Bilby, R.E., Brian R. Fransen, and Peter A. Bisson. 1995. 


Incorporation of nitrogen and carbon from spawning coho 


salmon into the trophic system of small streams: evidence 


from stable isotopes. Can. J. Fish. Aquat. Sci. (In 


Press). 





	Highest levels of enrichment of the stream biota with the heavier 


isotopes occurred in the early spring, shortly after carcasses had 


decomposed.  Juvenile coho salmon exhibited a doubling in rate of growth 


and a 40% increase in condition factor following spawning.  Age 0 


cutthroat trout in a nearby stream without salmon exhibited no change in 


growth rate during the winter and condition factor did not improve until 


late winter.


	Proportion of nitrogen contributed by spawning salmon varied among 


trophic categories, ranging from about 17% in collector-gatherers to 


more than 30% in juvenile coho salmon.  Carbon contributed by spawning 


salmon ranged from 0% in the foliage of riparian plants and shredders to 


34% in juvenile coho salmon.


	Coho salmon, which usually spawn at lower densities than pink and 


sockeye salmon, are not known to represent an important vector of 


nutrient or organic matter transport from marine to freshwater 


ecosystems.  But because low levels of primary and secondary 


productivity are characteristic of many headwater streams in the Pacific 


Northwest, even modest inputs of certain nutrients or organic matter of 


high nutritional values may be of consequence in maintaining trophic 


productivity.


	Differences in isotope ratios between the two sites increased 


during the winter, while carcasses were present in the stream and 


remained at high levels through early spring, shortly after the 


carcasses were fully decomposed and while salmon fry were emerging from 


the streambed gravel.





	Juvenile fish more than doubled in weight following the arrival of 


the adult salmon in the system, gaining 3.6 g during the winter of 1992-


1993 and 3.2 g during 1993-1994.





	Condition factor of the juvenile coho salmon in Grizzly Creek 


decreased between June and October, achieving the lowest values of the 


year at this time.  Condition factor in December increased, 


corresponding with the presence of spawning salmon and continued to 


increase through March, the time of year when proportions of marine 


derived N and C are at peak levels in fish in the creek.





	Incorporation of marine-derived nutrients into the stream biota 


occurs while the nitrogen and carbon are in the organic form, prior to 


mineralization.





	Spawning salmon may be important in maintaining production 


regardless of the relative availability of nutrients from other sources.





	Larger size has been shown to substantially increase the over 


winter survival rate of juvenile coho salmon and larger smolt size has 


been related to increased marine survival in both coho salmon and 


Steelhead.  Reductions in winter and marine survival will reduce the 


number of returning adult salmon and further suppress the productivity 


of the freshwater system, thus creating a feedback process which 


progressively depresses the capacity of the freshwater system to support 


fish.





	However, fewer returning salmon, and the nutrients they carry, 


also has consequences for the productivity of stream ecosystems.  Large-


scale declines of coho populations in many coastal watersheds may 


represent significant losses of trophic productivity and nutrient 


capital in headwater streams, and reduced export of nutrients and 


organic matter from these streams may impact production in aquatic 


ecosystems downstream from areas in which spawning occurs.  The 


influence of spawning salmon on the trophic productivity of small 


headwater streams may be especially important since heavy shading limits 


autotrophic production throughout the year.  Decreased productivity may 


be self-perpetuating if the capacity of a watershed to produce future 


generations of coho salmon is progressively diminished by fewer and 


fewer returning adults.





	


Bilby, R.E. et al. 1995