Docket OW-2002-0049 DCN#: 6-5046F 

Jirxk S. N. Decker, W.Dey, J. Young, and 0. Dirorx, 

ASA AnalvJiJ and EPRI 

A Review-of ImpingementSurvival Studies at SteamElectric Power Stations 

A REVIEW OF' IMPINGEMENT SURVIVALSTUDIES 

AT STEAM-ELECTRIC POWER STATIONS 

Steven M. Jinks', Nancy Decker', William Dey', John Young', Douglas Dixon2 

1

ASA Analysis & Communication, Inc. 

291 County Route 62 

New Hampton, NY 10958 

2EPFU 

3412 Hillview Avenue 

Palo Alto, CA 94304 

Abstract.-EPA has recently proposed draft $316(b) regulations for existingpower producing facilitiesthat 

contain performance requirements based, in part, on reducing fish and shellfish impingement mortality at 

the cooling water intake structure by 80% to 95% relative to a baseline consisting of a shoreline intake with 

no impingement controls. Analyses of the potential for focal species to survive impingement for both 

existing and alternativeintake design and operation will be important for demonstrating compliance with 

this requirement. Recognizing this fact, EPRI has recently sponsored a review of the historical studies on 

impingement survival. The majority of impingement survival studies were conducted between the mid1970sand mid-1980s, as part of the initial surge of activity in response to the requirements of $316(b) of the 

Federal Water Pollution Control Act Amendments of 1972. The review included studies at 31 steamelectric plants located in 15 states and the province of Ontario, covering all four of the major waterbody 

types for which USEPA has proposed 93 16(b) performance requirements. Various biological, CWIS, and 

water body factors have been shown to influence impingement survivalrates, but no generally applicable 

mechanistic models for predicting impingement survival have been developed. Results of the review 

indicate that over half of the taxonomic families of fish and shellfish studied to date have the potential for 

impingement survival rates of 70-80% or higher with adequate screen design and operation. Reported data 

also indicate that modifying screenwash operation to a continuous mode is one of the most effective means 

for enhancing impingement survival. Uses and limitationsof the historical studies and available summaries 

and a database of key information from the historical studies are discussed. 
Introduction 

In April 2002, USEPA proposed CWA $316(b) regulations for existing utility and non-utility power 

producing facilities with cooling water flows greater than 50 million gallons per day. These proposed 

regulations would establish national performance standards for BTA based, in part, on reducing fish and 

shellfish impingement mortality at the cooling water intake structure by 80% to 95% relative to a baseline 

consisting of a shoreline intake with no impingement controls. These required reductions could be achieved 

through a combination of intake system design and operational controls, implemented in whole or in part to 

reduce impingement mortality, or through environmental enhancements or restoration. However, a facility 

may also qualify for a site-specific determinationof BTA if it can demonstrate that the costs to the specific 

facility are significantly greater than considered by USEPA in establishing the standard or if the costs to the 

facility of complying with the standard are significantlygreater than the associated environmentalbenefits. 

Impingement mortality is a function both of the number of organisms impinged and their rate of mortality 

from exposure to the impingement process (Text Box 1). Estimates of impingement mortality rates for the 

primary species impinged at existing facilities will, therefore, be an important input for assessing 

compliance with the new regulations, whether that assessment is focused on demonstrating compliance with 

the performance requirements for reducing impingement mortality or on balancing risk reduction benefits 

versus costs of alternative fish protection measures. 

The probability of death from impingement, or impingementmortality rate, has been studied at a number of 

existing steam-electric stations since the mid-1970’s in response to the requirements of §316(b) of the 

Federal Water Pollution Control Act Amendments of 1972. The purpose of these studies was generally to: 

1) document the actual impingement mortality rate at an existing intake so that estimates of impingement 

losses, and associated potential for adverse environmental impact (AEI), could be more accurately assessed; 

and/or 2) evaluate the reduction in impingement mortality rate, and impingement mortality, achieved by 

changes in screen design and operation’. Typically, these studies used the proportion of organisms 

surviving as the measure of organism response to the impingement stress2,and are therefore generally 

referred to as “impingement survival’’ studies. 

Recognizing that existing information on impingementmortality would provide valuable background for 

planning and conducting future studies and assessmentsto address the new regulatory requirements, EPRI 

sponsored a review of prior impingement survival studies to serve as an information resource for a diversity 

of users involved in the regulatory process, including scientists, engineers, managers, and lawyers working 

for the utility industry, regulatory and resource management agencies, academic and private consultants, and 

environmental advocates. The specific purposes of this review were to: 

0 

Identify and summarize impingement survival studies conducted to date; 

Facilitate access to impingement survival study reports; 

Identify factors potentially influencing impingement survival; and 

Discuss important considerations for using impingement survival data in BTA assessments. 

Impingement survival study reports available for this review were identified and accessed from several 

sources including: 1)the EPRI Intake Systems Database maintained by Alden Research Laboratories; 2) the 

library of ASA Analysis & Communication; 3) a search of the open literature using the DIALOG system; 4) 

The latter studies were often part of SPDES permitting agreementsto test alternative intake screen technology. I 

2 The impingement mortality rate is simply equal to (1 -proportion surviving). 
questionnaires soliciting impingement survival information that were sent by EPRI to its members; and 5) 

direct requests to several power companies thought to have completed impingement survival studies. The 

results of the review of these impingement survival reports are being published in full in Evaluating the 

Effects of Power Plant Operations: Summary of Impingement Survival Studies, EPRI Report 1007821 (EPRI 

2003). This paper provides an overview of that report and summarizes some of the results of the review. 

Text Box 1. Definitions of Kev Terms 

Impingement Mortality Rate-A measure of the sensitivity of the organisms to impingement exposure fix., 

probability of dying as a result of impingement). Typically measured in site studies as theproportion of 

organisms surviving impingement,or impingementsurvival rate fix.,Impingement Mortality Rate = 

(I -Impingement Survival Rate). 

Impingement Mortality-Impingement loss, or the number of organisms of each species killed by 

impingement. Impingement loss is afunction of both the exposure (numbers impinged)and sensitivity 

(mortalityrate as a result of impingement)of the organisms. 

Sensitivity-An organism’s tolerance or ability, when exposed to a stress, to resist ejrects or to maintain its 

physiological state within normal homeostatic bounds. 

Summary of the Impingement Survival Studies 

Sixty-five impingement survival source documents were obtained for the review. The majority of these 

source documents are reports on studies funded or conducted by electric generating companies at steamelectric facilities. The review summarized the general methodology used in the impingement survival 

studies, the species, waterbodies, and screen designs covered by the studies, and the survival rate estimates 

obtained from the studies. 

General Methodology Used in Prior Studies 

Debris and organisms in the cooling water withdrawn into steam-electricpower plant intakes are usual�y 

filtered, first with fixed bar racks (typically about 8-10 cm spacing) and then with rotating traveling screens 

(typically 0.95 cm, but at some facilities as small as 0.5 mm spacing). A screen-wash spray system washes 

organisms and debris that impinge on the traveling screens into a sluiceway, which discharges the wash 

water to the source waterbody. Impingement survival studies monitor the post-impingement survival rate of 

fish and macroinvertebrates that are washed from the intake traveling screens. The detailed methods that 

have been used to conduct this monitoring vary somewhat among facilities and, sometimes, among years at 

a given facility. Variations in methods over time generally reflect attempts to refine and improve 

impingement survival estimates by reducing handling and holding stresses, or to accommodate testing of 

alternative intake screen technology (Muessig et al. 1988). Nevertheless, the large majority of studies have 

used similar methods that allow description of a general approach for monitoring impingement survival, 

which is provided below. 

Data Collection 

For impingement survival study, impinged organisms generally were collected s;om the screenwash water 

using a dip-net or a basket type of device containing a mesh opening equal to, or slightly smaller than, that 
of the traveling screen panels. The collection location was either at a point along the sluiceway (often at a 

debris collection pit designed to allow removal of large pieces of debris before discharge to the waterbody) 

or at the point where the sluice water discharges to the waterbody (Figure 1). At some sites, modifications 

were made to the sluiceway system itself to bypass impinged organisms to collection pools or tanks for use 

in the impingement survival studies (ECSI and LMS 1996,Davis et al. 1988). Sampling gear consisted of 

metal mesh baskets, with or without net liners, floating live-pens, or angled screen flumes. Brief sampling 

intervals (e.g., 15-30minutes) or continuous dip-netting from the sluiceway or collection device were often 

used to minimize damage from the collection process (e.g., Tatham et al. 1977; NUSC 1986). Most studies 

either focused sampling on peak periods of impingement or sampled seasonallyto obtain data representative 

of the majority of impingement at the facility. 

Immediately following collection, impinged organisms are categorized as live or dead, or as live, stunned; 

or dead. Data on length or age classification is often also taken at the time of sample collection. These 

initial survival data have typically been taken on all fish species collected in the samples, as well as on 

important shellfish species. Most impingement survival studies have also collected data on delayed, or 

latent, mortality from impingement by transferring organisms initially alive to a holding facility and 

monitoring mortality (number dead) at pre-established intervals over an extended period of time. Typically, 

mortality observations are made at least once during each 24-hour period of the extended survival study to 

avoid data loss through decay, scavenging, etc. To optimize the use of holding facilities and monitoring 

efforts, these “extended” survival studies have often been limited to focal species that are frequently 

impinged and/or of commercial, recreational, or other importance in the waterbody. Holding facilities used 

for the extended survival studies have consisted either of land based flow-through tanks supplied with water 

by pumping from the source waterbody or floating live pens maintained in the source waterbody near the 

cooling water intake. Extended survival observations have typically ranged from less than 24 hours up to 

108 hours, although at least one study monitored mortality of impinged fish for up to 204 hours (EA 1979a). 

To distinguish impingement effects from the effects of sampling and holding, some studies have conducted 

survival tests on several fish species using controls collected by seining or box trapping in the source 

waterbody (Serven and Barbour 1981; Muessig et al. 1988; EA 1983). Field-collected fish were generally 

held for a day or more to allow recovery from collection stress prior to their use as controls, after which they 

were introduced to the collection device (collection controls) andor directly into the holding facility 

(holding controls). 

Data Analysis 

The majority of studies conducted to date have used the proportion of impinged organisms remaining alive 

as the principal statistic for survival analysis and the binomial distribution as the basis for calculating 

variance about the measured proportion surviving. The alternative approach has been to use failure analysis 

methods such as the Kaplan-Meier survivorship function (ECSI and LMS 1996). The definitions and 

formulae used to calculate impingement survival proportions and associated standard errors are reported in 

Muessig et al. (1988). 

Initial survival is calculated for each species and age group studied by dividing the counts of living 

organisms (“alive” = “live” plus “stunned”) by the total numbers collected. The “stunned” category used in 

many studies has generally had two purposes. First, it provided a basis for representatively sub-sampling 

3 In some studies, a “damaged” or “stressed” category was used instead. The distinctionbetween these categories is apparently 

based on whether behavioral observation (e.g. non-equilibrium, struggling movements) or visual inspection for injuries was used 

to classify fish. The text uses the term “stunned” to mean any of these categories. 

3 
both undamaged and damaged fish for use in extended survival studies when too many organisms were 

collected to allow them all to be transferred to the holding facilities. Second, if no extended survival studies 

were conducted, it provided an indicator of the potential for delayed mortality. In effect, initial survival 

estimates calculated using counts of “live”, rather than the “live” plus “stunned”, in the numerator adjust 

survival for delayed mortality by assuming that all stunned organismswill die from damage caused by 

impingement. 

Extended survival is the conditional probability of surviving both initially and during the extended-survival 

monitoring period, calculated by multiplying the initial survival proportion by the proportion of initially 

living organisms that survive to each extended observation interval (Table 2-1). While some of the prior 

impingement survival studies reported extended survival estimates for each observation interval used in the 

study, many chose not to report extended survival for intermediateobservation intervals, and instead 

presented estimates of initial survival and extended survival determined at the last observation interval (e.g., 

96-hr extended survival). 

The observed survival resulting from impingement, collection, and holding can be adjusted for control 

survival (reflectingthe effects of collection and holding) to obtain estimates of survival resulting from the 

effects of impingement alone. Control-adjusted impingement survival estimates are calculated by dividing 

the initial or extended survival proportion by the corresponding proportion of controls surviving. However, 

control-adjusted impingement survival estimates are available in relatively few studies and for only a few 

species, largely as a result of the difficulty of obtaining suitable control fish. Therefore, unadjusted 

extended survival is typically used as the best available estimate of impingement survival. To the extent 

that such estimates include mortality from collection and holding stresses, which they probably often do, 

they overestimate the mortality rate fiom impingement. 

Coverage of the Studies 

The majority of the impingement survival studies were performed between the mid-1970s and mid-l980s, 

as part of the initial surge of activity in response to the requirements of 5316(b) of the Federal Water 

Pollution Control Act Amendments of 1972. State Pollution Discharge Elimination System (SPDES) permit 

conditions that required testing of intake system alternatives for reducing impingement mortality provided 

the impetus for many of the studies, particularly during the 1980s and 1990s. 

The source documents reported impingement survival studies at 31 power plants located near the Atlantic, 

Pacific, and Gulf coasts of the U.S., on the Great Lakes, and on the upper Mississippi and Columbia River 

basins. More than half of the studies have been at plants located in the mid-Atlantic and southern New 

England states (Figure 2). Altogether, impingement survival has been studied for at least 31 steam-electric 

plants located in 15 states and the province of Ontario. The faciliti’es at which studies were conducted are 

located on 22 different waterbodies covering all four of the major waterbody types for which USEPA has 

proposed $316(b) performance requirements (USEPA 2002), as follows: 

0 Freshwater streams and rivers4 facilities located on 3 waterbodies; 

Great Lakes-5 facilities located on 2 waterbohes; 

Tidal rivers and estuaries-1 6 facilities located on 13 waterbodies; and 

Oceans4 facilities located ora 4 waterbodies. 

Impingement survival data is available for three basic types of vertical traveling screens: angled, dual-flow, 

and through-flow, as follows: 

Angled screens-5 reports covering studies at 4 facilities; 
Dual-flow screens-6 reports covering studies at 5 facilities; and 

Through-flow screens-55 reports covering studies at 23 facilities 

Three facilities, Danskammer Point, Oswego, and Roseton, have studied survival using single-flow and 

either dual-flow or angled screens. The majority of studies have tested some form of modification to screen 

design and/or operation intended to enhance impingement survival. 

The historical studies report data for over 300 different taxa, most identified to &e species level. Grouped 

by waterbody type there are 55 taxa represented fiom facilities on fieshwater streams and rivers, 39 from 

facilities on the Great Lakes, 184 from facilities on tidal rivers and estuaries, and 85 from facilities located 

at coastal ocean sites, of course with some overlap of taxa among waterbody types. The majority of species 

were collected in relatively low numbers in these studies, mostly because only a portion of the resident 

species are highly susceptible to impingement at any given facility. For comparison, there were about 180 

taxa where 10 or more organisms were collected in an individual study. 

Information Summaries 

Source documents were reviewed for selected types of information, which, if available, was compiled in a 

hierarchical database (Figure 3). EPRI (2003) provides tables listing the facilities and dates of studies, 

waterbodies, screen designs, and taxa that are covered in previous impingement survival studies. The EPRI 

report also contains tables of impingement survival rate estimates by species and by screen design and 

operation. To provide the broadest reasonable amount of survival information, impingement survival rate 

estimates available in the source documents were included in the database, if those estimates were based on 

ten or more organisms collected. The database contains several types of survival rate estimates, including 

the initial proportion surviving (&=(live + stunned)/totalcollected), initial proportion surviving assuming 

stunned are dead (Sidd=live/totalcollected), and extended survival (S,) at the end of the latent effects holding 

period. 

Factors Influencing Impingement Survival 

The survival of impinged organisms depends on the nature and magnitude of stresses imposed on them 

during impingement and on the tolerance of the organisms to those stresses. Theoretically, the magnitude of 

the various potential stresses of impingement, such as physical impact and abrasion, acidosis and 

neurological shock, suffocation, desiccation and thermal shock, depend on CWIS design and operation; 

while the sensitivities of impinged organisms vary depending on biological and waterbody characteristics. 

The impingement survival source documents were reviewed for information that would assist in 

characterizing the influence of theoretically important biological, CWIS, and waterbody factors on 

impingement survival rates. Examples of the information available from prior studies are provided below, 

and discussed more fully in EPRI (2003). 

Biological Characteristics 

The biological variables that could affect impingement survival include species type, developmental stage 

and size, and physiological condition. Studies at operating power plants have shown that impingement 

survival is strongly influenced by the inherent sensitivityof species to impingement stresses. The 

morphological, physiological and behavioral characteristicsof each species affect its sensitivity to 

impingement stress, although the relationships between these characteristics and impingement survival have 

not been quantitatively defined. In general, species types that are found to be hardy in terms of their 

resistance to collection and handling stress (e.g., crabs, killifish, catfish) are also tolerant of impingement 
stresses, while those that are difficult to collect and keep alive (e.g., herrings, anchovies, smelts) tend to be 

sensitive to impingement. 

It may be helphl to qualitatively consider species characteristics in the context of each of the sources of 

impingement stress discussed above when evaluatingthe potential for species to survive impingement at a 

CWIS (e.g., for planning site studies). For example, species possessing heavier skeletal structure, thick 

scales or bony scutes, thick protective slimes, or hard exoskeletonswould be more likely to resist physical 

injury and desiccation than would species that have light skeletons, and thin scales that shed easily. 

Similarly, species that are better adapted to low oxygen conditions or are able to extract some oxygen 

directly from the air are less likely to experience suffocation. Behavioral characteristics of the species may 

also have important influences on impingement survival. Some species may be responsive to local 

hydraulic conditions and, in the case of pelagic species, may tend to maintain a position up in the water 

column in the intake flow upstream of the screens. Such prolonged swimming may lead to systemic stress 

from oxygen debt and acidosis (Powers 1977), and increased mortality when the fish become exhausted and 

are impinged. Prior studies provide little information on the specific relationship between various species 

characteristics and impingement survival. However, studies conducted at the Millstone Nuclear Power 

Station examined the aggregate effects of body type and habitat preference on impingement survival (NUSC 

1986, 1987). Crustaceans and demersal fish species generally showed much higher impingement survival in 

these studies than either pelagic fish species or squid. 

The variation in extended impingement survival rates among taxonomic families that has been observed in 

prior studies is illustrated in Figure 4, which shows the mean and standard deviation of extended survival 

rate estimates reported for 36 taxonomic families under continuous screenwash condhons. The mean and 

standard deviation of the survival rate estimates reported for each taxonomic family for all screenwash 

conditions are presented in Table 1. Within family and screenwash frequency groups, survival rate 

estimates vary due to differences among facilities, and among other study conditions such as length of the 

extended survival observation and season. However, the survival rate estimates appear to reflect the relative 

species tolerances to impingement that would be expected based on the nature of the impingement stresses 

and the biological characteristicsof the species. For example, families with relatively high survival rates 

consist mostly of macroinvertebrates with hard exoskeletons, fish generally inhabiting shallow, turbid 

waters and known to be easily held in captivity such as killifish and minnows, demersal species and species 

tolerant of low dissolved oxygen levels such as flounders, catfishes and sunfishes, and species that are 

heavily scaled or armored, such as pipefishes and sculpins. Families with low survival rates are mostly 

characterized by soft-bodied pelagic forage species such as anchovies, herrings and smelts. Survival rate 

estimates for about two-thirds of the taxonomic families exceed 50 percent when screenwash is continuous. 

Twenty-eight out of the forty-seven families for which data have been reported, or about 60 percent, appear 

to have the potential, given adequate screenwash frequency, for impingement survival rates greater than 

about 70 to 80 percent. 

Within species, changes in sensitivityto the physical stresses of impingement should be most evident during 

distinct developmental transitions that significantly alter physical characteristics and/or physiological 

mechanisms (e.g., osmoregulation). The source documents contained several examples of the potential 

influence of such transitions on impingement survival rate. For example, studies of impingement survival 

on fine-mesh intake screens show that the survival rate of fish increased sharply as larvae transition to the 

juvenile stage, possibly reflecting the additional protection afforded by scale and skeletal development at 

this stage (EA 1979b). Likewise, the absence of a hard exoskeleton during molting increased the sensitivity 

of decapod crustaceans to impingement, resulting in more moderate survival rates of juveniles and adults 

6 

7 
during the molting season than at other times of the year (NtJSC 1987; CP&L 1985; Tatham et al. 1978; 

Serven and Barbow 1981). 

On the other hand, no consistent relationship between size or age of juvenile and older fish and 

impingement survival is apparent in the prior studies. Extended impingement survival rates have in some 

cases tended to increase (CP&L 1985; ECSI and LMS 1996) or decrease (ECSI and LMS 1996) with size of 

the fish. Other studies have found little consistent trend in extended impingement survival rates among size 

classes (Serven and Barbow 1981) or among age groups (Muessig et al. 1988) of several species. The 

ability to observe consistent trends in survival due solely to size or age may be confounded by other factors 

influencing survival, including seasonal differences in debris loadings or cooling system operation (e.g., 

cooling water flow rates), size-related differences in handling and holding mortality, and environmental 

variables influencingthe physiological state of the organism prior to impingement. 

Environmental factors that affect the organism’s physiology and condition are also likely to influence its 

sensitivity to impingement stresses. Nutrition, disease, and reproductive state (Con Ed 1986) may influence 

impingement survival rate, but relatively little information is available from prior studies regarding these 

factors. 

CWIS Characteristics 

The impingement survival realized by each species and life stage may be greatly influenced by intake screen 

design and operating conditions. Physical stresses present during impingement are influenced by 

screenwash frequency, screen rotation speed, and screen modifications intended to reduce stress associated 

with fish separation and handling. A review of the biological effectiveness, engineeringpracticability, and 

costs of fish protection systems, including active screening systems, has been presented in detail in three 

EPRI reports (EPRI 1986,1994,1999a). 

For vertical traveling screens there is generally a substantial increase in organism survival associated with 

decreased time between screen washes, with continuous screen rotation providing the highest survival (Kmg 

et al. 1978; Tatham et al. 1978). When screens are stationary for long periods of time, impinged organisms 

may become moribund in repeated attempts to free themselves and may suffocate against the screen. Inthe 

studies that contained data for various screenwash frequencies, extended impingement survival rate for most 

species decreased, often very substantially, as time between screen washes increased (Figure 5). However, 

the survival rate for several hardier species (e.g., tessellated darter, three-spine stickleback,hogchoker) was 

very high, regardless of the length of interval between screen washes. 

The duration of organism impingement on the traveling screens is also directly related to rotation time of the 

screen, or the in other words, the time of travel required before impinged fish reach the screenwashheaders. 

This travel tive is determined both by the speed of screen rotation and the elevation or height of the screen. 

Faster rotation and/or khorter screens would be expected to decrease stress. Screen elevation above the 

intake decking may also affect the length of the drop that fish experience from the screen to the screenwash 

sluiceway, which could potentially also impact impingement suwival. 

Several studies have examined the relationship between the speed of screen rotation and impingement 

survival. Studies at the Dunkirk, Mystic, and Brunswick generating stations indicated faster screen rotation 

speeds generally resulted in higher impingement survival rates (Table 2) (Beak 1988; SWEC 1981; CP&L 

1985). Studies on Ristroph-modified dual-flow traveling screens at the Roseton generating station at screen 

speeds of 9.8 ft/min and 19.7 Wmin found no definitive trend in survival between the two screen speeds 

7 
(Normandeau 1995). Six of ten fish species with 20 or more organisms collected had higher survival at the 

faster screen speeds, while 3 fish species and blue crab had higher survival at the slower screen speed. 

A number of physical modifications to screen systems have been developed specificallyto protect fish and 

other aquatic organisms. Screen systems employing fish buckets4(or troughs), continuousoperation, and 

low pressure washes with fish returns to the waterbody are typically referred to as Ristroph screens, after the 

original developer of the modified screen. Because fish protection measures evolved or were added over the 

years, later versions of the modified screens are sometimes referred to as modified Ristroph screens. 

One of the last modifications to the Ristroph design was to add a recurved lip to the screen fish buckets. 

Fletcher (1990) showed in laboratory studies that, in selected species, struggling behavior resulted in a 

downward movement along the screen mesh that directed them to the fish-lifting bucket attached to each of 

his experimental screen panels, Redesigning the bucket with a recurved edge creates a calm zone, where fish 

were found to seek shelter and remained in the bucket as the screen’s rotation carried them upward to the 

water surface and spraywash fish removal system. 

The effectiveness of modified screen systems in reducing impingement mortality compared to conventional 

screens has been evaluated at the Oyster Creek, Salem, Arthur Kill, and Roseton generating stations. Both 

modified and unmodified screens were single-flow (or through-flow) at the first two stations, and dual-flow 

at the Arthur Kill station. The study at Roseton compared survival on dual-flow modified screens to that on 

single-flow conventional screens. At the Oyster Creek’, Arthur Kill, and Roseton stations, impingement 

survival rates measured on modified screens were compared to those from unmodified, conventional 

screens. Studies at the Salem station, evaluated the benefits of progressive improvements in the fish 

protection measures incorporated into existing Ristroph screens. 

Overall, the comparisons indicate that the Ristroph modifications are effective in improving the survival 

rates, especially of species that are sensitive or moderately tolerant of impingement stresses (Table 3). 

However, the incremental fish protection benefits of Ristroph modifications, versus continuously operated 

conventional screens, may vary widely among species and power plant sites, 

Other CWIS factors, such as screen approach velocity, intake configuration, and the proximity of the 

screenwash discharge and fish return to the cooling water intake may influence impingement survival rates. 

However, the potential affect of these factors on survival rate has not been examined in any detail in the 

studies reviewed. Screen approach velocity and intake configuration could affect the behavior and energy 

expenditures of fish prior to encountering the screen surface, as well as the damage incurred during 

impingement. The proximity of the fish return to the cooling water intake, together with the hydrodynamics 

of the waterbody in the vicinity of the station, influences the magnitude of dish reimpingement after return to 

the waterbody. Multiple impingement exposures may tend to reduce survival as a result of cumulative 

stress and injury. 

WaterBody Characteristics 

Many of the impingement survival studies report survival rates separately for various time periods or 

seasons during the year, though only a portion of those studies have attempted to explicitly address the 

relationship of survival to season or to seasonal changes in environmental conditions (Tatham et al. 1978; 

“Buckets” are essentially troughs mounted along the bottom edge of each screen panel that hold a few inches of water. Their 

purpose is to reduce escape and reimpingement of impinged fish and keep them immersed as they are lifted from the waterbody. 5 The unmodified screens at Oyster Creek were operated intermittently(up to 2 hours between screenwashes).Unmodified 

screens at Arthur Kill and Roseton were apparently operated continuously, as were the Rstroph screens at all these plants. 

8 
Muessig et al. 1988; Con Ed 1986; Beak 1988; NUSC 1987; ECSI and LMS 1996; LMS 1991; EA 1986; 

Normandeau 1995;Reider 1984). A variety of water body characteristics that vary seasonally or over 

shorter time periods in response to weather conditions could potentially affect the mortality of impinged 

organisms. Such factors include: loadings of debris and other organisms that may damage fish; water 

temperature; dissolved oxygen and turbidity; and, in the case of estuarine sites, salinity. 

Intake screen loadings of debris and organisms with hard exoskeletons (e.g., crabs) appear to cause an 

increase in"injuryand death, reducing survival of impinged fish (Landry and Strawn 1974). Occurrence of 

debris and its blockage of intake screens is a highly site-specific factor. Some researchers have noted lower 

survival of impinged fish species when large masses ofjellyfish are present (NUSC 1987) or when the 

numbers and activity of crabs on the intake screens is high (Tatham et al. 1978). The potential for injury 

from physical contact with plant materials and man-made debris dwing impingement underlies the design of 

screens that include separate fish and debris spraywash removal troughs. 

A number of studies have examined the relationship between impingement survival and ambient water 

temperatures. Seasonal water temperatures near the upper or lower temperature tolerance limit of the 

species may increase their sensitivity to the subsequent stress of impingement, thereby lowering 

impingement survival relative to that observed at other times of the year. The elevation of metabolic rate as 

temperatures increase to their summer maximums may exacerbate the physiological stresses of 

impingement, including increasing the rate of oxygen starvation of tissues and suffocation. Very low 

temperatures have been observed to increase mortality due to handling and rearing stress as a result of 

increased osmoregulatory dysfunction (Wedemeyer 1972;Miles et al. 1974). In temperate zones, many 

species are unable to fully adapt physiologicallyto the wide changes in water temperatures that occur 

seasonally. Although many of these species adapt behaviorally by seasonal migration to habitats with more 

favorable temperatures, some portion of their populations often reside in waters quite close to the limits of 

their thermal tolerance. As a result, these organisms may be more susceptible to impingement mortality or 

may even be susceptible to natural seasonal mortality, especially in particularly cold winters or hot 

summers. For example, Lankford (1997) has shown that Atlantic croaker, which uses bays and estuaries 

during summer and fall months, can become highly stressed and more susceptible to impingement when 

water temperatures are lowest. Lajeone and Monzingo (2000) reported that the vast majority of fish 

(gizzard shad and freshwater drum)were either dead or moribund prior to their arrival on the Quad Cities 

generating station screens. 

One may expect that the affect of water temperatures on impingement survival will vary among ecosystems 

and species, depending on the adaptations of species to their thermal environments, Variation in 

temperature relationships observed in field studies can also result from covarying factors that may affect 

survival, such as increasing fish size during the growth season and, in the case of estuaries, salinity. It is 

therefore not surprising that reported temperature relationships vary considerably among the impingement 

survival studies. However, the general pattern of affect on fish that emerges when these data are examined 

collectively is one of highest impingement survival over some intermediate ambient temperature range, with 

the potential, depending upon species, for decreasing survival at temperatures above and below this range6 

(Muessig et al. 1988; EA 1986; LMS 1991;NUSC 1987; Normandeau 1995). 

The limited data available suggest that this intermediate temperature range may be relatively narrow for 

pelagic species more sensitive to impingement and relatively broad for demersal or littoral species more I 

tolerant of impingement (EA 1986;NUSC 1987; Normandeau 1995; LMS 1991). For example, at the 

6 It should be noted that those studies that have explored the relationship between impingement survival and temperature have not 

found such a relationship for all species. This may be due to variation among species andor to limitations of the data. 

9 
Roseton station, impingement survival of blueback herring, bay anchovy, and alewife steadily declined as 

temperature increased above about 16°C; from the 20 to 40 percent range below 16°Cto near zero percent 

at 22 to 28°C (Normandeau 1995; LMS 1991). In contrast, impingement survival of brown bullhead 

decreased only slightly at temperaturesup to 28°C. Most of the observations ofreduced impingement 

survival at low water temperatures (less than about 4.5 to 7" C) suggest that the effect may be confined to 

temperatures near the low temperature tolerance limits of the species, with the increased impingement 

mortality caused by the thermally stressed condition of the organisms at the time of impingement. 

Some impingement survival studies have not explicitly examined survival relationships to temperature, but 

have observed differences in extended impingement survival at various times of year (Tatham et al. 1978; 

Reider 1984; Beak 1988). Those observations that span warm and cold seasons, summarizedbelow, are not 

inconsistent with the typical seasonal cycles of ambient water temperature in temperate zones of the U.S. 

and the temperature patterns discussed above. In general, impingement survival rates observed in these 

studies for a given species have been lowest in summer and highest in spring and fall, with intermediate 

survival rates in winter. 

The reduced impingement survival rate at higher summer water temperatures that has been observed for 

some species may, in part, reflect the influence of seasonal changes in dissolved oxygen levels. The 

concentration of dissolved oxygen in water is determined by the interaction of several biological and 

physical processes, which typically results in a seasonal pattern of highest dissolved oxygen concentrations 

in the winter and lowest during the summer. The correlation of increasing water temperatures with 

decreasing djssolved oxygen levels makes it difficult to distinguishmgthe independent effects of dissolved 

oxygen on impingement survival rate. One study used discriminant analysis to examine the importance of 

temperature, dissolved oxygen, and other factors on impingement survival (Normandeau 1995). Dissolved 

oxygen was identified as a primary factor influencingthe impingement survival of blueback herring and a 

secondary factor influencing the survival of alewife. For both of these species, impingement survival rate 

was significantly higher at dissolved oxygen concentrations of 10012 mg/l than it was at 7-8 mg/l. 

Water salinitymay be an important factor influencing impingement survival in the brackish water regions of 

estuaries, where salinity vaxies seasonally in response to changes in fieshwater discharge and tidal height. 

Low to moderate levels of salinity reduce the energy input required for osmoregulation, and thereby act as a 

general stress ameliorator (Bowser and Buttner 1991; Kane et al. 1990; Palawski et al. 1985). Impingement 

survival studies in brackish water regions of the Hudson River suggest that the stress reducing effects of salt 

result in higher impingement survival of some species when brackish water is present than during 

freshwater periods (ORU 1977). For example, at the Bowline Point generating station, extended survival of 

white perch and striped bass increased in proportion to the logarithm of conductivity (Muessig et al. 1988). 

In studies at the Indian Point station, extended impingement survival of striped bass, white perch, weakfish, 

bay anchovy, and blueback herring was higher in August and September when water salinity was 6 to 8 ppt 

than it was later in the year, when salinity decreased to less than 1 ppt (Con Ed 1986). 

Impingement Survival and §316(b) Compliance 

Compliance with the $3 16(b) regulations soon to be established for existing power plants will likely require 

NPDES permit applicants to assess the level of fish and shellfish protection provided by existing and 

alternative intake technologies and operational measures, and in some cases may also compel applicants to 

assess the ecological significance of entrainment and impingement at their generating facilities as part of a 

cost-benefit analysis. These assessments will typically require quantitative estimation of fish losses from 

impingement, and impingementmortality rates will be needed as one of the inputs for calculating those 

10 
losses. Adequate characterization of impingement mortality rates for focal species impinged at the existing 

and alternative intakes is obviously important for the demonstrating compliance since the historical 

impingement survival data show that mortality rates are strongly influenced by biological and CWIS 

characteristics,but can be very low for many species, given adequate screen design and operation. 

Estimates of impingement mortality rates for use in the compliance demonstration may be obtained from 

site-specific studies conducted at the facility concerned or from published reports on impingement survival 

studies previously conducted elsewhere. Existing impingement survival data may be useful in a variety of 

ways in the BTA determination, including: 

Screening of intake alternatives-as part of this screening, existing impingement survival studies can be 

used to identify intake alternativesthat would have potential for reducing impingement losses and 

prioritize them for more detailed evaluation. 

Selection of focal (critical or representative) species-ane criterion for selection is susceptibility to 

effects of the intake. Existing impingement survival data can be used to help assure that the focal 

species selected for assessment adequately represent a range of species’ sensitivitiesto impingement. 

Detailed evaluations of fish protection benefits from intake alternatives-prospective analysis of 

potential benefits prior to selection and installation of alternatives, which necessarily requires 

impingementmortality rate estimates derived from prior survival studies at other facilities, is required to 

assure that economicallyjustifiable and environmentallybeneficial actions are taken. 

Detailed evaluations of fish protection benefits from the existing intake-evaluations of impingement 

losses for the existing intake using survival estimates appropriately selected from studies at other sites 

may help to identify facilities or species with low potential for impact and to evaluate whether feasible 

alternatives are likely to provide appreciable additional fish protection benefits? 

Defining additional data needs-the prior studies provide background information that is valuable for 

selecting impingement mortality rate estimates appropriate for the species, CWIS and waterbody 

characteristicspresent at a facility and help to identify additional information needed for adequately 

demonstrating compliance with the BTA requirements. 

During the planning stage of the 5316@) compliance assessment, available impingement survival 

information should be compiled and evaluated as part of the development of an analysis plan. Thisprocess 

may identifjr additional data needed for the assessment, and should result in an analysis plan that includes 

approaches for addressing uncertainties identified. Impingement mortality rate estimates used in the 

assessment should be based on survival data that is representative of the sensitivity of the impinged species, 

the design and operation of the existing CWIS and any alternatives under consideration, and the waterbody 

conditions during primary periods of impingement. 

Relevance of Mortality Rate Estimates to Site Conditions 

Since the inherent stress tolerance of different species types is one of the greatest influences on 

impingement survival rate, impingement mortality rates for the specific focal species at issue should be 

used, when available. However, the level of uncertainty associated with using mortality rate estimates for 

congeneric species, or in some cases even species of the same taxonomic family, may be acceptable when 

species-specificdata are unavailable. 

7 Later verification of the assumptions used in the evaluations, including impingement mortality rates, can still be required. 

11 
Mortality rate estimates should be consistent with the screenwash frequency normally used at the existing 

facility, and with that planned for alternative technologies and operations, since screenwash frequency is 

one of the most important CWIS factors affecting impingement survival. Screen travel time may be an 

additional considerationfor evaluating mortality of sensitive and moderately tolerant species during 

continuous screenwash, particularly when screen speed is relatively slow (e.g., 2 or 3 ft/min). Ristroph 

modifications appear to increase impingement survival rates for sensitive and moderately tolerant species. 

However, in comparing Ristroph screens to conventional screen alternatives, assessors should take care to 

use mortality rates that reasonably represent the incremental improvement of each alternative. For example, 

the incremental benefit of Ristroph screens over conventional screens operated intermittently at very slow 

rotation speeds ougbt to be greater than their incremental benefit over conventional screens operated 

continuously and at relatively high rotation speeds. 

Many of the impingement survival studies report variations in survival at different times of year. The most 

substantial changes seem to be associated with high summer ambient temperatures, or very cold winter 

temperatures, whch result in higher mortality rates. In estuaries, survival rates appear to be substantially 

higher when water is brackish than when it is fresh. Since prevailing environmental conditions may vary 

from year to year, inter-annual variation in survival rate may also be expected. No generally applicable 

mechanistic models have been developed for predicting changes in impingement survival rate based on 

environmental conditions. Therefore, assessors must select or collect data that are reasonably representative 

of conditions that occur during the primary periods of impingement of the focal species. For many species 

quantifylngvariations in impingement survival rates during all seasons or over all environmental conditions 

may be impractical and unnecessary, since the large majority of impingement occurs during only a portion 

of the year. 

Uncertainty in Impingement Survival Study Methods 

The methods used to collect impingement survival data and calculate mortality rates may introduce 

uncertainties in the assessment of impingement losses. Some of these uncertainties reflect inherent practical 

limitations that are not easily addressed by study design, and may either overestimate or underestimate 

actual impingement mortality rates, including: 

No correction for collectionholding mortality-relatively few studies have conducted control tests on 

the mortality due to stress of collection, handling, and holding for latent effects. As opposed to 

laboratorybioassays where standard test organisms are readily available for use in testing, obtaining 

control organisms from the wild for testing is difficult and often impractical for many species. For many 

species, the holding stresses alone are sufficient to cause mortality, even when holding protocols 

conform to standard bioassay practice. Mortality rate estimates that are unadjusted for control mortality 

may overestimate the actual mortality rate from impingement. 

No interaction of handlingholding and impingement stresses-studies that adjust mortality rate 

estimates for control survival assume that the probability of mortality from impingement and 

collectionholding are independent stxesses that do not interact. To the extent that these stresses are 

interactive, this assumption overestimates the impingement mortality rate. 

No accounting for fish mortality from stresses prior to impingement-impingement survival studies 

generally assume that all dead fish that are collected have been killed by impingement. To the extent 

that some portion of the impinged fish may have died from natural causes or other anthropogenic 

stresses, this assumption overestimates the impingement mortality rate. Such overestimation could be 

substantial in cases where large fish kills have occurred, such as fkom low winter temperatures or 

12 

0 
0 

disease outbreaks. In such cases, the majority of impinged fish may be dead or moribund prior to their 

arrival on the power station screens (LaJeone and Monzingo 2000). Distinguishing other sources of 

mortality by recording the physical condition of dead impinged fish (e.g., eye opacity and 

decomposition) and documenting local or regional fish kills may be an important consideration for 

design of future studies. 

Screenwash efficiency- impingement survival studies should assure that all fish are being 

representatively sampled. Impingement mortality may be underestimated to the extent that fish are 

carried over the screen or are trapped within the return system. Proper design and maintenance of the 

screenwash system and monitoring of the fish transport pathways can minimize biases from low 

screenwash and collection efficiency. 

Potential for predation-increased susceptibilityto predation is a potential source of impingement 

mortality that has not been measured by impingement survival studies. To the extent that predation 

effects occur, impingement mortality rates would be underestimated by these studies. The potential for 

predation may depend on screenwash system design factors that affect attraction and immediate access 

of predators, such as fish and birds (eg., enclosed fish sluice, location and depth of return). Other 

factors to consider in the assessment are the seasonal patterns in abundance and feeding rates of 

predators in relation to the primary impingement season(s). 

13 
REFERENCES 

Beak Consultants, Inc. 1988. Dunkirk Station Biological Studies, Final Report, January-December 1987. 

Prepared for Niagara-Mohawk Corporation. Lancaster, NY. 

Bowser, P.R. and J.K. Buttner. 1991. General Fish Health Management. NRAC Bull. No. 111-1992. 

Northeastern Regional Aquaculture Center, University of Massachusetts,N. Dartmouth, MA. 

Carolina Power & Light Company - Biology Unit (CP&L). 1985. Bmnswick Steam Electric Plant 1984 

Biological Monitoring Report. New Hill, NC. 

Consolidated Edison Company of New York, Inc. (Con Ed). 1985. Biological Evaluation of a Ristroph 

Screen at Inhan Point Unit 2. Consolidated Edison Company of New York, hc. New York, NY. 

Con Ed. 1986. Survival of Fish Impinged on a Ristroph-type Traveling Screen at the Indian Point 

Generating Station, Summer and Fall, 1985. New York, NY. 

Davis, R.W., J.A. Matuousek, M.J. Skelly, and M.R. Anderson. 1988. Biological Evaluation of Brayton 

Point Station Unit 4 Angled Screen Intake. In: Proceedings: Fish Protection at Steam and Hydroelectric 

Power Plants, Electric Power Research Institute, Palo Alto, CA. EPRI CS/EA/AP-5663-SR. 

Ecological Analysts, Inc. (EA). 1979a. Bowline Point Impingement Survival Studies 1975-1978Overview 

Report. Prepared for Orange & Rockland Utilities, Inc., Pearl River, NY. 

EA. 1979b. Evaluation of the Effectiveness of a Continuously Operating Fine Mesh Traveling Screen for 

Reducing Ichthyoplankton Entrainment at the Indian Point Generating Station. Prepared for Central 

Hudson Gas & Electric Corp., Consolidated Edison Company ofNew York, Inc., Orange and Rockland 

Utilities, Inc., Power Authority of the State of New York. Middletown, NY. 

EA. 1983. Moss Landing Power Plant Cooling Water Intake Structures 316(b) Demonstration. Prepared 

for Pacific Gas & Electric Company, San Francisco, CA. 

EA. 1986. Entrainment and Impingement Studies at Oyster Creek Nuclear Generating Station. Prepared 

for GPU Nuclear Corporation. Sparks, MD. 

Electric Power Research Institute (EPRI). 1986. Assessment of Downstream Migfant Fish Protection 

Technologies for HydroelectricApplication. EPRI Report No. AP-4711. Palo Alto, CA. 

EPRI, 1994. Research Update on Fish Protection Technologies for Water Intakes. EPRI Report No. TR104122, Project No. 2694-01. Palo Alto, CA. 

EPRI. 1999a. Fish Protection at Cooling Water Intakes. EPRI Report No. TR-114013. Pa10 Alto, CA. 

EPRI. 2003. Evaluating the Effects of Power Plant Operations: Summary of Impingement Survival 

Studies. EPRI. Palo Alto, CA. Report 1007821 

1 
Environmental Consulting Services, Inc and Lawler, Matusky, and Skelly, Inc. (ECSIC and LMS). 1996. 

1995 Supplemental Impingement Studies with an Assessment of Intake-Related Losses at Salem 

Generating Station. Prepared for Public Service Electric and Gas Company, Hancocks Bridge, NJ. 

Fletcher, I.R. 1990. Flow Dynamics and Fish Recovery Experiments: Water Intake Systems. Trans. Am. 

Fish. SOC.119: 393-415. 

Kane, A.S., R.O. Bennett, and E.B. May. 1990. Effect of hardness and salinity on survival of striped bass 

larvae. N. Amer. J. Fish. Management 10:67-71. 

King, L.R., J.B. Hutchison, and T.G. Huggins. 1978. Impingement survival studies on white perch, striped 

bass, and Atlantic tomcod at three Hudson River power plants. In: Fourth National Workshop on 

Entrainment and Impingement (L. Jensen, ed.), Ecological Analysts, Inc., Melville, NY.Bp. 21’7-233. 

LaJeone, L.J. and R.G. Monzingo. 2000. 316(b) and Quad Cities Station, Commonwealth Edison 

Company. Envir. Sci. Policy 3(Suppl. l), S322. 

Landry, A.M. and K. Strawn. 1974. Number of individuals and injury rates of fishes caught on revolving 

screens at the P.H. Robinson Generating Station. In: Proceedings of the Second Entrainment and Intake 

Screening Workshop (L. Jensen, ed.), Electric Power Research Institute, Palo Alto, CA. Pp. 263-271. 

Lankford, T.E., Jr. 1997. Estuarine recruitment processes and stock structure in Atlantic croaker, 

Micropogonias undulatus (Linnaeus). Ph.D. dissertation. University of Delaware. 

Lawler, Matusky, and Skelly Engineers (LMS). 1991. Roseton Generating Station Intake Debris Screen 

Post-Impingement Survival Evaluation Study. Prepared for Central Hudson Gas and Electric 

Corporation, Poughkeepsie, NY. 

Miles, H.M., S.M. Loehner, D.T. Michaud, and S.L. Salivar. 1974. Physiological responses of hatchery 

reared muskellunge (Esox masquinongy) to handling. Trans. Am. Fish. SOC.103:336-342. 

Muessig, P.H., J.B. Hutchison, Jr., L.R. King, R.J. Ligotino, and M Daley. 1988. Survival of fishes after 

impingementon traveling screens at Hudson River power plants. In: Science, Law, and Hudson River 

Power Plants: A Case Study in Environmental Impact Assessment. American Fisheries Society 

Monograph 4. 

Normandeau Associates, Inc. (NA). 1995. Roseton Generating Station 1994Evaluation of Post 

Impingement Survival and Impingement Abundance. Prepared for Central Hudson Gas and Electric 

Corporation, Poughkeepsie, NY. 

Northeast Utilities Service Company (NUSC). 1986. The Effectiveness of the Millstone Unit 1 Sluiceway 

in Returning Impinged Organisms to Long Island Sound. Waterford, CT. 

WSC. 1987. The Effectiveness of the Millstone Unit 3 Fish Return System. Waterford, CT 

Orange and Rockland Utilities, Inc. (ORU). 1977. Bowline Point Generating Station Near-Field Effects of 

Once-Through Cooling System Operation on Hudson Ever Biota, Pearl River, NY. 

2 
Palawski, D., J.B. Hunn, and F.J. Dwyer. 1985. Sensitivityof young striped bass to organic and inorganic 

contaminants in fresh and saline waters. Trans. Am. Fish. SOC.114:748-753. 

Powers, D.A. 1977. Physiology of fish impingement: role of hemoglobin and its environmental modifiers. 

In: Third National Workshop on Entrainment and Impingement (L. Jensen, ed.), EcologicalAnalysts, 

Inc., Melville, NY. Pp. 241-254. 

Reider, R.H. 1984. Alternative Screen Wash Suwival Study at the Monroe Power Plant April-September, 

1983. 

Semen, J.T. and M.T. Barbour. 1981. C.P. Crane Power Plant: Impingement Abundance and Viability 

Studies, Final Report January - December 1980. Prepared for Baltimore Gas and Electric Company, 

Baltimore, MD. 

Stone & Webster Engineering Corporation (SWEC). 1981. Final Report, Biological Evaluation of a 

Modified Traveling Screen, Mystic Station - Unit No. 7. Prepared for Boston Edison Company. 

Tathm, T.R., D.L. Thomas, and G.J. Miller. 1978. Survival of fishes and macroinvertebrates impinged at 

Oyster Creek Generating Station. In: Fourth National Workshop on Entrainment and Impingement (La 

Jensen, ed.), Ecological Analysts, Inc., Melville, NY. Pp. 235-243. 

USEPA. 2002. National Pollutant Discharge Administration-Proposed Regulations to Establish 

Requirements for Cooling Water Intake Structuresat Phase II Existing Facilities; Proposed Rule. 67 Fed. 

Reg. 17,122 (Apr. 9,2002). Washington, D.C. 

Wedemeyer, G. 1972. Some physiological consequences of handling stress in the juvenile coho salmon 

(Onchorhynchus kisutch) and steelhead trout (Salmo gairdneri). J. Fish. Res. Board Can. 29: 1780-1783. 

3 
AIternative impingement survivaI Screen-wash 

sampling locations Screen-wash j ceway Traveling screens 

trash pit 

Waterbody ~ ~ ,.-,.. 

I ,!,, 5,\ ,, 

Figure 4 Typical impingement survival study collection locations along the screenwash sluiceway system. 

4 
Figure 2 Locations of Impingement Survival Studies (numerals indicate number of facilities studied in each 

statelprovince). 

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Table 2 Extended Survival of Fish by Screen Speed at the Dunkirk, Mystic, and Brunswick Power Plants 

Mean Percent Survival (96 hr) 

During Period of Peak Abundance 

Power Plant (screen speeds) Species SizelStage Low Medium High 

Dunkirk (18, 54 ft/min) Emeraldshiner Juvenile 89 96 

IIIMystic (2.5, 7.5-10, 15 fffmin) Rainbow smelt 

Rainbow smelt 

Adult 

Large 

39 

11.0 31.3 

59 

40.0 

Small 22.5 58.3 66.7 

Alosa spp. Large 0.8 0 0.5 

Small 6.7 23.4 47.1 

Winter flounder All 96.8 100 98.6 

Brunswick (2.5, 6.5-10 ft/min) Atlantic croaker c25 mm 9.6 28.9 

>25 mm 35.6 36.0 

spot c25 mm 7.6 31.O 

Bay anchovy 0 0 

11 
Table 3 Summary of Mean Extended Impingement Survival Observed in Studies Comparing Conventional 

and Modified Screens 

Extended Survival (Proportion) 

Conventional Ristroph ModifiedStation Species Screens Screens Ristroph Screens

Oyster Creek 

Salem 

Arthur Kill 

Roseton 

Bay anchovy 

Atlantic silverside 

Winter flounder 

Sand shrimp 

Weakfish 

White perch 

Bay anchovy 

Atlantic croaker 

spot 

Alosa sp. 

Alewife 

Atlantic herring 

Atlantic silverside 

Bay anchovy 

Blueback herring 

Butterfish 

Menhaden 

Mummichog 

Northern searobin 

Seahorse 

Striped killifish 

Three-spine stickleback 

Weakfish 

White perch 

Winter flounder 

Alewife 

American shad 

Atlantic tomcod 

Bay anchovy 

Blue crab 

Bluegill 

Brown bullhead 

Gizzard shad 

Hogchoker 

Pumpkinseed 

Spottail shiner 

Striped bass 

White catfish 

White perch 

.06 .I9 

.77 .80 

1.o .93 

.92 .95 

.61 .82 

.87 .95 

.29 .45 

.51 .85 

.81 .93 

.40 .80 

.05 .95-.99’ 

.oo -22-.40 

.51 .98-.99 

.oo .41-.52 

.I5 .79-.96 

.I8 .72-.76 

.05 .71-.76 

.95 .80-.92 

.82 .90-.97 

1.o 1.o-I .o 

.91 .87-.96 

.99 I.o-1.o 

.42 .92-.97 

.63 .85-.90 

.41 .97-.97 

.oo .oo 

.oo .01 

.08 .20 

.oo .oo 

.96 .96 

.83 .98 

.65 .84 

.05 .I0 

.92 .97 

.73 .94 

.49 .68 

.24 .43 

.75 .84 

.28 .46 

1 Range for the two modified screens tested.