Cross-shelf distribution of Euphausia pacifica in the Oregon coastal upwelling zone: field evaluation of a differential transport hypothesis

Abstract

The frequent upwelling events characteristic of Oregon’s summer may make nearshore retention challenging for early, non-migrating euphausiid life stages inhabiting the surface layer. In contrast, later, vertically migrating life stages spend more time below the strongest offshore flow and are moved offshore less rapidly. We hypothesize that population stage structure will vary in predictable ways as a result of differential transport, a function of occupied depth (and therefore life stage). MOCNESS (Multiple Opening/Closing Net and Environmental Sampling System) samples have indicated that early Euphausia pacifica calyptopes (stages 1 and 2) are non-migrators, whereas late furcilia (stages 4–7) are migrators. We develop a metric (migrator fraction, FM) of relative densities of these larval stages within a sample to examine potential differential transport with alongshore winds on short timescales (0.5–7 days), for data collected during upwelling seasons from 2000 to 2002 at two stations off Oregon. FM and northward winds had a significant negative relationship in 2002 at the inshore station, indicating differential transport. Upwelling condition-classified sampled densities at nearshore and offshore stations also suggested differential cross-shelf transport, but trends were not always consistent with the hypothesis, suggesting the cross-shelf distribution of non-migrators was not consistent throughout the study period. Several other factors, such as spawning, might also contribute to significant trends observed in FM. The results of this analysis suggest that relationships exist between E. pacifica larval distributions and wind events in the Oregon upwelling region, but that the distributions may be too complex to predict at the event scale without more sophisticated tools, such as biological-physical coupled models.