Like Pacific salmon (Oncorhynchus spp.), nonnative American shad (Alosa sapidissima) have the potential to convey large quantities of nutrients between the Pacific Ocean and freshwater spawning areas in the Columbia River Basin (CRB). American shad are now the most numerous anadromous fish in the CRB, yet the magnitude of the resulting nutrient flux owing to the shift from salmon to shad is unknown. Nutrient flux models revealed that American shad conveyed over 15,000 kg of nitrogen (N) and 3,000 kg of phosphorus (P) annually to John Day Reservoir, the largest mainstem reservoir in the lower Columbia River. Shad were net importers of N, with juveniles and postspawners exporting just 31% of the N imported by adults. Shad were usually net importers of P, with juveniles and postspawners exporting 46% of the P imported by adults on average. American shad contributed <0.2% of the total annual P load into John Day Reservoir, but during June when most adult shad are migrating into John Day Reservoir, they contributed as much as 2.0% of the P load. Nutrient inputs by American shad were similar to current but far less than historical inputs of Pacific salmon owing to their smaller size. Given the relatively high background P levels and low retention times in lower Columbia River reservoirs, it is unlikely that shad marinederived nutrients affect nutrient balances or food web productivity through autotrophic pathways. However, a better understanding of shad spawning aggregations in the CRB is needed.
K E Y W O R D Saquatic food webs, lake and reservoir management, nonnative species, nutrients, salmon, shad
| INTRODUCTIONAnadromous fish are vectors of nutrient transport between marine and freshwater food webs. This phenomenon has been well documented for Pacific salmon (Oncorhynchus spp.; Holtgrieve & Schindler, 2011;Rex & Petticrew, 2008;Scheuerell, Levin, Zabel, Williams, & Sanderson, 2005;Vanni, 2002;Wipfli, Hudson, & Caouette, 1998), but is less explored for other anadromous fish. Pacific salmon are large-bodied nutrient vectors that contribute 3.0% of their body mass as nitrogen (N) and 0.36% of their body mass as phosphorus (P) in the form of excretions, gametes and carcasses to freshwater areas where they spawn (Brock, Leavitt, Schindler, & Quay, 2007). These nutrients can benefit progeny by increasing the productivity of rearing grounds but can also influence the productivity of entire ecosystems at multiple trophic levels. The decline of once great salmon runs in the Columbia River Basin (CRB) has reduced salmon-derived nutrient inputs to spawning grounds. Columbia River Basin salmon once numbered from 11 to 16 million per year and imported millions of kg of N and hundreds of thousands of kg of P annually (Gresh, Lichatowich, & Schoonmaker, 2000). These nutrient inputs have decreased by 90% with an associated decline in the productivity of salmon spawning grounds. This loss in productivity could inhibit ongoing salmon restoration efforts. In response, salmon restoration programmes annually supplement sp...