2003 Evaluation of Juvenile Fall Chinook Salmon Stranding in the Hanford Reach of the Columbia River

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Published: 2003

Pages: 37

Author(s): Paul Hoffarth, Andrew Fowler 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 2003 Evaluation of Juvenile Fall Chinook Salmon (Oncorhynchus tshawytscha) Stranding in the Hanford Reach of the Columbia River. The 2003 evaluation was the seventh 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 in previous years macroinvertebrates, of the Hanford Reach.

The objectives of the 2003 evaluation were to: determine if start and end dates for implementation of the juvenile fall chinook salmon protection operations meet observed emergence and susceptibility to entrapment and stranding; estimate the number of juvenile fall chinook salmon stranded (mortalities) and entrapped in isolated pools (at risk) due to reductions in discharge from Priest Rapids Dam within the designated sampling area during emergence and rearing; and to evaluate the effectiveness of operation guidelines (2003 Interim Protection Plan) on reducing mortality of fall chinook in the Hanford Reach.

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 PNNL and WDFW prior to the 1999 field season and was implemented during the 1999 to 2001 evaluations. 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 kcfs to 400 kcfs. The study area was reduced in 2002 and 2003 to the area from Locke Island (Rkm 600.2) to Hanford Slough (Rkm 584.5).

Emergence of naturally spawned juvenile fall chinook salmon in 2003, as calculated under the terms of the Vernita Bar Settlement Agreement, was estimated to start February 20 and end April 27. The 2003 Interim Protection Program began February 28 and ended June 5, when 400ºC temperature units were accumulated following the estimated end of emergence. Random sampling to assess the effectiveness of the 2003 Interim Protection Program began March 20 and ended June 14.

Hourly discharge from Priest Rapids Dam averaged 117.0 kcfs during the period the 2003 Juvenile Fall Chinook Interim Protection Plan was in effect. Hourly discharge ranged from 70.4 kcfs to 229.9 kcfs. Mean daily flow fluctuation from Priest Rapids Dam during the this period was 33.3 kcfs with 32 days of relatively stable flows (fluctuations < 20 kcfs), 54 days with flow fluctuations between 20 kcfs and 60 kcfs, and 12 days with flow fluctuations greater than 60 kcfs.

A total of 193 random plots encompassing 39,548 m2 were sampled in 2003. Random plots contained 152 juvenile fall chinook salmon including 133 stranded and 19 entrapped individuals. Fish were first encountered in random plots on March 20 and last found on May 31, 2003. The estimated number of juvenile fall chinook salmon stranding and entrapment mortalities within the 2003 sample area was calculated to be 154,853 with a 95% confidence interval between 83,903 and 225,802. No additional mortalities were attributed to this estimate by re-visitation of entrapments. Juvenile fall chinook salmon placed at risk of mortality from stranding and entrapment was calculated to be 164,643 with a 95% confidence interval between 91,093 and 238,192. Of the chinook found stranded and entrapped, 44.7% were observed at flow levels between 50 and 120 kcfs though only 21.9% of the flow fluctuations occurred at these levels.

Sampling was only conducted on those days when a fluctuation was of sufficient magnitude and duration to dewater shorelines in the study area, 23 miles downstream, and during normal working hours (8:00am â€" 4:00pm). Sampling was conducted on 50 of the 87 days during the evaluation period (March 20 â€" June 14). During the 73 days from the first chinook encountered in random sampling (March 20) to the last chinook recorded (May 31), 38 days had flow fluctuations capable of producing fall chinook stranding and entrapment in the study area. There were 101 random plots sampled on 26 weekdays and 59 random plots sampled on 12 weekend days during this time frame. Stranded/entrapped chinook were found in 35.6% (21 of 59) of the plots sampled on the weekends and in 16.8% (17 of 101) of the random samples during weekdays. There were almost double the number of chinook recorded stranded/entrapped on the weekends (98) compared to the weekdays (54).

Juvenile fall chinook salmon collected in random plots had a mean fork length of 40.3 mm and ranged from 35 mm to 50 mm. These results are similar to data from previous years indicating that juvenile fall chinook salmon are less susceptible to stranding and entrapment with increased size. Minimum fork lengths for chinook sampled along nearshore areas in the Reach continued to be less than 40 mm through the final survey on June 23, however the composition of newly emergent fry (<42 mm) in the sample had decrease markedly by June 9 (7.2% of collection).

The 2003 Juvenile Fall Chinook Interim Protection Plan was in effect for 98 days during the emergence and rearing period for fall chinook in the Hanford Reach. Of the 84 operational constraints established by the plan, 49 targets were met with daily flow fluctuations below the maximum. There were 70 weekdays during the protection plan and 14 weekends. Weekday constraints were met on 43 (61%) days and weekend constraints were met on 6 (43%) of 14 weekends. Fluctuations were outside of target by less than 5 kcfs on 9 constraints, 5 weekdays and 4 weekends.

Rock Island previous weekday discharge was used to predict Priest Rapids discharge and to set constraints for weekdays. Weekend forecasts for Chief Joseph Dam plus side flows were used to predict weekend flows for Priest Rapids Dam and set weekend constraints. Use of previous day flows from Rock Island and weekend forecasts from Chief Joseph were used with the goal of accurrarately predicting daily mean discharge for Priest Rapids Dam and thus maintaining flow fluctuations in the Hanford Reach that will reduce stranding and entrapment. Overall, constraints set using these predictive methods accurately met or were more restrictive on 75 of the 84 targets (89%) during the protection plan in 2003. Constraints were identical on 55 predictions, more restrictive on 20 occassions, and were not restrictive enough on 9 occassions. Weekday constraint were accurately predicted on 45 (64%) days, 17 (24%) of the daily constraints were more restrictive, and 8 (11%) daily constraints were not restrictive enough based on Priest Rapids actual mean daily discharge. Chief Joseph weekend forecasts accurately predicted weekend flows for Priest Rapids on 10 (71%) of the weekends, 3 (21%) constraints were more restrictive than would have been using Priest Rapids Dam daily discharge to set constraint, and 1 (7%) constraint was not restrictive enough based on Priest Rapids actual.

Constraints were set in accordance with the 2003 Protection Plan based on predicted daily discharge at Priest Rapids Dam with the goal of maintaining daily fluctuations within the Hanford Reach at levels where mortality of juvenile fall chinook is reduced. There protection plan covered 98 days during emergence and rearing of juvenile fall chinook in 2003. During this period there were 27 days when fluctuations occurred above the target maximums. Fluctuations above these maximums most often occurred when mean daily flows were between 80 kcfs and 110 kcfs (34.8%), 110 kcfs and 140 kcfs (34.5%), and above 170 kcfs (100%) (Table 9). The number of days outside of target is less than the number of constraints outside of target for the protection plan due to the fact that constraints set by the protection plan used Rock Island prior weekday flows and Chief Joseph forecasts that were often more restrictive than the constraints would have been if based on actual Priest Rapids discharge for the day. Additionally, weekend constraints as set by the protection plan were set for a 48- hour period rather than individual weekend dates.

Fall chinook fry production on the Hanford Reach was estimated to provide a rough estimate of the population affected by flow fluctuations from Priest Rapids Dam. Fall chinook fry production on the Hanford Reach in 2003 was estimated to be between at 13.8 and 33.4 million emergent fry. The 2003 production estimate was based on year 2002 Hanford Reach adult fall chinook escapement, female composition of the escapement, fecundity of hatchery fall chinook salmon at Priest Rapids Hatchery, egg retention of fall chinook salmon in the Hanford Reach, and an egg to emergence survival rate of 30% (Healey 1998). An additional fry estimate was produced using aerial redd counts for fall chinook in the Hanford Reach conducted by PNNL.