Effectiveness of eugenol sedation to reduce the metabolic rates of cool and warm water fish at high loading densities

Effects of eugenol (AQUI-S^®20E, 10% active eugenol) sedation on cool water, yellow perch Perca flavescens (Mitchill), and warm water, Nile tilapia Oreochromis niloticus L. fish metabolic rates were assessed. Both species were exposed to 0, 10, 20 and 30 mg L⁻¹ eugenol using static respirometry. In 17°C water and loading densities of 60, 120 and 240 g L⁻¹, yellow perch controls (0 mg L⁻¹ eugenol) had metabolic rates of 329.6–400.0 mg O₂ kg⁻¹ h⁻¹, while yellow perch exposed to 20 and 30 mg L⁻¹ eugenol had significantly reduced metabolic rates of 258.4–325.6 and 189.1–271.0 mg O₂ kg⁻¹ h⁻¹ respectively. Nile tilapia exposed to 30 mg L⁻¹ eugenol had a significantly reduced metabolic rate (424.5 ± 42.3 mg O₂ kg⁻¹ h⁻¹) relative to the 0 mg L⁻¹ eugenol control (546.6 ± 53.5 mg O₂ kg⁻¹ h⁻¹) at a loading density of 120 g L⁻¹ in 22°C water. No significant differences in metabolic rates for Nile tilapia were found at 240 or 360 g L⁻¹ loading densities when exposed to eugenol. Results suggest that eugenol sedation may benefit yellow perch welfare at high densities (e.g. live transport) due to a reduction in metabolic rates, while further research is needed to assess the benefits of eugenol sedation on Nile tilapia at high loading densities.