A proposal to remove four dams on the lower Snake River, in the northwestern United States, must be regarded as a "large experiment," which may not help endangered chinook salmon stocks recover.

But unless that chance is taken, chinook stocks in the river seem doomed to extinction. That’s the conclusion indicated by three researchers who used computer models to predict long term population trends for the spring and summer runs of chinook, which are in serious decline in the Snake River.

They found that the key to restoring chinook stocks lies not in improving in-river survival, but in boosting the survival rate of smolts in the estuary.

The dams, surprisingly, are thought to significantly impact what happens in the estuary, hundreds of miles away.

It has long been argued by conservationists and fishermen that the best thing to help the chinook recover would be simply to remove the four Snake River dams, which both impede upstream spawning runs and contribute to juvenile mortality in downstream migrations.

Others have argued the fish can be saved by rigorous mitigation attempts - through barging smolts downstream, improving upstream fish ladders and flushing water during migration periods, to help salmon.

But Peter Karelva and Michelle McClure, of the National Marine Fisheries Service, and Michelle Marvier, of the Department of Biology, Santa Clara University, found that although mitigation efforts are helping, chinook runs will continue to fall towards extinction unless more smolts survive the transition from freshwater to saltwater.

The researchers said dams in the Columbia River Basin have contributed to severe declines in wild salmon runs, with the four lower Snake dams altering spawning habitat and elevating adult and smolt mortalities during migrations.

They also say that without mitigation efforts over recent decades - which have focused on getting spawning salmon above the dams, and barging salmon smolts below them - wild stocks would have almost certainly been wiped out by now.

Those two points would seem to indicate that if fisheries managers could improve survival in the river, by doing a better job of helping salmon by-pass the dams, the runs might rebound.

But even a 100% survival rate in the rivers won’t do the trick, the researchers warn in their paper, Recovery and Management Options for Spring/Summer Chinook Salmon in the Columbia River Basin.

The researchers found that even if a perfect survival rate was obtainable in the river, an event they said was impossible, Snake River chinook would still be doomed.

"Remarkably, even if every juvenile fish that migrated downstream survived to the mouth of the Columbia, and every returning unharvested adult fish survived to reach the spawning grounds, the index stocks would continue to decline. Thus, management aimed solely at improving in-river migration survival cannot reverse the SRSS (Snake River Spring/Summer) chinook decline."

The researchers looked at other life stages and found that the early ocean/estuarine period was crucial to the recovery of chinook stocks.

A modest 9% increase in survival during the ocean/estuarine period could put chinook on the road to recovery.

The researchers concluded that dam removal could play an important role in improving survival during that period, because salmon that were exposed to more natural river conditions might be more robust by the time they reached the ocean.

"Because SRSS chinook salmon spawn in the upper reaches of Snake River tributaries, dam breaching is unlikely to affect available spawning habitat or first-year survival but could improve estuarine survival considerably," they state. "Although survival of juvenile fish during barging is quite high, barging might reduce the subsequent survival of barged fish relative to those that swim downstream.

Breaching the lower Snake River dams would mean the end of fish transportation operations and would therefore eliminate any delayed mortality from transportation. Additionally, the removal of four of the eight dams encountered by Snake River salmon might increase the physiological vigor of salmon that swim downriver, thus improving survival during the critical estuarine phase.

If this indirect mortality were 9% or higher, then dam breaching could reverse the declining trend of SRSS chinook salmon."

The researchers say that "policy-makers may have to view the decisions they make as large experiments, the outcomes of which cannot be predicted but from which we can learn a great deal pertaining to endangered salmonids worldwide."

Although the researchers do not say so, the paper indicates that if the experiment isn’t tried, Snake River salmon will eventually become extinct, no matter what efforts are made to pass fish around the existing dams.