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Ecology and salmon related articles

Hypotheses for Delayed or Extra Mortality

of Snake River Spring/Summer Chinook

Idaho Fish & Game Report to the Director 5/1/98

Why has barging not worked to recover salmon?

The PATH Preliminary Decision Analysis Report on Snake River Spring/Summer Chinook (Marmorek and Peters 1998; Appendix A, p. 94) defines delayed or extra mortality as any mortality occurring outside the juvenile migration corridor that is not accounted for by:

  1. underlying spawner-recruit relationships;
  2. estimates of direct mortality within the migration corridor; or
  3. common year effects affecting both Snake River and downriver stocks (delta model only).
A central issue of the competing hypotheses is whether smolt transportation has mitigated the effects of the hydropower system. The following three points are critical to understanding the issue:
  1. survival rates of Snake River spring/summer chinook stocks decreased more after the hydropower system was completed than did survival rates of downriver stocks (above 1-3 dams);
  2. most Snake River smolts have been transported for the past two decades; and
  3. a relatively small proportion of smolts die directly during barge or truck transportation.
The "hydro" hypothesis attributes most of the differential decline in Snake River salmon survival to increased direct and delayed mortality of juvenile migrants due to the hydropower system (p. 38; Table 4.1-2; Appendix A, p. 95-99).

Both the "BKD" or "stock viability" hypothesis and the "regime shift" hypothesis attribute most the differential decline of Snake River salmon survival to factors other than the hydropower system (p. 39; and Table 4.1-2; Appendix A, p. 105-109). It follows that if delayed mortality due to the hydropower system is low, then the "non-hydro" factors must have had a systematically worse effect on Snake river salmon since the dams were constructed (Table 4.1-2).

These three alternative hypotheses considered in the Preliminary Decision Analysis (p. 38-39) were:

  1. Hydro-related

    The completion of the Federal Columbia River Power system in the late 1960s through the mid-1970s and it subsequent operation, have increased the direct and delayed mortality of juvenile migrants, resulting in considerably sharper declines in survival rates of Snake River spring and summer chinook stocks (over the same time period), than of similar stocks which migrate past fewer dams and are not transported. This hypothesis follows from Conclusion 3a.2 of the PATH FY96 Conclusions Document:

    We are reasonably confident that the aggregate effects of the hydrosystem have contributed to reduced survival rates of Snake River stocks (from spawners to adults returning to the mouth of the Columbia River), during the post-1974 period, as compared to the pre-1970 period. Hydrosystem effects include both direct (e.g., turbine mortality) and indirect effects (e.g., delayed mortality, due to such mechanisms as changes in estuary arrival times).
  2. "BKD" or Stock Viability Hypothesis

    The hypothesis proposes that

    1. the viability of Snake River stocks declined as a direct or indirect result of the hydrosystem construction in the 1970s;
    2. current extra mortality is not related to either the hydrosystem or climate conditions; and
    3. extra mortality is here to stay, even if hydrosystem direct mortality is reduced and/or the climate improves.
    One hypothesis to account for decreased stock viability is that hatchery programs implemented after construction of the Snake River dams increase either the incidence in the level of bacterial kidney disease (BKD) within the wild population or its severity. In both cases, mortality increased in juvenile fish after they exited the hydropower system as compared to earlier years (or as compared to downstream stocks for the same time period). Under this hypothesis, it is unlikely that the increased rate of mortality from BKD would change back to a more favorable condition in the near future. Another stock viability hypothesis is that low stock sizes have led to increased predation rates on juveniles and insufficient nutrients from returning adults' carcasses to support growth of parr.

    BKD is one possible means by which stock viability may have been reduce. Occasional changes in underlying stock viability may cause some or all of the delayed mortality to remain, even if direct mortality is reduced. The consequence of falling into this category (i.e., "delayed mortality is here to stay") is that it is unknown when or if the impacts will switch back to a [more] benign state. For modeling purposes, we consider this the worst case, which is that these factors will stay in the present less favorable state.

  3. "Regime shift" Hypothesis

    Extra mortality is not related to the hydropower system but is due instead to an interaction with a long term cyclical climate regime shift with a period of 60 years. This regime is believed to have shifted from good to poor during brood year 1975, and is expected to return to above average conditions in 2005. The signatures of a recurring pattern of interdecadal climate variability are widespread and detectable in a variety of Pacific basin climate and ecological systems. These cyclical changes affect ocean temperatures and currents which affect distributions of predators and prey; and broad scale weather patterns over land masses which then affect temperatures, rainfall, snowpacks, and subsequent flows. The changes in conditions could affect various stocks to different degrees with the effect on Snake River stocks being systematically different from lower river stocks. There is nothing we can do to change these patterns, but they are expected over time to provide more favorable and less favorable conditions to species located in different areas.

Their Status and Recovery Options
Report to the Director Idaho Fish & Game 5/1/98
Issue Paper: Hypotheses for Delayed or Extra Mortality of Snake River Spring/Summer Chinook

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