Every summer I receive a few e-mails from curious anglers asking if I noticed bumps or cysts under the skin of our stillwater stocked trout. These type of question also appear on many of the fishing message boards and forums. As the water warms, we see these bumps appear on a lot of our trout pretty much every summer and disappear once the water starts to cool. So what are these bumps, are they harmful to the trout and are they harmful to anglers handling these fish or more importantly, consuming these fish? Click on the pick of this trout caught at Muir Lake (courtesy of Phil Rowley) for a closer look.
I noticed these bumps more frequently about six years ago. With the lack of precipitation we've seen around the Edmonton area, the lakes have been dropping and warming in the last six years. This combination not only causes more stress on the trout but appears to increase infection of a flat worm parasite which mistakenly uses the trout as it's host. I say mistekenly because these trout are not native to these bodies of water but rather stocked every spring.
Back when I first noticed these bumps becoming more profuse, I sent off an e-mail to Stony Plain area fisheries biologist, Stephen Spencer. He explained that the bumps were caused by a flat worm parasite. Cercaria (the larval form of the parasite) is actually intended to affect it's true host the snail. Once in the snail, it relies on consumption by a bird where it then matures into its adult stage and lays eggs in the bird's gastrointestinal tract. The eggs are then dropped back into the water through feces and the cycle repeats. The trout (like humans) are not its intended host and other than some ugly bumps (and itching for us - see "Swimmers Itch"), the parasite itself really doesn't harm the trout or humans but the bacterial infections caused by an overwhelming amount of parasites could be lethal to trout.
A couple years ago, A.S.R.D did put a warning out about these bumps for anglers eating fish showing signs of these parasites. Although these infected fish are not harmful to humans, it was thought to be a good idea to cut out any black spots (bacterial infections) caused by the flat worms and for the flesh to be cooked thoroughly.
Most years, Edmonton's local trout lakes see these parasitic bumps on our trout but some years they've been very prominent. This is one of those years. They aren't pretty and don't feel very nice when handling the trout. On years like this, it is beneficial to the trout to decrease your fishing activity until the water starts to cool again as there is a coalition to very warm water and numerous bumps under the skin of the trout. If the water is that warm, think how stressed these trout must be.