Is environmental enrichment beneficial for laboratory animals? A systematic review of studies in zebrafish

Environmental enrichment (EE) consists of a series of interventions that are carried out in the home environment to provide greater exposure to sensory stimuli with the objective of mimicking the natural habitat for the animals housed in the laboratory, offering a more complex environment like those found in nature. Some studies have shown the positive effects of EE on zebrafish housed in a laboratory environment. However, this evidence is still very recent and accompanied by contradictory results. Furthermore, there is great variability in the protocols applied, and in the conditions of the tests, tanks, and materials used for creating an EE environment. This substantial variability can bring many uncertainties to the development of future studies and hinder the reproducibility and replicability of research. In this context, the main objective of this study was to carry out a systematic review of the literature aiming to provide an overview of the EE protocols used in zebrafish. We conducted a systematic review following PRISMA recommendations. The literature search was performed in PubMed, Scopus, and Web of Science databases and the studies were selected based on inclusion/exclusion criteria. We performed data extraction and risk of bias analysis of the studies included. A total of 901 articles were identified in the databases and 27 of these studies were included in this review. Among these studies, the effect of EE was evaluated as two different proposals. (1) to improve animal welfare and (2) as an intervention for the prevention of some disorders. Although the zebrafish EE protocols presented a series of experimental differences, the results showed that the benefits of the EE for zebrafish were robust. According to the results described here, the use of EE in the zebrafish home tank provides better welfare and may reduce sources of bias in scientific experiments, such as high-stress levels and fighting events.