Survival of Snake River Salmon & Steelhead
Data compiled July 2004 by
Skip to Executive Summary

The following is a generalized example of where salmon mortality occurs. In all cases we will be talking about Snake River salmon & steelhead. Our chief example will follow the spring chinook, making comparison with Snake River sockeye or other species when appropriate.


5000 eggs per spring chinook adult pair.

On average, Snake River spring chinook lay about 5,000 eggs. Studies in Idaho indicate that well over 90% of the eggs will be fertilized by the male (or males). In the gravel, some eggs will perish because they were not perfectly formed, others will fall prey to microorganisms like fungus or parasites. Additionally, young fish and developing eggs are eaten by birds and by other fish. Of the 5000 eggs laid by the female chinook, about 8% survive to migrate downstream towards the ocean. On average, 400 smolt migrate per spring chinook adult pair.


Snake River Chinook Salmon in-river Survival 1964-2010.

Juvenile Migration to Hydrosystem Corridor

For Steelhead and spring/summer chinook about 60% survive to the beginning of the hydrosystem corridor at Lewiston, Idaho. The survival rate to here depends on the distance traveled but typically falls in the 50 - 70% range. Sockeye survival to the first reservoir is a bit lower with a survival rate in the 30-40% range.

Juvenile Survival through the Hydrosystem Corridor of eight dams and reservoirs

Based on averages from 1994 through1999, juvenile survival through the Hydrosystem Corridor is 7.4% for fall chinook, 40.3% for spring/summer chinook and 41.5% for steelhead.

Mortality in Reservoirs
Reservoirs formed behind the dams slow water flows, alter river temperatures, and provide habitat for predators, all of which may result in increased mortality. Salmon mortality occurs mostly in the reservoirs (40%), while mortality at dams is about 20%.

Through the first reservoir, 95-96% of the steelhead and spring/summer chinook reach the first dam (Lower Granite). The situation is much worse for the fall chinook with only 60% surviving to Lower Granite dam.

During late June through August, summertime flow augmentation provides measurable survival benefit for juvenile fall chinook. It is far from conclusive, however, if this survival benefit is due to water temperature, turbidity or flow rate. The next few years of returning adults (which migrated downstream as juveniles during low flow augmentation years), will greatly increase our understanding of flow augmentation effects (see

Mortality by Predacious Fish
Survival past predacious fish is estimated to be about 92.7% - 94%. The higher survival rate is attributed to a sport reward fishery of Northern Pikeminnow costing Bonneville Power Administration ratepayers (primarily Washington electricity users) $1 million annually.

Collection and Transportation by Barge
During migration season, salmon and steelhead are collected, transferred and transported by specially designed barges where they are released below Bonneville dam, the last dam before the ocean. For decades, the Army Corps of Engineers has suggested that collection and barging provides adequate salmon survival. The recent "Comparative Survival Study" by Fish Passage Center (11/26/3) suggests otherwise:


Predation in estuary below Hydrosystem corridor

Caspian Terns consume 10.4% of spring/summer juvenile chinook, 5.4% of juvenile Fall Chinook and 18% of juvenile steelhead. Cormorants consume approximately 3% and gulls consume another 1% of juvenile salmonids. Comparable to the predation by birds, it is estimated that harbor seals may consume 14.4% of juvenile chinook.

Delayed Mortality (see

Indirect evidence suggests that the hydropower system causes delayed mortality in salmon. While direct mortality from hydropower eliminates 25 to 73 % of juveniles and adults, Snake River fish may experience 37% to 68% "additional mortality," or delayed mortality from the effects of the hydrosystem corridor.

Ocean Conditions
Ocean conditions, availability of food, and an abundance of predators brings an estimate of 94% - 98% mortality in the 2 to 3 years spent in the ocean.


Adult survival to Hydrosystem Corridor

Mortality by Seals and Seal Lions
Returning adult salmon face predation by seals and sea lions at an estimated mortality rate of 1% to 1.5%.

Harvest Impacts allowed by NOAA Fisheries

For spring chinook and sockeye, allowable impacts depend on expected run sizes. In recent years fishery managers have allowed a 31.3% impact on Snake River fall chinook, up to an 11% impact on spring/summer chinook, and 17% impact on steelhead.

Adult survival through Hydrosystem to Spawning Grounds

Based on data collected on returning adult chinook and steelhead, 72.2% survive upstream migration through the Hydrosystem Corridor. For adult chinook that survived all the way up through Hydrosystem Corridor about 80% - 90% return to their homeland spawning beds.


In the case of Snake River spring chinook, an adult female lays about 5000 eggs.
5000 eggs per adult spring chinook pair.
400 smolt migrate, assuming 8% survival.
200 - 280 smolt survive to first reservoir, assuming 50% to 70% survival.
81 - 113 survive the Hydrosystem Corridor as juveniles, assuming 40.3% survival.
57 - 80 survive bird and seal, sea lions predation in the estuary, assuming 71.2% survival.
18 - 51 survive delayed effects of the Hydrosystem, assuming 32% to 63% survival.

0.36 - 3.0 survive years in the ocean to begin upstream migration, assuming 2% to 6% survival.
0.32 - 2.7 survive seal and seal lion predation and harvest, assuming 87.5% to 88% survival.
0.23 - 1.9 survive the Hydrosystem Corridor as adults, assuming 72.2% survival.
0.18 - 1.7 adult progeny survive to spawning grounds, assuming 80 to 90% survival.


The available data indicates with 95% confidence that Snake River salmon and steelhead populations continue to trend downward. Note that while the 2000 FCRPS Biological Opinion establishes lambda as the best measure of population trend, the proposed draft Biological Opinion looks to abandon this approach.

Estimated return rates: Adult to Adult
1.76 survive to spawning grounds per adult pair Snake River fall chinook.
1.62 survive to spawning grounds per adult pair Snake River spring/summer chinook.
1.42 survive to spawning grounds per adult pair Snake River steelhead.
0.36 survive to spawning grounds per adult pair Snake River sockeye.


Four Lower Snake River dams in southeastern Washington

Lower Granite, Little Goose, Lower Monumental and Ice Harbor



For further information, questions or comments contact

Visit to stay current on the plight of Idaho's wild Salmon & Steelhead.