Shifts in condition and distribution of eastern North Pacific flatfish along the U.S. west coast (2003–2010)

• We examined condition and distribution of ten flatfish species along the U.S. west coast.
• Both were compared with large-scale environmental indices and fluctuating biomass levels.
• Based on AIC, condition was most closely related to environmental effects rather than biomass.
• Distributional shifts were usually best described by models with environmental effects and biomass.
• Flatfish were generally displaced southeast as conditions cooled and biomass declined.

Flatfish condition indices and distribution were examined along the U.S. west coast (55–1280 m) in relation to environmental variability and biomass using data from ten frequently occurring species collected in annual groundfish surveys from 2003 to 2010. The study was conducted during a period characterized by a cooling trend in the northern California Current system and by declining biomass for flatfish in general. Annual condition indices for six species (arrowtooth flounder, Dover sole, English sole, Pacific sanddab, petrale sole, and rex sole) were significantly related either to large-scale climatic indices (Pacific Decadal Oscillation, Multivariate El Niño–Southern Oscillation Index, North Pacific Gyre Oscillation) and/or annual biomass levels. Condition was most closely related to environmental effects rather than either biomass alone or both variables, with condition typically higher during cool climatic conditions. A similar analysis revealed that changes in distribution (measured as variation in annual catch-weighted mean latitude, longitude, depth and temperature) tended to be best described by models incorporating environmental effects and biomass rather than either variable alone. Linear trends in the center of distribution along a southeast–northwest axis were significant for seven species (arrowtooth flounder, deepsea sole, Dover sole, flathead sole, Pacific sanddab, petrale sole, and slender sole) with a tendency for flatfish to be displaced towards the southeast as environmental conditions shifted from warm to cooler conditions and biomass declined. A spatial distribution analysis indicated that for the majority of species (80%) the greatest magnitude of displacement (km) occurred when the centers of biomass were compared between environmental phases (average annual displacement 34 km) rather than changing biomass levels (average displacement 24 km). Taken together both approaches revealed that environmental changes and variation in biomass play significant roles in flatfish distribution.