Caged seaweed aquaculture as habitats for epifauna: A case study demonstrating the importance of cages rather than cultured seaweed

Seaweed aquaculture can have various environmental effects, with the role of providing habitat for surrounding organisms being of growing interest. Aquaculture systems typically consist of components such as ropes, cages, and cultivated seaweed itself. Each component possesses distinct characteristics and attributes, potentially influencing its function as a habitat. However, limited research has explored the differences in habitat provision among these various components. We conducted a field experiment to assess the habitat function of a seaweed aquaculture system involving the red alga Solieria pacifica, comparing the effects of ropes, cages, and cultured seaweed. Three treatments were tested: (1) a rope suspended without additional structure, (2) a rope suspended with an empty cage, and (3) a rope suspended with a cage containing cultured seaweed, representing the typical aquaculture setup. In all the treatments and sampling months, epifaunal assemblages are generally predominated by tube-dwelling amphipod crustaceans. Epifaunal abundance and diversity were significantly greater in the treatment with the rope and empty cage compared to the rope-only treatment, suggesting that cages provide an important habitat for epifauna. Interestingly, there was no significant difference in epifaunal diversity or assemblage structure between treatments with and without cultured seaweed. Furthermore, epifaunal abundance was higher in the absence of seaweed, indicating that cultured seaweed had limited habitat function and may even deter some epifauna. This case study offers valuable insights, demonstrating that structurally complex cages can provide more significant habitats for epifauna than the cultivated seaweed itself, highlighting an important consideration in aquaculture design.