Measurements of natural mortality for Pacific sardine (Sardinops sagax)

The northern stock of Pacific sardine (Sardinops sagax) in the California Current Ecosystemis periodically assessed to provide harvest guidelines for the United States and Canadian fisheries. The assessment model incorporates the estimates of abundance from acoustic-trawl method (ATM) and other surveys and many parameters including a constant value for the instantaneous natural mortality coefficient (M). Here, the ATM-estimated length-structured sardine abundances for 2006-2011 are combined to directly estimate M. The stock was dominated by the 2003, 2004, and 2005 year classes (2003-2005 cohort) from 2006 to 2010 and by the 2009 and 2010 year classes in 2011. Insignificant recruitments during 2006-2010 allowed the 2003-2005 cohort to be tracked andMto be estimated. The bootstrap distributions of estimatedMencompass the assessment-assumed value of 0.4(-year). The measurements suggest thatMis high for a new cohort, declines during mid-life, then increases as the fish approach their longevity. ThemeanMfor the entire period, 0.52, is 30% higher than the one currently assumed for the assessment, reflecting the steeper decline in the ATM-estimated biomass versus that in the assessment model.