Natural mortality estimates of juvenile Pacific herring (Clupea pallasi) in Prince William Sound, Alaska

The densities of young-of-the-year and 1-year-old Pacific herring, Clupea pallasi, in Prince William Sound, Alaska, were estimated using acoustic surveys from June 1996 to March 1998. Four bays were surveyed with acoustic transects that were repeated three times in 24 h. Species composition and size structure were determined from net collections. Averaging over the 24-h period allowed the best use of all data, as observations between the three replicates were similar but sporadic, possibly resulting from the different seasonal day-night cycle in these northern latitudes. The average instantaneous natural mortality rates for young-of-the-year Pacific herring were 0.009 (standard deviation (SD) = 0.002) and 0.016 (SD = 0.012) for the 1996 and 1997 cohorts, respectively. The average instantaneous natural mortality rates for 1-year-old Pacific herring were 0.003 (SD = 0.007) and 0.008 (SD = 0.005) for the 1995 and 1996 cohorts, respectively. Combining our estimates with those in the primary literature for other life history phases of Pacific herring indicated a progressive decrease in instantaneous natural mortality with age. This study presents the first direct estimates of natural mortality for juvenile herring.