Aim: Species ranges provide a valuable foundation for resolving biogeographic regions, evolutionary processes, and extinction risks. To inform conservation priorities, here we develop the first bioregionalization based on reef fish abundance of the Hawaiian Archipelago, which spans nearly 10° of latitude across 2,400 km, including 8 high volcanic islands in the populated main Hawaiian Islands (MHI), and 10 low islands (atolls, shoals, and islets) in the remote northwestern Hawaiian Islands (NWHI)..

Location: The Hawaiian Archipelago.

Taxon: Fishes (276 taxa).

Methods: We compiled 5,316 visual fish surveys at depths of 1-30 m from throughout the Hawaiian Archipelago. Geographic range (km2) for each species was measured as extent of occurrence (EOO) and area of occurrence (AOO). PERMANOVA and PCO were used to investigate drivers of fish assemblage structure. Distance-based multivariate analyses were used to evaluate the relationship between fish assemblage structure and predictor variables including latitude, reef area, temperature, chlorophyll-a, wave-energy, and human population density.

Results: Distinct fish assemblages exist in the MHI and NWHI, with two additional faunal breaks driven primarily by endemic species abundance. Latitude explained 37% of the variability in fish assemblages, with reef area accounting for an additional 9%. EOO showed a significant correlation with latitude. Endemics comprised 52-55% of the numerical abundance at the northern end of the archipelago but only 17% on Hawai‘i Island in the extreme south. Maximum size and activity regime (day vs. night) explained the most variation in the abundance of endemics.

Main conclusions: The Hawaiian fish assemblages are strongly influenced by endemic species, affirming the archipelago as a biodiversity hotspot of high conservation value. The higher abundance of endemics in the NWHI may represent preadaptation to oceanic (oligotrophic) conditions. Resolution of distinct bioregions across the archipelago provides a better understanding of reef fish macroecology, with implications for management at the archipelago scale.

Fish surveys were conducted throughout the Hawaiian Archipelago from 2007 to 2016 by the US National Oceanic and Atmospheric Administration’s Ecosystem Sciences Division, using a stratified random sampling design (Heenan et al., 2017). A total of 5,316 surveys were conducted at 2,108 locations, across a depth range of 1 – 30 m. Only forereef locations were used to control for differences in habitat types. Each survey consisted of 2 divers, collecting data at adjacent survey areas using a modification of the paired stationary point count (SPC) method (Heenan et al., 2017). For the SPC, the divers conducted simultaneous counts in estimated 15 m diameter cylinders extending from the substrate to the limits of vertical visibility. Each SPC consisted of two components: a 5-minute species enumeration period in which divers recorded all species present or moving through their cylinder, followed by a tallying portion, in which divers systematically recorded the number and size (total length to nearest cm) of each taxon on their list. The tallying portion was conducted as a series of rapid visual sweeps, with one species grouping counted per sweep.

Heenan, A., Williams, I.D., Acoba, T., DesRochers, A., Kosaki, R.K., Kanemura, T., Nadon, M.O. & Brainard, R.E. (2017). Long-term monitoring of coral reef fish assemblages in the Western central pacific. Scientific Data, 4(1), 1-12.