Stress in farmed fish is associated with poor feeding, slow growth, disease, and mortality. Hence, fish stress level monitoring is critical to improve aquaculture productivity and welfare. The stress hormone, cortisol, is a reliable indicator of fish stress; however, blood sampling is highly invasive. Alternative sampling methods, such as fin, scale, mucus, as well as cortisol released into the surrounding water, have been suggested as less-invasive means to assess fish stress. As part of this project we seek to understand the relationship between cortisol in these different biological samples, and in the water, in response to a stress challenge in Asian Sea bass (Lates calcarifer). Initial results using handling stress demonstrate an expected increase in plasma, fin, and mucosal cortisol peaking within 2-4 hrs. Water cortisol analysis is still ongoing. Further studies will manipulate water quality and stocking density to determine their effects on Asian Sea bass cortisol release and assess the reliability of water cortisol as a stress indicator.
A stress trial was conducted in a 9000 L recirculating system (3000 L tank) with 35 g Lates calcarifer at stocking densities of 3.28 kg/m3. Water (100 ml) and biological samples from 4-5 fish were sampled during and after handling stress (chasing and air exposure) (Fig 1). Samples were tested via ELISA (Cusabio kit). In a pilot experiment, we observed a post-stress elevation of plasma, fin, and mucus cortisol, peaking between 2-4 hrs (Fig. 1). Mucus cortisol stayed significantly elevated at 24 hrs, unlike plasma and fin. Scale cortisol did not show significant changes. Finally, plasma cortisol is most strongly correlated to fin cortisol on a fish-by-fish basis (R = 0.56). Overall, we show that fin and mucus cortisol are feasible as less-invasive cortisol sampling methods. Water cortisol analysis by ELISA and HPLC is ongoing. Further investigation will be performed to optimize cortisol sampling and quantification, and to establish the relationship between water and tissue cortisol with different stressors, including disease, water quality, and stocking density.