Distributional Ecology of Benthic Megaepifauna and Fishes in Gorda Ridge Axial Valley

Abstract

Using bottom photographs and videotapes taken by various institutions in 1984 to 1986, we describe the taxonomic composition and patterns of distribution on two geographic scales and the relative abundance of the benthic epifauna and fishes in Gora Ridge axial valley. Gorda Ridge runs approximately northeast-southwest 200 to 300 km off northern California and southern Oregon and lies within the U.S. Exclusive Economic Zone. Suspension feeding invertebrate taxa occur in all rocky and sedimentary environments. Crinoids appear dominant at the northernmost stations, and ascidians are dominant in the central Gorda Ridge region. In the sedimented Escanaba Trough hexactinellid sponges and gorgonian soft corals are present. Suspension feeding epifauna tend to occur nonrandomly with even distributions predominant. Assemblages of invertebrates and fishes associated with rock and sediment differ in composition and abundance. Deposit feeding organisms become relatively more abundant and are interspersed with stalked suspension feeders at the southern stations of the sediment-filled southern Escanaba Trough. Many species associated with sediments are similar to those found on the surrounding abyssal plains. Biomass is unevenly distributed. The axial valley fauna is abundant and diverse, undoubtedly supported by organic materials of continental origin, including runoff from the Columbia River. In addition, scattered hydrothermal venting provides energy sources for adjacent chemosynthesis-based food webs. The enclosing valley walls and rough topography appear to constrain currents and cause turbulence that maintains the high concentrations of suspended particles observed near the valley floor.