2002 Evaluation of Juvenile Fall Chinook Salmon Stranding on the Hanford Reach of the Columbia River

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Published: October 31, 2002

Pages: 38

Author(s): John Nugent, Paul Hoffarth and Wendy Brock

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Hanford Reach Salmonid Entrapment Research

The Hanford Reach stretches from Priest Rapids Dam 82 kilometers downstream to Richland, Washington. The topography, river dynamics, and climate of the area create a unique habitat for wildlife and fish populations. The Hanford Reach supports the larger of the only two remaining healthy naturally spawning fall chinook salmon (Oncorhynchus tshawytscha) populations in the Columbia River System. This population is a primary source of ocean and freshwater sport, commercial, and in-river tribal fisheries and is a primary component of the Pacific Salmon Treaty between the United States and Canada. River flows for this section of the Columbia River are controlled by discharge from Priest Rapids Dam. Flow fluctuations from Priest Rapids Dam can occur rapidly due to changes in hydroelectric power generation, irrigation, water storage, and flood control. These fluctuations have been observed to cause stranding and entrapment of juvenile fall chinook salmon on gently sloped banks, gravel bars, and in pothole depressions in the Hanford Reach area of the Columbia River.

Stranding of juvenile fall chinook salmon occurs when the fish are trapped on or beneath the unwatered substrate as the river level recedes. Entrapment occurs when the fish are separated from the main river channel in depressions as the river level recedes. Fish mortality in entrapments occurs from stranding, thermal stress, and piscivorous, avian, and mammalian predation.

The impact of river fluctuations due to operation of hydroelectric facilities on rearing salmonids has been assessed on numerous Columbia River tributaries and other river systems but limited research has been conducted on the Hanford Reach prior to 1997. In 1997, the Washington Department of Fish and Wildlife (WDFW) was contracted through the Bonneville Power Administration (BPA) and the Grant County Public Utility District (GCPUD) to perform an evaluation of juvenile fall chinook salmon stranding on the Hanford Reach. The multi-year study was developed to assess the impacts of water fluctuations from Priest Rapids Dam on rearing juvenile fall chinook salmon, other fishes, and benthic macroinvertebrates of the Hanford Reach and for directing the future management of flows from Priest Rapids Dam.

Executive Summary

The Washington Department of Fish and Wildlife (WDFW) in cooperation with the Grant County Public Utility District (GCPUD) and Pacific Northwest National Laboratory (PNNL) performed the 2002 Evaluation of Juvenile Fall Chinook Salmon (Oncorhynchus tshawytscha) Stranding on the Hanford Reach of the Columbia River. The 2002 evaluation was the sixth year of a multi-year study to assess the impacts of water fluctuations from Priest Rapids Dam on rearing juvenile fall chinook salmon, other fishes, and benthic macroinvertebrates of the Hanford Reach. The field effort was performed from February 20 through June 15.

The objectives of the 2002 evaluation were to determine the start and end dates of the juvenile fall chinook salmon protection program, estimate the number of juvenile fall chinook salmon killed and placed at risk within the designated sampling area during the protection program, and evaluate the impacts of flow conditions and protection measures. The 2002 Hanford Reach Juvenile Fall Chinook Salmon Interim Protection Program is provided in this report. The program set stricter flow fluctuation limits at lower flows and more operational flexibility at higher flows.

A sampling plan to estimate the total number of juvenile fall chinook salmon killed or placed at risk due to flow fluctuations was designed by Pacific Norwest National Laboratory (PNNL) and WDFW prior to the 1999 field season and was implemented in 1999 through 2001. The plan was developed for the portion of the Hanford Reach defined by the SHOALS bathymetry data (Rkm 571.3 to Rkm 606.9) along the shorelines exposed by flows of 40 to 400 kcfs. In 2002, the sample area was reduced to approximately half that of the previous years. The reduced area was selected to include the section of the river where the majority of the stranding and entrapment was observed in previous years. This area stretches from Rkm 584.5 to Rkm 600.2.

River and meteorological conditions on the Hanford Reach varied prior to and during the 2002 juvenile fall chinook salmon emergence and rearing period (March-June). Air temperatures were much warmer than normal during the winter prior to the emergence and rearing period, which contributed to warmer than usual river temperatures. Cooler than normal air temperatures during the emergence and rearing period slowed increasing river temperatures to near average. Precipitation was lower than normal during most of the emergence and rearing period, attaining only 66% of normal. Solar radiation levels, a good indication of cloud cover, were also slightly below average for most of the emergence and rearing period. Mean monthly river flows ranged from 74.8 kcfs in March to 219.9 kcfs in June.

Emergence of wild juvenile fall chinook salmon in 2002, as calculated under the terms of the Vernita Bar Settlement Agreement, was estimated to start on March 17. Population index surveys were initiated on February 20 to account for possible early emergence. Implementation criteria were met on March 19 and the 2002 Interim Protection Program began March 21. The program ended on June 4 when 400ºC ATU’s following the estimated end of emergence were attained. Random sampling plan to assess the effectiveness of the 2002 Interim Protection Program began on March 22 and ended June 15.

Priest Rapids Dam discharges averaged 131.2 kcfs from March 21 through June 4. Hourly discharge from the Dam ranged from 50.8 to 293.8 kcfs. Mean daily flow fluctuation during this period was 47.1 kcfs.

A total of 194 random plots encompassing 47,234 m2 (508,439 ft2) were sampled within the reduced sample area in 2002. Random plots contained 188 juvenile fall chinook salmon including 89 stranded and 99 entrapped individuals. Field crews recorded 91 direct mortalities consisting of the 89 stranded and 2 thermal induced fatalities. Projected mortalities were estimated at 93 based on revisitation the next day to determine if the entrapments drained or reached lethal temperatures (>24ºC). Fish were first encountered in random plots on March 23 and last found on June 9. The majority (92%) of juvenile fall chinook salmon were sampled during the month of April. Only two other species of fish, sculpin (Cottus spp.) and threespine stickleback (Gasterosteus aculeatus), were found stranded or entrapped in random plots.

The estimated total number of juvenile fall chinook salmon stranding and entrapment mortalities within the reduced sample area in 2002 was calculated to be 67,409 with a 95% confidence interval between 28,623 and 106,195. The number of mortalities estimated by revisitation of entrapments was 70,903 with a 95% confidence interval between 31,517 and 110,288. Juvenile fall chinook salmon placed at risk of mortality due to stranding and entrapment was calculated to be 144,249 with a 95% confidence interval between 28,813 and 259,685.

Juvenile fall chinook salmon collected in random plots had a mean fork length of 40.7 mm and ranged from 32 to 45 mm. The majority of juvenile fall chinook salmon found in random plots in 2002 were located upstream of Rkm 595 in the Locke Island/White Bluffs Slough area (80.3%) and at flow levels between 50 and 120 kcfs (87%).

An estimated 21.4 million fall chinook salmon fry were produced on the Hanford Reach in 2002. Sampling to assess juvenile fall chinook salmon abundance and fish size began on February 20 and ended on June 19. A total of 5,550 juvenile fall chinook salmon were seined during this period. Juvenile fall chinook salmon were collected in every weekly sample from March 1 to June 19 but peak abundance occurred from March 27 to May 29. The largest catch of the season was obtained on May 15 when 1,739 individuals were sampled.