize those rates to our wild fisheries. What we need to understand though, before running off with this "sky is falling" message is that in wild fisheries it appears that the disease is most detrimental to the more vulnerable fish. Again, I'm not a biologist but native species such as mountain whitefish don't seem to be as resilient as the formerly introduced and now wild trout although in many of the journals I read, trout in their first year appeared to be more vulnerable than more mature trout.
I think it's also worth noting that trout surviving the disease can develop an immunity to it. What this means is that some fisheries may have already had the disease and those fish may have developed resistance to it. In 1996, PKD was found in the Madison and had been blamed for whitefish kills as well as the South Fork of the Snake in 2012-2013. Although I don't think we should take the incident on the Yellowstone now lightly, I also would caution any kind of dooms-day reporting. Both the Madison and the South Fork have high trout counts as they have survived their outbreaks.
As for what species of trout are most at risk, I didn't find any evidence that one is more vulnerable than another. There are examples of brown trout being affected in Switzerland as well as rainbows and salmon in other European countries and now, cutthroat in North America as we are seeing some cutties being affected in the Yellowstone. Again however, it would appear that the fish that are being impacted the most tend to be the most vulnerable, which are the whitefish and the younger classes of trout.
I read a ton of articles and journals about the disease and the one thing I didn't find is an explanation of how the parasites are spread from river to river however, it would seem likely that boats or wading shoes or anything else that can pick up the hosts of the parasite could certainly spread it to other waters. If we could take any positives from this is that it's a glaring example of why we need to clean our gear and maybe it brings that point home to all anglers and guides before something even more detrimental is spread.
I do feel bad for those folks in Livingston and Bozeman that are having to deal with the economic impact of the Yellowstone closure. These folks are resilient as well. One advantage they have over some of the rest of us is that they are used to having to travel at other times of the year when the Yellowstone and other rivers are blown out. Having said that, it obviously brings concern to those of us that guide on the Missouri since they very well could bring the parasite to the us if it hasn't been spread to the Mo already. It does warrant some discussion of how we might reduce the spread of such outbreaks.
I realize it's not a very popular topic but there has been discussion over some kind of home-water restrictions for commercial use in the past due to how busy rivers like the Mo and the Horn get during certain times of the year. I'm not waving the banner for this yet but does this situation now bring some credence to the argument? Is this something we should talk more about?
Keep 'em where they live...
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