Whole-body cortisol is an indicator of crowding stress in adult zebrafish, Danio rerio

Plasma cortisol levels have been used to evaluate the stress response in a variety of cultured fish species. However, little is known about the stress response of zebrafish, Danio rerio, despite its extensive use as a laboratory research organism. Due to its small size, evaluation of whole-body cortisol has provided a means to assess zebrafish stress levels Understanding the role of crowding on zebrafish whole-body cortisol would allow researchers to optimize various rearing procedures. The aim of this study was to determine the zebrafish whole-body cortisol response to crowding stress. Baseline cortisol values have not been established for zebrafish; therefore, we compared specific treatment groups acclimated to similar conditions. We crowded zebrafish at a high density (40 fish/L) for 3 h (acute stress), or 5 days (chronic stress). Crowding resulted in a four-fold increase in mean whole-body cortisol level in both groups compared to zebrafish maintained at a much lower density (0.25 fish/L). Additional experiments demonstrated that the cortisol response to crowding was modulated by fasting, feeding and density. In large glass aquaria (76 L), fasted, crowded fish (40 fish/L) had significantly higher cortisol compared to fasted, control fish (0.2 fish/L). Furthermore, weight was inversely related to cortisol level in fasted, crowded fish held in large glass aquaria. For fed fish, crowding did not significantly increase cortisol level, suggesting an interaction between feeding and crowding. In small tanks (4 L), crowding (40 fish/L) did not significantly increase cortisol compared to control fish held at 4 fish/L. Our results suggest that whole-body cortisol is a useful indicator of crowding stress in fasted, adult zebrafish. Understanding how crowding and other environmental conditions affect zebrafish fitness could aid in optimizing zebrafish growth, health and reproduction as well as improving the consistency and reproducibility of in vivo studies that use this popular vertebrate model.