New research published in the September Issue of the peer-reviewed journal Ecology demonstrates the need for comprehensive methods of revitalizing depleted salmon stocks.
The study by scientists James M. Helfield and Robert J. Naiman of the University of Washington shows that the viability of salmon populations and terrestrial ecosystems are mutually dependent, and therefore calls into question traditional single-species approaches to fisheries management, endangered species legislation, and ecological restoration.
The findings come at a time when Pacific salmon have disappeared from or are in serious decline throughout most of their historical spawning range in North America.
Salmon benefit from the “riparian vegetation” (named after the plants’ proximity to the river’s natural banks) which provides many of the necessary conditions for successful spawning. The riparian plants are a source of shade which helps to regulate the temperature of the spawning grounds. In addition, the plants are a source of large woody debris which retain sediments and help create sheltered areas within the river that can limit mortality among incubating salmon embryos and young fish, particularly during the winter.
“This study allows ecologists to see that the relationship between riparian vegetation and salmon is a two-way street,” Naiman said. “As a main limiting factor for terrestrial plant growth in many northern and temperate forests, Nitrogen is of vital importance to plants, and the Nitrogen derived from spawning salmon is an essential addition to the ecosystem.”
Helfield and Naiman compared the amount of Nitrogen in riparian vegetation adjacent to spawning sites with sites near regions without salmon along two rivers on Chichagof Island in southeast Alaska. They found that plants in spawning sites contained a higher level of Nitrogen than did their reference counterparts. Differences between spawning sites and reference sites were significant in all plant species studied except for red alder, a species that obtains most of its usable Nitrogen from the air.
The presence of Salmon may explain why trees in the riparian zone are often larger than trees farther away from the riverbed. For example, in Helfield and Naiman’s study, basal growth rates of Sitka spruce located at spawning sites were more than triple that of reference sites. The researchers note that this means the large woody debris that is essential for in-stream habitat can be produced in less than a century along salmon rivers as opposed to taking over three centuries along rivers without salmon.
Because of the mutually dependent relationship between salmon and riparian vegetation, a decline in salmon could cause changes in the riparian forest. Those changes may in turn adversely impact the salmon, thereby speeding up a negative process. Helfield and Naiman say that scientists and policymakers working to ensure a healthy salmon population must be cognizant of this linkage within the ecosystem.
“The traditional approach of focusing on just getting more salmon into the rivers is not going to work. We have to devise more innovative ways to ensure the viability of the riparian and river ecosystems,” said Helfield and Naiman. “This study is a clear demonstration of the complex series of interactions that take place within any ecosystem, and we must be mindful of not upsetting these exchanges through our restoration efforts,” they continued.
The Ecological Society of America (ESA) is a scientific, non-profit, 7,800-member organization founded in 1915. Through ESA reports, journals, membership research, and expert testimony to Congress, ESA seeks to promote the responsible application of ecological data and principles to the solution of environmental problems. ESA publishes three scientific, peer-reviewed journals: Ecology, Ecological Applications, and Ecological Monographs. Information about the Society and its activities is published in the Society’s quarterly newsletter, ESA NewSource, and in the quarterly Bulletin. More information can be found on the ESA website: http://esa.sdsc.edu
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