Moore, J. W., S. A. Hayes, W. G. Duffy, S. Gallagher, S. Ricker, and D. Wright. Nutrient fluxes and the recent collapse of coastal California salmon populations
Pacific salmon are well known for transporting nutrients from the ocean to coastal freshwaters. However, migratory salmon move nutrients both in and out freshwaters during the different parts of their life-cycle. There have been surprisingly few quantifications of the actual import and export of nutrients by salmon from coastal freshwaters. We used a mass-balance approach to quantify recent changes in phosphorus fluxes in 6 coastal California watersheds that have recently experienced dramatic decreases in salmon populations. Semelparous Chinook and coho salmon imported 8.3 and 10.4 times more phosphorus, respectively, than they exported, while iteroparous steelhead imported only 1.57 times more than they exported. The semelparous species whose life-histories led them to import more nutrients were also the species that decreased the most dramatically. In addition, the relationship between import and export was nonlinear, with export being proportionally more important at lower levels of import. This pattern was driven by two density-dependent processes—smolts were larger and disproportionately more abundant at lower spawner abundances. Thus, density-dependent processes decrease net import disproportionately rapidly as salmon populations decline. In fact, in two of the study streams, we calculated that salmon drive net export of P in the most recent year and model fitting suggested that this net export can occur in two additional streams. In other words, salmon may reverse their ecological function at low abundances. Differential vulnerability and density-processes can exacerbate the loss of net nutrient import as salmon populations decline.