2024 Survival Rate Of Migrating Juvenile Salmon In Columbia/Snake Rivers? Hard To Say With Yet Another Year Of Low Detection, Tagging Rates

Increased spill levels at Snake and Columbia river dams, along with lower water flow in the rivers, hampered the ability of scientists to tag and detect juvenile salmon and steelhead as they migrated downstream in 2024.

This is the sixth straight year that detection at the dams has been made more difficult by the level of spill in the rivers, an issue that was exacerbated as spill levels rose to a total dissolved gas saturation cap that was set at 125 percent. Spill at Snake River dams in 2024 was 58,900 cubic feet per second, far above the 2006-2024 mean of 38.1 kcfs. It was the second highest mean spill discharge on record, according to a preliminary NOAA Fisheries’ 2024 survival estimates memorandum.

The low detection and tagging rates, which were below average, reduced the sample size of fish available for sample estimates of both hatchery and wild juveniles from Lower Granite Dam on the Snake River, as well as at other dams downstream in the Snake and Columbia, according to the memo. That, it said, caused imprecise survival estimates in many river reaches.

Overall, the memo shows that in most river reaches, 2024 survival estimates were generally below average for both hatchery and wild Snake River Chinook salmon and steelhead in the river reach between Lower Granite Dam on the Snake River and Bonneville Dam on the Columbia River.

While the survival estimate for the Lower Granite project (head of reservoir to the tailrace) was above average for the combined hatchery and wild groups of Chinook and steelhead, survival for the overall reach from the Snake River Trap to Bonneville was still below average for Chinook and only about equal to average for steelhead.

Travel times for juvenile Chinook for most of the 2024 season were short. Flow in 2024 was moderately below average, yet even then Chinook travel times resembled high-flow years, and April 2024 was among the shortest ever seen for that month.  On the other hand, steelhead travel times were not especially short and were what would be expected for the intermediate flow conditions seen in 2024.

The unusually short travel times observed for Chinook in 2024 may be due to the warmer than average water temperatures this spring, especially in April, the memo added.

The proportion of smolts transported in 2024 was the lowest in the history of this project, slightly lower than the previous low in 2021, according to the memo

Titled “Preliminary survival estimates for the passage of spring-migrating juvenile salmonids through Snake and Columbia River dams and reservoirs, 2024,” the memo released by NOAA every year summarizes conditions in the Snake and Columbia Rivers and lists NOAA’s preliminary estimates for survival of PIT-tagged juvenile salmon and steelhead that passed Columbia and Snake river dams during the 2024 spring outmigration. The memo also analyzes travel time for Chinook salmon and steelhead, as well as the proportion of fish transported from Snake River dams downriver to below Bonneville Dam (https://pweb.crohms.org/tmt/agendas/2024/1120_2024_Preliminary_Survival_Estimates_Memo_final.pdf).

The Nov. 8, 2024 memo was transmitted to Ritchie Graves, Columbia Hydropower Branch Chief at NOAA, from Scott Hecht of NOAA’s Fish Ecology Division of its Northwest Fisheries Science Center in Seattle. It includes key data for assessing the impact of federal hydropower operations on 13 species of salmon and steelhead listed under the Endangered Species Act. NOAA has been providing these estimates since 1993.

According to the memo’s preliminary results, fewer wild Chinook salmon and steelhead smolts were PIT-tagged at Lower Granite in 2024 than in years prior to 2020. In addition, the higher spill levels resulted in a lower number of fish that entered the dam’s juvenile bypass system where PIT-tagged fish would have been detected. More fish passed through spillways where detection is difficult. In fact, the number of fish detected in the bypass system was the second lowest on record, and the number detected in the spillway was the lowest since spillway detection began in 2020.

“An even more consequential limiting factor for survival estimation in 2024 was low detection probabilities at dams downstream of Lower Granite Dam,” the memo said. “A combination of high rates of spill with low flow resulted in extremely low rates of bypass system passage, which equate to low probabilities of detection.  Detection rates in 2024 were close to the lowest on record at nearly all dams. Also, the detection rate dropped at Lower Granite dam relative to 2020-2023. Low detection rates resulted in imprecise survival estimates for most reaches.”

To determine survival downstream of Bonneville Dam, the study uses data from an estuary trawl as well as alternate sources, such as PIT-tags deposited by predacious birds as well as newer automated detections sites at Pile Dikes 5, 6, 7 and 8. The avian information will not be available for several months.

Preliminary Survival Estimates: Snake River Chinook Salmon

• PIT-tagged yearling Chinook have been released every year – 1993 through 2024 – from seven Snake River hatcheries (Dworshak, Kooskia, Lookingglass/Imnaha Weir, Rapid River, McCall/Knox Bridge, Pahsimeroi and Sawtooth hatcheries). From these hatcheries to Lower Granite Dam, the annual mean estimated survival has remained steady. In 2024 survival was 65.3 percent, near the 2023 mean of 65.3 percent. The 1998-2024 mean is 65.1 percent. “Estimated survival from release to Lower Granite Dam was the highest on record for fish from Kooskia hatchery, but the lowest on record for fish from Lookingglass hatchery,” the memo said.
• Survival of yearling Chinook (hatchery and wild) was above average in the Lower Granite to Little Goose and Little Goose to Lower Monumental reaches, but below average in the Lower Monumental to McNary reaches. Survival was also below average in the McNary to John Day reach, but above average in the John Day to Bonneville reach. The memo noted that most of the estimates are imprecise.
• Longer reach survival was “moderately” below average. Lower Granite to McNary for yearling Chinook was 67.8 percent, from McNary to Bonneville survival was 67 percent and from Lower Granite to Bonneville it was 46 percent.
• Looking at just the Lower Granite project (head of reservoir to tailrace), mean survival was a high 96.2 percent for yearling Chinook (wild and hatchery). The long-term mean is 91.9 percent and is one of only two estimates higher than the long-term mean for this reach since 2018, according to the memo.
• The combined yearling Chinook salmon survival estimate from the Snake River trap to the Bonneville tailrace was 44.3 percent, which is below the long-term mean of 48.1 percent.
• Survival for wild Snake River yearling Chinook from the Lower Granite tailrace to the McNary tailrace was 66.0 percent and survival from the McNary tailrace to the Bonneville tailrace was 48.2 percent. Survival for the longer reach from the Lower Granite tailrace to the Bonneville tailrace was 31.8 percent, far below the long-term mean of 46.8 percent.
• Survival of wild Chinook from the Snake River trap to Bonneville was 34.4 percent, below the long-term mean of 44.1 percent. However, the memo said that the difference is not “statistically different from it due to the imprecision of the estimate. Both low detection rates and the small sample size of smolts tagged at the trap contributed to the poor precision of these estimates.”

Preliminary Survival Estimates: Snake River Steelhead

• Survival for Snake River steelhead (hatchery and wild) was close to average in the Lower Granite to Little Goose reach and above average in the two remaining reaches of the Snake River in 2024, though the precision of these estimates was extremely poor, the memo said. Estimated survival was well below average in both the McNary to John Day and John Day to Bonneville reaches. Both estimates were imprecise.
• Survival from Lower Granite to McNary for Snake River wild and hatchery steelhead was above average (81.9 percent), but below average for the long reach from Lower Granite to Bonneville (55 percent).  Estimated survival from Lower Granite to Bonneville was 43.5 percent, which is below the long-term average of 49.6 percent, the second lowest estimate since 2007, but not statistically significant due to poor precision. The overall steelhead survival estimate from the Snake River trap to the Bonneville tailrace was 46.9 percent, which was close to the long-term mean of 46.8 percent.
• Wild Snake River steelhead survival from the Lower Granite tailrace to the McNary tailrace was well above average at 89.6 percent. Long-term, the mean is 65.8 percent, but, again, precision was poor, the memo said. Mean survival from the McNary tailrace to the Bonneville tailrace was 32.3 percent. The long-term mean is 63.8 percent.
• Wild steelhead survival from the Lower Granite tailrace to the Bonneville tailrace was 29.0 percent, far below the long-term mean of 42.8 percent.  The survival estimate from the Snake River trap to the Lower Granite tailrace was 91.3 percent, also below the long-term mean of 94.8 percent. The survival estimate from the Snake River trap to the Bonneville tailrace was 26.5 percent, also far below the long-term mean of 41.6 percent.

Preliminary Survival Estimates: Snake River sockeye salmon

• Estimated survival of Snake River sockeye salmon (hatchery and wild) from the tailrace of Lower Granite to the tailrace of Bonneville was 27.9 percent, below the long-term mean of 40.4 percent.

Preliminary Survival Estimates: Upper Columbia River

• Estimated survival for upper Columbia River hatchery yearling Chinook from the McNary tailrace to the Bonneville tailrace was 71.5 percent, which is below the long-term mean of 79.4 percent.
• Estimated survival for hatchery steelhead from the upper Columbia River in 2024, between the McNary tailrace and the Bonneville tailrace was 62.9 percent, below the long-term mean of 76.2 percent, but the difference was not statistically significant due to poor precision.
• Estimated survival in 2024 of Columbia River sockeye (hatchery and wild) from the lower Wenatchee River screw trap to the Bonneville tailrace was a low 12.2 percent. The long-term mean is 27.7 percent.

Transport and Travel Time

• The memo says that 6.4 percent of non-tagged wild spring-summer Chinook from the Snake River basin were transported, while 8.4 percent of non-tagged hatchery spring- summer fish were transported. Some 5.5 percent of wild steelhead were transported, while 6.2 percent of hatchery steelhead were transported. All the fish were transported either from Lower Granite Dam or downstream from Little Goose or Lower Monumental dams. This was the lowest rate of transfer since 1993.

Travel times for Chinook were quite short for much of 2024, and in both the first half of April and in the middle of May Chinook travel times were close to the shortest on record, the memo said.  Steelhead travel times were not as short and most were generally close to or slightly shorter than travel times observed in recent low-flow years. “There was a noticeable increase in travel time for both species in late May which may be related to especially low flows observed at that time,” the memo said.

For Background, see:

— CBB, October 8, 2024, 2024 DRAFT ANNUAL SALMON SURVIVAL REPORT: SMOLT-TO-ADULT RETURN RATES WON’T MEET REGIONAL GOALS UNDER NON-BREACH ALTERNATIVES, HTTPS://COLUMBIABASINBULLETIN.ORG/2024-DRAFT-ANNUAL-SALMON-SURVIVAL-REPORT-SMOLT-TO-ADULT-RETURN-RATES-WONT-MEET-REGIONAL-GOALS-UNDER-NON-BREACH-ALTERNATIVES/

— CBB, January 5, 2024, WITH HIGHER SPILL AT DAMS, DETECTING SURVIVAL OF PIT-TAGGED JUVENILE SALMON, STEELHEAD EXTREMELY ELUSIVE, CREATING KEY DATA GAP, HTTPS://COLUMBIABASINBULLETIN.ORG/WITH-HIGHER-SPILL-AT-DAMS-DETECTING-SURVIVAL-OF-PIT-TAGGED-JUVENILE-SALMON-STEELHEAD-EXTREMELY-ELUSIVE-CREATING-KEY-DATA-GAP/

— CBB , November 1, 2022, IS HIGHER SPILL AT DAMS HELPING JUVENILE SALMON PASSAGE? WITH CONTINUED LOW DETECTION RATES OF TAGGED FISH, HARD TO SAY, HTTPS://CBBULLETIN.COM/IS-HIGHER-SPILL-AT-DAMS-HELPING-JUVENILE-SALMON-PASSAGE-WITH-CONTINUED-LOW-DETECTION-RATES-OF-TAGGED-FISH-HARD-TO-SAY/

— CBB, November 4, 2021, NOAA: HIGHEST SPILL LEVELS EVER FOR JUVENILE SALMONIDS MAKES SMOLT DETECTION DIFFICULT IN 2021, IMPRECISE REACH SURVIVAL ESTIMATES, HTTPS://CBBULLETIN.COM/NOAA-HIGHEST-SPILL-LEVELS-EVER-FOR-JUVENILE-SALMONIDS-MAKES-SMOLT-DETECTION-DIFFICULT-IN-2021-IMPRECISE-REACH-SURVIVAL-ESTIMATES/

— CBB, November 12, 2020, “Missing Data: Covid, High Spill, Low Fish Detection Results In Lack Of 2020 Juvenile Salmonid Survival Estimates From Upstream Dams To Below Bonneville,” https://cbbulletin.com/missing-data-covid-high-spill-low-fish-detection-results-in-lack-of-2020-juvenile-salmonid-survival-estimates-from-upstream-dams-to-below-bonneville-2/

— CBB, Jan. 2, 2020, NEW FISH DETECTION SYSTEM AT LOWER GRANITE DAM TRACKS PIT-TAGGED JUVENILE SALMON/STEELHEAD THROUGH SPILLWAYS, https://cbbulletin.com/new-fish-detection-system-at-lower-granite-dam-tracks-pit-tagged-juvenile-salmon-steelhead-through-spillways/

— CBB, Sept. 26, 2019, MEMO OFFERS PRELIMINARY 2019 JUVENILE SALMON/STEELHEAD SURVIVAL ESTIMATES THROUGH COLUMBIA/SNAKE DAMS; SOCKEYE SHOW IMPROVEMENT, https://cbbulletin.com/memo-offers-preliminary-2019-juvenile-salmon-steelhead-survival-estimates-through-columbia-snake-dams-sockeye-show-improvement/

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