Environment and Cohort Strength of Dover Sole and English Sole

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Abstract
We examined relationships between cohort strengths of Dover sole (Microstomus pacificus) and English sole (Parophrys vetulus) off the Columbia River and variables that could hypothetically influence spawning success or survival. These variables included measures of spawning capacity, monthly means of oceanographic and Columbia River factors, and measurements of short‐term weather variability. For Dover sole, spawning capacity was not significantly related to cohort strength. A correlation model incorporating two oceanographic factors, upwelling in early summer and offshore divergence the next winter, explained 65% of the cohort strength variation. Upwelling may have influenced food availability following yolk sac absorption, and divergence may have influenced the location of larval settling. For English sole, factors that had unusual values coincident with the unusually large 1961 cohort were not well correlated with the strengths of the other cohorts. Unique weather variability in the winter of 1960–1961, characterized by high storm frequency but low average wind speed, may have contributed to the 1961 success by stimulating plankton production. For the other cohorts, early fall (prespawning) values of up‐welling, barometric pressure, and sea surface temperature showed significant correlation with cohort strength and accounted for 84% of cohort strength variability. During upwelling, these three factors were associated with bottom temperature; therefore, we propose that colder bottom temperatures, which may have delayed spawning or improved egg condition, were directly linked to stronger cohorts.