The Value of a Net‐Cage as a Fish Aggregating Device in Southern California

There is an urgent need for primary data that can be used to quantify the value of marine aquaculture facilities that also describe influences on the surrounding natural ecosystem and its wild fish communities. Divers completed 360 transect replicates below a net‐cage and at nearby and distant rocky reefs off Catalina Island, California, estimating the species abundance and size class of all conspicuous fishes in the water column. We observed 10,234 fishes aggregating below the net‐cage with a mean annual density of 142 (SE ± 30) per 100 m² and diversity H′ 2.29. At the adjacent reference reef, we counted 8452 fishes with a mean annual density of 117 (SE ± 20) and H′ 1.45 and at Reference Reef 2, located 500 m away, there were 8958 fishes with a density of 124 (SE ± 20) and H′ 1.13. The total density of fishes was the same between the study sites (ANOVA; P = 0.24); however, the spatial density of fishes in the water column was different between the net‐cage and reference reefs. Additionally, the density of juvenile fish was significantly lower at the net‐cage (ANOVA; P = 0.02), while the density of subadult and adult fishes was higher (ANOVA; P = 0.03). Feeding guild analysis ranked the net‐cage highest (3507), while Reference Reef 1 (2149), and Reference Reef 2 (2241), had lower values, a relationship often found between artificial and natural reefs. This suggests the importance of available trophic resources to the adult population of wild fishes, and has implications in the artificial reef attraction–production debate. The results of this study not only reproduce a pattern of fish attraction to net‐cages in the marine environment reported in previous studies but also demonstrate a relative habitat value compared in both time and space, of the fish community at a net‐cage compared with vibrant kelp bed communities.