Because of their large populations and body sizes, Pacific salmon are a major source of food for terrestrial and aquatic organisms associated with spawning streams, from bears to bacteria.   Pacific salmon spend most of their life cycles as top predators in the nutrient-rich North Pacific Ocean, where they incorporate carbon, nitrogen, phosphorus, and other micronutrients into their body tissues. 

Until recently, these processes had really only been studied in the more pristine areas of the Pacific Northwest.  Dr. Peter Moyle, of UC Davis, and I wanted to see if pathways for salmon-derived materials still functioned in highly modified salmon stream ecosystems of California's Central Valley.
These tissues provide an important nutrient and energy subsidy to oligotrophic streams where they spawn and eventually die. Spawning salmon release nutrients into streams through normal metabolic processes, release of gametes, the consumption of salmon flesh by numerous organisms or the decay of carcasses. 
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A large female Chinook salmon in the Mokelumne River (Click on image to enlarge).  Note the large puncture wounds indicating a narrow escape from a sea lion.
Recently, the use of stable isotopes to trace trophic pathways has revealed that marine-derived nutrients from Pacific salmon are transferred not only into aquatic ecosystems, but into adjacent terrestrial ecosystems. The large difference in heavy isotope (e. g., 15N and 13C) composition of salmon tissue relative to freshwater or terrestrial values has been used to estimate the proportion of salmon-derived N or C in animal tissues, invertebrates, biofilms, and plant communities.  Thus, stable isotope analysis provides a means for tracing marine elements from spawning salmon through the trophic systems of the streams they use, as well as those of adjacent terrestrial systems. Marine derived nitrogen (MDN) has been found in riparian vegetation tissue far from river banks. Observations of MDN signatures in bird guano, adult aquatic insects, and terrestrial vertebrate scat and urine suggest a pathway of nutrient transport from salmon tissue to terrestrial systems beyond simple transport through root systems of trees.
Salmon carcasses play an important role in the behavior and ecology of many terrestrial consumers. Densities of many vertebrates increase around streams in which salmon are spawning as they move in from surrounding areas to feed on the salmon. This can have a profound effect on salmon nutrient dispersal. Terrestrial scavengers may move 58% to 90% of all salmon biomass to land, sometimes hundreds of meters from the stream, providing a transport mechanism for as much as 85% of MDN uptake of riparian foliage within 0.5 km of the spawning channel.
The salmon subsidy results in an increase in the abundance and growth rates of aquatic invertebrates and fish and riparian tree production. In this context, salmon nutrients are part of a positive feedback loop, because juvenile salmon grow faster and have higher survival rates when their invertebrate prey are abundant in streams and riparian areas. The complexity of stream habitat, important for both juvenile and spawning salmon, also increases when large, salmon-fertilized trees fall into the water.
Using a combination of tissue samples, salmon surveys, and infrared motion detector cameras along the regulated Cosumnes and Mokelumne rivers, we found some interesting results: 

We observed 14 terrestrial vertebrate species, including “herbivores” such as mule deer (Odocoileus hemionus) and western gray squirrels (Sciurus griseus), feeding directly on salmon carcasses. Although the lower Mokelumne River (LMR) drainage has lost large vertebrate scavengers, such as bears (Ursus spp.), wolves (Canis lupis), and to a great extent, bald eagles (Haliaeetus leucocephalus), domestic animals, such as cats (Felis domesticus) and dogs (Canis domesticus) and commensal animals, such as turkey vultures, opossums, and raccoons, continue to transport nutrients to adjacent agricultural fields, including vineyards. The distinct differences in carcass consumption rates and scavenger numbers (mainly turkey vultures) between the Mokelumne and Calaveras rivers suggest that the nutrient transport system observed in more pristine systems continues to function in highly modified systems if the salmon runs also continue. In fact, we detected significant amounts of marine derived nutrients in riparian foliage, including vineyards, along the regulated lower Mokelumne River (LMR) that supports a robust salmon run.

If we use the peak estimate of 600 000 California Central Valley Chinook salmon escapement for the year 2001, and our average N estimate, California runs may actually contribute as much as 6000 metric tons of biomass, 1800 metric tons of C, and 337 metric tons of N to the Central Valley annually.

Managing regulated rivers for salmon has benefits far beyond simply providing fish for anglers. The marine derived nutrients of salmon can positi vely affect both natural riparian systems and agricultural crops, with considerable economic benefit; 600 000 salmon could be worth US$186 000-1.06 million as N fertilizer alone.

If you'd like to learn more about this, our study will be published soon in Ecological Applications (Winter 2006).