Reef effect of vertical relief on the shelter-seeking and foraging processes of juvenile fat greenling (Hexagrammos otakii) in a mesocosm experiment

Artificial reefs with sufficient vertical relief can support secondary fish production by attracting pelagic fishes and/or providing both shelter and food resources for them. Although vertical relief is a widely investigated factor in reef design, little is known about how the shelter-seeking and foraging processes of benthic fishes respond to relief changes. With a laboratory mesocosm experiment using hollow blocks to construct different combinations of reef volume and relief, we tested the effect of vertical relief of artificial reefs on a benthic juvenile fish (fat greenling, Hexagrammos otakii). The volume of the experimental reef depended on the number of blocks, and the reef relief was changed by the stacking pattern of the blocks at each volume. We measured the aggregation, sheltering, and foraging effects of each reef structure, respectively, based on fish distribution around the reef, their shelter-seeking behavior under disturbance, and their foraging rate. Our results demonstrate that increasing the vertical relief while maintaining the reef volume had a positive effect on the aggregating juvenile fat greenling but decreased the sheltering effect of the reef. The relationship between the foraging effect and relief was nonlinear, and high relief would also limit the foraging rate of juvenile fat greenling. We suggest that a reef needs to have sufficient relief to provide habitat for juvenile fat greenling, as well as sufficient horizontal cover to shelter the fish during a disturbance. Not only limited resources (e.g., food and shelter) but also the shelter-seeking and foraging processes of the benthic fish could have a potential impact on secondary production. Our findings highlight the importance of considering the aggregation, sheltering, and foraging effects of relief on benthic fishes in the field observation and the design of artificial reefs.