Cohort analysis of Euphausia pacifica from the Northeast Pacific population using a Gaussian mixture model

Abstract Euphausia pacifica cohorts were identified from biweekly zooplankton samples collected on the Newport Hydrographic Line (Newport, Oregon, USA) from 2001-2011. Cohorts were identified using a Gaussian mixture model and tracked over time from the juvenile stage through adulthood. Initial size mode at the juvenile stage was typically 4-5mm and final size modes ranged from 12-18mm. In total, 28 cohorts were identified, of which 22 appear to be complete cohorts that were tracked from start to finish during the 11-year study period. Of these 22 cohorts, 19 were tracked for ≤1.5 years. The three cohorts tracked for >2 years grew more slowly than other cohorts, though their final size modes were similar. These three cohorts were associated with delayed upwelling and moderate chlorophyll concentrations, suggesting that their extended duration and slower growth were related to suboptimal environmental conditions. Growth rates calculated from cohort size modes decreased overall as animals reached adult size. Cohort analysis captured some instances of negative growth, particularly after animals reached a total length of 10mm, similar to instantaneous growth rates (IGR) measured in a previous study. Survivorship curves were created from eggs and larvae for each year from 2001-2005. Survivorship was similar among years except in 2005 when upwelling and subsequent spawning were delayed by one month. Based on the survivorship curves, the E. pacifica juvenile stage lasts an average of six months and the total life span in the study area is approximately two years. Successful identification and tracking of cohorts suggests that euphausiids at station NH25 are representative of the overall population dynamics of Euphausia pacifica in the shelf-break region off the Oregon Coast.