Posted by: Dan | November 9, 2008

Salmon and the Dams They Cross

I’d been meaning to mention this article for a few weeks, Rethinking Dam: Pacific Salmon Recovery May Rest on Other Factors. The story of Chinook and Steelhead salmon has been one of the best known migration stories. It has also been one of the iconic stories of species decline.

Approaching the confluence of the Snake and Columbia Rivers in 1805, Lewis and Clark marveled at the “almost inconceivable multitudes” of salmon hurling themselves through the roiling waters, but worried at the “emence numbers” of bruised, battered, and dead fish farther upriver. The American explorers, unaware that Pacific salmon die after an orgy of spawning, were mystified at the frenzied spectacle of life and death before them.

An estimated 16 million salmon once swam the ancient migration route from river to ocean and back again, sustaining then-abundant bear, eagle, mink, and river otters along the way and releasing nutrient-rich compounds to replenish Columbia Basin streams and forests as their spent bodies decayed. By 1930, decades of overfishing, unchecked mining, logging, and engineering projects pushed wild stocks into serious decline—just as the US government resolved to fully exploit the hydroelectric and irrigation potential of the Columbia with a massive system of dams.

Today, just under 2% of salmon retrace the improbable journey of their forebears; 13 Columbia salmon stocks are listed as threatened or endangered. Most researchers and environmentalists believe that the extensive network of dams poses the biggest threat to salmon recovery. But a new study by David Welch, Carl Schreck, and colleagues suggests that factors other than dams may also be vital to the survival of ocean-going juveniles (called smolts).


These individuals are tagged before they are released as stocked salmon in the Snake and Thompson Rivers. Their movements are monitored by an impressive system called the POST array.

Surprisingly, smolts fared just as well negotiating the heavily dammed Columbia as they did going down the free-flowing Fraser. Comparing the rivers section by section, Chinook smolts traversing the dammed system actually had higher survival rates than their cousins in the Fraser. Adjusting estimates to consider the distance and time smolts had to migrate to reach the river mouth revealed that average survival rates were much higher for both species from the Snake River than for those in the undammed Fraser. In fact, no matter how they analyzed the data, the researchers reported, “survival is not worse in the Columbia despite the presence of an extensive network of dams.”

The only thing I don’t understand is how migrating down the dams is a greater obstacle than migrating up them. Anyone know?

Don’t miss the video included in the supplementary data, and the TOPP website (Tagging Of Pacific Pelagic). They show real time data and timescale data of individual white sharks, tuna and other large predators. (HT: Rob at Bayblab)

  • Welch DW, Rechisky EL, Melnychuk MC, Porter AD, Walters CJ, et al. (2008) Survival of Migrating Salmon Smolts in Large Rivers With and Without Dams . PLoS Biol 6(10): e265 doi:10.1371/journal.pbio.0060265

Responses

  1. I’m writing from the upper Columbia. The danger in migrating down dams is getting munched in the turbines. Its a huge issue affecting mortality rates – even any scales scraped off affects the fish. Douglas County Public Utility in WA state installed a very expensive fish screen and slide to pass juveniles downstream around the dam… various opinions on that… the slide ends in mid air and depending on the water levels that year, there can be quite a drop, which can temporarily disorient the smolts and leave them vulnerable to seagull predation. check out http://www.wenatcheevalley.org/tours_dams.html
    for the less controversial info.

  2. Thanks very much for the explanation!

    That makes sense for the problem of getting down the dams, but what about getting back up when returning as adults to spawn – isn’t that just as much an obstacle, if not more? Or are there ways that the salmon can circumvent the dams (via artificial assistance) on their upward journey?

  3. Sorry, I think I answered my own question, by re-reading the Columbia River Dams link you shared: Fish Ladders:

    The Wells Hydroelectric Project was built with fish ladders on both ends of the dam to facilitate the upstream migration of adult salmon and steelhead. One of the ladders is equipped with a trapping mechanism to aid biologists in the study of fish and to help enhance propagation of the fish

    Apparently fish ladders are nothing new, with the idea going perhaps back even to 17th-century France, and is said to be a very successful approach.


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