From salmon to shad: Shifting sources of marine‐derived nutrients in the Columbia River Basin

Abstract

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...

Risk assessment for the reintroduction of anadromous salmonids upstream of Chief Joseph and Grand Coulee Dams, Northeastern Washington

“…Further analyses could simulate the effect of differing run sizes of anadromous salmonids on nutrient loads given adult run sizes. Similar analyses were recently done for nonnative American Shad using historical and current run sizes of Pacific Salmon, American Shad run sizes, juvenile population estimates from dam indices, and a long-term nutrient monitoring site downstream of McNary Dam (Haskell, 2017). These analyses indicated that the nutrient balances are a product of existing nutrient loads, reservoir retention times, adult run sizes, and the spatial extent of spawning.…”

Quantification of land–sea nutrient fluxes supplied by allis shad across the species’ range

“…These numbers for allis shad were also lower than the nitrogen and phosphorus loadings reported for other diadromous species. By comparison, Haskell (2018) found that the closely related American shad (Alosa sapidissima) imported over 15 000 kg N and 3 000 kg P annually in the John Day Reservoir (JDR) in the lower Columbia River over the 1997-2015 time period. Considering that the JDR has a surface area of 222.6 km 2 , the total amount of nitrogen and phosphorus associated with American shad spawning runs would be 67.38 kg.N.km 2 .year -1 and 13.48 kgP.km 2 .year -1 .…”

“…3.1. Quantifying nutrient imports Following the approaches of Haskell (2018) and Barber et al (2018), it was assumed that adults were not feeding after entering rivers in order to only account for marine-derived nutrient inputs. A fish that has successfully migrated to a river after growing and maturing at sea is likely to: (i) die before reaching the spawning grounds and reproducing, (ii) die after reproduction (i.e.…”

“….0067 for females (Haskell 2018). Likewise, the excretion rate of 2.17 10 -6 µg N and 𝜂 𝑃 𝑓 𝜏 × 24.7171 10 -5 µg P g wet fish mass -1 .h -1 were based on results for alewife (Post and × × Walters 2009)(Table 1).…”

Linking functional response and bioenergetics to estimate juvenile salmon growth in a reservoir food web

“…In 2015, nearly 140 million hatchery reared juvenile salmon were released into the CRB, but there are many more juvenile shad. In John Day Reservoir alone, as many as 112 million juvenile shad are produced annually, with many more originating in other Snake and Columbia River reservoirs and the Columbia River Estuary [ 15 ]. Juvenile shad consume up to 80% of the available zooplankton production in John Day Reservoir during the time that subyearlings are also migrating seaward [ 16 ].…”