Environmental fluctuation and cohort strength of Dover sole (Microstomus pacificus and English sole (Parophrys vetulus) Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/ws859j146

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  • Models were developed that described cohort strength of female Dover sole (Microstomus pacificus) and female English sole (Parophrys vetulus) in stocks off the Columbia River as functions of environmental variables. The environmental variables considered were those that could hypothetically influence spawning success or survival. They included a spawning power index, monthly mean measurements of different oceanographic and Columbia River factors, and short-term measurements of weather variability. Cohort strength was calculated by using Pope's cohort analysis. For Dover sole, only upwelling in June and July, which may regulate food availability following yolk sac absorption, and offshore divergence the next December and January, which may regulate the location of larval settling, explained significant cohort strength variation. A model incorporating these two variables explained 64.7 percent of cohort variation. Although the Dover sole stocks were clumped around the Columbia River, it appeared that Columbia River factors had less influence on recruitment than did these two oceanographic factors. Spawning power was also not significantly related to cohort strength over the range observed. For English sole, the factors that were associated with the great success of the 1961 cohort were apparently unique to that cohort; thus, they could not be examined statistically. From data examined here, however, it appeared that unique weather variability, characterized by high storm frequency but low average wind speed, may have been partly responsible. For the other cohorts, variation in early fall (prespawning) values of upwelling, barometric pressure, and sea surface temperature explained 72.8 percent, 83.8 percent, and 49.0 percent, respectively, of the cohort variation. During upwelling these three factors are associated with bottom temperature; therefore, it was proposed that bottom temperature variation, which may regulate time of spawning or egg condition, was the factor most directly linked to English sole cohort strength. Colder bottom temperatures were associated with stronger cohorts. Principal components of the environmental variables were derived and regressed against recruitment. The first two principal components explained 65.9 percent of the variation in Dover sole recruitment. For English sole, the third principal component explained 76.9 percent. Because many environmental variables probably influence recruitment, and because every environmental variable considered contributes to each principal component, models incorporating principal components could predict recruitment more accurately than models which include only one or two environmental variables.
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