Monday, September 12, 2016

Oh, That Nasty Copepod

Back on January 12, 2012 I had my first encounter with copepods. Out at Lake Camanche I kept catching Rainbow Trout with those slimy spots on their sides. I didn’t know what they were until Aaron at check-in told me that was from where copepods had hung on the trout and dropped off when the water got colder.

Image result for copepod

On the last post Pat Konoske made a comment regarding copepods in the Moccasin Creek hatchery.
Rather than having you go back to that post, here is the information. “It used to be that every few years the hatchery would use brook trout as a sort of bio-filter. Apparently, for an unknown reason, placing brook trout in the raceways helps control/clear out a parasitic copepod to which rainbow trout (and other trouts) are susceptible to, something more common to West Coast trout (and brook trout is native to the east…). From what a hatchery biologist told me, when these copepods attach themselves to brook trout they don’t produce eggs, as they do when attached to rainbow trout.”

So I thought I’d go directly to our resident Biologist (and super flyfisher girl), Marisa and see what she knows about copepods. She sent me a couple articles which I’ve gleaned some interesting information from. 

Rainbow trout, along with several other west-coast fish species, are susceptible to a parasitic copepod called Salmincola californiensis. The tiny, shrimplike parasites, which are about the size of a pencil eraser, attach to fishes’ gills, where they leave eggs and complete their life cycle. Apparently from my experience in 2012, they also attach to the side of trout.

Copepods in Gills
Gross, huh?

When copepods are in a hatchery, they attach to trout gills in so many numbers that they weaken the fish, making them more prone to disease, and even causing the fish to suffocate. A study at a California hatchery (could have been Moccasin Creek) concluded that placing brook trout upstream from rainbow trout somehow filters copepod larvae from the water supply, reducing infestation. 

Another reason is that in a stream a copepod has little chance of coming into contact with a host that is needed to complete its life cycle. More space for the trout to move and more distance between the fish and the parasite. A free swimming copepod has a very short lifetime, about 48 hours. In a hatchery, a copepod gets loose, many fish to choose from. You get the idea. 

One theory is that copepod larvae are specific to certain west-coast fishes (of course, look at the name Salmincola californiensis) and cannot complete their life cycle on other species, such as east-coast-native Brook Trout.

One article that Marisa sent me has to do with trying to kill the copepods in a hatchery with a variety of chemicals. Without going into a lot of detail (it’s about 5 pages long), it’s next to impossible without draining the pond, scrubbing it clean, leaving it dry for several months and then restocking.

So here is my conclusion. 

  • They are ugly little buggers. 
  •  They are trout murderers.
  •  They are a menace to society. 
  •  They are next to impossible to kill once they are on a trout. 
  •  They are ugly. Wait, I already said that.
  •  They ruin a good two pound trout by putting little slimy spots all over it. I’m not going to eat   one even though they don’t affect the fish. 
  •  We need to find a good copepod assassin.

·      I hope you enjoyed this little excursion into the realm of copepods. 

Fishing Thursday.



  1. Bravo "brook trout"
    Very interesting piece Mark.

  2. I had to skip that one picture...too early in the a.m. for that kind of stuff... But thanks for the expansion on these slimy little buggers. The sad thing is that brook trout haven't been raised at Moccasin Creek for years. But when they were (often during winter), being brook trout, they ate and ate and ate, meaning when stocked for opening day they would be close to two pounds.

    1. There is nothing in this world that would stop me from catching a 2 lb Brook Trout.

  3. That is disgusting! Those little stinkers! I am an early fisherman and it will never change. Now, I just need to train myself to leave the river early too. Haha

  4. Gill Maggots is what they look like . feel sorry for the fish

  5. (shuddering at the photo) ... are these parasites related to the ugly things that infest saltwater cutthroat trout that live in the more isolated and sort of stagnant "backwater" areas of Puget Sound?