In the study, they apply a population model to two scenarios, with and without delayed transportation mortality. Population growth anchors the model. All elements of the population matrix are specified except first-year survival, s₁, for which they solve (s₁ = 0.022 for their index stock). All unspecified mortality, and hence all uncertainty, is apportioned to s₁.

In the model of Kareiva et al., without delayed mortality and with dam breaching, stocks would continue to decline. If first-year survival, s₁, were misapportioned and actually higher, however, second-year survival, s₂, would have to be adjusted downward. Estimates of first-year survival within the Columbia basin are available (2-6), and range from 0.023 to 0.15. Using an average value, s₁ = 0.075, we calculate that the s₂ value cited by Kareiva et al. would have to be multiplied by 0.29. If this adjustment applies to barging or dam-passage survival, then we calculate that dam breaching could reverse the decline of these salmon stocks.

Kareiva et al. conclude that dam breaching would recover salmon if delayed (indirect) mortality were significant. They do not, however, present this conclusion in the abstract, dismissing it as speculative simply because indirect mortality is difficult to evaluate. Although their model can be interpreted as supporting dam breaching, or reasonably modified to support dam breaching, Kareiva et al. would have us believe that "modest reductions" in mortality, such as simultaneously doubling survival rates s₁ (survival throughout natal streams of the Snake River basin) and s_e (survival in the estuarine/early ocean environment), may be preferable to removal of man-made obstacles. Model interpretations not based on a full range of reasonable alternatives should be judged with caution.

Jeffrey M. Dambacher
Department of Fisheries and Wildlife
Oregon State University
Corvallis, OR 97331, USA
E-mail: dambacherj{at}fsl.orst.edu
Philippe A. Rossignol
Department of Entomology
Oregon State University
E-mail: rossignp{at}bcc.orst.edu.
Hiram W. Li
Department of Fisheries and Wildlife
Oregon State University
and Oregon Cooperative Fish and
Wildlife Research Unit
U.S. Geological Survey
Corvallis, OR 97331, USA
E-mail: Hiram.Li{at}orst.edu
John M. Emlen
Western Fisheries Research Center
U.S. Geological Survey
6505 NE 65th Street
Seattle, WA 98115, USA
E-mail: john_emlen{at}usgs.gov

REFERENCES

1. P. Kareiva, M. Marvier, M. McClure, Science 290, 977 (2000) [Abstract/Free Full Text] .
2. R. L. Major and J. L. Mighell, Wash. Fish. Bull. 67, 347 (1969) .
3. T. C. Bjornn, College of Forestry, Wildlife and Range Sciences Bull. 27 (Univ. of Idaho, Moscow, ID, 1978).
4. R. B. Lindsay et al., Study of Wild Spring Chinook Salmon in the John Day River: Final Report (Oregon Department of Fish and Wildlife, Portland, OR, 1985).
5. R. B. Lindsay et al., Spring Chinook Salmon in the Deschutes River, Oregon: Information Report (Oregon Department of Fish and Wildlife, Portland, OR, 1989).
6. M. J. Bradford, Can. J. Fish. Aquat. Sci. 52, 1327 (1995) .

Citing higher egg-to-smolt survival values from several sources, Dambacher et al. suggest that we have apportioned too much mortality to the first year of life in the freshwater environment (1). In fact, the range of first-year survival rates we estimated (1.5 to 4.1%) falls within the range of documented values [high estimate of 15% (2); low estimate of 1.1% (3)]. In addition, the single study that reported a first-year survival rate greater than 10% (2) estimated survival only for the subset of juveniles that remained in the study area before smoltification. Finally, it is important to recognize that we derived our estimates of first-year survival from data collected for the specific stocks and period of interest.

Second, the suggestion by Dambacher et al. that the conclusions of our report should be fundamentally altered--because mortality must be apportioned elsewhere--is biologically tenuous. They propose that second-year survival should be reduced to 0.0038 (the value of our s₂ estimate, multiplied by 0.29). However, s₂ is not a completely free parameter, but is given by the equation s₂ = [zs_z + (1 - z)s_d]s_e, where z is the proportion of smolts transported downstream in trucks and barges, s_z is their survival, s_d is the survival of instream migrants, and s_e is survival in the estuary and during entry into the ocean (1). Widely accepted estimates of z = 0.729 and s_z = 0.98 (4), coupled with our estimate of s_e = 0.017, imply that for s₂ to equal 0.0038, s_d would have to have a negative value (-1.81)--an obvious impossibility. Indeed, the most recent data from PIT-tag studies suggest that s_d is actually substantially higher than the value we used in our model (6); we purposely selected the lower estimate derived from PATH models (5) to give the dam-breaching option its best chance of showing effective results.

Of course, s₂ = 0.0038 can be achieved by assuming a sufficiently low s_e. But s_e is survival below all of the dams; hence, any argument about the merits of dam breaching must return to the point we made [figure 5 of (1)] regarding mortality outside of the migration corridor attributable to the hydropower system. Thus, the conclusions reported in (1) remain solid: reducing mortality in the first year of life and in the estuarine/early ocean phase can substantially improve the population growth rates of these stocks.

Peter Kareiva
Northwest Fisheries Science Center
National Marine Fisheries Service
2725 Montlake Boulevard East
Seattle, WA 98112, USA
Michelle Marvier
Department of Biology
Santa Clara University
Santa Clara, CA 95053, USA
Michelle McClure
Northwest Fisheries Science Center
National Marine Fisheries Service

REFERENCES

1. P. Kareiva, M. Marvier, M. McClure, Science 290, 977 (2000) .
2. T. C. Bjornn, College of Forestry, Wildlife and Range Sciences Bull. 27 (Univ. of Idaho, Moscow, ID, 1978).
3. D. Fast, J. Hubble, M. Kohn, B. Watson, Yakima River Spring Chinook Enhancement Study: Final Report (Yakima Indian Nation Fisheries Resource Management, 1991).
4. National Marine Fisheries Service, DRAFT Biological Opinion: Operation of the Federal Columbia River Power System (2000) (available at www.nwr.noaa.gov/1hydrop/hydroweb/docs/2000/2000Biop.htm).
5. D. Marmorek, C. Peters, I. Parnell, Eds., Plan for Analyzing and Testing Hypotheses (PATH): Final Report for Fiscal Year 1998 (ESSA Technologies, Vancouver, Canada, 1998).
6. J. Williams, S. Smith, W. Muir, North Am. J. Fish. Manage., in press.