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Draft 1999 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: December 07, 2001

Number of Pages: 213

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


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), 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 1999 Evaluation of Juvenile Fall Chinook Salmon (Oncorhynchus tshawytscha) Stranding on the Hanford Reach of the Columbia River. The 1999 evaluation was the third 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 5 through September 29.

The objectives of the 1999 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. The information will be used to develop a model for determining susceptibility of juvenile fall chinook salmon to stranding and entrapment due to flow fluctuations. WDFW subcontracted U of I and SPC to assess the effects of flow fluctuations on the benthic macroinvertebrate communities and USGS/BRD to study the effects of heat stress on the survival, predator avoidance ability, and physiology of juvenile fall chinook salmon.

River and meteorological conditions on the Hanford Reach during the 1999 juvenile fall chinook salmon emergence and rearing period (March–July) were marked by above average river flows, below normal ambient air temperatures, below normal precipitation, and above average solar radiation levels. Priest Rapids Dam (Rkm 639.1) discharges averaged 161.4 kcfs from March 8 through June 30. Hourly discharge ranged from 61.9 to 261.3 kcfs. Mean daily fluctuation during this period was 42.1 kcfs.

Scanning Hydrographic Operational Airborne Lidar Survey (SHOALS) system bathymetry data collected by the United States Army Corps of Engineers (COE) in 1998 on 35.1 km2 of the Hanford Reach from Rkm 571.3 to Rkm 606.9 was processed in 1999. SHOALS data was used in conjunction with the Modular Aquatic Simulation System 1D (MASS1), a one-dimensional unsteady flow model for the Hanford Reach (Richmond and Perkins 1998), to characterize the Hanford Reach at stage discharges from 40-400 kcfs.

The Hanford Reach produced an estimated 8,405,936 fall chinook salmon fry in 1999. Juvenile fall chinook salmon were first captured in nearshore areas on March 5 and last sampled July 21. Peak abundance was observed between April 28 and June 2 with the largest catch of the season occuring on May 13. Juvenile fall chinook salmon with fork lengths at or below 42 mm (emergent fry) comprised 30% or more of the fish sampled each week through May 26. Fish with fork length greater than 59 mm (size threshold thought to be less susceptible to stranding or entrapment) began to appear in nearshore samples on May 5 but did not occur in large numbers until June 2.

Field crews located 1,026 stranded and entrapped juvenile fall chinook salmon in random plots in 1999. Fish were first encountered in random plots on March 20 and last observed June 12. The majority of stranded and entrapped fish were sampled during the weeks of March 21-27, April 4-10, April 11-17, and May 23-29. These time periods coincided with lower flows (<120 kcfs) and large flow fluctuations (>80 kcfs).

Stranded and entrapped juvenile fall chinook salmon had a mean fork length of 45.6 mm and ranged from 36 mm to 66 mm. Indivduals less than 60 mm comprised 96.9% of the juvenile fall chinook salmon measured. Fish were found throughout the SHOALS defined study area in a variety habitats and flow bands but the highest concentrations were found at the island complex areas of Locke Island (600-605 Rkm) and 100 F Islands (590- 595 Rkm) at flows of 80-120 and 120-160 kcfs in random plots with gravel to cobble substrates, low substrate embeddedness, and absent to medium vegetation density.

The estimated total number of juvenile fall chinook salmon stranding and entrapment mortalities within the SHOALS defined study area (Rkm 571.3 to Rkm 606.9) in 1999 was calculated to be 125,695 with a 95% confidence interval between 50,724 and 200,666. Juvenile fall chinook salmon placed at risk of mortality due to stranding and entrapment was calculated to be 381,897 with a 95% confidence interval between -347 and 764,141.

Other fish species found stranded and entrapped in 1999 included northern pikeminnow (Ptychocheilus oregonensis), threespine stickleback (Gasterosteus aculeatus), smallmouth bass (Micropterus dolomieui), sculpin (Cottus spp.), mountain whitefish (Prosopium williamsoni), sucker (Catostomus spp.), bluegill (Lepomis macrochirus), lamprey (Lampetra spp.), peamouth (Mylocheilus caurinus), dace (Rhinichthys spp.), and walleye (Stizostedion vitreum). The stranding of lamprey in 1999 is of special interest because pacific lamprey (Lampetra tridentatus) and river lamprey (Lampetra ayresi) are listed as federal species of concern and river lamprey are also designated as a Washington State candidate species. Lamprey were also found stranded in 1998.

Long-and short-term studies of the effects of dewatering on artificial substrates, indicate water level fluctuations affect nearshore community structure, density, and biomass of macroinvertebrates in the Hanford Reach. Longterm tests on the effects of fluctuations clearly show that benthic macroinvertebrates within the river fluctuation zone were severely limited in density and biomass compared to the communities on continually inundated areas. Total invertebrate density was approximately 4 times higher on bricks never de-watered than on substrates exposed only 1 to 24 hours. Mean total invertebrate density and biomass were reduced by 59% and 65%, respectively, from substrates exposed up to 24 hours to substrates never dewatered. Effects of short-term exposure scenarios revealed that a dramatic decrease in survival was found with even short duration exposures to air. Artificial exposure tests revealed that survival of macroinvertebrates on substrates exposed to air decreased dramatically with increasing duration of exposure, with only 50% survival after 1 hour of exposure. Changes in discharge and water levels also catastrophically entrained macroinvertebrates into the drift outside of behavioral diel periodicity.

USGS/BRD thermal tolerance tests showed thermally-stressed juvenile fall chinook salmon had little direct mortality and no increased vulnerability to predation. However, these fish showed transient increases in plasma concentrations of cortisol, glucose, and lactate, and a dramatic (25-fold higher than controls) and persistent (lasting 2 weeks) increase in levels of liver hsp70. It is not known what the consequences of exposure to multiple, cumulative stressors may be to the fish.

An emergency management team (EMT) consisting of WDFW and YN personnel was organized in 1999 to monitor primary fall chinook salmon rearing areas to identify flow fluctuation events that pose risks (imminent drainage of entrapments, lethal water temperatures) to large numbers of entrapped juvenile fall chinook salmon. The EMT monitored 119 entrapments from April 17 to June 21. A total of 8,240 juvenile fall chinook salmon were seined from these entrapments. Field crews recorded 166 direct mortalities at the time entrapments were sampled. Projected mortalities were estimated at 428 based on drainage or lethal temperatures monitored in entrapments. Criteria for emergency action were reached on four days (April 17, May 18, May 22, and May 23). GCPUD provided additional water to re-inundate (or increase river elevations) on each of these occurrences except May 22.

Based on the results of the 1999 evaluation, the Hanford Policy Group recommended, with the exception of eliminating the rewetting of entrapment zones after large fluctuations, that the operation constraints imposed in 1999 should be repeated in 2000. Operational constraints recommended for the 2000 juvenile fall chinook salmon emergence and rearing period include limiting flow fluctuations from Priest Rapids Dam to a range of 40 kcfs on a daily basis (60 kcfs on a daily basis during flow augmentation for outmigrating juvenile fish under NMFS Biological Opinion) when weekly average flows are less than 170 kcfs at Priest Rapids Dam and restricting flows to an hourly minimum of 150 kcfs when weekly average flows are greater than 170 kcfs at Priest Rapids Dam.