Geology and Geomorphology of Coral Reefs in the Northwestern Hawaiian Islands

The Northwestern Hawaiian Islands (NWHI) comprise a portion of the middle of the 6,126 km long Hawaiian–Emperor seamount chain, considered to be the longest mountain chain in the world Grigg (1983) (Fig. 13.1). Located in the middle of the North Pacific Ocean, the Hawaiian Islands have been referred to as the most geographically isolated archipelago in the world. The islands are ~3,800 km from the nearest continental landmass, the west coast of North America. The nearest other island, Johnston Atoll, is located almost 900 km southwest of the NWHI. A distance of 1,500 km separates the island of Hawai’i at the southern end of the archipelago from the next nearest island, Kingman Reef in the Line Islands. Kure Atoll at the northwestern end of the Hawaiian Islands is the closest point in the archipelago to the northernmost of the Marshall Islands, 2,000 km to the southwest.

Because of their physical isolation and low levels of marine biodiversity, Hawaiian reefs feature high levels of endemism. Approximately a quarter of the species present are found nowhere else, and the Hawaiian Archipelago has approximately twice as many endemic coral species as any other area its size in the world (Fenner 2005). This unique area is home to over 7,000 marine species (Hawai’i DLNR 2000), the critically endangered Hawaiian monk seal (Monachus schauinslandi, Fig. 13.2), and provides nesting grounds for 14 million seabirds and 90% of Hawai’i’s threatened green sea turtles (Chelonia mydas) (NWHI MNM 2006). Reefs in the NWHI are dominated by top predators such as sharks and jacks, which make up more than half of the overall fish biomass (Fig. 13.3). In contrast, top predators in the heavily fished main Hawaiian Islands (MHI) make up approximately 3% of the overall fish biomass, which is more typical of coral reefs worldwide (Friedlander and Demartini 2002; Maragos and Gulko 2002).

Although resources in the Northwestern Hawaiian Islands have been exploited by humans since their discovery by ancient Polynesians, their isolation has afforded some protection and helped preserve their coral reef ecosystems. Additionally, most of the atolls and reefs of the NWHI have been afforded some level of environmental protection for almost a century. As a result, the NWHI are the only large-scale coral reef ecosystem on the planet that is mostly intact, a marine wilderness that provides us with insights on what other coral reefs may have been like prior to human exploitation (Fenner 2005).