Suitability of clove oil for the euthanasia of four ornamental fish species: Goldfish ( Carassius auratus ), Bristlenose catfish ( Ancistrus sp.), Midas cichlid ( Amphilophus citrinellus ) and Three-spot gourami ( Trichopodus trichopterus )

Euthanasia of ornamental fish is usually performed by immersion in an anaesthetic bath. However, it may be difficult for private fish owners to access anaesthetics, and alternative methods are limited. Clove oil is commonly available in drugstores and its use for fish euthanasia has been reported. However, there is a lack of scientific literature supporting its suitability for this purpose and of available species-specific protocols. Our study aimed to assess the suitability of clove oil to euthanise goldfish ( Carassius auratus ), bristlenose catfish ( Ancistrus sp.), Midas cichlid ( Amphilophus citrinellus ) and three-spot gourami ( Trichopodus trichopterus ). Time to loss of reflexes and to cessation of opercular movements were recorded. Aversive reactions and histological gill lesions were compared with positive and negative controls, and reversibility of euthanasia attempts was assessed. Opercular movements did not reliably cease in goldfish, even under high clove oil concentrations (1000 mg/L). Clove oil caused loss of indicators in Midas cichlid at a concentration of 1000 mg/L, and in bristlenose catfish and three-spot gourami at a concentration of 400 mg/L. Five Midas cichlid recovered after treatment with clove oil. Clove oil caused severe gill lesions to goldfish and Midas cichlids. The results show that clove oil is not suitable for humane euthanasia of goldfish and Midas cichlid. Additionally, some studies have questioned the ability of clove oil to induce loss of consciousness in fish. Therefore, we recommend avoiding euthanasia with clove oil when suitable alternatives are available and using a secondary killing method, as euthanasia attempts are not always irreversible.