Species composition and community structure of pelagic nekton off Oregon and Washington under variable oceanographic conditions

Dramatic changes in the physical and biological conditions off Washington and Oregon, USA, have occurred since 1998, including extreme El Nino (warm) and La Nina (cool) years, high and low Columbia River flow years, a major intrusion of subarctic water, and a low oxygen event on the shelf. The occurrence of contrasting environmental conditions provided an excellent opportunity to examine pelagic nekton distributions and their abiotic and biotic associations. Pelagic surface trawl surveys conducted during June and September from 1998 to 2002 off northern Washington to central Oregon revealed a nekton community dominated by Pacific herring (33.5 % of total catch), Pacific sardines (29.9%), and northern anchovy (12.3%). Between 1998 and 2002, species composition shifted from a community dominated by southern species (mackerels and hake) to one dominated by northern species (squid, smelts, and salmon), but the transition was gradual, and small pelagic species (sardines, herring, and anchovy) showed no consistent trends in abundance over time. Species diversity/evenness was highest in September 2002. Cluster analysis identified 7 species and 6 station clusters. Subyearling Chinook salmon, market squid, Pacific sardine, yearling coho salmon, and Pacific saury were the strongest indicator species for 5 of these cluster groups. Cluster group distributions differed between both inshore/offshore and north/south. A 3-dimensional ordination explained 55 % of the total variance with bottom depth, distance from shore, and sea-surface temperature correlated with the first axis, latitude with the second axis, and surface salinity, surface density and stability with the third axis. Our results suggest that the habitats occupied by pelagic nekton species expand and contract in relation to the dynamic nature of the California Current and are affected by changing ocean conditions at both seasonal and interannual periodicities.