Haven't tried this myself - but I know formalin has been used diluted to remove lice from salmon, with success...   

I've used formalin too, but it was for lethal sampling.  This would be nonlethal and must be government approved.  I believe the products above are used in aquaculture and are approved for use on fish for consumption.  I just have no experience with them.

If you are trying to get the parasites to 'drop off' of the fish you might want to consider MS-222 (Finiquel).  While performing some efficacy studies with Parasite-S, we found we could not use MS-222 to euthanize the fish because it would cause gyrodactylus to drop off, making it difficult to get accurate parasite counts. We were working with freshwater fish, but I would think it would work for saltwater fish too. The Parasite-S tended to kill the parasites in place for the most part. If you want the fish alive, just be sure to use the anesthetic dose rather than the euthanization dose.

It depends on the type of parasites you want to remove. Freshwater is probably your best all-rounder for marine bugs. Pop the fish in for 5 or so minutes. Most ectoparasites will fall off in this time. More elaborate techniques are available and I agree with Susan that anaesthetic is also useful for monogeneans, but you will have trouble with useful removal concentrations nearing or exceeding euthanasia for fishes. I found that 2-Phenoxyethanol is particularly good for this. If you have the time you can intubate under anaesthetic with a praziquantel formulation at 150 mg/kg body weight. You should get quantifiable results after 4 hours for monogeneans, but not crustacean ectoparasites.

Thanks for the input.  Yes, I simply want the ectoparasites to drop off (for collection).  I can't use MS-222 because these fish will be released.  I've done this before with formalin, but the fish were euthanized afterwards, so human consumption was not a concern.  My understanding is that the products I listed in the original question were approved for fish to be sold for consumption.  I like the freshwater suggestion, but it would likely kill the host.  I might simply need to lobby my collaborators to change opinion on euthansia!

Hi Charles

The freshwater dip shouldn't kill the host as it is only a temporary osmotic shock. That said, if the hosts are moribund or otherwise compromised then any procedure is going to increase the risk of mortalities. What is the target host species? If you are dealing with a commercial interest then freshwater is probably going to be the most acceptable method as there are obviously no issues with chemical use approvals and retention times. I have used this method repeatedly on a large diversity of species including some very sensitive ones (and juveniles) with good success. Your concerns should be covered under a university ethics approval, which I would imagine would be acceptable to your collaborators? Under all methods they would need to acknowledge some possible mortalities, but that these would be avoided were possible. If I can be of any future assistance please just shout - I am more than happy to help.

Best regards, David

Thanks David and Susan, I will definitely reach out to you if we proceed with this project.  Appreciate the input!