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Food Availability, Density Dependence and the Dynamics of Natural Mortality in a Temperate Marine Fish

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Daniel Okamoto
Russell J. Schmitt
Sally J. Holbrook
Daniel C. Reed

Understanding mechanisms that regulate natural mortality in populations is a central goal in ecology and resource management.  Yet how fluctuating resources and density dependence affect mortality in marine fish remains elusive.  Reasons for this include difficulties in assessing natural mortality and food supply, as well as obstacles to evaluating density dependence from annual censuses.  To address such issues, we collected age-structured data using spatially replicated series of annual surveys of black surfperch (Embiotoca jacksoni) on Santa Cruz Island, California, along with estimates of habitat, densities of prey and densities of competitors.  We demonstrate substantial interannual variability in adult mortality that challenges the commonly made assumption that natural mortality remains relatively constant through time.  Mortality varied between eighty and one percent in different years.  This tremendous range was primarily driven by the interactive relationship between food availability and fish density.  This indicates there was strong bottom-up and density dependent regulation in this population that arose from food densities that varied from year to year by orders of magnitude.  Such quantitative understanding of the relationship between fluctuations in food supply and mortality in fish populations can lead to more robust management, particularly by enhancing the ability to forecast periods of elevated natural mortality.

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Background Photo by: Nicole Hansen - Jornada (JRN) LTER