Understanding spatial heterogeneity of rocky reef demersal fish assemblages: Links to conservation plans in the Juan Fernández Archipelago Marine Protected Area in the Southeastern Pacific.

Spatial heterogeneity linked to fish assemblages has been key to advancing conservation plans and assessing marine resources in remote ecosystems. The Juan Fernández Archipelago is a remote and highly heterogeneous geologic hotspot located on the Nazca plate (SE Pacific), comprising three islands, numerous seamounts and guyots, and a highly rugged coastal marine shelf. Hundreds of rocky reefs of various sizes characterize this insular coastal shelf, supporting diverse finfish populations of interest for the Robinson Crusoe Island artisanal fishing fleet. Two hydroacoustic surveys were conducted on the insular shelf of Robinson Crusoe and Santa Clara Islands. Reef distribution and morphometric attributes (length, height, depth strata, and geographic zones) were characterized across the shelf seascape. To identify traits that correlated with the presence and acoustic backscatter of demersal fish aggregations, a set of statistical models was implemented. Results indicate that reefs were present up to ~200 meters in depth around the islands, with the northern region hosting small and tall reefs, while the southern region had the fewest reefs but the largest. The findings indicated that 75% of the fish aggregations observed outside of reefs were within 280 m of them. Also, there was a higher probability of fish aggregations occurring in depths up to 150 m (~90%) given the presence of reefs, compared to the relatively flat seafloor (~20%). Reef-scale analysis showed that the longer and taller the reef, the higher the acoustic backscatter from fish. These findings highlight the ecological value of reefs for fish populations throughout the insular shelf seascape. In addition, this study highlights the usefulness of small-scale hydroacoustic surveys as a monitoring tool to assess the association between ocean-floor spatial heterogeneity and demersal fish assemblages, and the implementation of effective periodic surveys within the Juan Fernández Marine Protected Area.