Chronic handling stress in zebrafish Danio rerio husbandry

The zebrafish (Danio rerio, Hamilton, 1822) is one of the most important fish model species in scientific research, with millions of fish housed in laboratory animal facilities around the world. During husbandry, it is necessary to regularly handle the fish, which could cause short- and long-term stress, possibly affecting both fish welfare and experimental outcomes. In two experiments, the authors studied effects of transferring adult zebrafish, by chasing them with a net and/or exposing them to air (netting) on different endpoints: cortisol levels, reproduction parameters and behavioural parameters. They used realistic chase and air-exposure times to mimic normal zebrafish husbandry and investigated the potential to habituate to handling stressors. Finally, the potential welfare improvements of a nutritional reward after handling were studied. All types of handling induced a stress response, but the authors did not find a correlation with the intensity of the stressor. Realistic (short) handling routines also caused stress, both after the first time and after regular handling over a long period of time. Cortisol levels peaked after 15 min, were still elevated after 30 min and dropped to resting level after 60 min. This should be taken into account by researchers when carrying out measurements or behavioural trials within an hour after handling. There is a minor potential benefit of nutritional rewards that may contribute to a faster recovery of normal behaviour. They did not find evidence of habituation to chasing and netting stress. Taking the stress response after handling into consideration will improve fish welfare and health and minimise husbandry-associated sources of variation.