Fish/Shellfish Research and Management - Fish/Shellfish Research
Date Published: May 29, 2002
Number of Pages: 55
Author(s): John Nugent, Todd Newsome, Paul Hoffarth, Michael Nugent, Wendy Brock and Michael Kuklinski, Jr.
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.
The Washington Department of Fish and Wildlife (WDFW) in cooperation with the Bonneville Power Administration (BPA), Grant County Public Utility District (GCPUD), and Pacific Northwest National Laboratory (PNNL), performed the 2001 Evaluation of Juvenile Fall Chinook Salmon Stranding on the Hanford Reach of the Columbia River. The 2001 evaluation was the fifth year of a multi-year study to assess the impacts of water fluctuations from Priest Rapids Dam on rearing juvenile fall chinook salmon. The field effort was performed from March 14 through June 28.
The objectives of the 2001 evaluation were to collect basic information on the physical parameters of the Hanford Reach, evaluate the extent of stranding and entrapment of juvenile fall chinook salmon and other fish species, and identify critical habitat zones. PNNL will use this information to develop a model for determining susceptibility of juvenile fall chinook salmon to stranding and entrapment due to flow fluctuations. The overall goal will be to develop a long term agreement for the protection of juvenile fall chinook during emergence and rearing.
River and meteorological conditions on the Hanford Reach during the 2001 juvenile fall chinook salmon emergence and rearing period (Marchâ€“July) were marked by below average river flows, above normal river temperatures, near normal ambient air temperatures, and below average solar radiation levels. Priest Rapids Project discharges averaged 70.9 kcfs from March 21 through June 10 (range 37.5 kcfs to 152.2 kcfs). Mean daily fluctuation in discharges from Priest Rapids Dam during the Protection Program (March 26 â€“ June 10) was 23.2 kcfs (range 0.7 kcfs to 84.5 kcfs).
Emergence of juvenile fall chinook salmon in 2001, as calculated under the terms of the 1988 Vernita Bar Settlement Agreement (GCPUD 1988), was estimated to start on April 1. Population index surveys were subsequently initiated on March 21 to account for possible early emergence. Implementation of the 2001 Interim Protection Program began March 26. The Protection Program ended on June 10 and evaluation field activities were continued through June 28.
A total of 434 random plots encompassing 86,526 m2 (931,388 ft2) were sampled in 2001 between April 13 and June 28, 2001. Flows were relatively stable through May 21 with limited fluctuations greater than 10 kcfs. Though few in number, fluctuations occurring during this early period of emergence and rearing often resulted in large numbers of stranded/entrapped juvenile fall chinook salmon. From May 6 through May 10, increased numbers of juvenile fall chinook salmon were found in random plots as flows gradually decreased from 72.8 kcfs to 52.6 though no significant fluctuations occurred during this period. By May 28, juvenile fall chinook salmon found in random samples had decreased though daily flow fluctuations had risen in accordance with the criteria in the protection plan indicating reduced susceptibility.
Random plots contained 3,313 juvenile fall chinook salmon in 2001. Field crews recorded 3,238 direct mortalities consisting of the 316 stranded and 2,922 thermal induced fatalities. Fish were first encountered in random plots on April 13 and last found on June 22. The majority of juvenile fall chinook salmon mortalities were sampled during the month of April (2,278). The estimated total number of juvenile fall chinook salmon stranding and entrapment mortalities in the study area in 2001 was calculated to be 1,628,878 with a 95% confidence interval between â€“286,153 and 3,543,910. Juvenile fall chinook salmon placed at risk of mortality due to stranding and entrapment was calculated to be 1,663,636 with a 95% confidence interval between â€“252,186 and 3,579,458. Juvenile fall chinook salmon were found throughout the SHOALS defined study area at a variety flow bands but the highest concentrations were found at the island complex areas of Locke Island (600-610 Rkm) at flows of 40-80 kcfs.
An estimated 27,979,577 fall chinook fry emerged in the Hanford Reach in 2001. Sampling to assess juvenile fall chinook salmon abundance and fish size began on March 14, two weeks prior to the estimated start of emergence on April 1 and ended on June 27. A total of 37,036 juvenile fall chinook salmon were seined during this period. Peak abundance was observed from April 18 to May 23. Juvenile fall chinook salmon with fork lengths at or below 42 mm made up a minimum of 25% of the fish seined in the Hanford Reach through May 23 and fish of this size remained in the samples until June 27.
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. 2001), began to appear in the samples on April 11 but were not collected in considerable numbers (>5%) until June 13.
The Emergency Management Team monitored entrapments in primary fall chinook salmon rearing areas from March 26 to June 28. A total of 27,639 juvenile fall chinook salmon were seined from 63 entrapments. Juvenile fall chinook salmon were observed in an additional ten entrapments that were too large too seine. A total of 98 entrapments were monitored in 2001. Field crews recorded 7,927 direct mortalities at the time entrapments were sampled. In addition to juvenile fall chinook salmon, thousands of resident fish were reported entrapped.
Juvenile fall chinook were susceptible to stranding/entrapment from March 14, date first found fish in index samples, through the end of index sampling on June 27, minimum chinook fork lengths less than 60mm. Based on chinook â€œat riskâ€ in random samples and population abundance from index sampling, the primary period when operations at Priest Rapids Dam were most likely to have significant impacts to the juvenile fall chinook population was from April 11 through May 30. Data from 1999, 2000, and 2001 all indicate decrease susceptibility beginning at roughly 200 temperature units Celsius after the estimated end of emergence, May 27 in 2001. This trend was observed in chinook â€œat riskâ€ and in the length frequency data for all three years. In 2001, the number of juvenile fall chinook salmon at risk decrease dramatically after May 28 though daily fluctuations in discharge from Priest Rapids were relatively high.
The joint fish managers, consisting of WDFW, the Columbia River Intertribal Fish Commission, the Tribes of the Columbia River Basin, the Oregon Department of Fish and Wildlife, NMFS, and USFWS, and the power managers, consisting GCPUD, BPA, the United States Bureau of Reclamation, and the Mid-Columbia Public Utility Districts (Chelan and Douglas Counties), should continue to work together through the Hanford Policy Group meetings to refine annual interim protection plans to protect emergent and rearing juvenile fall chinook salmon in the Hanford Reach area of the Columbia River until a permanent agreement can be adopted. A permanent agreement will need to allow adaptive management options for fish management and hydropower as conditions change.
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