A Geological and Ecological Reconnaissance off Western Oahu, Hawaii, Principally by Means of the Research Submarine "Asherah"

Abstract

In November 1965 a combined geological and ecological reconnaissance of the sea floor off western Oahu was undertaken using a variety of methods and techniques to maximize both the range and reliability of the information obtained. Bottom topography and fish concentrations were surveyed with a precision echo sound recorder for which the transducer was towed in a streamlined housing below the research ship. Photographic bottom surveys were also made with an automatic stereo-camera system, and some bottom dredging and trawling were undertaken to secure samples of the bottom and the biota. Direct visual observations were also made using a small research submarine largely in the depth range of 25- 180 meters. The dominant geological features were a series of submerged , wave cut, largely sand covered terraces separated by rocky escarpments. The major terraces were an upper one terminating seaward at approximately 60 meters, an intermediate one from 70 to 120 meters, and a deep one beginning from a shoreward depth of 180 meters or deeper. Patterns of littoral sand movement were observed to be southerly in the region between Kaena Point and Kepuhi Point with a substantial movement offshore. It was estimated that approximately 10,000 cubic yards of calcareous sand move seaward and are deposited annually on the inner portions of the deep terrace. Associated with the escarpments were large and discontinuous aggregations of fish and, on the upper and intermediate terraces, extensive beds of the clam Pinna muricata. The observed patterns of distributions may be a response to the localized accumulation of food. Organisms which make nocturnal vertical migrations in adjacent deep water may be swept shoreward by surface currents and become trapped on the terraces. The collection of planktonic organic material in the thermocline where the water increases rapidly in density with depth may be a mechanism for the localized accumulation of particulate food of value to the clams. The simultaneous use of a variety of observational techniques in an area provided non-identical and independent observations of the same situations . Th is served to confirm the information obtained and to add new and significant detail.