Ecology and salmon related articles

Smolt Transportation

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

Has smolt transportation compensated for effects of dams?

Conclusion: Transportation alone does not appear sufficient to overcome the negative effects on survival of salmon caused by development and operation of the hydroelectric system.

The Independent Scientific Group (Williams et al. 1996) concluded that the "concept that we can engineer our way out of the present crisis [is] at odds with the prevailing scientific thought." "Furthermore, transportation alone does not appear sufficient to overcome the negative effects on survival of salmon caused by development and operation of the hydroelectric system."

Transported smolts are released alive, but survive poorly to return as adults. Delayed mortality of transported smolts appears to be related to conditions within the hydropower system. Several possible mechanisms have been identified from the literature that may explain delayed mortality of smolts that are transported (as well as those that migrate through the hydropower system). These include: altered saltwater entry timing which is poorly synchronized with the physiological state of the smolts; stress from crowding and injury (including descaling) during bypass, collection, holding and transport; increased vulnerability to disease outbreak due to stress and injury; and increased vulnerability to other stressors in the environment or to predation, particularly northern squawfish.

Smolt-to-adult return rates (SAR) of transported wild spring/summer chinook smolts have been consistently less than the 2% to 6% interim goal defined in PATH. The SARs of transported wild smolts have shown no evidence of an increasing trend over the past two decades.

Not only have SARs remained extremely low, there is no indication that the gap has narrowed between performance of Snake River stocks and lower river stocks, as might be expected if transportation and hydrosystem improvements were merely masked by generally poor ocean conditions for all stocks. In the 1996 Retrospective Analysis, Deriso et al. (1996) estimated the differential instantaneous mortality rate ("mu") between Snake river and similar downriver populations. The differential mortality between upriver and downriver stocks did not decrease over time; the geometric mean of mu by period was 1.3, 1.3, 0.6, 1.3 and 2.0 for 1972-1974, 1975-1980, 1980-1984, 1985-1989, 1990-1992, respectively. The differential mortality increased significantly as water velocities decreased during the smolt migration. Examination of the data in Fig. 2 (not show on this web site) does not support a hypothesis that migration conditions (including transportation) were continually improving for Snake River stocks compared to lower river stocks.

Hypotheses and models that suggest transportation has effectively reduced hydrosystem mortality must point to some other factor that selectively causes mortality for Snake River fish beyond that affecting lower river stocks. This is because passage models which assume low delayed mortality of transported fish do not match the spawner and recruit patterns of both Snake River and downriver populations after the hydropower system was completed. Some additional explanation is then needed to resolve this discrepancy. These hypotheses have pointed at some factor (independent of effects of the hydropower system), such as BKD, genetic viability, or some unidentified climate factor, being selectively worse for Snake River fish than for downriver stocks since the hydropower system was completed. Biological mechanisms to support these alternative hypotheses have not been postulated.

There may be relative benefits to transportation compared to allowing smolts to migrate through the hydroelectric system, but these are not conclusive. Regarding relative benefits of transportation, Mundy et al. (1994) concluded that while transportation appears to improve the relative survivals of certain salmon and steelhead from the Snake River Basin under certain combinations of dam operations and river flow conditions, it removes only part of the mortalities attendant to passage through the hydroelectric system. Recent studies have raised further questions about even these relative benefits.

Emerging PIT tag data for wild spring/summer chinook smolts raise questions about the perceived relative benefits of transportation (IDFG 1998). Typically, transportation has been justified primarily because smolts avoid some of the direct mortality due to the dams, and based on NMFS studies indicating that adult return rates have been higher for transported smolts than for smolts that migrate through the hydropower system. In past transportation studies, smolts were bypassed, collected, held in raceways, tagged and assigned into "transport" and "control" study groups. The "controls" pass through mechanical bypass systems an average of two and on half times during their migration through the four lower Snake dams. Mechanical bypass systems are used at the dams to collect smolts for barging and can be quite stressful. Emerging PIT tag data suggest that the more times smolts are bypassed the poorer the adult return. Transported smolts returning as adults in 1997 did not do any better than smolts migrating in the reservoirs in 1995 which passed the dams via the spillways or turbines. The estimated survival of these true inriver migrants (0.38%) was actually higher in 1997 than that of transported fish (0.26%), although the difference was not statistically significant.

In 1998, the Independent Scientific Advisory Board (ISAB) was asked by the Implementation Team to evaluate transportation as a management option for the 1998 smolt migration season. The ISAB (1998) recommended a spread-the-risk approach involving barges, spill and other measures intended to enhance downstream passage survivals throughout the entire spectrum of the salmon and steelhead emigration.

Their Status and Recovery Options
Report to the Director Idaho Fish & Game 5/1/98
Issue Paper: Smolt Transportation

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