Cortisol is a readily measured bio-indicator of stress in most organisms, also fish. However, the blood cortisol level may be instantly influenced by handling-stress at sampling. Thus, quantification of cortisol in feces present the potential noninvasive measurement. In the past two decades, the analysis of cortisol levels in extracts of faeces has been reported by few authors, but so far with limited success.
The aim of this study was to determine the appropriate measurement of cortisol in salmon feces and plasma sample using an enzyme-linked immunosorbent assay (ELISA).
In this study, 10 salmons were achieved monthly from the same local salmon farm net pen for almost one year. Both feces and blood samples were collected and centrifuged and the supernatants of feces and the plasma were applied for comparative tests. Two commercially available ELISA kits were used for cortisol evaluation. Cortisol was extracted using ethyl ether according to the instructions. Both extracted blood and feces samples and raw samples were tested for cortisol level.
The preliminary results using both kits showed that the cortisol in plasma could be enriched by the extraction procedure, but the cortisol in feces could not. In the supernatant of feces there is much higher level of cortisol than in the extracts. In the following experiments, the cortisol was tested directly in supernatant of feces and extracts of plasma. The mean value of cortisol every month was 7.8-17.9 ng/ml in feces and 7.9-51.4 ng/ml in plasma. During the month with serious disease problems, the average cortisol level in plasma was significantly elevated to 246.3 ng/ml in feces and 164.4 ng/ml in plasma.
The results of this study indicated, the supernatant of salmon feces could be a proper resource for direct cortisol test using a ELISA method. Furthermore, the cortisol in feces might reflect the real stress level in salmon, not the instant release of cortisol in blood related to handling. In addition the evaluation of feces is non-intrusive compared with the blood test.