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With the rapid development of aquaculture and the gradual decrease in fishery resources, aquatic animal welfare (AAW) has received increasing attention from animal protection organisations, government departments, biologists, fish culturists and consumers. Although environmental enrichment (EE) can potentially benefit AAW, the relevant results are sharply mixed, and the drivers for these discrepancies are largely unclear. In this study, we conducted a series of meta‐analyses to overcome this knowledge gap. We firstly compiled a full data set, consisting of 1171 effect sizes from 147 studies across 82 species and then performed a multilevel mixed‐effects model to evaluate the overall effect size and conduct the subsequent meta‐regression analyses. Overall, our results showed that aquatic animals from a physically enriched environment had significantly improved AAW compared with their counterparts from barren environments. For moderators, specific welfare traits, animal taxa, animal stages, enrichment types and enrichment colours showed strong positive EE effects on AAW. Subsequently, we constructed a fish data set and reran the meta‐analysis, yielding results substantially similar to those obtained using the full data set. Finally, we performed a multi‐model inference to identify the importance ranking of potential moderators. Results showed that enrichment type, animal taxon and welfare category are the best moderators of the direction and magnitude of EE effects on AAW. These results provide insights into the possible drivers of EE effects on aquatic animals with important implications for aquaculture, fisheries, conservation, research and aquarium, providing evidence‐based guidance for future animal welfare theories and practices.

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Intraspecific aggression is detrimental to body/fin damage, physiological stress, and other problems in aquaculture. Environmental enrichment has been proposed to have positive effects on fish aggressive behavior, physiological stress, and fish welfare, but there are mixed results. Here, we examine the impact of physical enrichment levels (i.e., the intensity of physical enrichment) on aggression in black rockfish (Sebastes schlegelii) and fat greenling (Hexagrammos otakii). Generally, with the increase in the enrichment level, the frequency of the aggressive behavior of black rockfish gradually decreased. In contrast, a non-monotonous effect of the enrichment level on aggression was observed for fat greenling, with low and intermediate levels leading to no or more aggression, while a high enrichment level reduced aggression. After three days, the high-level enrichment groups in both rockfish and greenling reached social stability (i.e., a relatively stable social structure indicated by lower aggression), while aggression in the other groups continued increased. These results show the significant regulatory effect of enrichment levels on the aggressive behavior in both black rockfish and fat greenling. This study may promote the development of environmental enrichment measures, and it provides useful information for reducing fish aggression and improving fish welfare in aquaculture.

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