Growth of Euphausia pacifica in the upwelling zone off the Oregon coast

Growth rates and intermolt periods for Euphausia pacifica were determined from individual growth rate (IGR) experiments carried out over a 7-year period (2000–2006) on euphausiids collected off the Oregon Coast. Adult growth rates averaged 0.020 mm total length (TL) day−1 during the summer upwelling season and 0.011 mm TL day−1during the winter downwelling season. Average intermolt period (IMP) was 8 d during upwelling and 11 d during downwelling. Growth rates and IMPs were highly variable, and were not related to surface chl-a concentration or length of the euphausiid. Negative growth occurred at all times of year. Negative growth rates during winter downwelling were most likely related to poor feeding conditions, while negative growth rates during the summer upwelling season were likely a result of the euphausiids investing ingested energy towards reproduction rather than somatic growth. Growth rates from this study using the IGR method were generally lower than E. pacifica growth rates from other studies that used the cohort analysis method. Since negative growth rates are difficult to capture using cohort analysis, we calculated a seasonal growth rate using only the positive values from our experiments to compare with cohort analysis growth rates. These seasonal positive growth rates were 0.08 mm TL d−1 (2.4 mm month−1) for the upwelling season and 0.04 mm TL d−1 (1.2 mm month−1) for the downwelling season, equivalent to weight-specific growth rates of 0.0135 d−1 (upwelling) and 0.0089 d−1 (downwelling), and are comparable to rates obtained in other studies using cohort analysis. Highest annual growth rates during upwelling were measured in 2004 (warm) and 2006 (cold), suggesting that E. pacifica is less affected by warm and cool periods than other types of zooplankton, such as copepods.