Linking Phytoplankton Phenology to Pink Salmon Productivity Along a North-South Gradient

Biological ocean productivity is believed to be an important determinant of Pacific salmon (Oncorhynchus spp.) population dynamics, yet mechanisms linking phytoplankton dynamics to salmon productivity are poorly understood. In this study, we investigated whether spatial and temporal components of phytoplankton dynamics in the Northeast Pacific Ocean could explain long-term changes in productivity of 27 pink salmon (O. gorbuscha) populations. Specifically, we used spatial covariance functions in combination with hierarchical spawner-recruit models to quantify relationships among satellite-derived chlorophyll-a concentrations, initiation date of the spring phytoplankton bloom, and salmon productivity. For all variables, positive spatial covariation was strongest at the regional scale (0-800 km) with no covariation beyond 1500 km. Spring-bloom timing was significantly correlated with salmon productivity for both northern (Alaska) and southern (British Columbia) salmon populations, although the correlations for those two regions were opposite in sign. An early spring bloom was associated with higher productivity for northern populations and lower productivity for southern populations. Furthermore, the spring bloom initiation date was a better predictor of salmon productivity than mean chlorophyll-a concentration. Our results suggest that changes in spring bloom timing resulting from natural climate variability or anthropogenic climate change could potentially cause latitudinal shifts in salmon productivity.