US Fish & Wildlife Service


Date of this Version



Published in A response by State, Tribal, and USFWS technical staff to the 'D'
analyses and discussion in the Anadromous Fish Appendix to the U.S.
Army Corps of Engineers’ Lower Snake River Juvenile Salmonid
Migration Feasibility Study, 1-50, (1999)


In the Anadromous Fish Appendix of the US Army Corp of Engineers (USACE) Environmental Impact Statement on the Lower Snake River Hydrosystem Alternatives for recovery of Snake River salmon and steelhead (hereafter referred to as "A-Fish"), the National Marine Fisheries Service (NMFS) suggested that transportation effectiveness of spring/summer chinook may have improved markedly in recent years. The NMFS conclusion was based on estimates of 'D'-values (the differential delayed survival rate between transported fish and fish that migrated in-river) for 1994-1995 (NMFS, 1999). NMFS suggested, if 'D' is high (estimated in A-Fish at 0.8) and extra mortality of in-river and transported smolts is unrelated to the hydropower system, transportation options may meet recovery standards as well or better than natural river options. NMFS also suggested that further studies could reduce the uncertainty about true values of 'D' and provide greater confidence to make a decision on the alternative management action needed to recover listed Snake River salmon and steelhead. In this analysis, we demonstrate that the evidence is compatible with a wide range of ‘D’ values, but only a small portion of this distribution is as high as the A-Fish estimate. We also present evidence that the extra mortality of in-river fish is related to the hydrosystem.

We analyzed a suite of plausible assumptions used in the calculation of 'D'. Based on our analysis of the 1994-1996 PIT-tag data, there is a wide range of possible 'D'-values. The NMFS' estimate falls at the upper end of this distribution (90th – 95th percentiles). Alternative 'D'-values, based on what we believe to be more reasonable assumptions, were closer to 0.48. Because 'D' is a modeled value (and not a measurement, as implied in the A-Fish), it is very sensitive to the suite of assumptions made and how the data are grouped. 'D' estimates were most sensitive to: (1) whether or not fish that were transported from downstream collection/transport sites (Lower Monumental (LMO) and McNary (MCN) dams) were included in the group of fish used to estimate transport smolt to adult return rates (SAR); and (2) how reach survival rate estimates were extrapolated down to Bonneville Dam (BON). In 1994 the ‘D’-value estimated using four collection projects was much lower than two collection projects. However, in 1995 and 1996 the difference in ‘D’ using two and four collection projects was not as dramatic as in 1994. Therefore, the estimated high ‘D’-values are mainly driven by this single assumption for one year. Based on past and proposed future transportation operations, it is unclear why fish transported at the lower two projects were excluded from the NMFS analysis.

Transported fish are subjected to stress, injury, and crowding at the collection projects. In addition, the physiological state of fish may be poorly synchronized with the time of saltwater entry for transported fish. These factors could explain the higher delayed mortality experienced by transported fish as suggested by a consistently estimated ‘D’ value that is less than 1.

We disagree with the NMFS assertion that “ongoing direct experiments that contrast the return rates of tagged fish that pass through the hydrosystem versus the return rates of transported fish can resolve this question in a clear and unambiguous manner”. While a few components of the 'D'-value estimate are measurable, the sensitivity analysis highlights differences in assumptions
and uncertainties that are not likely resolvable in the near term. In addition, low numbers of returning adults and small numbers of smolts for wild spring/summer chinook salmon may hamper reducing the uncertainty in estimates for reach survival rates and SARs for a non-detected group. Therefore, data are unlikely to perfect our understanding of 'D' or eliminate the uncertainty in the most influential assumptions.

The hypothesis of extra or delayed mortality due to hydrosystem passage has an empirical basis, as well as biological rationale. Based on recent PIT tag data we also found evidence that delayed mortality of both in-river and transported smolts was related to hydropower. More specifically, the evidence suggests that, at least for collected and bypassed smolts, there is a difference between the patterns of direct passage survival rates and SARs. Smolts first detected and transported from the downstream projects (LMO and MCN) had lower SARs than smolts collected and transported from higher up in the system. Similarly (as reported in the A-Fish), SARs of in-river smolts decreased as the number of times the fish were collected and bypassed increased. These pieces of information provide evidence that the Snake River spring summer chinook extra mortality is related to the juvenile migration hydrosystem experience.

Based on results from life-cycle modeling (Marmorek and Peters 1998b), transport based management options lead to a high likelihood of recovery only when ‘D’ is high and the source of extra mortality is not related to the experience during hydrosystem passage. However, when extra mortality is hydrosystem related (which our analysis supports), the natural river options are still the most likely management action to recover these stocks, even if ‘D’ is high (which our analysis does not support). Simply studying ‘D’, if that were possible, without determining the source of extra mortality, yields little additional insight into effects of the different management actions on Snake River spring/summer chinook recovery. Given the dangerously low level of
these populations, we do not believe it is prudent to make management decision on the configuration and operation of the Snake and Columbia hydrosystem for the next 5-20 years (i.e. delaying a decision preserves status quo configuration), based solely on one optimistic assumption about the effectiveness of past and current hydrosystem operations.