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2000 Evaluation of Juvenile Fall Chinook Salmon Stranding on the Hanford Reach of the Columbia River

Category: Fish/Shellfish Research and Management - Fish/Shellfish Research

Date Published: May 29, 2002

Number of Pages: 90

Author(s): John Nugent, Todd Newsome, Mike Nugent, Wendy Brock, Paul Hoffarth and Paul Wagner

DESCRIPTION:

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 Bonneville Power Administration (BPA), Grant County Public Utility District (GCPUD), Pacific Northwest National Laboratory (PNNL), University of Idaho (U of I), Streamside Programs Consultation (SPC), United States Geological Survey Biological Resources Division (USGS/BRD), and Yakama Nation (YN) performed the 2000 Evaluation of Juvenile Fall Chinook Salmon (Oncorhynchus tshawytscha) Stranding on the Hanford Reach of the Columbia River. The 2000 evaluation was the fourth year of a multi-year study to assess the impacts of water fluctuations from Priest Rapids Dam on rearing juvenile fall chinook salmon, other fish species, and benthic macroinvertebrates. The field effort was performed from March 13 through August 28.

The Hanford Reach experienced slightly warmer to near normal air temperatures and wetter than normal conditions during the 2000 juvenile fall chinook salmon emergence and rearing period (March–July). Solar radiation levels, a good indication of cloud cover, were above the 20-year mean (1980-1999) each month during this time period with the exception of March. River flows during juvenile fall chinook salmon emergence and rearing period were below the 10-year mean flows (1990-1999) for each month with the exception of April when flows were 30.6 kcfs above the previous 10-year mean.

Emergence of wild juvenile fall chinook salmon in 2000, as calculated under the terms of the 1988 Vernita Bar Settlement Agreement (GCPUD 1988), was estimated to start on March 20 and population index surveys were subsequently initiated on March 13. Implementation criteria were met on March 19 and the 2000 Interim Protection Program began March 21. Random sampling to assess the effectiveness of the 2000 Interim Protection Program began on March 20 and ended June 25. The protection program continued through June 26.

Priest Rapids Dam (Rkm 639.1) discharges averaged 147.7 kcfs from March 21 through June 26 in 2000. Hourly discharge from the Dam ranged from 62.1 to 293.2 kcf. Mean daily fluctuation during this period was 50.0 kcfs. The primary period of susceptibility of juvenile fall chinook salmon to stranding in 2000 based on fish recorded as “mortalities” and “at risk” in random samples and length frequency distribution from index sampling appears to be from the start of emergence to May 21. Mean daily flow fluctuation from Priest Rapids Dam during the primary period of susceptibility was 46.5 kcfs with 9 days of relatively stable flows (fluctuations < 20 kcfs) and 33 days of flow fluctuations greater than 40 kcfs including 8 days of flow fluctuations greater than 80 kcfs.

A total of 709 juvenile fall chinook salmon were sampled from random plots in 2000 including 138 stranded and 571 entrapped individuals. Field crews recorded 156 direct mortalities consisting of the 138 stranded and 18 thermal induced fatalities. Projected mortalities were estimated at 625 based on revisitation of previous sites to determine if the entrapments drained or reached lethal temperatures (>24°C). Fish were first encountered in random plots on March 24 and last found on June 2. The majority of juvenile fall chinook salmon were sampled from March 26-April 15 and April 23-May 6.

The estimated total number of juvenile fall chinook salmon stranding and entrapment mortalities in 2000 was calculated to be 72,362 with a 95% confidence interval between 34,270 and 110,454. The number of mortalities estimated by revisitation of entrapments was 209,997 with 95% confidence interval between –20,483 and 440,476.

Juvenile fall chinook salmon placed at risk of mortality due to stranding and entrapment was calculated to be 255,222 with a 95% confidence interval between 17,743 and 492,701. Juvenile fall chinook salmon collected in random plots had a mean fork length of 41.7 mm and ranged from 33 to 86 mm. Individuals less than 60 mm comprised 99.2% of the juvenile fall chinook salmon measured. Juvenile fall chinook salmon were found throughout the SHOALS defined study area at a variety of flow bands but the highest concentrations were found at Locke Island (595-605 Rkm) and the downstream end of 100 F Islands (585-590 Rkm) at flows of 120-200 kcfs.

An estimated 16,293,584 fall chinook salmon fry were produced on the Hanford Reach in 2000. Sampling to assess juvenile fall chinook salmon abundance and fish size began on March 13, just prior to the estimated start of emergence on March 20 (Carlson 2000), and ended on June 26. A total of 5,624 juvenile fall chinook salmon were seined during this period. Juvenile fall chinook salmon were collected from six index locations once per week during this period. Peak abundance was observed from April 24 to May 29. The largest catch of the season was obtained on May 29 when 870 individuals were sampled. Juvenile fall chinook salmon with fork lengths at or below 42 mm comprised 30% or more of the fish seined in the Hanford Reach until May 15 and fish of this size remained in the samples through June 19. Juvenile fall chinook salmon with fork lengths greater than 59 mm, the size threshold that individuals are thought to become less susceptible to entrapment (Nugent et al. 2001a and 2001b), began to appear in the samples on April 24 but were not collected in considerable numbers until May 23.

The emergency management team (EMT) monitored entrapments in primary fall chinook salmon rearing areas from March 20 to June 24. A total of 10,705 juvenile fall chinook salmon were seined from 158 entrapments during this time period. Many of the same entrapments were sampled on multiple days in conjunction with EMT monitoring. Field crews recorded 311 direct mortalities at the time entrapments were sampled. Projected mortalites were estimated at 4,451 based on drainage or lethal temperatures monitored in entrapments. Criteria for emergency action were reached on 10 days (March 27, April 2, April 6, April 15, April 27, April 30, May 2, May 6, May 26, and June9) in 2000. GCPUD provided additional water to re-inundate (or increase river elevations) entrapments on six of these days (March 27, April 2, April 6, April 15, April 27, and June 9).

Minimum numbers of fish other than fall chinook salmon were sampled during the implementation and evaluation of the Interim Protection Program in 2000 (March 13-June 26). Spring chinook salmon (Oncorhynchus tshawytscha) and at least 10 other species of fish were collected in nearshore sites and random plots during the spring and early summer sampling period. Resident species found consisted of mountain whitefish, northern pikeminnow, peamouth, redside shiner, sculpin, smallmouth bass, sucker, threespine stickleback, dace, and yellow perch. Spring chinook salmon, peamouth, smallmouth bass, dace, and yellow perch were not represented in random plots.

In 2000, the summer and early fall sampling program began on July 11 and ended August 28. Species collected in nearshore sites during this time period consisted of American shad, common carp, peamouth, northern pikeminnow, dace, redside shiner, undetermined minnow species, sucker, threespine stickleback, bluegill, smallmouth bass, undetermined bass species, and sculpin.