Wai‘anae Ecological Characterization

Pelagic and Deep Sea Communities

The seafloor off the Wai‘anae coast quickly drops from shallow-water coral reef habitat to the deep seafloor and blue water in a distance of a few miles. This area is termed the pelagic zone, defined as the waters of the open ocean. The pelagic zone includes those species that live and feed in the upper layers of the water column above the seafloor and away from the shallow waters of the coast.

The pelagic zone is further divided into three zones (Garrison 1999). The epipelagic zone extends from the surface downward as far as sunlight penetrates during the day. In Hawaiian waters, this is approximately 200 meters. This zone is well mixed due to surface wind and waves, creating uniform temperature, salinity, nutrient concentrations, and other water characteristics. The mesopelagic zone extends from 200 to 1000 meters. Here the temperature gradient is even and gradual with little seasonal variation. This zone contains an oxygen minimum layer and usually the maximum concentrations of the nutrients nitrate and phosphate. The bathypelagic zone continues to the seafloor 5000 meters below the surface, and is characterized by very cold water and very high pressures. Few creatures are found at these depths.

Off the Wai‘anae coast, the seafloor beyond the reef begins at approximately 100 meters depth, and continues to a depth of 5000 meters. Sand and sediment cover limestone and volcanic rock where currents do not scrub the seafloor bare. Ledges and slopes off Ka‘ena Point are prime habitats for a number of bottomfish species. Numerous pinnacles rising from the seafloor attract fish and other species as well. A massive and ancient landslide of the Wai‘anae mountain range exists 100 miles offshore (approximately 160 kilometers from the coast) in 4000 meters of water (approximately 2.5 miles deep) (Coombs and others 2004). Deep sea communities are few and far between in this cold, continual darkness.

Pelagic Species

The spinner dolphin pods (Stenella longirostris) off of the Wai‘anae coast are perhaps the best known residents to visitors and inhabitants of the area. They can be seen resting in the shallow waters off the coast every morning. These marine mammals are actively feeding at night and in the early morning hours. Tuna and billfish are also seasonal inhabitants off the coast. Seasonal runs of yellowfin ahi (Thunnus albacares), wahoo (Acanthocybium solandri), and the schooling inshore pelagic species big-eyed scad or akule (Selar crumenophthalmus) lure fishermen in huge numbers to the area. Fishery data indicate that tuna runs appear to occur between May and August, while billfishes are found more frequently during September through December. Mahi mahi are most common in August through November (Glazier 1999). Squid, flying fish, sea turtles, and monk seals compose the other large animals found in the pelagic zone.

Mesopelagic Boundary Communities

An important but elusive community occurring offshore is known as the mesopelagic boundary community (Benoit-Bird 2003). This is a dense assemblage of species that migrate from the ocean depths to near the surface in regular patterns offshore of our islands. This community is composed of small fishes (less than 4 inches or 10 centimeters long), shrimps, and squids, the most abundant of which are the myctophids, or lanternfishes. They serve as an important food resource for many animals including spinner dolphins, bottomfish, tunas and billfish. The mesopelagic boundary community undergoes daily, vertical migrations: 400 to 700 meters (approximately 1300 to 2300 feet) deep during the day, and near surface to 400 meters (1300 feet) deep at night. They undergo horizontal migrations as well, moving offshore during the day and returning to within 0.5 kilometer (0.3 mile) of shore at night.

Deep Sea Communities

Beyond the coral reef is a seafloor dominated by sand and bare rock. Few species live in this ocean desert. In areas of steep slopes or strong currents, sand and sedimentation can be minimized. If the seafloor is rugose (irregularly formed rock with pockets or holes for refuge), the area is often habitat for deepwater snappers, such as opakapaka (Pristimomoides filamentosus) and onaga (Etelis coruscans). These species are found between 100 and 400 meters depth (approximately 330 to 1300 feet deep). While there is a lot of ocean to explore, most have a very small home range that is determined by the suitability of these rare habitats (Haight and others 1993). Many of these species form schools and are favorite targets for fishermen who have the right gear to fish this deep.

Deep sea precious corals also inhabit this dark realm. These animals are very different from those commonly seen close to shore. First, they live in complete darkness. The coral at the surface requires sunlight, so the zooxanthellae (symbiotic algae living inside the body of the coral polyp) can photosynthesize and provide food for both itself and its coral host. Precious corals grow long thin branches in the shape of a fan or a small narrow bush. These branches would break apart immediately in the surface wave action, but are the most efficient method to trap food floating by in the gentle deep-sea currents. These corals exist sporadically in deep waters throughout Hawai‘i, and, like the snappers, require specific currents and seafloor structure to settle and grow. Because they live so deep in the ocean, it is difficult and expensive to find and study these beautiful creatures. Scientists have discovered one bed of pink coral in 1000 meters of water (just over 0.6 mile deep) off of the Wai‘anae coast (Grigg 1993). We await future discoveries as we dive deeper and longer into our ocean.

References Cited

Benoit-Bird, K.J. 2003. Dynamics of the Hawaiian Mesopelagic Boundary Community and Their Effects on Predator Foraging. Ph.D. Dissertation. University of Hawaii. 272 pp.

Garrison, T. 1999. Oceanography: an Invitation to Marine Science (3rd Edition). Wadsworth Publishing Co. Belmont, California.

Coombs, M.L., D.A. Clague, G.F. Moore, and B.L. Cousens. 2004. "Growth and collapse of Waianae Volcano, Hawaii, as revealed by exploration of its submarine flanks." Geochemistry, Geophysics, Geosystems 5(8): 10.

Glazier, E.W. 1999. Social Aspects of Hawaii's Small Vessel Troll Fishery. Phase II of Joint Institute for Marine and Atmospheric Research (JIMAR) Social Aspects of Pacific Pelagic Fisheries Program. University of Hawaii at Manoa. 280 pp.

Grigg, R. 1993. "Precious coral fisheries of Hawaii and the U.S. Pacific Islands." Marine Fisheries Review 55(2): 50-60.

Haight, W., D. Kobayashi, and K. Kawamoto. 1993. "Biology and management of deepwater snappers of the Hawaiian archipelago." Marine Fisheries Review 55(2): 20-27.

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