MEPS 419:171-184 (2010)  -  DOI: https://doi.org/10.3354/meps08848

ABSTRACT: This study described the phenology and patterns of variability of forage fish and mesozooplankton populations near the Columbia River plume. Our objective was to identify the timing of the seasonal appearance of forage fish and to characterize the temporal patterns of abundance in relation to ocean conditions including zooplankton availability. Observations were collected at 2 stations in 2008 and 2009 using 200 kHz bio-acoustic moorings, and with bi-weekly net sampling surveys conducted at nearby stations to measure total fish density and relative species composition. Acoustic time series results revealed that the seasonal timing of acoustic schools, representing the forage fishes northern anchovy Engraulis mordax, whitebait smelt Allosmerus elongatus, and Pacific sardine Sardinops sagax, occurred abruptly, with specific dates of appearance identified at each station in mid-May in both years. Both 2008 and 2009 represented very similar years for the timing and strength of wind-driven coastal upwelling. The timing of forage fish appearances in these years was linked with ocean temperature and salinity data collected at a nearby oceanographic buoy but was poorly correlated with mesozooplankton abundance, which was highly variable and fluctuating over a period of hours to days. Fluctuations in zooplankton and forage fish populations drive many trophic interactions, including juvenile salmon, seabirds, and large predators (e.g. adult salmon) that rely on the timing and abundances of these populations and have significant implications for ecosystem based management.

KEY WORDS: Forage fish · Zooplankton · Acoustics · Phenology · Upwelling · Columbia River plume · California Current · Sardines · Anchovies