General · 8th December 2017
A large number of chum this year have succumbed to the parasitic attacks of freshwater leeches in Hyacinthe Creek. Bill Dubois noticed pre-spawn chum as well as spawned-out chum covered in the small black leeches, and contacted some members of Quadra Island Salmon Enhancement Society as well as other concerned citizens. They have been tentatively identified (by photograph and behaviour) as a salmonid leech Piscicola salmositica. It's usually the slower swimmers – especially spawned-out dying fish – that get the brunt of the leeches. But according to Hyacinthe Creek residents, the leeches seem to be killing even bright, pre-spawn chum. For some reason chum seem to be more sensitive than coho, according to a DFO veterinarian who got back to us via email.
So far, we don’t have clear understanding of why this is happening; while salmon-specific leeches are a natural phenomenon, they are unusual in these numbers. Leeches aren’t typically fatal to salmon, but in high numbers they can cause anaemia, and in some cases can transmit a blood-dwelling parasite, Cryptobia sp. The parasite is not typically lethal, but possibly damaging in combination with other stress factors. Salmonid leeches are known to occur periodically in some waterways, with higher populations in some years. Their populations tend to increase over the spawning season, as host numbers increase. Perhaps we could be seeing more leeches due to higher water temperatures?
If anyone knows of other creeks and rivers where this is occurring, please get in touch with QISES members so we can follow up on this and learn more about it. We’ve heard that Nitinat is having a 'banner year' this year, but that they usually have some occurrence in the chum.
Also, if anyone has university or other contacts who might be interested in examining the leeches (for example, we could have them tested to see if they are carrying Cryptobia parasites), please contact QISES. Bill DuBois has preserved some in his freezer, but if we want any fresh samples, we’ll have to act soon!
Editor's note: Check out QISES' new salmon counting system in the DI this week – Issue 667 is hot off the press!