CHAPTER 1: BACKGROUND FACTS ABOUT COLUMBIA BASIN SALMON
We know much more about about salmon than most species of fish. The scientific facts we know about what makes salmon thrive are the facts that ought to be guiding our decisions about salmon recovery programs. At least since the Enlightenment, people have applied science to solve problems. This book proceeds from the premise, increasingly unfashionable, that the salmon problem is a problem that can be solved by applying basic scientific knowledge.
From a scientific perspective, growing salmon is not a problem different in kind than farming any other species. Given ocean conditions, the production of the salmon crop will always be highly variable. But we can measure when farming techniques are working, and when they are not.
Salmon are found throughout the higher, cooler latitudes around the world. In vast forested regions around the world, small gravel-bottomed streams have the potential to support juvenile fish, but do not offer sufficient food to sustain adult fish. Anyone who has ever hiked in the mountains east of the Cascades knows that the mountain streams are clear and cold. The crystal clarity of the water means that the water itself is deficient in the micro-organisms that form the bottom of the food chain to support large numbers of big fish.
Salmon take advantage of this underinhabited habitat by using the stream gravel to deposit their eggs. They dig nests in the gravel which are called “redds”. Over the course of several days, a chinook salmon will typically dig a long groove in the gravel, depositing four or five egg pockets in a line running upstream, and covering them as she goes.1 Salmon have literally thousands of eggs, meaning that but for enormous mortality throughout the life cycle, the world would be quickly awash in salmon. Zoologist Ernest Mayr has pointed out that on average only two of any pair of animal’s offspring successfully reproduce.2
The type of spawning habitat needed by salmon has been the subject of long and serious scientific inquiry. It has also been the subject of much popular and political attention, as various salmon recovery plans attempt to remake salmon habitat to the specifications of the authors of those plans. Chinook salmon can accept a wide range of spawning habitat, in terms of the depth of the water and its velocity, and the size of the gravel.3
The most important factor for chinook salmon appears to be good subsurface flow of water through the gravel, which allows the eggs to “breathe”. Because chinook salmon have the largest eggs (and thus the lowest ratio of surface area to volume), good subsurface flow is critical for them. Biologists theorize that spawning salmon sense where gravel has good subsurface flows, and concentrate their redds there, avoiding other areas where the gravel looks fine to human observers.
Once the eggs are deposited in the redds, they are relatively safe from predators. However, one of the largest sources of mortality in the redds is flooding. The flooding can scour out the redds entirely, or bury them in silt that restricts the ability of water to percolate through the gravel.4 Very high flows can cause survival rates for eggs to fall by a factor of ten or more.5 Alternatively, redds can dry out if river levels drop, whether the cause is natural or the product of river regulation by dams. Some juvenile salmon have a tendency to burrow in and hide under gravel in the river beds, and sometimes, juvenile salmon trapped by falling water levels can dig deep to keep wet.6
There is some evidence that salmon limit themselves to spawning in river reaches with a gradient of 3% or less.7 Ironically, most of the streams in the Pacific Northwest currently under federal protection as salmon habitat, particularly upland streams at higher elevations, have larger gradients, and are at best sub-optimal salmon habitat.8
M. C. Healey, “Life History of Chinook Salmon”, reprinted in Pacific Salmon Life Histories 321 (UBC Press 1991).
2 Cited in G. Easterbrook, A Moment on the Earth 144.
5 “High water tied to salmon egg decline”, The Spokesman-Review, May 14, 1997 (Reporting on WDFW research: “On the Cedar and Skagit, the survival rate is about 20% when flows are modest. During the biggest floods, it drops to 2% or less. In seasons with moderately high flows, survival hovers between 5% and 12%).
6 B. Brown, Mountain in the Clouds 93.
7 J. Palmisano, Informal Comments to the NWPPC on the ISG’s Report, April 15, 1997, at 3.
This Web page was created using a Trial Version of HTML Transit 3.0.