Graduate Thesis Or Dissertation
 

Seasonal change in the abundance and spatial distribution of a meiobenthic assemblage on the open Oregon coast and its relationship to the diet of O-age flatfishes

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  • The shallow, wave-swept sea floor off the central Oregon coast is used by 0-age flatfishes as a nursery ground. This dissertation is based on a series of projects designed to: 1) identify the dominant meiobenthic nematodes present in a coastal nursery area; 2) describe the important temporal and spatial scales of nematode variability at this site; 3) discuss the role of sediment disturbance in structuring nematode assemblages; and 4) relate seasonal and spatial differences in the distributions of meiobenthos to observed changes in the diet of flatfishes. The 19 numerically dominant nematode species found at the study site showed significant fluctuations in density over a 15 month period, yet their rank order based on abundance did not change. Four mechanisms capable of producing seasonal changes in nematode abundance without altering species proportions are discussed: winnowing from sediments, non-selective predation, seasonally variable food resources, and compensatory links between birth and death rates among species. Significant differences in the abundance of nematodes were found over horizontal distances of kilometers and centimeters. Sediment disturbance contributes to the generation of these spatial patterns. Over broad scales there were two distinct faunal groups associated with different water depths. The division between these assemblages was closely correlated with the threshold depth at which sediments are influenced by passing waves. On small scales (<0.25 m²), the distributions of the numerically dominant species varied seasonally. In the winter, frequent storm activity mixes the bottom sediments and randomly distributes the fauna over the sea floor. In the late spring and summer, physical disruption of sediments is minimal and biological factors, e.g., attraction between males and females, lead to aggregation. Food habits of juvenile English sole were a function of location of capture within the study area, season, and fish length. Diets of fish less than 35 mm SL varied greatly both between seasons in the same year and between years. Diets of English sole captured in trawls obtained at the same depth and different depths were similar in January 1979 but were highly variable in May 1979. These temporal and spatial differences in feeding are thought to be related to seasonal changes in the abundance and spatial distributions of benthic prey.
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