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1 September 2006 Factors controlling the age structure of Margaritifera falcata in 2 northern California streams
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Freshwater mussels are long-lived and relatively stationary organisms that form annual rings, so they provide excellent opportunities for studying how population properties such as age structure relate to environmental influences on recruitment and mortality. We investigated population dynamics of the freshwater pearl mussel, Margaritifera falcata, in 2 northern California Coast Range rivers, the South Fork Eel and the Navarro. We used field observations and inverse theoretical modeling to assess age structures and identify factors that control population demographics. Distinctly different mortality rates for young (<20 y) and old (≥20 y) mussels were observed in the South Fork Eel, and recruitment and mortality both were related to river discharge regimes. Margaritifera falcata recruited more successfully during low-discharge than during high-discharge years, and a 2-fold increase in discharge caused a 60% decline in recruitment success. Active annual recruitment (˜1100–1800 ind./y) was observed in the South Fork Eel, whereas recruitment has been low or nonexistent during the past 30 y in the Navarro. This difference probably is a consequence of the influence of 2 non-mutually exclusive factors: landuse history and host-fish abundance. The Navarro watershed has extensive timber harvesting and orchard/vineyard agriculture and has had severe declines in fish populations, whereas large sections of the South Fork Eel pass through protected land and the river has retained its historical fish populations. Investigation of the factors that shape local M. falcata population age structures yielded insights into the influence of environmental variables and history on mortality and recruitment that will aid conservation of this endangered family.

Jeanette K. Howard and Kurt M. Cuffey "Factors controlling the age structure of Margaritifera falcata in 2 northern California streams," Journal of the North American Benthological Society 25(3), 677-690, (1 September 2006).[677:FCTASO]2.0.CO;2
Received: 21 July 2004; Accepted: 8 May 2006; Published: 1 September 2006

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