Anaesthetic performance of AQUI-S and 2-phenoxyethanol in fish for field applications

Abstract Ethical handling of fish in ecological research requires anaesthetic methods that are effective, practical, and adaptable for a diversity of field conditions. Estuarine environments, in particular, present challenges due to their fluctuating water chemistry. To improve our understanding of anaesthetic performance for field conditions, we compared AQUI-S, a commonly used clove oil-based anaesthetic, and 2-phenoxyethanol, a cost-effective alternative. First, we tested their influence on water pH across a range of baseline pH levels. We then conducted controlled trials on a representative Australian estuarine species (common hardyhead; Atherinomorus vaigiensis) , assessing induction, recovery, and euthanasia outcomes across increasing concentrations. Finally, we synthesised literature data across 49 species to assess broad trends and model environmental and biological drivers of anaesthetic response. AQUI-S induced anaesthesia rapidly but caused slower and more variable recovery, and slight declines in water pH. In contrast, 2-phenoxyethanol produced slower induction but offered faster, more consistent recovery, a broader safety margin (i.e., the difference between induction and euthanasia doses), and no measurable effect on water pH. Our multi-species models indicated that temperature and life history stage modulate AQUI-S responses, while pH influences the induction times of 2-phenoxyethanol. While both AQUI-S and 2-phenoxyethanol are effective anaesthetic agents, trade-offs between induction speed, recovery consistency, and environmental sensitivity can guide their optimal application for field research. Graphical abstract