|Abstract or Summary
- Northern anchovy (Engraulis mordax) are a dominant forage fish in the California Current large marine ecosystem (CCLME). However, little is known about northern anchovy abundance, distribution, age structure, or population fluctuations relative to ocean conditions in the eastern boundary upwelling system off the U.S. West Coast. This thesis includes three primary studies of northern anchovy in the CCLME off Oregon and Washington using data collected during four National Marine Fisheries Service (NMFS) surveys (1977-2006): inter- and intra-annual variation in anchovy distribution and its relationship to oceanography, growth rates of larval and juvenile anchovy that indicate the timing of major spawning events and changes in mortality rates, and analysis of whole lipids and fatty acid profiles to determine food sources in years of contrasting oceanographic conditions. First, I quantified the relationship between northern anchovy abundance with environmental variables at two spatial and temporal scales: 1) mesoscale, including sea surface temperature (SST), salinity (SSS), density (SSD), chlorophyll a, distance from shore, and depth; and 2) macroscale, including Pacific Decadal Oscillation Index, Multivariate El Niño Southern Oscillation Index, timing of the Spring Transition, and abundance of cold-water zooplankton. Anchovy densities increased significantly from 1999-2004, and decreased significantly from 2005-2006. SST and proximity to shore were the most consistent parameters explaining anchovy distribution. Year-class strength was highly correlated with, and presumably driven by, the abundance of cold-water copepods. Second, I characterized recruitment for northern anchovy by conducting microstructure analysis of saggital otoliths from late larval and juvenile life history phases collected in September 2006. I identified a protracted spawning period for northern anchovy ranging from June-August 2006. Juveniles that were spawned and hatched early in the summer 2006 had higher growth rates, but smaller back-calculated sizes-at-age during the larval phase, when compared to congeners spawned later that summer. Finally, I determined total lipid content and fatty acid signatures of northern anchovy and three other forage fish species during two contrasting periods of oceanographic conditions (summers of 2005 and 2006): Pacific sardine (Sardinops sagax), Pacific herring (Clupea harengus pallasii), and whitebait smelt (Allosmerus elongatus). Forage fish lipid levels were lowest in 2005 and increased in 2006. Fatty acid biomarkers in 2005 indicated that the food web was based mainly on dinoflagellates, corroborating observations of delayed coastal upwelling and low primary productivity in the CCLME. In 2006, fatty acids reflected higher levels of diatom feeding and zooplankton carnivory. The results of these studies confirm that in the CCLME, northern anchovy are sensitive to even small environmental perturbations, which is important because this work provides metrics for evaluating climate-mediated, bottom-up ecological processes affecting anchovy survival.