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MEMORANDUM

Tetra Tech, Inc.

400 Red Brook Blvd., Suite 200

Owings Mills, MD 21117

phone 410-356-8993

fax 410-356-9005

DATE: November 6, 2013 (revised May 14, 2014)

TO: Lisa Biddle and Paul Shriner

FROM: Henry Latimer, Blaine Snyder, and Ann Roseberry Lincoln

SUBJECT: Holding times to evaluate post-impingement mortality

Clean Water Act §316(b) requires an examination of impacts on the aquatic community

from impingement and entrainment related to withdrawal of cooling water. As part of 

evaluating the impact that a cooling water intake structure (CWIS) has on the aquatic 

community, the number and types of organisms (fish and shellfish) impinged and 

entrained by that structure are often quantified. Depending on the types of screens, screen 

wash systems, fish return systems, and operations at the facility in question it may also be 

useful to quantify impingement survival rates. Because impingement survival rates are 

influenced by the physical design and operations of the CWIS as well as by the species 

and conditions within the source water (e.g., high vs. low debris load), impingement 

survival is typically facility-specific. Therefore, the measurement of impingement 

survival rates can be useful in developing a more refined understanding of mortality

associated with impingement at a facility. This memorandum addresses suggested 

holding times to use in evaluating post-impingement latent mortality of fish and shellfish.

McLaren and Tuttle (2000) held fish for 96 hours post-impingement to evaluate latent 

mortality associated with impingement on 1 mm Ristroph-type traveling screens at the 

Somerset Station (a 281 million gallons per day (mgd) intake, 625 mega-watt (MW) coalfired power plant on Lake Ontario). Survival of impinged fish did not include evaluation 

of effects of passage through the 300 meter (m) fish return pipe. Similarly, Thompson 

(2000) reported that fish and shellfish were held for 96 hours post-impingement to 

evaluate mortality associated with impingement and returned to the source water at the 

Carolina Power & Light Company’s Brunswick Steam Electric Plant in North Carolina. 

Ronafalvy et al. (2000) held fish for 48 hours post-impingement to evaluate mortality at 
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the Salem Generating Station in New Jersey. More recently, Bigbee et al (2010) 

evaluated fish for post-impingement survival for 48 hours at a newly installed modified 

rotary disk screen (screen opening of 6.1 mm2) at the North Omaha Station, a coal-fired 

generating station on the Missouri River in Omaha, Nebraska. Also, a recent study by

Shaw (2010) demonstrated that post-impingement survival of laboratory-reared catfish 

could be successfully monitored for seven days at the Barry Steam Generation Plant on 

the lower Mobile River, Alabama.

A holding time of 24 hours post-impingement was used by Black (2007) to evaluate the 

mortality of various fish species in a laboratory setting. This study was also different 

from some of the facility studies in that the laboratory setting allowed for fish not subject 

to impingement to be used as controls to quantify mortality associated with holding 

conditions. Laboratory studies such as this one fail to completely replicate conditions in 

the field such as survival under varying temperatures and additional physical damage 

related to debris loading on the traveling screen. Further, this study did not consider a 

potentially large source of stress on impinged organisms i.e., passage through a fishreturn system. Although laboratory studies are necessary and appropriate for use in 

evaluating the ability of various technologies to reduce stresses to fish and shellfish 

related to impingement, survival data from studies such as this should only be used to 

evaluate the relative magnitude of stress related to each technology for a given species. 

Because of the site-specific nature of impingement mortality and additional stresses to 

which fish and shellfish are exposed on site (e.g., temperature, debris, etc.), laboratoryderived survival data should not be used to estimate impingement survival at a given 

facility.

A review of post-impingement mortality studies is available in the literature. EPRI (2003) 

evaluated 65 different impingement survival studies conducted at 29 different power 

generation facilities in North America. This review determined that these studies used 

post-impingement holding times ranging from less than 24 to 108 hours (with one study

that held organisms for over 200 hours). The plurality of these studies (approximately 30 

of 65) held fish for 96 hours to evaluate post-impingement mortality (approximate values 

are used here as data were presented graphically). It also appears that 11 studies only

evaluated instantaneous mortality and 10 studies evaluated mortality at 48 hours postimpingement. The remaining studies held fish for an intermediate period between 24 and 

84 hours post-impingement.

Using data from two different studies that collected mortality data (corrected using 

control organisms), EPRI (2003) also evaluated the percentage of latent mortality

observed at various time intervals. This evaluation indicated that most latent mortality

effects can be observed within 96 hours post-impingement. Further, it appears that for 

some species only approximately 60% of latent mortality is accounted for at 48 hours 

post-impingement. For more fragile species, the majority of mortality associated with 

impingement is likely to occur within the first 24 to 48 hours.

Table 1 below combines data from multiple studies and shows percent survival by life 

stage across several ranges of holding times. The first column of percent survival in the 
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table shows the percentage of fish that initially survived impingement plus holding times 

ranging from 0 to 8 hours; the second column shows the percent of fish that initially

survived impingement plus holding times between 8 and 12 hours; etc. The intervals 

result from natural groupings in the available study data. In general, the data show 

differences in survival over time for different life stages. Additionally, for some life 

stages and for fragile species, lower holding times appear sufficient to capture the 

majority of latent mortality.

Table 1. Percent survival of fish across multiple studies and holding times.

Life Stage

Sum of 

Number

Collected

Average Percent Survival

Initial + 0-8 

hours holding

Initial + 8-12 

hours holding

Initial + 24-36 

hours holding

Initial + 60-84 

hours holding

Initial + >84 

hours holding

Adult 2,555 85.11 75.18 60.79 44.13

Juvenile 1,584 99.35 85.73 78.51

Post Yolk Sac 966 95.63 47.01 44.12

YOYA31,474 70.16 63.05 53.23 38.87 16.85

Grand Total 36,579 79.42 69.81 59.47 40.24 16.85

A YOY – young of the year

It appears that a holding time between 18 and 96 hours is warranted. An 18 hour 

minimum would be required to evaluate latent mortality of impingement samples 

dominated by life stages most likely to experience latent mortality, while a maximum 

holding time of 96 hours would be required to ensure that the majority of initial and latent 

mortality associated with impingement is observed, especially for more robust life stages

and species. Holding times longer than 96 hours are not recommended due to the 

increased mortality resulting from stresses associated with holding (and/or feeding) 

conditions.

As a comparison, use of a 96 hour exposure period has been recommended to account for 

latent mortality in acute toxicity tests with fish (e.g., Ruesink and Smith, 1975). This 96 

hour exposure period/holding time is consistent with the implementation of EPA’s acute 

whole effluent toxicity (WET) test methods for fish (EPA 2002). Acute toxicity testing 

methodologies for fish used in NPDES permit compliance WET testing typically require 

an exposure period of 96 hours, although the methodology allows for use of exposure 

periods between 24 and 96 hours and exposures of 48 hours are sometimes used. Further, 

some States (e.g., California) require the use of 96 hour WET testing in evaluation of 

potentially toxic discharges (CRWQCB – San Diego, 1995). The evaluation of postimpingement survival is slightly different than the evaluation of mortality related to 

exposure to toxic materials as the exposure period for WET testing is integral to the 

amount of stress to which the organisms are exposed (e.g., exposure to toxins continues 

for the entire test period) whereas the stress of impingement and transit through a fish 

return system is discrete and occurs prior to holding. However, the goals of WET testing 

and post-impingement survival studies are similar enough that the holding times used 

should be similar.
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Conclusions

Some organisms are killed outright by impingement and passage through the fish-return 

system, while others are injured. Holding organisms for a set period following 

impingement and, ideally, passage through the fish-return system allows quantification of

any latent mortality associated with these injuries. Various holding times have been 

reported in the scientific literature and in industry reports. Based on reviews of these 

studies, we conclude that a minimum post-impingement holding time of 18 to 96 hours is 

necessary to adequately evaluate latent mortality associated with impingement and 

passage through a fish return system, with shorter holding times likely being sufficient 

for fragile species, and longer holding times required for more robust species. Longer 

holding periods are possible, but are likely associated with substantially increased effort 

and scientific uncertainty. Further, a 96 hour holding time is in agreement with existing 

EPA toxicity testing guidance regarding short-term chronic impacts to fish. Caution

should be taken to ensure that the conditions in the holding tank systems do not cause 

additional stress (e.g., predation, aggressive interactions, low dissolved oxygen, high 

ammonia, etc.) which could relate to additional mortality that could be erroneously

interpreted as the result of impingement. If possible, control tanks should be included 

with similar organisms that have not been subject to impingement, and that control

should be used to evaluate baseline mortality in the holding tanks. The control data 

should be used to normalize the observed mortality of fish/shellfish that have been 

subject to impingement and passage through the fish return.
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Literature Cited

Bigbee, DL, RG King, KM Dixon, DA Dixon, and ES Perry. 2010. Survival of fish 

impinged on a rotary disk screen. North American Journal of Fisheries Management 30: 

1420-1433.

Black, J.L. 2007. Laboratory evaluation of modified traveling screens for protecting fish 

at cooling water intakes. Graduate Thesis, University of Massachusetts Amherst.

California Regional Water Quality Control Board (CRWQCB), San Diego Region. 1995. 

Water quality control plan for the San Diego Basin (9). CRWQCB, San Diego Region, 

San Diego, CA.

Electric Power Research Institute (EPRI). 2003. Evaluating the effects of power plan 

operations on aquatic communities: Summary of impingement survival studies. EPRI, 

Palo Alto, CA. Report 1007821.

McLaren, J.B. and L.R. Tuttle, Jr. 2000. Fish survival on fine mesh traveling screens. 

Environmental Science & Policy 3 (Supplement 1): 369-376.

Ronafalvy, J.P., R.R. Cheesman, and W.M. Matejek. 2000. Circulating water traveling 

screen modifications to improve impinged fish survival and debris handling at Salem 

Generating Station. Environmental Science & Policy 3(Supplement 1): 377-382.

Ruesink, R.G. and L.L. Smith. 1975. The relationship of the 96-hour LC50 to the lethal 

threshold concentration of hexavalent chromium, phenol, and sodium pentachlorophenate 

for fathead minnows (Pimephales promelas Rafinesque). Transactions of the American 

Fisheries Society: 567-570.

Shaw, WW. 2010. Effects of traveling screen operation on impingement and survivability

and latent health of impinged catfish and Barry Steam Generation Plant. Graduate Thesis, 

Auburn University.

Thompson, T. 2000. Intake modifications to reduce entrainment and impingement at 

Carolina Power & Light Company’s Brunswick Steam Electric Plant, Southport, North 

Carolina. Environmental Science & Policy 3(Supplement 1): 417-424.

U.S. Environmental Protection Agency. 2002. Methods for measuring the acute toxicity

of effluents and receiving waters to freshwater and marine organisms, fifth edition, 

October 2002. EPA-821-R-02-012. Office of Water, Washington, DC.