The work carried out as part of this study aimed to reduce the overall number of zebrafish used in experiments by validating the use of the non-lethal measure of stress as well as to refine the way zebrafish are currently cared for in laboratories.
Zebrafish have become one of the world’s most experimented-on animal. They are ideal for research as they can be kept at high density, have relatively low maintenance costs, and have a fast generation time. With increasing numbers of fish being used in scientific studies, further research is required to ensure appropriate conditions are used to promote good conduct and correct housing as well as guaranteeing scientifically valid results.
Zebrafish are known to experience stress in a way that is directly comparable to humans. Under stressful conditions, the stress response is initiated leading to release of the stress hormone, cortisol, into the bloodstream. This, in turn, affects the fish physiology and behavior. Additionally, zebrafish are a naturally gregarious species and are known to experience isolation stress if separated from a group, however, after experimentation, they are often isolated for recovery.
The focus of our work was to determine whether individual fish which were exposed to a stressful experiment would recover faster if they were reintroduced to a familiar group or if they were allowed to recover in isolation. Furthermore, we wanted to validate a non-invasive measure of cortisol, water-borne cortisol, to show it is an accurate measure of physiological stress in zebrafish by comparing it to the lethal whole body cortisol measure usually used.
Fish were exposed to a stressor and then either returned to a group tank, put into a pair, or left in isolation to recover. They were then monitored for indicators of behavioral stress and water cortisol levels measured at 1, 2, and 3 hour time points, and whole body cortisol was collected at the end of the experiment.
There were clear behavioral differences between the individuals, with those in groups showing much faster recovery to normal stress levels by the end of the experiment. When we measured both water-borne and whole body cortisol levels, these also corresponded to the behavioral stress measures. Interestingly, individuals who were put into pairs after exposure to stress had the worst recovery. This is likely to be due to a disruption of the social hierarchy and the stressed individual being shown more aggression from its tank mate.
Taken as a whole, the results from this experiment indicate that group housing is important for zebrafish both before and after experimentation has taken place, with faster recovery seen in group-housed fish. This is not only important for the welfare of laboratory zebrafish but also for the results we can collect from them. If less stressed, these individuals will be displaying more natural behaviors and have a more normal physiology. In addition, we showed that there is a strong positive correlation between whole body and water-borne cortisol levels. This will allow checking the welfare of zebrafish in a non-lethal manner and reduces the number of individuals required for time series experiments.
These findings are described in the article entitled The impact of social context on behaviour and the recovery from welfare challenges in zebrafish, Danio rerio, recently published in the journal Animal Behaviour. This work was conducted by Lewis J. White, Jack S. Thomson, Kieran C. Pounder, and Lynne U. Sneddon from the University of Liverpool, and Robert C. Coleman from the University of Chester.