Time to rethink anaesthetics?


A study into the effects of chemicals commonly used in euthanasia and anaesthesia of fish has found some of them to be less humane than previously thought, says Nathan Hill.

A while back I wrote a comprehensive piece on the methods of despatch for dying fish. At the time I consulted many authorities in the fish world, trying to collate the few universally acceptable techniques as advised by researchers, hobbyists and even within the field of commercial aquaculture.

Alas — or rather I should say fortunately — the scientific community has decided to spend some time investigating the effects of anaesthetics on fishes, and have found that what we traditionally recommended may in fact be unwise, at least in the case of Zebra danios.

In a nutshell, the time has come to reassess the use of anaesthetics in fish euthanasia, and for good reason. Though we hobbyists are more interested in keeping our fish alive, it should be remembered that some fish are at the pioneering front of medical research, mainly because of their fast rates of growth and maturation. If studying the occurrence of cancer, say, it’s handier to have thousands of test subjects all developing symptoms rapidly, than waiting months at a time for a slower organism to develop.

Because of such a high turnover, it follows that the vast number of fish – estimated to be around 500,000 in number – will be humanely destroyed after research is complete. Given that we hold increasingly high standards of welfare across all domains, but especially in the field of scientific research using animals, it only seems right that current methods are frequently assessed and updated.

In the Plos one open access papers 'Do Fish Perceive Anaesthetics as Aversive' and 'Conditioned Place Avoidance of Zebrafish (Danio rerio) to Three Chemicals Used for Euthanasia and Anaesthesia' evidence is offered to suggest that in the presence of some chemicals at least, Zebra danios show a consistent aversion to some chemicals used in their humane destruction and sedation. This is especially relevant for researchers, given that around half of those 500,000 test subjects are Danio rerio.

In the first paper, tests were carried out using a range of potential sedative chemicals, including quinaldine sulphate, Isoeugenol, Benzocaine, MS222, 2 Phenoxyethanol, Propoxate, Lidocaine hydrochloride, 2,2,2 Tribomoethanol, and Etomidate.

Fish were offered a choice of swimming lanes, one free of chemical tainting, the other containing one of the anaesthetics to be tested (along with control substances, of course). For the fish lover the results weren’t promising.

At 50% of its effective dose rate, MS222 elicited aversive responses in Zebra danio, with the fish overwhelmingly preferring to spend their time in the ‘clean’ lanes. The implication of this is that not only can they detect the chemical, but it has some noxious quality that they wish to avoid. Given that some literature cites MS22 as an irritant, the role of the chemical as a ‘humane’ method of fish control is called into question.

In fact, of the nine tested substances, quinaldine sulphate, isoeugenol, benzocaine, MS222, 2 phenoxyethanol, propoxate, and lidocaine hydrochloride all resulted in aversion by the fish.

As everyday fishkeepers, we won’t have access to MS222 as it requires veterinary prescription, but for the home aquarist, the concern in all of this is the aversion to 2 phenoxyethanol. Those who have had to euthanise or anaesthetise fish in the past may have had dabblings with this chemical, under the guise of Aqua-Sed. It is, after all, one of the chemicals that has been long established as suitable for humane destruction.

During the tests, fish were shown to significantly increase their swimming speeds when subjected to 2 phenoxyethanol, which, as most aquarists will know, is suggestive of a classic 'fight or flight' response.

The conclusion of the research is that MS222 cannot be considered best practice for humane destruction of fish, and in addition the authors cite that there could be a better and more humane alternative to 2 phenoxyethanol. Further, they suggest that these chemicals should only be used in exceptional circumstances with Zebra danios.

So what are the options from here? Well, the second paper reports on similar experiments involving MS222, Aquacalm (a Canadian fish sedative made from metomidate hydrochloride), and that classic UK staple of clove oil. These were again tested against Zebra danios as the industry standard species.

This experiment involved preferential areas for fish in aquaria both with and without the stimuli of sedatives (the fish were trained first to swim through tunnels to either dark or light compartments, giving them a choice of sides where they were happier — in this case on the light side, with inherent aversion to the dark).

What was discovered was that despite the training, the fish quite staunchly refused to re-enter the usually preferred side of the set up once they associated that side with MS222. Even after the chemical had been removed, and the directly aversive stimuli no longer present, they opted to remain on the side that had previously been their non-preferred choice.

However, by comparison, when exposed to clove oil and Aquacalm, only a tiny, negligible minority of fish eschewed their formerly preferred side, suggesting that clove oil has considerably less ground as an aversive substance. That’s not to say that there’s no aversion at all, and in the former tests, utilising isoeugenol — a derivative of clove oil’s 'plainer' eugenol — still provoked an aversive reaction.

So do I sedate or not?

Well thankfully, none of us aside public aquarists need concern ourselves with MS222. Given the results as they stand, I would be keener at this stage to use clove oil (adequately mixed) to sedate or kill a fish than 2 phenoxyethanol, but this last chemical is still preferable to the plethora of barbaric methods still frequently touted on forums and ill researched websites. Either way, nothing is set in stone and further research will be required to see where this is all going.

From my own perspective, I would probably hold at this time that despite the revulsion factor involved, blunt trauma by an experienced aquarist followed by 'pithing' (destruction of the brain using a fine wire) is likely the most humane method of despatch currently available. But if you’re inexperienced, or dealing with something fiddly (how does one club the head of something like a Neon tetra?) then sedative is still likely the kindest thing you can do for your fish. It sure beats dying a slow death of bacterial necrosis or skin boring parasites, I’m sure…

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