To me, one of the most vexing problems with fish keeping is knowing when to euthanize a fish and how best to do it humanely. Euthanasia can be conducted in a variety of ways – physically (e.g., a blow to the head, decapitation), chemically (e.g., clove oil, Benzocaine, Tricaine Methanesulfonate), and naturally (e.g., freezing). Killing a fish humanely is quite subjective because not everyone agrees on what “humane” means. Ultimately, the decision comes down to you and whether your fish’s discomfort matters to you or not.
There have been multiple studies conducted on euthanasia in animals, especially for scientific research. In fact, there have been many studies on fish euthanasia. So what do the scientists say? Well, as you might expect, they don’t all agree.
One of the best articles I’ve read on the subject comes from a study conducted by some Australian researchers in 2010. In their study, they examined the differences between euthanasia on the small bodied, tropical fish species Nematolosa erebi, which grows to an average maximum length of 15″ or 40cm. You can look it up, but it’s a rather unremarkable species that resembles a tetra (just much bigger). Nonetheless, the article employed two standard euthanasia techniques, which the authors compared to determine quickest time to death and least amount of stress. Using controlled experiments, the authors compared the euthanasia effects of a Benzocaine solution and an ice-slurry.
Surprisingly, at least to me, the authors concluded that the ice-slurry resulted in less observable stress and quicker death compared to the Benzocaine method. There are many variables that can contribute to the results – fish size, fish type (warm water vs cold water species), and even the species itself. I wouldn’t conclude that the results easily translate to cichlids but neither would I dismiss it.
I read the actual article from the Journal of Fish Biology, but a quick search found a downloadable copy from Penn State University.
Reference: Blessing, J. J., Marshall, J. C. and Balcombe, S. R. (2010), Humane killing of fishes for scientific research: a comparison of two methods. Journal of Fish Biology, 76: 2571–2577. doi:10.1111/j.1095-8649.2010.02633.x