Do collectors still dump live fish into formalin?

It has been a long time since I graduated from college. I am no longer associated with professional ichthyologists and no longer collect and preserve fishes for University collections. But 50 years ago we were dumping live fish directly into formalin in the field. This always seemed cruel to me, as judged by the violent reactions of the fish to this. I became so disturbed by this that I eventually just refused to do it. One thing that often happens with small specimens so treated is that their gills flare out and heads bend upward into an unnatural position. Recently I was looking through a book of darters and noted that many of the color pictures had specimens distorted this way. The authors had taken pictures in the field to capture the vivid colors of these fishes but killed them before attempting to photograph them. So engrained was this killing method that the authors sacrificed the quality of the images to handle the fish this way. You can see here on iNaturalist thousands of pictures of live fish taken in the field. Those taken live in water usually have erect fins (important for counting rays) and the colors vivid.

My question is do people still do this? Do they dump fish directly into formalin or alcohol?

Even more disturbing to me was the practice of using rotenone in streams to collect fish. In streams really?? Do ichthyologists still do that?

I do understand the value of preserved specimens. I have spent many hours in fish collections sorting and identifying. But have we gotten any better in our acquisition methods?


I’m not too familiar with fish euthanasia/specimen prep. A while ago there were certainly some euthanasia methods that were considered ok that are not currently allowed (across many taxa). I do know some ichthyologists who have collected by immersing (pretty small) fish in liquid nitrogen flasks in the field. This seems to cause near instantaneous death and likely is a good method of preservation for certain types of specimens/data.

In your example, I don’t know that the conclusion that

is necessarily correct. Dead specimens can lead to better pictures since the subject isn’t moving, reducing blur/enhancing detail. Standardized postures are also often used in guides to facilitate comparisons. And other types of euthanasia might also lead to that characteristic posture, not just formalin immersion.

As for rotenone, a Google scholar search shows that there are recent pubs based on rotenone sampling, so I would guess that it is used in some contexts at least.


They’re killed not for pictures, but for specimens, and most pictures of fish on iNat are from alive specimens, including darters, there’re special containers that allow to get amaing pictures. I also don’t think fast kill like that is any worse than just slow dying of caught for food fish.


I’ve recently read a 2007 paper that suggests the following for bivalves:

There is no single technique for relaxing all species of bivalves.
We have had varying results relaxing bivalves by adding
menthol crystals, magnesium chloride (7% in seawater), or dilute
ethyl alcohol (approximately 3%) to native seawater until
the specimens are not responsive to touch. Members of the
family Veneridae can be especially difficult to relax, and we
have had to resort to a combination of the above methodologies
to properly relax them.
When a specimen is nonresponsive to touch it can then be
fixed in 5% buffered formalin (a handful of sodium borate [Borax]
per gallon sample—not scientific but the reality of fieldwork).
Formalin should be buffered to prevent decalcification,
especially of small shells that can rapidly dissolve. Most bivalve
specimens should not be allowed to remain in formalin for
more than 48 hours, or severe damage can occur to the shell.
As usual, use care with formalin; this carcinogen should only
be used outdoors or in well-ventilated conditions.

Coan, Eugene & Valentich-Scott, Paul. (2007). Bivalvia, in: The Light and Smith manual – Intertidal invertebrates from central California to Oregon.

I’m not sure how unpleasant or painful this would be. I do think that if there were a non-chemical way to get the specimens dead and open without mutilating the animal and/or shell, someone would have discovered it.

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Immersion in boiling water is effective for mollusks, but muscle contraction is inevitable in all but the smallest species so it’s not good for dissections.

And I can’t speak for vertebrates, but in the invertebrate collections I’ve worked with we don’t use formalin. All ethanol now.

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The method of euthanasia that you described would not be acceptable by Institutional Animal Care and Use Committees today. Scientists follow guidelines published by societies such as the American Fisheries Society and which are approved by their IACUC. It’s all very tightly regulated in most countries.


I agree. Many years ago when I did research in an Australian university, all vertebrate (and even some invert) research had to have Animal Ethics approval. For fish, the anaesthetic MS222 (tricaine methanesulphonate) was required for any operations and/or euthanasia.

But on a purely philosophical note, I have often wondered whether a slow lingering death (of fish suffocating after being caught) is better or worse than a rapid but more painful death?? [I am not a recreational fisher].

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I don’t have too much extra to add. Perhaps contacting an Ichthyological department and asking about both. If you see a specimen on iNat that seems to conform to that ‘look’, contacting the person may help clarify things.
I would appear that rotenone may be still used to sample fish populations. This paper from 2008 seems to promote it’s use - Rotenone: An Essential but Demonized Tool for Assessing Marine Fish Diversity | BioScience | Oxford Academic ( This may be because rotenone is ‘organic’ (bean extract) and because of it’s relatively short life span (5 days).
I don’t have much involvement with fish, so for more clarification it may be good to contact a research facility. I don’t know where you live, but there is a Freshwater Institute in Winnipeg that may be able to answer your concerns - Fisheries and Oceans Canada ( There is a contact link at the bottom of the page. Good luck!

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I think you misread me melodi_96. I was not suggesting that people on iNat were killing fish. In fact, I was praising how good many of these photos are without killing fish. I understand the need for maintaining fish collections of preserved specimens. I have worked with them. My question was in regard to the method of killing fish, and also I was questioning the need to kill them to photograph them in one particular book. I apologize if I was unclear.


Yes, sorry, you mentioned book an I missed that! I think it’s still used as we were thought in university about this method (though formalin isn’t used now because it degrades dna).

@cthawley I may have been unfair to the authors of the darter book which was written some time ago. They were professional ichthyologists of high caliber and I meant to be pointing at the old collecting culture as I remember it. And yes there is at least one aspect of photographing bottom fishes that is very hard with live specimens. In live specimens, the lower fins tend to get folded beneath the fish, making it hard to count rays and see patterns in those fins. That alone could account for the decision to kill the fish, and the choice of formalin as a killing agent… well maybe that is a thing of the past.

From yours and other replies, I am happy to see that people are opting for more humane methods.


@Marina_Gorbunova that is an excellent point about DNA. I suppose all new preservation methods must take that into account.

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Is slow chilling a reasonable form of euthanasia for fish?

A fish vet (koi specialist) gave me advice when I had a large (14” long) injured goldfish. He suggested putting it into a container of cold water and putting it in the refrigerator and, later, the freezer.

(Coda: Actually, I didn’t end up euthanizing the fish. The vet gave me a medicine to dump in the pond and my fish recovered… only to be eaten by an ambitious raccoon a year later).


I do know that the ice fishers up here can basically flash freeze fish on the ice if it’s cold enough. I don’t ice fish, so don’t know the temperature at which this happens. Below -10C?

Thanks for your concern! My job entails a lot of research based on fish collections, and unfortunately, yes, there are still some people doing that. I do notice that younger generations luckily oppose more and more to that, although I have also visited research teams were immersing living fish in formaldehyde or dissecting living fish was so common practice that students did not question it, as they had never seen anything else.
Personally, I make sure that fish killed for collecting purposes on “my” field trips are sacrificed in a humane way, e.g. MS222, clove oil, or, when chemical anaesthesia is not an option for some reason or another, a cut behind the head. And I can only hope many students/collaborators follow in these steps…


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