ICHTHYOPLANKTON ABUNDANCE AND COMPOSITION OFF THE CENTRAL OREGON COAST FROM 1997-2000 AND THE EFFECTS OF THE 1997-98 EL NIÑO

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2011-02-16

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ICHTHYOPLANKTON ABUNDANCE AND COMPOSITION OFF THE CENTRAL OREGON COAST FROM 1997-2000 AND THE EFFECTS OF THE 1997-98 EL NIÑO. Ashley A. Emerson In the southern hemisphere trade winds over the Pacific Ocean normally blow towards the west causing water in the western Pacific to be approximately 1/2 meter higher in Indonesia than along the coast of South America. During an El Niño Southern Oscillation (ENSO) event these trade winds relax, allowing ocean waters in the southern hemisphere to shift back towards the east (Peru). This shift then pushes water north along the coast of the Americas, affecting the ocean currents off the coast of Oregon. Under normal conditions, upwelling occurs along the coast of Oregon, characterized by the movement of nutrient-rich water from greater depths carried up towards the shallow coastal waters. Under normal conditions in the summer, coastal transport is southward and moves offshore. During an ENSO event coastal transport (off Oregon) reverses, becoming northward and onshore. El Niño is known to alter the distribution, composition, and abundances of copepods. It is thought to similarly affect ichthyoplankton larvae. This study investigated these ideas by analyzing the effects of the 1997-98 El Niño on ichthyoplankton off the central Oregon coast. Samples used in this study were collected at a station five nautical miles from the coast along the Newport Hydrographic line (or at station NH05). At this relatively nearshore sampling station, I expected to observe an increase in the overall abundance of larvae and in the abundance and occurrence of offshore larvae during the El Niño. I also anticipated an increase in species diversity during this period due to the mixing of standard (coastal) species with unusual (offshore) species moved by the ocean currents. The data did show that Engraulis mordax, a particular offshore species of interest, was found significantly more during El Niño sampling as I had expected. The occurrence of offshore larvae was also significantly greater during El Niño samples. There was a significant interaction between the effects of season and El Niño on the abundance of offshore species. While species richness analyses did not reveal any significant differences in the number of species present in El Niño and non-El Niño periods, similarity analyses revealed differences in species identity and community composition. The data showed no significant increase in overall larval abundance or offshore larval abundance during El Niño, but they did suggest that abundance patterns changed seasonally.

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