Zebrafish have become an increasingly popular model organism in basic biological research. According to the European Commission's latest report on the number of animals used in research in the EU, the number of fish increased by 28.5% from 2008 to 2011 (310,307 more fish)  . The UK Home Office reported that fish were the second most used group of animals in research in 2015 in the UK (14%, 294,000 procedures), with zebrafish representing 50% of all fish species . The growing use of fish in research is often regarded as a major achievement to replace mammalian model organisms, notably rodents . However, the increasing relevance of zebrafish as an animal model creates an urgent need for techniques and methods that guarantee that any research with zebrafish can be carried out according to the same scientific and animal welfare standards as the laboratory rodents they are often replacing.
Legal methods to euthanize fish
From the moment zebrafish become free‐feeding larvae, any intervention on them is regulated by EU legislation. This means that any procedure done in these animals from about 5 days after fertilization must comply with the EU Directive 2010/63/EU on the protection of animals used for scientific purposes . This includes euthanizing animals, which are no longer needed, whose suffering needs to be ended or whose organs are collected for further analysis. The European Directive lists only three methods for humanely killing fish: anesthetic overdose, concussion, or electrical stunning. Other methods can be used in unconscious animals.
Each of these methods has clear limitations for research and for animal welfare. The zebrafish's small size makes efficient …