Mucosal defences of salmonids in complex RAS environments

Defences at mucosal sites play a critical role in forming robust primary barriers of fish. Because of their close contact with the aquatic environment, these mucosal immune structures and mechanisms are under constant pressure from environmental challenges, especially in intensive aquaculture systems. As recirculating aquaculture systems (RAS) become more widely adopted, there is increasing interest in understanding how these controlled, complex, land-based environments affect mucosal defences in cultured species, particularly salmonids. Modern intensive RAS are highly intricate systems characterised by minimal water exchange and extensive water treatment processes to maintain water quality that supports the biological requirements of fish. However, the unique and dynamic nature of RAS environments exposes fish to a suite of physical, chemical, and microbial pressures that directly affect mucosal surfaces, including skin, gills, gut and olfactory organ. This review consolidates current knowledge on the primary environmental variables monitored in RAS and evaluates their known and potential impacts on the mucosal defences in fish, focusing on salmonids. While the health impacts of these parameters are documented to some extent, the specific consequences for mucosal health are unclear. The majority of these studies have focused on mucosal defences in the gills, with limited understanding of other mucosal organs. Moreover, current knowledge is largely based on histological evaluations, making it challenging to draw functional conclusions about the immunological processes and mechanisms influenced by RAS-related environmental factors. Given the integral role of mucosal barriers in disease resistance and immunity, deepening our understanding of how the RAS environment influences these surfaces is crucial for optimising production practices. As direct interfaces with the environment, mucosal surfaces provide valuable insights into rearing conditions, making their interaction with the RAS environment essential to ensuring fish health and welfare in intensive aquaculture.