Cortisol concentration in scales is a valid indicator for the assessment of chronic stress in European sea bass, Dicentrarchus labrax L

Abstract Chronic stress is known to impair fish health, performance, and welfare. Although physiological responses to acute stress have been well studied and reliable as well as easily quantified indicators, like circulating cortisol, are available in fish, the same is not true for chronic stress. Specifically, commonly used stress indicators cannot reliably identify the effects of chronic stress in fish. Recently, cortisol concentration in fish scales has been proposed as a novel marker of chronic stress. The purpose of the present study was to verify whether circulating physiological stress indicators and cortisol concentrations in fish scales are reliable markers of chronic stress in European sea bass. For that reason, a predictable, repeated chronic stress protocol was developed and applied. Results showed that, in the absence of an additional acute stressor, physiological indicators like plasma cortisol, glucose, lactate and osmolality were not able to differentiate non-stressed from chronically stressed fish. Only when fish were exposed to an additional stressor, cortisol response of the chronically stressed fish was blunted compared to controls. On the contrary, cortisol concentration in scales differentiate non-stressed from chronically stressed fish, being significantly higher in the later. The same was observed when previously identified as High cortisol Responding (HR) fish, but not Low Responding (LR) fish were tested. Collectively, current results confirm that circulating levels of stress indicators cannot mark chronic stress in E. sea bass, while cortisol concentration in fish scales is able to identify chronic stress situation.