Standard metabolic rate and daily activity patterns of Australoheros facetus (Jenyns, 1842)

We provide first-time estimates of the standard metabolic rates (SMR) of Australoheros facetus , a freshwater cichlid fish that lives in streams and lakes in South America. Individuals of A. facetus were sourced from the Naposta Grande river, located in southern Buenos Aires province, Argentina, during spring (September–November in the southern hemisphere) of 2023. The individuals of A. facetus were captured using traps, baited and set close to submerged vegetation on the margins of the river at shallow depths. The traps were left undisturbed for up to 45 minutes. Traps were then pulled up by hand, and the individuals were collected and transferred to aerated transportation tanks. The fish were allowed to recover in these acclimation tanks from the stress originated from the capture and transport for 1 month. Experiments were conducted following the guidelines established by the EU Directive 2010/63/EU and the Argentinean Law n° 14346 on “The protection of animals”, as well as recommendations of the institutional committee for the use and care of laboratory animals (CICUAL). We quantified SMR and aerobic scope and daily activity patterns, finding this fish to be active during the day and sluggish during the night. Its metabolic rate is within the ranges expected for cichlids (SMR: 104.48 mg O2 kg–1h–1) and increases slightly during the daylight hours (126.25 mg O2 kg–1h–1). Having information on the metabolic rates and aerobic scope of this species provides another piece to the puzzle of the biology of A. facetus , complementing the research on oxidative stress and social behavior that has already been published and future research efforts. Furthermore, it constitutes progress towards the development of predictive models on the effects of increasing water temperatures in a global warming scenario.