[3] Good Hydro Survival For Juvenile Salmon
NOAA scientists made their annual pilgrimage to the Northwest Power and Conservation Council last week to announce their latest survival findings for fish traveling through the federal hydro system. The news was mainly good, if you like dams.
Fish survival in the Snake River above the hydro system was pretty close to what they found in the hydro system itself. Last spring, 58 percent of the inriver juvenile spring chinook made it from Idaho all the way past Bonneville Dam, the best results since they started pit-tagging fish and sending them downstream, they said.
But the region may not be able to wring much more fish survival from dam modifications. The researchers said they couldn't correlate hydro survivals with adult returns and ocean conditions, which likely far override any tweaks at the concrete. They also mentioned another take-home point that feds seemed to have neglected in the past--the dams can't be blamed for all the juvenile fish death in the hydro system, much of it is perfectly natural.
This year's 58-percent survival is a bit better than last year's, said NOAA's Steve Smith who led Council members through the numbers. He said 2006 was the second-highest flow year since 1993, when the pit-tagged survival studies started.
Smith said the survival of hatchery fish to Lower Granite Dam still depends a lot on how far they have to swim, and the fish start dying as soon as they leave their particular hatchery, long before they encounter any dams. It's a process that continues through the reservoirs as well, after the fish enter the hydro system.
In 2006, average hatchery survivals were up from the year before--about 2/3 of the fish from the hatcheries have survived to Lower Granite in the past eight years.
Better conditions greeted fish once they entered dam country, as well, with more of them shooting over dam spillways in 2006 because more water was spilled this year than 2005, when little spill occurred during the early spring because of low flows.
In early spring, young fish spent nearly 30 days getting from Lower Granite all the way downriver to Bonneville Dam. However, as flows increased, the fish showed some of the shortest travel times researchers had ever seen, about 10 days, on average, Smith said.
In contrast, during the late spring of 2001, the second lowest flow year on record, the fish were still taking about 25 days to get through the system. Last year, travel times ended up about average.
Smith noted that the inriver migrants have made up a lesser portion of all fish that reached Lower Granite Dam over the years. Most have been barged--up to 90 percent in years like 2001, and in the 60 to 70 percent range when flows were about average.
Between Lower Granite and Little Goose dams, the next project downstream, that inriver group of spring chinook has averaged about 92 percent survival (1993-2004), and steelhead has averaged 90 percent.
From Lower Monumental to McNary, survival averaged about 84 percent for chinook and 71 percent for steelhead between 1995 and 2004. A large colony of Caspian terns near Pasco, Wash., on Crescent Island is likely responsible for eating a lot of steelhead, Smith reported. In 2001, 23 percent of the pit tags used for steelhead ended up on the island.
For 2006, Smith said, the 58-percent juvenile chinook survival to Bonneville Dam is the highest they have ever seen. For steelhead, the estimate was 37 percent, higher than the recent past.
Council Chair Tom Karier asked what was responsible for the higher survivals this year. Smith suggested that it could have been faster travel times due to a combination of higher flow and spill provided throughout the system. He said his agency thinks all those things are related.
But Smith said mortality through the hydro system was close to that measured outside of it. He "hadn't done the math yet," but if one calculated the "per-km" mortality from the hatcheries to Lower Granite, it would be very similar to the mortality found in the hydro system, he told council members.
Smith said the researchers found that each time a fish is detected passing through a dam bypass system rather than over a spillway or through a turbine route, about half a day is added to its journey through the hydro system. This works out to a couple of more days if the fish is routed through four bypass systems, which makes the trip 17 days long instead of 15.
Those extra days add up to more mortality, and researchers have been looking into the factors that affect fish travel time. Smith said information contained in the February 2005 tech memo on dam effects said travel time seemed to decrease as the season progressed, yet, after about May 1, flow seemed to be more important to reduce travel time than time of departure.
But for steelhead, the date the fish migrated seemed unimportant, while flow reduced travel time both early and late in the migration season.
Smith also used the occasion to make a few critical comments about the "broken-stick" survival model exhibited in the tech memo. Excluding low fish survivals from the low-flow year 2001, he showed a graph with no real connection between flow and survival. However when the 2001 data was added, the "broken stick" was apparent.
But Smith said the tech memo contained a "survival-versus-date" graph using the 2001 data that showed higher survival at relatively lower flows. The missing link in all this was a temperature factor.
He said findings developed while creating a new passage model for analyzing potential BiOp scenarios has caused them to go back and try to parse out reservoir survival of spring chinook from that at the dams. They have found several factors to consider, including the length of the reservoirs, temperature, a small negative influence of travel time on survival, and a positive linear relationship between flow and survival through the reservoirs.
"But the temperature effects appear to be larger," he told the Council. He said mortality of spring chinook is really high when waters reach a 14- to 15-degree C. range, "which is what we saw in 2001."
The scientists couldn't say why increased flows improved survival, but speculated that it may have something to do with increasing turbidity or improving fish travel time past potential predators.
A question arose about the value of more dam improvements. Smith said NOAA Fisheries has attributed about 17-percent fish mortality to the four lower Snake dams. He said if every fish could be routed through the most benign passage route (usually a removable spillway weir), mortality could be decreased to 9 percent. He added that this was not realistic.
Realistic improvements might improve overall spring chinook survival through the four-dam stretch to about 88 percent from the current 83 percent, Smith said.
Some Council members seemed a bit startled that the feds were showing a positive flow/survival relationship. Idaho's Jim Kempton pressed the researchers for more information.
Smith said the COMPASS model is addressing a number of possible hypotheses, while the tech memo had pointed to a "slight, still positive relationship between survival and flow." He said the new model may have a slightly greater influence of flow, but it may not be the hypothesis that ends up with the most support.
NOAA researcher John Williams reminded the Council that the researchers had already pointed out that "there is natural selection working on these stocks from the time they are released, or released from a hatchery, until they got to the dams--distance made a difference. When we empirically measure an estimate of survival, that's the empirical estimate of what the survival is, but we can't derive a hundred percent of all the mechanisms by saying it's just because it passed through the dam.
"Certainly," Williams continued, "some of those fish going downstream were going to be in the process of dying just like they were upstream of the dam."
Williams said salmon start out with 5,000 eggs, and only two adults are expected back from then, so "there's a continual process throughout the whole part."
Smith said the same principle applies to the analysis of barged fish--some of the fish they put in barges would have died if they had been left in the river.
The researchers said they didn't have enough time to present information on the latest transportation results, but if they had, it would have been another good news day for the Council.
NOAA Fisheries researcher Doug Marsh had pointed out at a November 2005 Corps of Engineers research review that adult wild chinook from the 2002 outmigration showed more than a 60-percent survival benefit from being barged at Lower Granite, with steelhead showing better than a 300-percent benefit.
Williams explained another element of NMFS' latest research--using coastal upwelling indices to help predict adult returns, a result suggesting "that the ocean conditions, to a large degree, are overriding anything that we're seeing in the way of changes in survival in the hydropower system."
Another issue Williams mentioned was the growing evidence that pit-tagged fish returning as adults are coming back at a lesser rate than untagged fish. He said smolt-to-adult returns of wild spring chinook from 1998 to 2002 were above 2 percent (a common yardstick for SARs needed for recovery), but if you looked only at pit-tagged returns, there was only one year that showed a SAR above 2 percent.
Williams also said using data from the Fish Passage Center's Comparative Survival Study, the unmarked portion of the run returned at a rate about 40 percent higher than the tagged population.
He cautioned that pit tags were good when looking at comparing fish going through two treatments, but were not an effective way to measure the "absolute" rate of return. He said the mechanisms are not known for this discrepancy, but the tags may be shed at some point, or less likely, unread as adults. He suspected that something about pit tags was decreasing the fitness of the fish.
NOAA scientist Bill Muir pointed out the importance of ocean conditions in the spring chinook's life cycle. He exhibited graphs that showed a fairly consistent range of juvenile survivals in the hydro system in recent years, ranging from about 26 percent in 2001, to 60 percent. SARs of those fish, on the other hand, ranged from about two one-hundredths of a percent in 1993 to about 1.6 percent in 2001--"a 75-fold variation."
Muir said there was no statistical relationship between the hydro survival of juveniles and their adult return rates. The same held for steelhead.
He noted that once the fish get through the hydro system, there is still a lot of mortality yet to happen. He said nearly 100 percent of the barged fish make it to below Bonneville, yet, on average, 99 percent die before they return.
As for latent mortality to fish that is attributed to their passage through the hydro system, since it occurs outside the researchers' realm, "it's a little hard to get a handle on," Muir said, adding that they need to develop a better understanding of it.
But he did say that researchers have found that the time when juvenile fish enter the ocean "seems to very important." Ultimate survival varied dramatically from one week's entry time to another, according to a graph he presented to the Council.
Muir said the last four dams added to the hydro system have significantly increased the time it takes them to reach the ocean. In average flow years, he said, it takes 10-12 days for a fish to get through it with four dams in place. Now with twice as many projects, it takes twice as long.
To reduce travel time, Muir said increased flows would help, but only in a limited way, because of the large reservoirs in the system. More removable spillway weirs would help, too, since fish use them during the day, but tend not to use regular bypass or turbine routes during daylight hours.
He said the region may be overfocused on the effects of flow within the hydro system where it can be measured, and that flows downstream may be adding "quite a bit of benefit." He said the benefit would be hard to quantify, "but I think it's still there." -B. R.
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Northwest Power and Conservation Council latest survival findings
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