The Relationship Between Respiration Rates and Electron Transport System Activity in Fish

Fishes contribute to the biological carbon pump, yet their overall role remains poorly constrained due to the difficulty of obtaining direct metabolic measurements and, consequently, is poorly understood. Electron transport system (ETS) activity is commonly used as a proxy for potential respiration, but its application requires an appropriate relationship between respiration (R, measured as oxygen consumption MO2) and ETS activity. Here, we examined the relationship between swimming activity, oxygen consumption, and ETS activity in juvenile Sparus aurata using swimming-tunnel respirometry. Oxygen consumption increased with swimming speed following a four-parameter sigmoidal model, whereas ETS activity remained independent of short-term changes in activity. Normalizing respiration by ETS produced R/ETS ratios ranging from 0.17 to 0.71, values consistent with those reported for zooplankton and micronekton. Lower ratios correspond to minimal aerobic demand and may represent quiescent behaviour, while higher ratios reflect elevated demands associated with active movement or feeding. These ratios are suggested for the assessment of respiration rates from ETS activity during diel vertical migration in the ocean to improve estimates of respiratory flux. However, methodological issues related to ETS activity in different body regions must be solved to enable reliable measurements.