Effects of Upwelling on Local Abundance and Distribution of Larval Alewife (Alosa pseudoharengus) in Eastern Lake Michigan

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Abstract
Intrusion of cold (usually < 10 °C), hypolimnetic water into the Lake Michigan nearshore zone (≤ 15 m) substantially changed local abundance and distribution of larval alewife (Alosa pseudoharengus) during the summers of 1977–79. During upwelling, larval fish densities were reduced to below 1000 larvae/1000 m3; when no upwelling occurred densities commonly exceeded 5000 larvae/1000 m3. Widespread distribution to depths of at least 15 m was found in the absence of upwelling, while during upwelling larvae were more concentrated in water ≤ 6 m deep. Additionally, at depths > 3 m during upwelling, alewife larvae were most abundant in bottom strata, a trend not observed in the absence of upwelling. Postulated mechanisms for reduced larval alewife abundance in inshore areas during upwelling include mortality of larvae due to thermal shock, and offshore or alongshore movement, either actively by larvae and potential spawning adults, or passively by larvae.Key words: Alosa pseudoharengus larvae, distribution, abundance, Lake Michigan, upwelling

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