Physical enrichment for improving welfare in fish aquaculture and fitness of stocking fish: A review of fundamentals, mechanisms and applications

In recent years, the issue of fish welfare in various aquaculture systems has garnered significant attention from animal protection organizations, government departments, conservation biologists, fish culturists, and the public. Fish welfare can be broadly classified into captive welfare, post-release fitness, and mental state in contexts of aquaculture and hatchery release. Multiple methods have been proposed to enhance specific aspects of fish welfare, and environmental enrichment (EE) has demonstrated immense potential for improving all three welfare aspects. In this review, we provide an overview of state-of-the-art research on the impact of physical enrichment (PE) on fish welfare, and subsequently analyze the potential mechanisms underlying the effects of PE, and suggest future research directions. Regarding fish captive welfare, most studies report positive results of PE, such as reduced aggressive behavior, blunted stress response, and increased growth performance. However, some studies have also reported no or negative effects. We discuss the sources of these discrepancies and further analyze the potential mechanisms of PE effects on aggression, metabolism, and growth. While preliminary studies have shown promising positive effects, research on the impact of PE on disease resistance and reproductive success in fish is limited. For fish post-release fitness, PE has exhibited great potential for improving the development of natural behaviors and increasing the individual fitness of released fish. However, the results are mixed, and most PE strategies that present improved effects on fish post-release fitness often include several EE types, with very few studies showing positive results from solely conducting PE. This review also provides an in-depth analysis of the potential mechanisms underlying the improved post-release fitness of fish, from the perspectives of adaptation, morphology, behavior, and physiology. Additionally, we discuss the possible existence of a sensitive period by analyzing the time-dependent effects of PE on fish behavior. The positive impact of PE on fish mental state has been consistently reported across studies. We also discuss potential issues associated with PE and present a decision-making framework to guide practical application and production. The synthesized knowledge and novel regulatory mechanisms presented in this review will contribute to the advancement of EE strategy and fish welfare theory in the aquaculture industry and hatchery release projects.