Stress assessment is crucial in aquaculture to ensure and improve the health and well-being of production animals, including fish. Assessing fecal cortisol and its metabolites (FCMs) is a less invasive and more labor-efficient method for monitoring the overall health and welfare of salmonids compared to plasma cortisol (PC) assessment. Determining the appropriate sampling timeframe is essential for accurate measurement . The primary objective of this study was to compare the results of our serial studies to further investigate the optimal sampling times for FCMs in farmed Atlantic salmon of different sizes. Blood, intestinal contents, and feces samples were collected before, during, and after a stress-inducing event, including net cleaning, sea lice counting and chasing . The PC and its metabolite levels in the mid-intestinal contents (MICMs) and FCMs were assessed by an enzyme-linked immunosorbent assay (ELISA) and LC-MS/MS techniques.
In a group of Atlantic salmon (1 kg) , blood, intestinal contents, or feces samples were collected before, during, and at 2 and 20 hours after a net cleaning event. The results showed a sequential increase in PC levels and FCMs following net cleaning. The elevation of FCMs was not observed at 2 hours , with a peak occurring at 20 hours post-net cleaning. There was no strong correlation between PC and FCM due to different time lag after acute stress.
Next, a fish group with an average weight of 438 grams was sampled before and at 1, 2, 4, 6, 12, 24, 48, and 72 hours after netting and relocation. The results showed that peak feces levels were observed 24 hours post-netting and relocation.
Both results underscore the importance of considering delayed and extended sampling patterns when evaluating acute stress with FCMs in aquatic environments. They suggest that 24 hours post-handling is an optimal time window for FCMs sampling in fish over 400 grams.
Conversely, in a fish group averaging 82 grams, an elevat ion in FCM levels were detected 40 minutes after chasing. FCM levels showed a significant correlation with PC levels in these fish. In another study, a fish group averaging 188 grams was subjected to suspected extra strain during a one-hour period of capture, sedation, and transport. After two hours of sampling, simultaneous elevations in PC and FCMs were observed in the same fish.
The most notable finding from these studies is that the secretion of FCMs exhibits a significant size-dependent pattern . In Atlantic salmon over 400 grams, a 24-hour delay for feces sampling was recommended for FCMs testing following acute stress. For fish under 400 grams, especially those under 200 grams, sampling might need to start earlier, and the sampling window appears narrower. Further investigation is required, considering fish size, temperature, and feeding regime.