Spatial patterns of endemism in shallow-water reef fish populations of the Northwestern Hawaiian Islands

Abstract

The spatial distribution and magnitude of endemism in shallow-water (<18 m) reef fishes of the 10 Northwestern Hawaiian Islands (NWHI) are described in terms of occurrence, and numerical and biomass densities, using a series of diver-observation surveys conducted during Sep- tember/October 2000, 2001, and 2002. Based on species-presence in our surveys, we found endemism to be equal (20.6% using all available data) for fishes in the NWHI and the Main Hawai- ian Islands (MHI; 20.9%). On average, percentage endemism was much higher based on the biomass (37%) and numerical densities (52%) of endemics, increased with latitude, and was especially pro- nounced at the 4 northernmost reefs that are the most ancient emergent geological features of the archipelago. Endemic reef fishes were appreciably smaller bodied than non-endemics within the NWHI. Median body size did not vary with latitude and longitude for either endemics or non- endemics, negating environmental effects. Reef fish populations at higher latitude reefs included larger proportions of young-of-year (YOY) recruits. YOY length frequencies did not differ for most species between northern and southern reefs, suggesting that a seasonal lag in spawning and recruitment at higher latitudes cannot explain the greater YOY densities observed there. Dispropor- tionate recruitment at higher-latitude reefs may be related to better growth and survivorship after settlement onto reefs, higher levels of within-reef and regional reseeding at higher latitudes, or other factors. Resolution of these issues will be difficult, but it is profoundly important to the future siting of no-take Marine Protected Areas (MPAs) within the NWHI Coral Reef Ecosystem Reserve and to the rational conservation and management of reef fish resources throughout the Hawaiian Archipelago.