2584 | wileyonlinelibrary.com/journal/jbi Journal of Biogeography. 2020;47:2584–2596. © 2020 John Wiley & Sons Ltd Received: 14 March 2020 | Revised: 1 August 2020 | Accepted: 4 August 2020 DOI: 10.1111/jbi.13966 RESEARCH PAPER Dominance of endemics in the reef fish assemblages of the Hawaiian Archipelago Alan M. Friedlander 1,2 | Mary K. Donovan 2,3 | Edward E. DeMartini 2 | Brian W. Bowen 2 1 National Geographic Society-Pristine Seas, Washington, DC, USA 2 Hawaiʻi Institute of Marine Biology, University of Hawai‘i, Kāneʻohe, HI, USA 3 University of California Santa Barbara, Santa Barbara, CA, USA Correspondence Alan M. Friedlander, Hawaiʻi Institute of Marine Biology, , P.O. Box 1346, Kāneʻohe, HI, 96744, USA. Email: friedlan@hawaii.edu Funding information The National Oceanic and Atmospheric Administration's Coral Reef Conservation Program; Hawaiʻi Department of Land and Natural Resources, Division of Aquatic Resources, Grant/Award Number: #C30597. Abstract Aim: Species ranges provide a valuable foundation for resolving biogeographical regions, evolutionary processes and extinction risks. To inform conservation priori- ties, 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 is- lands (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 through- out the Hawaiian Archipelago. Geographical range (km 2 ) for each species was meas- ured 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 as- semblage 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 preadapta- tion to oceanic (oligotrophic) conditions. Resolution of distinct bioregions across the archipelago provides a better understanding of reef fish macroecology, with implica- tions for management at the archipelago scale. KEYWORDS biodiversity, endemism, fish assemblage structure, Hawaiian Archipelago, latitudinal gradient, life-history attributes, marine conservation