How Much Do Dams Slow Down Juvenile Salmon?

Most proponents of the importance of flow also argue that the dams have substantially lengthened the time it takes juvenile salmon to migrate to the sea, so that hundreds of thousands of smolts die in the river that would have made it to the sea. (Whether or not they would have died anyway in the sea is not discussed.) One particularly influential paper on this subject, has asserted that juvenile stream-type salmon required 26 days to reach The Dalles from the Salmon River before construction of dams and 65 days after construction.46

But this research was later determined to suffer from a critical flaw. The researchers compared marked groups of fish, but they assumed that time of release had no effect on migration speed.47 Dr. William McNeil examined the data and found that “early migrating juveniles move slowly and late migrating juveniles move rapidly. The correlation between migration speed and release date is consistently direct and highly significant . . . . The correlation between migration speed and stream discharge, on the other hand, is equivocal.”48 Dr. McNeil notes that this result “contradicts the theory that migration is a passive behavior”.49

The newest research by the National Marine Fisheries Service has found that 91.2 percent of the variance in travel time for groups of juvenile salmon released in Lower Granite Reservoir and detected downstream could be explained by a simple model using only date and temperature and a year-specific constant.50 The same data showed "absolutely no correlation between flow exposure and median travel time".51

Dr. McNeil has also determined that the time of passage for juvenile salmon populations migrating down the Columbia River “remained consistent among years for four species of stream-type and one species of ocean-type juvenile salmon even though stream discharge fluctuated nearly two-fold annually.52 Records of the Seufert Brothers Company in The Dalles, going back to 1885, show that "there may have been more salmon in the earlier days than there are now, but the pattern for the runs, when they came and so on, is definitely the same".53

The only response to Dr. McNeil’s work that I have ever seen, offered by the Northwest Power Planning Council’s Independent Science Group, is that “the preponderance of thought clearly supports the links between flow and migration rate” and that “[t]his view is reflected in proposed salmon restoration plans”.54 But scientists are not supposed to uncover scientific truth by looking for the “preponderance of thought” or what the government says the truth is. They are supposed to look at data.

There are even claims that increasing the duration of the downstream migration causes fish to become “confused” or lose their urge to migrate. The story is that the transformation of juveniles from fresh to salt water is “delicate and easily disrupted” so that delays experienced while migrating through reservoirs cause fish to “lose their migratory drive”.55 When I couldn’t find any scientific evidence at all in support of this notion, I called up a scientist at the National Marine Fisheries Service in Seattle to ask him about it. He laughed and said he had had the same problem. When he checked the references, he found that they said precisely the opposite: that the salmon were flexible.56

Despite the absence of competent scientific evidence suggesting that juvenile migration delays have had a measurable adverse effect on salmon populations, death from delay is accepted as acknowledged truth and repeated over and over again in most popular books on salmon recovery.57

46 H. Raymond, “Effects of dams and impoundments on migration of juvenile chinook salmon and steelhead from the Snake River, 1966 to 1975”, Trans. Am. Fish. Soc. 108: 505-529 (1979).

47 W. McNeil, “Water Velocity and Migration of Juvenile Chinook Salmon in the Columbia River”, Sept. 26, 1994, at 5 (paper prepared for Hydro Review; later published).

48 Id. at 6.

49 Id.

50 S. Smith et al., "Survival Estimates for the Passage of Juvenile Salmonids Through Snake River Dams and Reservoirs, 1996", at 35.

51 Id. at 34. Flow did explain about 30 percent of the variance in travel time in a regression based on a different set of data derived from groups released at Lower Granite Dam.

52 W. McNeil, "Water Velocity and Migration of Juvenile Chinook Salmon in the Columbia River", at 8.

53 F. Seufert, Return to the River at 145 (emphasis added).

54 ISG, Return to the River 231.

55 B. Harden, A River Lost 71-72.

56 See also, NMFS, 1995 Biological Opinion on FCRPS Operations, Mar. 2, 1995, at 72 (“In his review on smolt transformation, Hoar (1976) provided clear evidence that juvenile fall chinook salmon once they begin to migrate are likely in a state of maturation that would allow a gradual (and possibly sharp) transition to full-strength seawater”).

57 See, e.g., J. Cone, A Common Fate 119; B. Harden, A River Lost 71-72; K. Petersen, River of Life, Channel of Death 107 (“young salmon do not swim to the sea”).

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