Sunday, January 11, 2009

The effect of dams on the survival of Pacific salmon smolts

A tiny salmon smolt, on its treacherous journey of a hundred miles or more to the sea, faces many dangers—-but new research suggests that the dams blocking its way may no longer be among them. A multi-year study using new tracking technology has demonstrated no survivorship difference between smolts in a dammed river and its undammed neighbor.

The study took place in the Snake-Columbia river system of Idaho and Washington, which has eight dams, and its dam-less neighbor to the north, the Thompson-Fraser river system of British Columbia. The dams in the Snake and Columbia rivers have often been blamed for killing the small salmon that swim through them, and ultimately for the decline in salmon stocks. Although dams were historically responsible for killing large numbers of salmon, in recent decades billions of dollars of investments have made dams much more salmon-friendly. The effect of the improved dams on salmon has not been compared with that of a damless river until now.

No matter how they analyzed the data or adjusted their detection methods, the scientists behind this study did not find a higher death toll among the salmon smolts in the dammed Snake-Columbia river than in salmon of the same species in the undammed river nearby.

“This result is surprising,” wrote the researchers in their October 2008 paper in Public Library of Science: Biology. “Dams are often implicated as major barriers to [salmon] recovery in the Columbia.”

With only two river systems included in the study, it cannot be proven that the results found were caused only by the difference in the number of dams, or if they resulted from some other dissimilarity between the two rivers. For example, it is possible that the salmon in the Snake-Columbia river system were of a stronger strain that was better able to survive the rough ride through eight dams en route to the sea.

However, if larger-scale studies confirm the result found by this study, the implications could be great for management of critically endangered salmon populations in the Northwest.

Salmon supply $1 billion dollars in income to fishermen on the Pacific coast and employ 60,000 people there. As a keystone species, they also provide valuable ecosystem services on which the survival of many other species hinges.

With those kinds of numbers at stake, and with billions of dollars available to spend, studies like this one are necessary to prevent taxpayer money from being wasted on expensive projects that have no effect on salmon populations, while more important conservation strategies remain undiscovered.

As the authors of the paper point out, their tracking showed smolt survivorship of as high as 20-30 percent from headwaters to ocean, but other studies have shown that the percent of salmon that survive long enough to turn back around and leave the ocean hovers around 0.5. This implies that the real focus for salmon conservation efforts should be the ocean, where the most salmon death may be occurring.

It is possible, however, that the dams were killing the smolts slowly rather than quickly, which would have not been detected by this study’s methodology.

“Our data do not address whether the possible delayed effects of hydropower system passage subsequently affects mortality after the fish leave the river for the ocean, currently a contentious issue,” wrote the authors. “We suggest that conservation efforts in the Columbia may be better directed towards understanding the effects of hydropower system passage on ocean survival.”

This study, and the larger-scale studies it will surely trigger, only became possible with the recent development of a massive acoustic tracking system permanently installed along 1,500 miles of western North America’s continental shelf. The Pacific Ocean Shelf Tracking (POST) Array uses an acoustic frequency that penetrates both salt and fresh water, and is powerful enough to detect a fairly small chip implanted in a fish hidden in a large body of water.

Before the POST Array was constructed, the only technology available to track the movements of small fish was a very short-range radio tagging system. This radio system is so weak that it only records fish two or three feet from the detector. While this works fine for dammed rivers-—the radio detectors can be installed in the fish channels through the dams—-it offers no reliable way record fish survivorship in wide-open, undammed rivers.

With this more powerful detection method in place, it should become easier for scientists to tell where and when salmon are dying. If it is confirmed by further study that hydroelectric dams should no longer be a point of concern for salmon conservation, all of those who rely on salmon for a salary or for a meal can breathe a small sigh of relief—-and turn their attention to the host of remaining dangers faced by these delicious, important fish.
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postscript. The paper is publicly available at http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0060265

4 comments:

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