Use of muscle activity indices as a relative measure of well-being in cultured sea bass Dicentrarchus labrax (Linnaeus, 1758)

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Aquaculture of sea bass is widely spread in the Mediterranean and employs a variety of husbandry protocols that need to be evaluated in terms of fish well-being. Behavioural tests can be used as operational indicators of short-term stress, because changes in swimming performance and/or muscle activity (e.g. electromyograms) can be interpreted as response associated with a wide variety of stressors. Diagnostic procedures, based on physiological telemetry, will thus enable appropriate mitigative strategies to be implemented to ensure the well-being of cultured fish. The objective of this study was to examine the contribution of two muscle types to the swimming activity of sea bass (Dicentrarchus labrax, L. 1758). Hard-wire technology was used to ‘fine-tune’ measures obtained using a physiological transmitter. Fine-tuning showed that the aerobic muscle displayed an increase in recruitment of muscle fibres relative to increasing swimming speed up to 0.6–0.7 ms−1 of the Ucrit, where the anaerobic (white muscle) activity started to exponentially augment with swimming speed, reaching up to ∼7 times its initial value. Intensity of electromyogram signals were described by logarithmic (red muscle): y = 0.5922Ln(x) + 1.2251 (R 2 = 0.9906) and exponential (white muscle): y = 0.0977e2.4723x (R 2 = 0.9845) relationships. Fine-tuning indicated that the two muscle types in the sea bass are involved in fuelling swimming activity below the Ucrit. Thus, scope for activity is not supported solely using aerobic metabolism, though the red muscle powers the majority of the swimming ability. Measurement of Ucrit displayed an average value of 3.43 BLsec−1 (SE = 0.12). Associated EMG values measured during the forced swim trials using an implanted bio-sensitive radio transmitter showed that EMG’s intensity increased, on average, 3.2 times between 0.2 msec−1 and the Ucrit velocity (∼1 msec−1). Above EMG values were fine-tuned using estimates obtained from direct monitoring of the red and white musculature. Overall, the results demonstrated that the scope for activity, previously thought to represent only aerobic metabolism, is composed of both aerobic and anaerobic pathways. Fine-tuning of physiological transmitters to measure activity of free ranging fish can therefore be utilised to monitor the proportion of the scope of activity utilised in response to external stressors. This proportion and the level of compensatory ability remaining could represent a measure of well-being in cultured fish.