Yesterday I caught a fairly large fish in the River Wharfe upstream of Bolton Abbey that had a fish lice latched onto its throat. I tried to get a photo but didn't quite manage it.

What is it?
Luckily, Fish Lice are not all that common. However, they are nasty when they do occur. They are often left unnoticed, which can be quite detrimental to the fishes health, they can be mistaken for algae or dirt. It reaches up to 5 mm in diameter. (0.2 inch)
The free swimming parasite is usually brought in on new fish or plants. They will multiply and attach to the fish, burrowing in slightly so they can feed off the fish. The feed by using a straw-like structure into the fish, injecting digestive enzymes (similar to a spider!) and then sucking out the liquefied flesh. This causes great irritation to the fish, and may cause dangerous bacterial infections.

The harm comes from major tissue damage (that can cause scarring) or as a secondary infection. The fish also become very stressed, and become quite susceptible to other parasites such as white spot.

Symptoms:
Being the largest fish parasite, it is quite obvious to the naked eye (if you're looking for them). They often reside in protected areas, such as behind the fins or near eyes and gills. The louse will appear clear and somewhat transparent and may be seen moving around. There may also be red markings, ulcers or inflamed areas where the lice have previously been.

Now these comments relate to fish in an aquarium, but to see this in the wild was a surprise to me.

Is this common? Does or has anyone else seen these on fish that they have caught?

I know that they are common on salmon. but to be on a trout so far up the river and an area where salmon to not get to seems disturbing.

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"A traditional wet fly addict"

Uk representative and Instructor for Fly Fishing Masters - www.flyfishingmasters.co.uk

Distributor for Terenzio Silk and Artificial Silk Fly Lines - Pay distributor retail price.

Don't get confused between argulus and sea lice. Argulus live in freshwater and are not brought in by salmon returning from the sea. The argulus are a very common parasite which you will regularly find on trout if you look close enough. You will find them more regularly in extended periods of low flow and high temperatures but generally they are not something to worry about as in the wild their numbers are rarely high enough to cause any damage. All part of nature.

When I was a boy, the sticklebacks in the local canal were crawling with them.

i caught a rainbow on a small stillwater early this year that had strange looking "mud lumps" attached to it - three i think. I handled it a bit ginger like before returning. later i read something about lice being on the bottom (?) and as it was cold i thought thats how fish must have come into contact. Regularly fish same venue and haven't come across it since.
One question tho - should i have tried to remove them or would this have done more harm than good?
grey

http://www.marlab.ac.uk/Uploads/Documents/Lice%20v3.pdf

Informative link on the 3 Argulus species now found, and spreading, in the British Isles.

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Save the softmouth trout

Many years ago I used to fish for barbel in the upper reaches of the river Swale (Yorkshire).

It was noticeable, particularly in June, July and August, that most of the fish caught invariably had a translucent type of 'lice' attached to their flanks and underbelly. Wherever the lice were on the fish, there was evidence of blood-red surface marks - presumably the result of lice feeding on the blood of the fish.

I always used to make a point of removing the lice with forceps and discarding them on the river bank, in the hope that it would relieve the fish of any distress which I was convinced it must have been in. It was always assumed that barbel were the most susceptible to these lice because they made contact with the river bed and ranunculus fronds (water crowfoot), both of which enabled lice to transfer to the fish. However, Chub, too, were vulnerable but fish such as Dace, which prefer to be nearer the surface, were very rarely seen with lice on them.