Should the ice slurry method be used to euthanase fish?

I had put a few posts in the past about recommended methods for fish euthanasia.

I was alarmed when I came across this article that was published only as recently as 2010 in the Journal of Fish Biology –

Humane killing of fishes for scientific research: a comparison of two methods. In the abstract it concluded that ice-slurry immersion was more humane than benzocaine overdose and that it should be accepted as a standard humane method for the species used and for other warm-water species. But when you read the paper, there are a few things I have major reservations about.

One of the opening statements was –

The aim was, therefore, to compare the behavioural stress response and the associated time taken to lose equilibrium and reach the death endpoint by a small to medium bodied Australian freshwater Clupeid bony bream (Nematolosa erebi),…..

Anyone who deals with anaesthesia should be aware that there are planes of anaesthesia and that the “excitatory phase” is one of the planes of anaesthesia. It’s not a behavioural stress response. It can be avoided by using higher doses of anaesthetic so that this phase can be shortened or “bypassed”. Behavioural stress response is not an adequate measure to come to such a conclusion. Heart rate and cortisol levels are some of the methods we could be using.

The paper also states the reason why several other authors have found disfavour with the ice slurry method:

The traditional method of immersing fish in an ice-slurry to achieve death by hypothermia relies on lowering the core body temperature resulting in anaesthesia and subsequent death. This has been approved by some fish euthanasia guidelines (Barker et al., 2002), but rejected by others (e.g. European Commission, DGXI – Working Party, 1996; Barker et al., 2002; Reilly et al., 2001; European Commission, DGXI – Working Party, 1997). Rejections of the method have been based firstly, on evidence that the method potentially causes an initial period of discomfort due to ice crystal formation both on the skin and within the body (Reilly et al., 2001), and secondly on a potentially long treatment time before achieving death (Van De Vis et al., 2003) because of the poor thermal conductivity of tissues surrounding the brain (European Commission, DGXI – Working Party, 1996). 

It sounds like the authors found good evidence to suggest that the ice slurry method should be aborted, but chose to discount the evidence.

The paper goes on to say…

It should be noted, however, that studies of the use of ice-slurry for fish slaughter (in the food industry) have often been undertaken on large cold-water species from the Northern Hemisphere, such as turbot (Psetta maxima), gilt-headed sea bream (Sparus auratus) and Atlantic salmon (Salmo salar) (Van De Vis et al., 2003; Morzel et al., 2003). Current Australian guidelines are based on these northern hemisphere studies (Reilly et al., 2001), but it is not clear if such assumptions are universally valid, particularly for small to medium bodied, warm-water fish. 

According to fishbase, the Australian freshwater Clupeid bony bream (Nematolosa erebi) is a subtropical species and has a preferred water temperature range of 15 – 25 degrees Celsius. Subtropical fish should by no means be considered “warm-water fish”. Its tolerance range is not shown on the database but I’d expect it to be much lower than this.

In conclusion,

• I would not support the ice slurry method of fish euthanasia.
• I am appalled that this paper passed the peer review process to become published.
• It pays to read the article in its entirety to check out its methodology before believing.

I shall dismount from my high horse now and get back to more menial tasks that I have to do today.