Marshall Steam Station 

2006-2007 Impingement Study and Assessment of 

Adverse Environmental Impact 

Final Report, 2009 

Cosponsor(s) 

Duke Power 

P.O. Box 1006 

Charlotte, NC 28201-1006 

Project Manager 

David E. Bailey 

EPRI Project Manager 

D. Bailey 

EPRI • 3412 Hillview Avenue, Palo Alto, California 94304 • PO Box 10412, Palo Alto, California 94303 • USA 

800.313.3774 • 650.855.2121 • askepri@epri.com • www.epri.com 
ii 
iii 

CITATIONS 

This technical report was prepared by 

ASA Analysis & Communication, Inc. 

5 Fairlawn Drive, Suite 205 

P.O. Box 57 

Washingtonville, NY 10992 

Principal Investigators 

W. Dey 

J. McLaren 

ASA Analysis & Communication, Inc. 

This Technical Report describes research sponsored by EPRI and Duke Energy Corporation. 

The Technical Report is a corporate document that should be cited in the literature in the 

following manner: 

Marshall Steam Station: 2006-2007 Impingement Study and Assessment of Adverse 

Environmental Impact, EPRI, Palo Alto, CA, 2009. 

v 

REPORT SUMMARY 

This report has been prepared for Duke Energy’s Marshall Steam Station to support Clean Water 

Act Section 316(b) requirements. It presents the results of sampling conducted during 2006-

2007 to document current levels of impingement of fish and other aquatic organisms at the 

facility’s cooling water intake. Sampling results are interpreted in terms of the status of Lake 

Norman, from which the cooling water is withdrawn, and its fish community. Impingement 

losses are interpreted as to whether they constitute adverse impact to the fish community of Lake 

Norman. 

Background 

Marshall Steam Station (MSS) is a 2,090-MW coal-fired steam-electric generating facility 

located on Lake Norman approximately 48 km (30 miles) north of the City of Charlotte, North 

Carolina. MSS consists of four operating units and uses once-through cooling for its steam 

condensers. Units 1 and 2 began commercial operation in 1965 and 1966, respectively. Units 3 

and 4 began operation in 1969 and 1970, respectively. During 2002-2007, the average annual 

capacity factor, based on Maximum Net Dependable Capacity, was 79 percent and average 

annual intake flow during 2002-2006 was 73 percent of the maximum design capacity, which is 

1,463 MGD. 

Impingement sampling was conducted weekly from 6 April 2006 until 30 March 2007. MSS has 

16 intake bays and fixed screens, with each fixed screen consisting of an upper and a lower 

removable panel. To conduct impingement sampling, a crane operator lifted the upper and lower 

screen panels, which were pressure-washed, and the cleaned panels were placed behind the next 

screen to be sampled to act as a back-up screen. Because this process was time-consuming, only 

four screens could be washed and sampled during each sampling event. Therefore, the sampling 

design had to incorporate a random selection process to determine which screens were to be 

washed and sampled weekly. Sampling and data recording adhered to strict Standard Operating 

Procedures prepared specifically for this study and facility. 

Impingement Sampling Results 

During impingement sampling, a total of 6,219 fish weighing a total of 30.2 kg was collected. 

Ten fish species were found in the impingement collections. No state or federally-listed 

threatened or endangered species were impinged. Threadfin shad and alewife dominated the 

collections, comprising 80.8 percent and 12.6 percent of the fish collected, respectively. For the 

12-month period from 1 April 2006 to 31 March 2007, an estimated 188,419 fish and 1,036 kg of 

fish biomass were impinged at MSS. This estimate was derived from the density of fish 

collected during each sampling event, expressed as number per volume of cooling water 

withdrawn at each of the four generating units, extrapolated over the full 12-month period. 
vi 

Results from a year-long impingement sampling program including all intake bays, directly 

comparable to the 2006-2007 program, are not available from previous years for comparison. 

However, biweekly impingement sampling at one screen per unit (four screens total) from April 

1974 to May 1975 yielded a total of 1,050,505 fish collected. Of those identified to species in 

1974-1975, threadfin shad were the most numerous, comprising 61.9 percent of the total 

collections. When combined with gizzard shad, shad unidentifiable to species, and unidentified 

clupeid species, these four taxa comprised 99.8 percent of the total impingement. While not 

directly comparable to the 2006-2007 data, the sampling results indicate a dramatic reduction in 

impingement rates at MSS from 1974-1975 to the present. Reductions in impingement of 

clupeid species (threadfin shad, alewife, and gizzard shad) appear to be the primary reason for 

this decline in impingement at MSS. 

During the 2006-2007 monitoring program, impingement rates were lowest during the spring, 

April-June, and highest in the late summer and early fall (August-October) or mid-winter 

(February). This seasonal pattern reflected the impingement of the two dominant species, 

threafin shad and alewife. Alewife primarily were impinged in late summer whereas threadfin 

shad were impinged throughout the fall and winter months, September-March. Impingement 

rates may have been influenced by water temperatures, lake levels, or DO concentrations. Peak 

impingement of threadfin shad occurred when daily mean water temperatures were 

approximately 6.7-7.8°

C (44-46°F) in February and early March, and the lake level was low. 

Peak alewife impingement occurred in August when DO concentrations were well below 4 

mg/L. Stress related to cold temperatures or low DO may have increased impingement rates for 

these two species. 

AEI Assessment Results

The potential for the estimated levels of impingement at MSS to be considered “adverse 

environmental impact” under §316(b) of the Clean Water Act was assessed. Adverse 

environmental impact was defined as an unacceptable reduction in: 

 biological integrity as measured in terms of aquatic community species composition, 

diversity, and functional organization in Lake Norman; or, 

 human use of the aquatic resources of Lake Norman, especially fishing opportunity or 

catch quantity or quality. 

The assessment used the U.S. Environmental Protection Agency’s well-established Framework 

for Ecological Risk Assessment. Potential adverse effects were addressed from two 

perspectives: prospective (predicted future risks) and retrospective (effects linked to present 

operation). The results were used in a “weight-of-evidence” approach to determine the overall 

risk of adverse environmental impact from operation of the MSS cooling water intake structure 

on Lake Norman. 

There was no evidence that impingement at MSS is causing adverse environmental impact in 

Lake Norman. This conclusion is based on a careful evaluation of all relevant lines of evidence 

including the following: 
vii 

1. Numbers of fish lost as a result of impingement are exceedingly small compared to likely 

sizes of fish populations in Lake Norman. For all but three species, removal rates amounted 

to less than 10 fish per day. For blue catfish, removal rates were less than 22 young juveniles 

per day. These young juveniles could be expected to yield only a single harvestable adult, on 

average. As with the other species, it is hard to imagine that such a removal rate would 

constitute an adverse environmental impact on Lake Norman. 

2. Total number impinged each year for the two most frequently impinged species, threadfin 

shad and alewife, amounted to less than 0.3 percent of the population in the lake. Both 

species are highly prolific spawners with high growth rates and short life spans. This high 

reproductive potential can easily compensate for the relatively small losses at MSS with no 

long-term effects. 

3. The aquatic community of Lake Norman has been relatively stable in terms of fish 

abundance and trophic structure, particularly in recent years. Fish abundance in pelagic areas 

has stabilized since 1997. There also has been no sustained trend in the size of the littoral 

zone catch since 1993, while species richness has increased with the addition of several new 

species. 

4. Intensive recreational fishery management and species introductions likely have been 

responsible for changes in the fish community experienced in the past. However, there is no 

evidence of changes related to operation of the MSS cooling water intake and impingement 

losses. 

5. Estimates of total fishery catch that could result from the fish impinged at MSS, had they 

survived, amount to only 254 lb (115 kg) per year, most of which was blue catfish. That 

amount translates to less than 1 lb (0.5 kg) per day. This level of harvest is less than what 

would be expected from a small number of anglers. 

6. Estimates of the total production foregone as a result of fish impinged at MSS amounted to 

just over 5,100 lb (2,313 kg) per year of biomass. Almost all of this was a result of threadfin 

shad and alewife, both of which are highly abundant species in Lake Norman. That biomass 

translates to less than 3 ounces per surface acre (210 g/ha) across the entire reservoir. 

7. The best estimate of the total economic value of fish impinged at MSS was $577 per year. 

Even considering the uncertainty in these estimates, it is highly unlikely that the economic 

value exceeds $800 per year. These values must be considered trivial compared to the likely 

total economic value of the recreational fishery in Lake Norman.

Keywords 

Clean Water Act §316(b) 

Economics 

Cooling water intake structures 

Fisheries 

Environmental impact assessment 

ix 

CONTENTS 

INTRODUCTION ............................................................................................................................ 1-1

Background ................................................................................................................................ 1-1

Report Organization ................................................................................................................... 1-2

PLANT AND SOURCE WATERBODY DESCRIPTIONS .............................................................. 2-1

Marshall Steam Station .............................................................................................................. 2-1

Cooling Water Intake Structure ............................................................................................. 2-1

Plant Operation and Water Withdrawal ................................................................................. 2-1

Lake Norman .............................................................................................................................. 2-2

Geomorphology and Aquatic Habitat ..................................................................................... 2-2

Water Quality ......................................................................................................................... 2-3

FISH COMMUNITY DESCRIPTION ............................................................................................... 3-1

Description of Available Information ........................................................................................... 3-1

Species Composition.................................................................................................................. 3-2

Protected Species ...................................................................................................................... 3-3

Target Species ........................................................................................................................... 3-4

Threadfin Shad ...................................................................................................................... 3-4

Alewife ................................................................................................................................... 3-5

Blue Catfish ........................................................................................................................... 3-6

White Perch ........................................................................................................................... 3-7

Channel Catfish ..................................................................................................................... 3-8

IMPINGEMENT MORTALITY ......................................................................................................... 4-1

Impingement Monitoring Programs ............................................................................................ 4-1

Current Impingement Sampling Program Results ...................................................................... 4-2

Species Composition ............................................................................................................. 4-3

Seasonal Patterns ................................................................................................................. 4-4

Variables Affecting Impingement Rates ................................................................................ 4-5

Environmental Variables ................................................................................................... 4-5

Species-Specific Abundance and Spatial Distribution ...................................................... 4-6
x 

Annual Losses ............................................................................................................................ 4-7

Annual Variability ................................................................................................................... 4-8

Intake Operation ............................................................................................................... 4-8

Environmental Variables ................................................................................................... 4-8

Fish Abundance ................................................................................................................ 4-8

Comparison of Annual Monitoring Programs .................................................................... 4-9

ASSESSMENT OF ADVERSE ENVIRONMENTAL IMPACT ........................................................ 5-1

Introduction ................................................................................................................................ 5-1

Assessment of Ecosystem Effects ............................................................................................. 5-3

Prospective Approach ........................................................................................................... 5-3

Retrospective Approach ........................................................................................................ 5-4

Fish Abundance ................................................................................................................ 5-5

Species Richness ............................................................................................................. 5-6

Trophic Structure .............................................................................................................. 5-6

Retrospective Summary.................................................................................................... 5-7

Effects on Human Uses.............................................................................................................. 5-8

Equivalent Loss Measures .................................................................................................... 5-8

Economic Value ..................................................................................................................... 5-9

Uncertainty in Economic Value .............................................................................................. 5-10

Conclusions ................................................................................................................................ 5-10

REFERENCES CITED .................................................................................................................... 6-1

APPENDIX A: METHODS TO ESTIMATE EQUIVALENT LOSS AND ECONOMIC 

VALUE OF FISH IMPINGED AT MARSHALL ............................................................................... A-1

Introduction ................................................................................................................................ A-1

Estimates of Target Species Equivalent Losses ........................................................................ A-1

Assignment of Age Categories for Impinged Individuals ....................................................... A-3

Life Table Information ............................................................................................................ A-3

Life Stage Durations .............................................................................................................. A-4

Natural Mortality Rates .......................................................................................................... A-4

Fishing Mortality Rates .......................................................................................................... A-5

Fishing Vulnerability Rates .................................................................................................... A-5

Weight at Beginning of Age ................................................................................................... A-6

Economic Valuation Methods ..................................................................................................... A-7
xi 

Direct Use Values .................................................................................................................. A-7

Market Values (Commercial Fishing) .................................................................................... A-7

Non-Market Direct Use Values (Recreational Fishing) .......................................................... A-8

Indirect Use Benefits ............................................................................................................. A-9

Non-Use Benefits .................................................................................................................. A-12

Adjustment for Other Species ................................................................................................ A-12

Uncertainty Analysis ................................................................................................................... A-13

Inputs to Direct and Indirect Use Benefits ............................................................................. A-14

Sensitivity Analysis ................................................................................................................ A-15

Monte Carlo Analysis ............................................................................................................. A-15
xii 

LIST OF FIGURES 

Figure 2-1 Location of MSS ...................................................................................................... 2-5

Figure 2-2 Aerial View of MSS .................................................................................................. 2-6

Figure 3-1 Sampling Zones and Locations for Lake Norman ................................................. 3-10

Figure 4-1 Seasonal Pattern of Impingement Rates for All Species Combined ...................... 4-11

Figure 4-2 Seasonal Impingement Pattern and Length Frequency of Impinged Alewife ........ 4-12

Figure 4-3 Seasonal Impingement Pattern and Length Frequency of Impinged Threadfin 

Shad ................................................................................................................................. 4-13

Figure 4-4 Seasonal Impingement Pattern and Length Frequency of Impinged Blue 

Catfish .............................................................................................................................. 4-14

Figure 4-5 Seasonal Impingement Pattern and Length Frequency of Impinged Channel 

Catfish .............................................................................................................................. 4-15

Figure 4-6 Seasonal Impingement Pattern and Length Frequency of Impinged White 

Perch ................................................................................................................................ 4-16

Figure 4-7 Daily Lake Level, Dissolved Oxygen Concentration, and Intake Water 

Temperature vs. Impingement Rate for All Species Combined, April 2006-March 

2007 ................................................................................................................................. 4-17

Figure 4-8 Daily Lake Level, Dissolved Oxygen Concentration, and Intake Water 

Temperature vs. Impingement Rate for Young-of-Year (YOY) and Yearling and 

Older (YAO) Threadfin Shad, April 2006-March 2007...................................................... 4-18

Figure 4-9 Daily Lake Level, Dissolved Oxygen Concentration, and Intake Water 

Temperature vs. Impingement Rate for Young-of-Year (YOY) and Yearling and 

Older (YAO) Alewife, April 2006-March 2007................................................................... 4-19

Figure 5-1 Annual Population Estimates of Pelagic Fish in Lake Norman Using 

Hydroacoustic Sampling, 1993-2006 ............................................................................... 5-12

Figure 5-2 Total Annual Electrofishing Catch for Bluegill, Redbreast Sunfish, 

Largemouth Bass, and All Species Combined at 30 100-m Transects in Lake 

Norman, 1993-1997 and 1999-2007 ................................................................................ 5-13

Figure 5-3 Total Annual Electrofishing Catch for Blue Catfish, Alewife, and Threadfin 

Shad at 30 100-m Transects in Lake Norman, 1993-1997 and 1999-2007 ..................... 5-14

Figure 5-4 Total Annual Electrofishing Catch for Channel Catfish and White Perch at 30 

100-m Transects in Lake Norman, 1993-1997 and 1999-2007 ....................................... 5-15

Figure 5-5 Species Composition in the Littoral Zone of Lake Norman as Determined by 

Electrofishing in 1993-1997 and 1999-2007 (Shading Denotes Species Presence) ........ 5-16

Figure 5-6 Percent Composition of Fish Trophic Guilds in the Littoral Zone of Lake 

Norman Based on Electrofishing Results of 1993-1997 and 1999-2007 ......................... 5-18

Figure 5-7 Estimates of the Range of Effects of Uncertainty in Each Input Parameter on 

Estimates of Annual Economic Benefit at MSS ............................................................... 5-19

Figure 5-8 Results of Monte Carlo Analysis of Uncertainty in Key Input Parameters on 

Estimates of Annual Economic Benefits at MSS .............................................................. 5-20
xiii 

LIST OF TABLES 

Table 2-1 Monthly Mean Cooling Water Flow Rate (MGD) and Percentage of Maximum 

Design Generation and Flow Rate at Marshall Steam Station, 2002-2007 ........................ 2-7

Table 3-1 Fish Species Collected in Lake Norman Fishery Surveys (Siler et al. 1981; 

Duke 2004a, 2004b, 2006, 2007) ..................................................................................... 3-11

Table 4-1 Fixed Screens Sampled for Impingement (Marked as “1”) by Unit and 

Sampling Date, 2006-2007 .............................................................................................. 4-20

Table 4-2 Estimated Total Number and Biomass of Fish Impinged at Marshall Steam 

Station, 1 April 2006 – 31 March 2007 ............................................................................. 4-22

Table 4-3 Total Number and Percent Composition of Fish Collected at Marshall SS, 6 

April 2006 – 30 March 2007 ............................................................................................. 4-23

Table 4-4 Total Biomass and Percent Composition of Fish Collected at Marshall SS, 6 

April 2006 – 30 March 2007 ............................................................................................. 4-23

Table 4-5 Number of Impinged Fish Collected* at Marshall Steam Station, 30 April 1974 

– May 1975 ...................................................................................................................... 4-24

Table 4-6 Number and Percent Composition of Taxa Other Than Fish at MSS, 6 April 

2006 – 30 March 2007 ..................................................................................................... 4-25

Table 4-7 Estimated Total Number and Biomass of Fish Impinged Monthly at all Units of 

MSS, 1 April 2006 - 31 March 2007 ................................................................................. 4-26

Table 4-8 Maximum Total Length (mm) Used to Distinguish Young-of-Year from 

Yearling Fish by Month .................................................................................................... 4-28

Table 4-9 Estimated Annual Impingement of Fish by Age Group at all Units of MSS, 1 

April 2006 - 31 March 2007 .............................................................................................. 4-29

Table 4-10 Comparison of Relative Abundance of Fish Species in the March-April 2006 

Littoral Zone Electrofishing Surveys and the 2006-2007 MSS Impingement 

Collections ........................................................................................................................ 4-30

Table 4-11 Estimated Total Number and Biomass of Fish Impinged Monthly at all Units 

of MSS for Full Flow Operations, 1 April 2006 - 31 March 2007 ...................................... 4-31

Table 5-1 Species Composition of the Purse Seine Catch in Lake Norman, 1993-2006 ....... 5-21

Table 5-2 Trophic Guild Assignments for Fish Species in Lake Norman ................................ 5-22

Table 5-3 Estimates of Potential Total Annual Equivalent Fishery Yield (lb) from 

Individuals Impinged for Each Target Species at MSS .................................................... 5-23

Table 5-4 Estimates of Potential Total Annual Production Foregone (lb) from Individuals 

Impinged for Each Target Species at MSS ...................................................................... 5-23

Table 5-5 Estimates of Economic Value of Fish Impinged at MSS ......................................... 5-24

1-1 

1 

INTRODUCTION 

Background 

ASA Analysis & Communication, Inc. has prepared this report for Duke Energy’s Marshall 

Steam Station (MSS) as part of the application process for a National Pollution Discharge 

Elimination System (NPDES) permit from the North Carolina Department of Environment and 

Natural Resources (NCDENR). Under the Clean Water Act §316(b), an NPDES permit 

applicant must demonstrate that the location, design, construction and capacity of its cooling 

water intake structure represents Best Technology Available (BTA) for minimizing adverse 

environmental impact. The primary impacts of concern under §316(b) are entrainment of 

smaller aquatic organisms into the cooling water system and impingement of larger organisms 

onto traveling screens in the cooling water intake. 

In an effort to establish and enforce consistent national standards under CWA §316(b), the U.S. 

Environmental Protection Agency (EPA) published a Phase II Rule [69 Fed. Reg. 41576 (9 July 

2004)]. The Rule applied to existing electric generating facilities that operate cooling water 

intake structures (CWIS) with a minimum design capacity of 50 million gallons per day (MGD), 

withdraw water from waters of the U.S., and use 25 percent or more of the water withdrawn for 

cooling purposes. MSS fits this definition for a Phase II facility. Compliance with the Phase II 

Rule was based on achieving performance standards for reduction of impingement mortality and 

entrainment set by the EPA on the basis of facility location. The Rule required that impingement 

mortality be reduced by 80 to 95 percent and entrainment be reduced by 60 to 90 percent, 

compared to a baseline level (i.e., the calculation baseline) specifically determined for the 

facility. Adhering to the Phase II Rule guidelines, MSS would not be subject to entrainment 

reduction performance standards because it is located on a reservoir that has a retention time 

exceeding seven days. 

In 2006, the Phase II Rule was challenged in the U.S. Court of Appeals for the Second Circuit, 

which reached a decision on 25 January 2007. The decision remanded several provisions of the 

Phase II Rule, including BTA determination, performance standards, cost-cost and cost-benefit 

analysis, compliance according to the Technology Installation and Operation Plan (TIOP), 

restoration as a compliance alternative, and “independent supplier” provisions. In response to 

this decision, EPA suspended the Phase II Rule [72 Fed. Reg. 37107 (9 July 2007)] and directed 

that permit requirements for cooling water intake structures at Phase II facilities be established 

on a case-by-case best professional judgment (BPJ) basis. However, the requirement that 

permitting agencies develop BPJ controls for cooling water intake structures that reflect BTA for 

minimizing adverse environmental impact (AEI) was retained. Recently the U.S. Supreme Court 

agreed to review the Second Circuit Court’s decision relative to the remand of the cost-benefit 

component of the original Phase II Rule. The Supreme Court decision is expected in 2009. 
Introduction 

1-2

To comply with the regulations, Duke Energy prepared a Proposal for Information Collection 

(PIC) for MSS that included the study design for impingement sampling and submitted the PIC 

to the NCDENR for review. The results of the impingement study are the subject of this report. 

Since the Phase II Rule was remanded after PIC and §316 is now being administered under BPJ, 

the analytical approach described in the PIC has been modified to be appropriate for a BPJ 

approach rather than for conformance to the remanded Phase II Rule. This report presents the 

results of sampling conducted at MSS during 2006-2007 in order to document current levels of 

impingement at the facility’s cooling water intake. Sampling results are interpreted in terms of 

the status of Lake Norman, the impoundment from which the cooling water is withdrawn, and its 

fish community. 

Report Organization 

Section 2 provides pertinent information on the structure and operation of the station’s cooling 

water intake structure, as well as a brief description of the geomorphology of Lake Norman and 

the aquatic habitat it provides. Section 3 describes the lake’s fish community composition and 

structure, particularly as related to susceptibility to impingement at the MSS intake. Section 4 

presents the results of the 2006-2007 impingement sampling program, compares these results to 

data collected during sampling programs conducted during 1974-1975, describes the seasonality 

and size distribution of impinged fish, and analyzes the possible influence of environmental 

factors and changes in the fish community on impingement rates. This information is used to 

characterize current baseline levels of impingement at MSS. Section 5 discusses the observed 

level of impingement mortality in terms of whether it constitutes adverse impact on the fish 

community of Lake Norman, using both a predictive approach and a retrospective approach. The 

predictive approach uses modeling to express impingement losses in terms of equivalent 

reductions in yield to the Lake Norman fishery and to estimate the economic value of the losses. 

The retrospective approach evaluates the composition of the current fish community to determine 

whether there is evidence of trends that could indicate impairments or instability related to 

annual impingement losses. 
2-1 

2 

PLANT AND SOURCE WATERBODY DESCRIPTIONS 

This section describes the cooling water intake structure of the Marshall Steam Station, its 

operation, and its cooling water source, Lake Norman. 

Marshall Steam Station 

Marshall Steam Station (MSS) is a coal-fired steam-electric generating facility located 

approximately 48 km north of the City of Charlotte, North Carolina in Catawba County on Lake 

Norman, a reservoir on the Catawba River (Figure 2-1). MSS consists of four coal-fired electric 

generating units rated at a combined net output of 2,090 MW. Units 1 and 2, rated at 385 MW 

net each, began operation in 1965 and 1966, respectively. Units 3 and 4, rated at 660 MW net 

each, began operation in 1969 and 1970, respectively. MSS uses once-through cooling for its 

condensers. 

Cooling Water Intake Structure 

MSS has a cooling water intake located at the end of a 2.1-km long, 81-ha cove (Figure 2-2). At 

the mouth of the cove is a skimmer wall that is 18.3-m deep at full pond and has a 3-m by 82-m 

opening for withdrawing cool, hypolimnetic water when the lake is thermally stratified. On the 

face of the intake are trash racks with 3-in. (76-mm) on-center spacing for the steel bars. Units 1 

and 2 each have three intake bays and two circulating pumps, and Units 3 and 4 each have five 

intake bays and three circulating pumps, for a total of 16 intake bays and 10 circulating pumps. 

Heated water is discharged into a 1.6-km long cove that is downlake from the intake cove and 

has a surface area of 30 ha. 

There are 16 sets (front and back) of fixed screens, with each screen consisting of upper and 

lower removable panels and with one screen set per intake bay. The screens have 3/8-in. (9.5-

mm) woven wire mesh. They are cleaned individually by removing them by crane, placing them 

horizontally over a steel debris collection pit that can be lifted by crane and emptied, and 

washing them with a high-pressure spray wash. Normally the screens are washed on an asneeded basis, but the screen washing process can be continuous during periods of heavy 

debris/fish loading. 

Plant Operation and Water Withdrawal 

Total generation and condenser cooling water pumping data are presented in Table 2-1 for 2002 

through 2007. Over this period, the average annual capacity factor, based on Maximum Net 
Plant and Source Waterbody Descriptions 

2-2

Dependable Capacity, was 79 percent. The average total annual intake flow rate for January 

2002 through December 2006 was approximately 73 percent of the maximum design capacity, 

2,264cfs or 1,463 MGD (Table 2-1). Annual flows ranged from 69 percent (2006) to 76 percent 

(2005) of the maximum design capacity. Typically, one cooling water circulating pump per unit 

is operated during winter months or during reduced plant loading, but all pumps are run during 

warm months (July-September) and periods of peak demand. 

Mean approach velocity at the fixed screens is 0.7 ft/s (0.21 m/s) at maximum drawdown lake 

level (El. 745.0 ft or 227.0 m) and maximum design flow for Units 1-4 (2,264 cfs or 1,463 

MGD). At full pond levels (El. 231.6 ft) and maximum design flow for Units 1-4, the velocity 

approaching the screens is 0.4 ft/s (0.12 m/s) (Alden and EPRI 2005). 

Lake Norman 

Geomorphology and Aquatic Habitat 

Lake Norman is an impoundment of the Catawba River formed by the Cowans Ford Dam 

(Figure 2-1). The Catawba River Basin lies in the south-central portion of western North 

Carolina and has its source in the eastern slopes of the Blue Ridge Mountains in Old Ford, 

McDowell County, North Carolina. From its source, the Catawba River flows eastward, then 

southward toward the City of Charlotte. The Catawba River Basin and the Broad River Basin 

join to form the headwaters of the Santee-Cooper River system. 

Lake Norman is located in the portion of the Catawba River Basin designated as the Middle 

Catawba River Subbasin 32, which lies within the North Carolina Piedmont physiographic 

region (Hogan 1981) and the Northern Inner Piedmont and Southern Outer Piedmont ecoregions 

(NCDENR 2003). The underlying rock in the Piedmont geologic province predominantly 

includes the Inner Piedmont and Charlotte belts. Rocks of the Inner Piedmont belt are 

metamorphic in origin and consist almost entirely of mica gneisses and schists, with lesser 

amounts of hornblende gneiss and granite (Hogan 1981). Rocks of the Charlotte belt are of 

plutonic and metamorphic origin and consist largely of granites, diorites, and gabbros. Soils are 

acidic, highly leached, and weathered clay subsoils, with slow percolation rates (Hogan 1981). 

Lake Norman and the Cowans Ford Dam are part of a system known as the Catawba Chain of 

Lakes formed by seven Duke Energy hydroelectric dams. The seven lakes consist of Lake 

James, Rhodkiss Lake, Lake Hickory, Lookout Shoals Lake, Lake Norman, Mountain Island 

Lake, and Lake Wylie. The lakes were created between 1904 and 1928, except for Lake 

Norman, which was created in 1963. Lake Norman is the largest reservoir in the Chain of Lakes 

and lies between Lookout Shoals Lake upstream to the north and Mountain Island Lake 

downstream to the south. 

The surface area of Lake Norman is 13,156 ha, its mean depth is 10 m, and it drains a 4,636-km2

watershed (NCDENR 2003). Lake Norman is 54 miles long from the tailrace of Lookout Shoals 

Lake downstream to Cowans Ford Dam (NCDENR and NCWRC 2001). MSS is approximately 
2-3

in the middle of this distance on what would be the west bank of the river. Major tributaries to 

Lake Norman are Lyle Creek and Buffalo Shoals Creek. 

Land use in Subbasin 32 is 54 percent forested, 31 percent pasture, 9 percent water, and 3 

percent each for urban and cultivated cropland. The primary use of Lake Norman is for cooling 

water at MSS and McGuire Nuclear Station and for hydroelectric generation at the Cowens Ford 

Dam. The secondary use is public recreation. 

Lake Norman provides an important recreational fishery for the region, particularly for striped 

bass, largemouth bass, and black crappie. Striped bass do not reproduce naturally in the 

reservoir and have been stocked annually (Van Horn 2001). The lake is managed for these game 

species and the forage species upon which they are dependent, such as shad species. Other game 

species include blue catfish, channel catfish, flathead catfish, white bass, white perch, yellow 

perch, white crappie, spotted bass, and several sunfish species such as bluegill. Several fishing 

tournaments are held on the lake for striped bass, largemouth bass, and catfish (e.g., blue catfish) 

by local and national organizations. 

Water Quality 

Lake Norman is characterized as being oligotrophic with low nutrient values and low algal 

production, and is classified as a water supply, WS-IV (NCDENR 2003, 2008). On the basis of 

long-term mean phytoplankton concentration, the lake might be classified as oligo-mesotrophic 

(Duke 2006). Water quality sampling by the NCDENR Division of Water Quality during MaySeptember 2007 recorded Secchi disc readings of 1.8-2.6 m, low total phosphorus concentrations 

(usually below detection limits), low to moderate chlorophyll a concentrations, and low ammonia 

and total organic nitrogen concentrations (NCDENR 2008). However, nitrite plus nitrate 

concentrations were elevated. 

Mean ambient surface water temperatures range from <7 °

C (44.6 °F) in January to 30-32 °C (86-

89.6 °F) in August (Duke 2004a). The lake thermally begins to stratify in April or May, and fall 

turnover occurs by November. In the summer, the thermocline is approximately at 15-20 m 

depth. Surface DO concentration typically ranges from 6 to 11 mg/L, but hypolimnetic DO 

concentrations usually are <5 mg/L from June to November and are near 0 mg/L in August to 

October (Duke 2004a). Anoxic conditions in the hypolimnion are more prevalent during 

particularly warm summers. Since the skimmer wall on MSS’s intake cove draws from depths 

below approximately 18.3 m, cool water with a depressed DO concentration is withdrawn from 

the hypolimnion during warm weather months. 

Water quality monitoring by Duke has shown pH ranging from 6.2 to 7.4, low surface turbidity 

(1.02-5.05 NTU), low conductivity (69-110 mhos/cm), and heavy metal concentrations that do 

not exceed state water quality standards, although iron and manganese can be released from 

bottom sediments during anoxic conditions (Duke 2004a). Surface DO concentrations uplake 

can exceed 110 percent saturation, as in 2002 (NCDENR 2003). Nutrients and chlorophyll a 

concentrations in Lake Norman remain low or less than detection limits. The biggest water 

quality hazard for the lake is the infestation of the exotic macrophytes Hydrilla and 

Myriophyllum, which can decrease fish habitat and impact recreation (NCDENR 2003). 
Plant and Source Waterbody Descriptions 

2-4

Macrophyte control using herbicides and triploid grass carp (Ctenopharyngodon idella) stocking 

has been conducted in 2006-2007 through a joint effort by Duke, the Lake Norman Marine 

Commission, and the N.C. Division of Water Resources (NCDENR 2008). There are no fish 

consumption advisories except for the statewide advisory for bowfin due to mercury 

contamination. 

The Catawba River Basin is the most densely populated river basin in North Carolina, with a 

population exceeding 1 million persons, inclusive of the City of Charlotte (NCDENR and 

NCWRC 2001). Because the river basin faces unusually high development pressures, a Catawba 

River Basin Natural Resources Plan has been prepared by the NCDENR and the North Carolina 

Wildlife Resources Commission (NCWRC), with input from multiple plan participants including 

Duke, for the protection and enhancement of the basin’s natural resources (NCDENR and 

NCWRC 2001). This Natural Resources Plan complements the Catawba River Basinwide 

Management Plan and assigns priorities for specific resources or issues. The upper portion of 

Lake Norman has been assigned the top priority (I) due to its lower level of development and 

more natural riverine environment and resources (e.g., shoreline and wetland habitats). The 

lower portion of Lake Norman has been assigned medium priority (II), with emphasis on its high 

recreational demand. Fish and wildlife habitat are to be protected and enhanced in both areas. 
2-5 

 Figure 2-1 Location of MSS
Plant and Source Waterbody Descriptions 

2-6 Figure 2-2 Aerial View of MSS 
Table 2-1 Monthly Mean Cooling Water Flow Rate (MGD) and Percentage of Maximum Design Generation and Flow 

Steam Station, 2002-2007 

Percent Net Capacity Generation 

Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec A

2002 79% 80% 76% 79% 65% 83% 86% 85% 78% 85% 66% 76% 2003 95% 91% 78% 52% 75% 82% 85% 86% 91% 80% 83% 97% 2004 94% 83% 82% 74% 71% 88% 85% 86% 77% 72% 74% 84% 2005 92% 89% 87% 83% 77% 85% 85% 91% 84% 77% 77% 79% 2006 88% 90% 60% 49% 71% 80% 82% 85% 72% 48% 46% 72% 2007 75% 94% 80% 76% 77% 85% 68% 86% 66% 86% 86% 87% 

Average 87% 88% 77% 69% 73% 84% 82% 87% 78% 75% 72% 83% 

Actual Plant Flow Rate (MGD) 

Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec A

2002 1,066 1,078 990 965 988 1,094 1,106 1,461 1,431 1,383 937 1,042 2003 1,073 912 840 641 1,014 1,092 1,102 1,253 1,463 1,200 1,028 1,084 2004 1,092 829 876 911 862 1,095 1,103 1,365 1,365 1,016 1,086 1,177 2005 843 851 1,002 1,028 1,004 1,093 1,146 1,363 1,444 1,287 1,219 1,072 2006 1,083 1,042 758 689 963 1,062 1,111 1,463 1,423 853 728 979 2007 1,016 1,082 968 990 N/A N/A N/A N/A N/A N/A N/A N/A 

Percentage of Maximum Flow Rate (2,264 CFS or 1,463 MGD) 

Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec A

2002 73% 74% 68% 66% 68% 75% 76% 100% 98% 95% 64% 71% 2003 73% 62% 57% 44% 69% 75% 75% 86% 100% 82% 70% 74% 2004 75% 57% 60% 62% 59% 75% 75% 93% 93% 69% 74% 80% 2005 58% 58% 68% 70% 69% 75% 78% 93% 99% 88% 83% 73% 2006 74% 71% 52% 47% 66% 73% 76% 100% 97% 58% 50% 67% 2007 69% 74% 66% 68% N/A N/A N/A N/A N/A N/A N/A N/A 

Average* 70% 64% 61% 58% 66% 74% 76% 94% 97% 78% 68% 73% *2002-2006 

3-1 

3 

FISH COMMUNITY DESCRIPTION 

This section describes the fish community in Lake Norman. The sources and types of available 

historical and recent information on the fish community are described first. The species 

composition and structure of the fish community then are discussed. Finally, the general life 

history of fish species commonly found in impingement sampling at Marshall Steam Station is 

discussed as background information that might provide insights on annual or seasonal 

susceptibility to impingement. 

Description of Available Information 

Duke conducted extensive ecological investigations of Lake Norman during 1973-1980 (Hogan 

and Adair 1981). These investigations, and particularly the fisheries studies, provide a 

description of the aquatic habitat and fish community of Lake Norman during a portion of its 

transformation from a riverine to lacustrine environment, which began when the Catawba River 

was impounded in 1963 to form Lake Norman. Fisheries surveys were conducted by Siler et al. 

(1981) from August 1973 through December 1980. 

Sampling methods included boat electrofishing, 27-m long variable-mesh (2.5 to 5.1-cm mesh) 

gill nets, rotenone for cove sampling, 2-m by 3-m or 1.5-m by 2-m mid-water Tucker trawls, and 

a 0.91-m diameter conical ichthyoplankton net (Siler et al. 1981). Boat electrofishing was 

quarterly at four to five locations in Lake Norman during 1974 through 1979 and monthly during 

June 1978 through May 1979. Gill netting was quarterly at three or four locations during 1974 

through 1979 and monthly at five locations during June 1978 through May 1979. Rotenone 

sampling in lake coves was conducted annually in late June or early August at three locations. 

Mid-water trawling at four locations was conducted monthly from July 1978 through April 1979 

and quarterly from April 1979 through April 1980. Ichthyoplantkon sampling was either 

biweekly or weekly at up to five locations. A special study was conducted on February 21, 1975 

to investigate the relative abundance of fish species within the intake cove by using boat 

electrofishing. 

Under an agreement with the NCDENR, Duke has performed annual physicochemical, 

macroinvertebrate, and fish community monitoring of Lake Norman since 1993 (Duke 2001, 

2003, 2004a, 2004c, 2005, 2006, 2007). The fish community is monitored using boat 

electrofishing and a combination of hydroacoustics, purse seining, and sometimes gill netting. 

For these fisheries assessments, Lake Norman is divided into five zones. Marshall Steam Station 

(MSS) is located in Zone 4 (Figure 3-1). Special sampling programs also have been conducted 

to monitor striped bass summer mortalities; age, species and size composition of forage fish; and 

cooperatively with North Carolina Wildlife Resources Commission (NCWRC) to monitor age, 
Fish Community Description 

3-2

growth and condition of striped bass by gill net sampling; and age and growth of crappies by trap 

net sampling. 

The spring electrofishing program samples three locations in late March and early April: near 

MSS in Zone 4, a reference area in Zone 3, and near the McGuire Nuclear Station (MNS) in 

Zone 1 (Figure 3-1). At each location, ten 300-m shoreline transects are sampled. These 

transects were found to be representative of habitat types found in Lake Norman, and have been 

sampled consistently since 1993. Electrofishing is conducted during daylight when water 

temperatures are expected to be 15-20°

C. 

Hydroacoustics sampling techniques and purse seining are used to assess the forage fish 

populations throughout the entire lake and its tributaries, and data are available for the period 

1993-2006 (Duke 2004b, 2007). Sampling is conducted at night during July, September, or 

November. For the hydroacoustics surveys, all six zones in Figure 3-1 are included. The 

hydroacoustics gear consists of a multiplexing 200-kHz digital transducer system with side-scan 

split-beam and down-looking dual-beam transducers. The side-scan transducer samples the top 2 

m of the water column, and the down-looking transducer samples the remainder of the water 

column1

. The output is integrated to estimate fish densities at 0.25-km intervals along sampling 

transects, and the estimated zonal densities are summed to estimate total lake standing crops. 

Species and size composition data are provided by purse seining at night using a 400-ft (121.9-

m) by 30-ft (9.1-m) purse seine with 3/16-in. (4.8-mm) mesh. 

Duke conducts an electrofishing survey in July in the vicinity of MSS in addition to the lakewide spring electrofishing survey described above. This summer electrofishing program samples 

five stations located in coves of Lake Norman in July during daylight hours (Duke 2004b). One 

sampling station is uplake from the MSS intake in Zone 4 and is used as reference data, two 

stations are near the MSS discharge in Zone 4, and two are downlake near the middle of Lake 

Norman in Zones 3 and 4. Shoreline segments that are 100-m in length are sampled on the right 

and left banks of each cove. 

Species Composition 

A composite of records of fish species collected during fisheries surveys of Lake Norman from 

1974 to 2006 indicate that 58 species and two hybrid varieties have been found in the reservoir 

since 1974 (Table 3-1). The total number of reported species decreased from 48 to 32 during the 

interval from 1974-1980 to 2004-2007. This decrease in species richness could be an artifact of 

differences in sampling methodology, but more likely is the result of the evolution of the fish 

community from a riverine community to a lacustrine community following the impoundment of 

the Catawba River. Examples of typically lotic species (Rohde et al. 1994, Pflieger 1997) that 

have not appeared recently in survey catches include cyprinids such as whitetail shiner and creek 

chub; catostomids such as white sucker, silver redhorse, V-lip redhorse, brassy jumprock, striped 

jumprock, and creek chubsucker; and ictalurids such as snail bullhead. Other species not found 

 

1

 Prior to 1996, only the down-looking transducer was used. 
3-3

since 1980 include the river carpsucker, smallmouth buffalo, yellow bullhead, and western 

mosquitofish. 

Recently, electrofishing has indicated that the numerically dominant shore-zone species in the 

sampled coves are bluegill and redbreast sunfish (Duke 2007). In terms of biomass, the 

dominant species are the spotted bass, largemouth bass, and common carp. Some species have 

only recently appeared in survey catches, such as alewife, white perch, goldfish, striped mullet, 

and spotted bass (Table 3-1). These species have not been stocked by the NCWRC and probably 

were introduced accidentally or intentionally by anglers. The spotted bass and white perch, in 

particular, have become established and support active recreational fisheries. White perch first 

appeared in 1998 and spotted bass in 2001. There is concern that spotted bass and white perch 

may be competing with largemouth bass and white bass, and could be responsible for declining 

populations of these latter two species (Duke 2007, NCWF 2008). 

The introduction of alewife to the lake in 1999 has affected the composition of the pelagic forage 

fish community in the lake in recent years. Prior to the year 2000, over 99 percent of the forage 

fish caught by purse seine during the hydroacoustic surveys were threadfin shad (Duke 2007). 

The percentage of the forage fish catch consisting of alewife increased to 25 percent in 2002, 

then began to decline again until it was approximately 2-5 percent in 2005 and 2006. Gizzard 

shad have constituted less than 0.5 percent of the forage base since 1993. 

Protected Species 

No fish species currently listed as federal or state endangered or threatened species have been 

collected in Lake Norman. There are, however, two fish species listed by the North Carolina 

Natural Heritage Program (NCNHP) for Catawba County as species of concern. These are 

highfin carpsucker (Carpoides velifer) and the Piedmont population of the Santee chub, 

(Cyprinella zanema pop 1). The highfin carpsucker is currently listed by NCNHP as a state 

species of concern with a global ranking of G4G52 and a state ranking of S23. The Piedmont 

population of the Santee chub is currently listed by NCNHP as a significantly rare species with a 

global ranking of G4T3Q4

 and a state ranking of S35. It is unlikely that either species would 

occur in Lake Norman since both species prefer lotic environments (Rohde et al. 1994). 

 

2

 G4G5=Apparently or demonstrably secure globally, although it might be quite rare in parts of its range (especially 

at the periphery). 

3

 S2=Imperiled in North Carolina because of its rarity or otherwise vulnerable to extirpation in the state. 

4

 G4T3Q=Species is apparently secure globally, but subspecies or variety is either very rare and local or found 

locally in a restricted area; taxonomic status is questionable. 

5

 S3=Rare or uncommon in North Carolina. 
Fish Community Description 

3-4

Target Species 

Among all fish species found in Lake Norman, a subset of species was selected for a detailed 

evaluation of impingement mortality presently occurring at the MSS cooling water intake. These 

species are most frequently observed in impingement collections, or are important because of 

their economic value or value to the ecosystem. 

The threadfin shad dominates the pelagic fish community in Lake Norman as well as 

impingement collections at the MSS intake (Section 4). Although much less abundant, the 

alewife, like the threadfin shad, is an important component of the forage base for the 

recreationally important fish species in Lake Norman. For these reasons, both threadfin shad and 

alewife were selected as target species for detailed evaluation. Other selected species include the 

blue catfish, white perch, and channel catfish. Blue catfish and channel catfish are traditionally 

important game species in Lake Norman, while the newcomer, white perch, had been added to 

the list of recreational species. These three species ranked third, fourth, and fifth in relative 

abundance in the 2006-2007 impingement collections at MSS (Section 4), and thus deserve 

further consideration. 

This section presents background information on the life history of these five species. 

Threadfin Shad 

Historically, the threadfin shad has been the most abundant pelagic species in Lake Norman and 

most abundant in impingement collections at MSS. Threadfin shad is an important forage base 

for several predator species that provide the recreational fishery of Lake Norman, including the 

striped bass, largemouth bass, spotted bass, catfish species, and crappie. Threadfin shad is an 

excellent forage species because of its size, which seldom will exceed 150 mm, and its great 

productivity. It is a planktivore, relying on filter feeding through its gill rakers. Threadfin shad 

were introduced into Lake Norman after its impoundment (Siler et al. 1981). 

Threadfin shad spawn when water temperatures reach 65-70°

F (18.3-21.1°C) and can have 

multiple spawning peaks through the spring, summer and early fall (Pflieger 1997, Smith 2002). 

In the late 1970s, clupeid larvae, which included threadfin shad and gizzard shad, were caught in 

ichthyoplankton samples in Lake Norman from April through August, peaking in May and June 

(Siler et al. 1981). Spawning primarily occurs from dawn to shortly after sunrise as spawning 

schools gather along the shoreline. The adhesive eggs are broadcast over submerged objects and 

hatch within three days. Young threadfin shad grow very rapidly (up to 25 mm per month), with 

many reaching maturity and spawning by the end of their first summer at 64 mm in length. 

Females generally grow larger and live longer than males, with a maximum age of about four 

years for females and three years for males. 

For habitat, threadfin shad prefer open water over sand, mud or debris. Threadfin shad tend to be 

more abundant in the upper portions of the lake rather than the lower portions, possibly due to 

differences in habitat or nutrient levels and productivity among in lake zones (Siler et al. 1981, 

Duke 2004b). The continued existence and relative abundance of threadfin shad, which is 

intolerant of water temperatures below 10 °C (Ruelle et al. 1977, Griffith 1978), in Lake Norman 
3-5

may be dependent upon the thermal input from MSS and McGuire Nuclear Station and the 

thermal refuges they provide during the winter. 

Alewife 

The alewife is a relative newcomer to the Lake Norman fish community, first appearing in 1999. 

Its introduction probably originated from angler releases, since it was not stocked by NCWRC. 

At the time of its appearance, threadfin shad dominated the forage fish base in Lake Norman. 

Threadfin shad continues to dominate pelagic forage species, but at its most recent peak 

abundance in 2002, alewife may have comprised numerically as much as 25 percent of this 

pelagic forage fish base. Alewife was the second-most frequently impinged species at the MSS 

intake in 2006-2007, behind threadfin shad (Section 4). 

The alewife is an anadromous clupeid that is indigenous to the Atlantic Coast from 

Newfoundland to South Carolina and spawns in coastal freshwater or brackish rivers and 

tributaries (Pardue 1983). Landlocked populations have long been established in the Great Lakes 

and several northern inland lakes such as the Finger Lakes of New York. Rarely has this species 

been introduced to southern lakes or reservoirs but Claytor Lake in Virginia is such an example. 

Studies at Claytor Lake, a mesothermal lake with temperatures ranging from 2°

C to 27°C (Kohler 

and Ney 1981, Nigro and Ney 1982), may provide insights into the general life history of the 

alewife in a southern reservoir such as Lake Norman. 

Nigro and Ney (1982) found alewives in Claytor Lake to be shorter-lived than alewives in 

northern landlocked populations, while achieving maturity earlier and having higher absolute and 

relative fecundity. They attributed these differences to a longer growing season in the south, 

rather than accelerated growth rates. Maturity was reached at approximately the same size as 

their northern counterparts. Eighty-six percent of age-1 females were mature at a median length 

of 160 mm total length (TL) while all age 2 and age 3 females were mature. Maximum first year 

growth was 130-160 mm. 

In Claytor Lake, as may happen in Lake Norman, alewives move inshore in large concentrations 

to spawn in the spring and early summer. Ripe males were first seen in the littoral zone when 

water temperatures were 13°

C, and spawning was first observed in early May at 16-18°C (Nigro 

and Ney 1982). Peak egg densities occurred in the third week of May at 21-23°

C, then declined 

during June through July. This extended spawning season was interpreted as possibly indicating 

fractional spawning, i.e., repeated egg deposition by individual females. 

Alewife eggs initially are demersal and adhesive, but when water-hardened they become pelagic 

and lose their adhesive properties (Pardue 1983). Egg incubation lasts from 36 hours to 6 days, 

depending on water temperature (Carlander 1969, Auer 1982). The yolk-sac is absorbed in 2-5 

days at a length of 5.1 mm (Pardue 1983) and larvae transform to the juvenile stage at 20 mm. 

Early life stages are positively phototaxic and pelagic. The principal food items are diatoms, 

copepods, and ostracods (Carlander 1969). Adults are size-selective planktivores feeding on 

cyclopoid copepods, and occasionally are piscivorous (Kohler and Ney 1981, Nigro and Ney 

1982). 
Fish Community Description 

3-6

Blue Catfish 

Blue catfish was the third-most frequently impinged species at MSS in 2006-2007 (Section 4). 

The blue catfish is native to the Mississippi, Missouri, and Ohio river basins and is considered a 

big river species (Graham 1999). It is the largest catfish in the United States. The largest 

recorded blue catfish weighed 315 lb (142.9 kg) and was caught in the Missouri River just below 

Portland, Missouri in 1866 (Graham 1999). During the mid-1800s, it was common to catch blue 

catfish weighing 125-200 lb (45.4-90.7 kg). The North Carolina record was caught in Lake 

Norman in 2004; it was 51.25 in. (1,302 cm) in length and weighed 85 lb (38.6 kg). Maximum 

age is 18 years. 

The blue catfish is an introduced species in Lake Norman, apparently being stocked by the 

NCWRC when Lake Norman was impounded. It was not detected until 1983, but its population 

has been growing rapidly ever since (Grist 2002). During this period, there has been a decline in 

the white catfish and snail bullhead populations. An active fishery has developed for blue catfish 

in Lake Norman, including tournaments specifically for this and other catfish species. Blue 

catfish dominate the catch and harvest of catfish species in Lake Norman, with 70 percent of the 

numbers caught and 77 percent of the biomass harvested (Grist 2002). North Carolina treats the 

blue catfish as a non-game species for which there is no catch limit, and the catch can be sold 

commercially. 

There are few technical reports on the life history of blue catfish. Adult blue catfish in the lower 

reaches of Lake Norman have been shown to move up to 34 km or more to the upper, more 

riverine portion of the lake in the spring, possibly to spawn, then return in the fall and winter to 

the lower lake (Grist 2002). Migratory movements in rivers can span several hundred kilometers 

with movement upstream in the spring and downstream in the fall in response to water 

temperature (Lagler 1961, Pflieger 1997). Spawning in the Ohio River has been reported to 

occur at water temperatures ranging from 68°

F to 86°F (20-30°C), with optimum temperatures of 

72-82°F (22.2-27.8°

C) (Simon and Wallus 2003). In Lake Norman, water temperatures of 72-

82°F would occur in June and July. 

Their spawning habits are believed to be similar to channel catfish (Pflieger 1997, Hubert 1999). 

Blue catfish are cavity nesters and deposit eggs in protected areas behind rocks, root-wads, 

depressions, undercut streambanks, or other areas with minimal currents. The male guards the 

eggs and fry. Egg diameters are between 3.0 and 3.3 mm, and fecundity ranges from 900 to 

1350 eggs/kg of body weight (Graham 1999). Egg incubation lasts 168 to 240 hours at 75 to 

79°

F (23.9-26.1°C) in the Ohio River (Simon and Wallus 2003). Eggs hatch in 7-8 days at 

temperatures between 21 to 24°

C (Graham 1999; Pflieger 1997). Blue catfish yolk-sac larvae are 

4.1 to 8.9 mm long when they hatch (Simon and Wallus 2003), and young may grow slower 

during their first year than channel catfish (Tidwell and Mims 1990). In Lake Norman, early 

growth rates and body condition are lower than in other impoundments (Grist 2002) but improve 

as the fish grow older and become more piscivorous. 

Graham and DeiSanti (1999) aged 2,389 blue catfish collected from 1988-1995 at the Truman 

Dam Tailwater, Osage River, Missouri. The mean back-calculated total lengths at annulus 

formation for the first eight years of life were as follows: 105 mm at age 1, 178 mm at age 2, 243 
3-7

mm at age 3, 309 mm at age 4, 371 mm at age 5, 371 mm at age 6, 426 mm at age 7, and 484 

mm at age 8. Blue catfish become sexually mature at about 381 mm in the Mississippi River 

near St. Louis (Graham 1999) and between 420-480 mm (about 6-7 years) in Lake of the Ozarks, 

Missouri (Graham and DeiSanti 1999). 

Blue catfish prefer open waters of large reservoirs and main channels, backwaters, and 

embayments of large flowing rivers where water is normally turbid and substrate varies from 

gravel-sand to silt-mud (Graham 1999). In Lake Norman, they are found at greater depths (e.g., 

>5 m) than channel catfish and prefer more open water (Grist 2002). Clupeids such as threadfin 

shad and alewife are their primary food, but they also feed on mollusks such as the Asiatic clam 

Corbicula and on stonewort Chara (Grist 2002). They likely compete with the stocked striped 

bass in Lake Norman for clupeids. 

White Perch 

White perch was the fourth-most frequently impinged fish species at MSS during 2006-2007 

(Section 4). Like several other species, it is not native to Lake Norman. It likely was introduced 

to Lake Norman by anglers, and first appeared in 1998. Since then, its population has grown 

rapidly, and it now supports a recreational fishery in Lake Norman. There is, however, the fear 

that its establishment has affected or will affect the fishery for other species, such as white bass, 

crappie, largemouth bass, and striped bass. While the white perch population has expanded, the 

white bass population has become decimated, possibly due to predation on the eggs or fry of 

white bass (http://www.ncwildlifederation.org/home/lakenormanfishery.htm) or through 

competition for food. There is also the fear that eventually white perch will become 

overpopulated and stunted. Most adult white perch in Lake Norman are 150-305 mm long and 

<0.5 lb (227 g). 

White perch is distributed geographically along the Atlantic coast from Canada to South 

Carolina. In coastal areas, it is semi-anadromous, leaving brackish water to spawn in freshwater. 

It has become established in inland lakes, including the Great Lakes, either naturally or through 

stocking in several states in the eastern and central US (Rohde et al. 1994,). It also recently has 

invaded the Mississippi River system (Irons et al. 2002) and the Arkansas River system as far as 

Oklahoma (Harris 2006). In the southeast, white perch has established populations in the Kerr 

Reservoir in Virginia and North Carolina, and the B. Everett Jordan Reservoir in North Carolina 

(Harris 2006). 

There is little or no information on the life history of white perch in Lake Norman. Based on 

studies elsewhere, spawning in freshwater lakes occurs in April and May (Stanley and Danie 

1983). In the Hudson River, peak spawning occurred at water temperatures of 16-20°

C (Klauda 

et al. 1988). Eggs are broadcast at depths of <12 ft (3.7 m). The demersal and adhesive eggs 

attach to vegetation and other firm substrates, and no parental care is given to the eggs or young. 

The eggs hatch in 30 hours at water temperatures of 20°

C, or may take as long as 96-108 hours at 

15°

C. The yolk-sac life stage lasts 3-5 days until a length of 3.5-4 mm is reached (Klauda et al. 

1988). Juveniles tend to be demersal in depths of 8-12 ft (2.4-2.7 m) (Stanley and Davie 1983). 
Fish Community Description 

3-8

In daylight, white perch school in open water, often over shoals or brush piles, and at night go 

inshore. Young white perch feed on zooplankton such as copepods, cladocerans, and ostracods 

(Prout et al. 1990). Adults consume benthic invertebrates and fish eggs, and when larger than 

80-150 mm are primarily piscivorous (Harris 2006). 

Males mature slightly earlier than females, beginning at age 2 when males are 100-110 mm TL 

and females are 110-120 mm TL (Klauda et al. 1988, Rohde et al. 1994). Most are mature by 

age 3-4. Maximum age is approximately 17 years but few are older than age 7. Fecundity 

ranges from approximately 5,000 to 300,000 eggs depending on female size (Stanley and Davie 

1983). In Kerr Reservoir, North Carolina, white perch reach a mean length of 104 mm at age 1, 

175 mm at age 3, 239 mm at age 5, 264 mm at age 7, and 303 mm at age 9 (Harris 2006). 

Angler exploitation of white perch on Kerr Reservoir begins at lengths of 150-200 mm TL. 

Channel Catfish 

A total of 57 channel catfish were collected during impingement sampling at MSS in 2006-2007, 

making them the fifth-most frequently impinged species (Section 4). Channel catfish is a longterm resident of Lake Norman and historically has supported commercial or recreational fisheries 

on the lake (Siler et al. 1981). 

The channel catfish is a very important and highly desirable commercial and recreational fish 

species throughout its geographic distribution and in North Carolina. It is native to the central 

part of North America (Smith 1985), but it has been stocked in rivers and reservoirs throughout 

the southeast region. It occurs in a variety of habitats and is especially characteristic of large, 

turbid streams and rivers that have a low or moderate gradient. In Lake Moultrie, South 

Carolina, channel catfish can reach up to 58 lb (26 kg) in size (Rohde et al. 1994). 

Channel catfish spawn in late spring and early summer when water temperatures reach 65°F 

(18.3°C) or more (McMahon and Terrell 1982, Pitlo et al. 2004). Spawning generally occurs 

over a range from 21°C to 29°C, with an optimal temperature of 27°C (Hubert 1999). Sustained 

temperatures in this range (21°C to 29°C) in Lake Norman would correspond to June and July. 

Several weeks prior to spawning, males select and clear suitable nest sites, usually consisting of 

secluded, dark areas such as hollow logs, drift piles, undercut banks, muskrat or beaver burrows, 

or rip rap. A single male often will spawn with several females (Rohde et al. 1994). Eggs are 

deposited in a gelatinous mass. Incubation lasts for 5.5 to 10 days at 75-82°F (23.9-27.8°C) 

(Holland-Bartels and Duval 1988). The male tends the nest while eggs hatch and stays there for 

about one week after hatching to guard the fry. Early growth is variable among year classes and 

apparently is dependent upon existing conditions (Pitlo et al. 2004). 

Adults may be found in many habitats including channels and large open areas, but prefer habitat 

with woody debris, bank cavities and moderate currents (Koel et al. 1998). In daylight they seek 

depths with cover and current. At night or in rising water levels they feed near or on the bottom 

in shallower depths. Young feed largely on aquatic insects and other benthic arthropods, but at 

100 mm length become omnivorous with fish making up a large part of their diet (Smith 1985). 
3-9

Channel catfish begin to mature at age 4 and 305 mm in length, and 75 percent of females are 

mature by age 6 (Pitlo et al. 2004). The average fecundity is 6,088 eggs per lb of body weight. 
Fish Community Description 

3-10

Figure 3-1 Sampling Zones and Locations for Lake Norman 

.
Table 3-1 Fish Species Collected in Lake Norman Fishery Surveys (Siler et al. 1981; Duke 2004a, 2004b, 2006, 2007

Family Scientific Name Common Name 1974-1980 1993-1999 2000-2003 

Lepistosteidae Lepistosteus osseus Longnose gar X X X Amiidae Amia calva Bowfin X Clupeidae Dorosoma cepedianum Gizzard shad X X X 

Dorosoma petense Threadin shad X X X Alosa pseudoharengus Alewife X Alosa aestivalis Blueback herring* 

Salmonidae Oncorhynchus mykiss Rainbow trout X Cyprinidae Cyprinus carpio Common carp X X X 

Carassius auratus Goldfish Hypognathus regius Eastern silvery minnow X Nocomis leptocephalus Bluehead chub X Notemigonus crysoleucas Golden shiner X X X Cyprinella analostana Satinfin shiner ? Cyprinella chloristia Greenfin shiner X X X Cyprinella galactura Whitetail shiner X Notropis hudsonius Spottail shiner X X X Cyprinella nivea Whitefin shiner X X X Semotilus atromaculatus Creek chub X Notropis procne Swallowtail shiner X X Pimephales promelas Fathead minnow 

Catostomidae Carpoides carpio River carpsucker X 

Carpoides cyprinus Quillback X X X Catostomus commersoni White sucker X Erimyzon oblongus Creek chubsucker X Ictiobus bubalus Smallmouth buffalo X Moxostoma anisurum Silver redhorse X Moxostoma macrolepidotum Shorthead redhorse X X X Moxostoma pappilosum V-lip redhorse X Scartomyzon sp. Brassy jumprock X Scartomyzon rupiscartes Striped jumprock X 
Fish Community Description 

3-12Table 3-1 continued 

Family Scientific Name Common Name 1974-1980 1993-1999 2000-2003 

Ictaluridae Ameirus brunneus Snail bullhead X 

Ameirus catus White catfish X Ictalurus furcatus Blue catfish X X Ameirus melas Black bullhead X Ameirus natalis Yellow bullhead X Ameirus nebulosus Brown bullhead X Ameirus platycephalus Flathead bullhead X Pylodictis olivaris Flathead catfish X X X Ictalurus punctatus Channel catfish X X X 

Mugilidae Mugil cephalus Striped mullet Poecilidae Gambusia affinis Western mosquitofish X Moronidae Morone chrysops White bass X X X 

Morone saxatilis Striped bass X X X Morone americana White perch X X 

Centrarchidae Lepomis auritus Redbreast sunfish X X X 

Lepomis gibbosus Pumpkinseed X X Lepomis gulosus Warmouth X X X Lepomis macrochirus Bluegill X X X Lepomis microlophus Redear sunfish X X X Lepomis cyanellus Green sunfish Micropterus salmoides Largemouth bass X X X Pomoxis annularis White crappie X X Pomoxis nigromaculatus Black crappie X X X Lepomis hybrid Sunfish hybrid X X Micropterus punctulatus Spotted bass X Micropoterus hybrid Black bass hybrid 

Percidae Etheostoma fusiforme Swamp darter X X 

Etheostoma nigrum Johnny darter ? Etheostoma olmstedi Tessellated darter X X X Perca flavescens Yellow perch X X X 

 Total species 48 26 29  Total hybrids 0 1 1 
4-1

4 

IMPINGEMENT MORTALITY 

This section presents the results of a one-year impingement monitoring program 

conducted at Marshall Steam Station (MSS) during 2006-2007 and earlier efforts to 

describe impingement in 1974-1975. The effects of seasonality and environmental 

variables, including lake level, dissolved oxygen (DO) concentration and water 

temperature, on impingement rates are discussed in the context of dominant fish species 

in the 2006-2007 impingement collections. 

Impingement Monitoring Programs 

Current impingement rates at MSS were evaluated by a monitoring program conducted 

during 2006-2007 by MACTEC Engineering and Consulting, Inc. (MACTEC). 

Complying with the requirements of the §316(b) Phase II Rule, Duke Energy submitted a 

Proposal for Information Collection (PIC) to North Carolina Department of Environment 

and Natural Resources (NCDENR) outlining a proposed plan for impingement data 

collection and analysis, and associated Quality Assurance/Quality Control (QA/QC) 

procedures (EPRI Solutions, Inc. 2006). Detailed descriptions of the sampling and 

QA/QC procedures used during the 2006-2007 monitoring program are presented in 

Standard Operating Procedures (SOP) prepared prior to the start of data collection. 

MSS has 16 intake bays and each of the 16 intake bays has a fixed screen consisting of an 

upper and a lower removable panel (Section 2). To clean or sample each screen, a crane 

operator had to lift the upper and lower screen panels and place them over a collection pit 

on the intake structure deck. The screen panels then were pressure-washed and the 

cleaned panels were placed behind the next screen to be sampled to act as a back-up 

screen. Because this process was time-consuming, only four screens could be washed 

and sampled during each sampling event. Therefore, the sampling design had to 

incorporate a random selection process to determine which screens were to be washed 

and sampled weekly. Fish impingement for all operating units had to be estimated by 

expansion of numbers collected from the sampled screens to all screens associated with 

operating units. 

Impingement sampling began on 6 April 2006 and continued weekly through 30 March 

2007. Prior to each sampling event, four fixed screens were chosen randomly from all 

screens associated with one or more operating cooling water circulating pumps (Table 4-
Impingement Mortality 

4-2

1). In a few cases6

, pumps at a particular unit only operated for a portion of the sampling 

day and estimated densities of impinged fish were based on the shortened time period 

rather than a full 24 hours. Individual screens were sampled from five to 19 times over 

the entire sampling program, and individual units were sampled from 39 to 61 times 

(Table 4-1). 

Prior to sampling, the selected screens were raised and cleaned, then replaced and 

allowed to accumulate impinged fish for approximately 24 hours. On the second day 

(after the 24-h accumulation), impinged fish were washed from the raised screens and 

collected. The back-up screen behind the cleaned screen was examined for any impinged 

fish that might have washed off the screen that had been lifted.7

 During backwash 

cleaning, the screen panels were placed over a collection pit on the intake decking, fitted 

with a sampling net and tarp collection system. Impinged specimens collected from the 

net were processed for species identification and length and weight measurements in 

accordance with the SOP. 

Earlier impingement monitoring had been conducted at MSS from 30 April 1974 to 27 

May 1975. Sampling was biweekly (once every two weeks). After clearing fish and 

debris, one screen per unit (four screens total) was allowed to accumulate fish and debris 

over a two-week period, then was raised and inspected for impinged fish. Impinged fish 

were identified to species (if possible), measured, and counted, and the degree of 

decomposition was noted. In cases of large numbers of fish for individual species such as 

threadfin shad, total numbers of that species were estimated from subsamples. 

Current Impingement Sampling Program Results 

For the 12-month period from 1 April 2006 to 31 March 2007, an estimated 188,419 fish 

and 1,036 kg of fish biomass were impinged at MSS (Table 4-2), based on the collection 

of 6,219 impinged fish (Table 4-3) and 30,229 g of fish biomass (Table 4-4). This 

estimate was derived from the density of fish collected during each sampling event, 

expressed as number per volume of cooling water withdrawn at each of the four 

generating units, extrapolated over the full 12-month period. To estimate the density at 

each unit on a particular sampling event, the collection totals were scaled up from the 

number of sampled screens to the total number of screens for the unit8

. The scaled-up 

collection total was divided by the total flow volume for that unit during the 24-h 

sampling period to estimate the impingement density for that unit and period. If no 

screens were sampled at a unit on a particular date when it was operating, the impinged 

fish density for that unit was estimated as the average of the sampled units on that date. 

 

6 8 June 2006 at Unit 1, 25 January 2007 at Unit 4, and 29 March 2007 at Unit 2

7 Only partially decomposed fish were observed, indicating they had been previously dead and thus were not 

counted.

8 For example, if three screens at Unit 3 were sampled on a particular date, the collection total was 

multiplied by 5/3 or 1.67, assuming that the randomly selected screens represented the average density of 

fish per screen. 
4-3

Total daily cooling water intake flow volumes for sampling days were obtained from 

plant operation records. 

Impingement density values for non-sampled days were estimated by interpolating 

between density values for the sampling days that were immediately previous and 

subsequent to the unsampled time interval, then multiplying the interpolated density by 

each total daily intake flow reported for the non-sampled days and summing the 

estimated impingement counts across the time interval: 

( / 2)( )

2

1

1 2

1

   

  

i

s s uj

n

j

Np DsiFsi D D F 

where 

Np = estimated total impingement during non-sampled period p and the sampled days 

bordering the period 

Dsi = organism density of sampling day i (1 = start, 2 = end) of period p 

Fsi = 24-hour cooling water flow volume on sampling day in period p 

Fuj = 24-hour cooling water flow volume on non-sampling day j in period p 

n = number of non-sampled days during period p 

Total annual impingement then was estimated by summing the estimated impingement 

during sampled days and non-sampled periods for each unit, then summing across the 

four units. 

Species Composition 

A total of 10 fish species were found in the impingement collections (Table 4-3). 

Threadfin shad and alewife dominated the collections, comprising 80.8 percent and 12.6 

percent of the fish collected, respectively. Together these two species accounted for 92.0 

percent of the estimated total annual impingement in terms of numbers (Table 4-2) and 

79.4 percent of the estimated total impinged biomass9

. When included with blue catfish, 

white perch, and channel catfish, these five fish species (described in Section 3) 

accounted for 99.3 percent of the estimated numbers impinged and 99.6 percent of the 

fish biomass impinged during the 12-month period. Threadfin shad was the most 

 

9 Note that the percentages of estimated total annual impingement for each species differ slightly from the 

percentages of all impinged fish collected because of the time- and flow-based extrapolation used to 

estimate total annual impingement.
Impingement Mortality 

4-4

commonly impinged species during the 1974-1975 monitoring program at MSS (Table 4-

5). 

The remaining seven impinged species (Table 4-3) included gizzard shad and four 

centrarchids (redbreast sunfish, bluegill, pumpkinseed, and white crappie), which 

combined represented <1 percent of the estimated total annual impingement. No federal 

or state-listed species were impinged. In addition to fish, the invertebrate Asiatic clam 

(Corbicula) and crayfish were found in impingement collections (Table 4-6). 

Seasonal Patterns 

Impingement rates for all fish species combined were lowest during the spring, AprilJune, and highest in the late summer and early fall (August-October) or mid-winter 

(February) (Figure 4-1 and Table 4-7), reflecting the seasonal impingement patterns of 

the two dominant species, threadfin shad and alewife. Alewife primarily were impinged 

in late summer (Figure 4-2) whereas threadfin shad were impinged throughout the fall 

and winter months, September-March (Figure 4-3). 

Impinged alewives were larger than impinged threadfin shad. The length frequency for 

alewives was bimodal, with modes at approximately 80 mm and 150 mm (Figure 4-2), as 

opposed to the unimodal distribution for threadfin shad with a modal length of 

approximately 60 mm (Figure 4-3). Based on these length frequency data and literature 

sources on length at age (Table 4-8), most impinged alewives were yearling and older 

fish, whereas approximately half of the impinged threadfin shad were young-of-year 

(YOY) fish (Table 4-9). 

Blue catfish were impinged primarily in the summer, July-August (Figure 4-4). The 

smallest blue catfish was 51 mm TL and the largest was 250 mm TL and weighed 101 g. 

The great majority of impinged blue catfish were yearling and older fish (Table 4-9), but 

YOY began to be impinged after mid-November. 

Channel catfish primarily were impinged during the spring and summer, from March 

through mid-August (Figure 4-5). They ranged in length from 52 to 114 mm TL and 

most appeared to belong to the 2005 year class and were classified as YOY until July 

2006, when they became yearling fish (channel catfish were considered to become one 

year old as of 1 July following the year of spawning; Table 4-8). Peak impingement rates 

were recorded in July and August (Figure 4-5). 

White perch primarily were impinged during the late summer, August-September, as 

yearling and older fish (Figure 4-6). The median length was 147 mm TL and the length 

range was from 58 to 199 mm. Based on literature values for white perch in an estuarine 

enviroment, almost all of these fish would be yearling or older fish (Table 4-9); growth 

rate information for white perch in Lake Norman currently is not available. 

The seasonality of impingement for other species can be summarized as follows (Table 4-

7): 
4-5

Bluegill—primarily winter, January-February 

Gizzard shad—September-October 

Pumpkinseed—June 

Redbreast sunfish—spring and fall 

White crappie—September 

Variables Affecting Impingement Rates 

Three categories of variables potentially affecting impingement rates are discussed: 

environmental variables, species-specific abundance, and spatial distribution. 

Environmental Variables 

Three potentially important environmental variables that could affect impingement rates 

are water temperature, dissolved oxygen concentration (DO), and lake level or elevation. 

Mean daily water temperature records from the MSS intake and daily lake level data were 

available for the sampling period from April 2006 through March 2007. DO readings 

were made by MACTEC sampling crews at the intake on the day of impingement 

sampling, usually in the morning between 0700 h and 1100 h. 

The mean daily intake temperature varied seasonally from a low of 44.3°F (6.8°C) on 13-

14 February 2007 to a high of 80.6°F (27°C) on 30 August 2006 (Figure 4-7). The lake 

level varied 4.5 ft (1.37 m) during the 12-month sampling interval from 1 April 2006 to 

31 March 2007, with a maximum elevation (759.0 ft or 231.3 m above mean sea level) on 

8 January 2007 and a minimum elevation (754.5 ft or 230.0 m amsl) on 19-20 February 

2007. Full pond is 760 ft (231.6 m) amsl. DO readings on impingement sampling days 

ranged from 0.4 mg/L on 11 August 2006 to 11.0 mg/L on 15 February 2007 (Figure 4-

7). 

The peak impingement rate for all species combined occurred on 15 February 2007, when 

the lake’s surface elevation and water temperatures were near their minimum levels for 

the year (Figure 4-7). Impingement on this date consisted primarily of yearling and older 

threadfin shad, but also included YOY threadfin shad and YOY alewife (Figures 4-8 and 

4-9). No other species were impinged on this date. DO levels at this time were near 

saturation. 

Minor peaks in impingement rates for YOY threadfin shad occurred on 19 October 2006, 

8 February 2007, and 1 March 2007 (Figure 4-8). The October peak coincided with the 

period of declining temperatures, but the water temperature was still moderate at 

approximately 70°F (21.1°C). The other two peaks occurred at winter low temperatures 

(approximately 46°F or 7.8°C) and reduced lake levels. Threadfin shad are stressed at 

water temperatures less than 10 oC (50o

F) and suffer complete mortality at 4°C (39.2°F) 

(Griffith 1978). 
Impingement Mortality 

4-6

Most alewife impingement was experienced in one of two periods: mid-summer or midwinter (Figure 4-9). The mid-summer impingement involved yearling and older fish, as 

well as YOY, while the mid-winter impingement involved the 2006 year class (YOY). A 

prominent peak in impingement of yearling and older alewife occurred on 10 August 

2006 when water temperatures were near 73°F (22.8°C), but DO concentrations at the 

intake were nearly anaerobic (0.39 mg/L). 

Stress related to low DO concentrations may have increased impingement rates during 

July and August. DO concentrations at the intake on impingement sampling days were 

<4 mg/L from 22 June until 14 September, with the lowest concentrations occurring on 4 

August (0.70 mg/L) and 10 August (0.39 mg/L). Cooling water for MSS is drawn 

beneath a skimmer wall at the entrance to the intake canal, thus drawing water with low 

DO concentrations from the lake’s hypolimnion. Fish occupying the canal at this time 

may succumb to the lowered DO levels and become impinged at the intake. Besides 

alewife and threadfin shad, fish species that showed increased impingement rates at this 

time included blue catfish (July-August, Figure 4-4), channel catfish (July-August, Figure 

4-5), and white perch (August-September, Figure 4-6). 

During the 1974-1975 monitoring programs at MSS, summer impingement rates were 

considerably lower than during the remainder of the year, particularly the winter (Table 

4-5). In that earlier study, cold water temperatures, particularly during DecemberFebruary, appeared to have had an influence on impingement rates, especially for 

threadfin shad. 

Species-Specific Abundance and Spatial Distribution 

Data on the relative abundance of individual species and their distribution in Lake 

Norman are available from recent annual electrofishing surveys of the littoral zone and 

hydroacoustic/purse seine surveys of the pelagic zone (Section 3). A comparison of the 

results of these surveys and the species composition of the 2006-2007 impingement 

collections provides insight into the role of species-specific abundance and spatial 

distribution on the susceptibility of species to impingement at MSS. 

As is typical for many deep reservoirs, the fish community of Lake Norman may be 

partitioned into at least two components: the near-shore, littoral zone community and the 

open water, pelagic community. The annual fisheries surveys target these two sometimes 

distinct communities, but relating the relative abundance of individual littoral vs. pelagic 

species becomes difficult because of the differences in sampling gear efficiencies in the 

two zones. 

Four clupeids have been identified in Lake Norman and are associated with the pelagic 

zone: threadfin shad, alewife, gizzard shad, and blueback herring. In 2006, threadfin 

shad appeared to be the dominant species in the pelagic zone, comprising 94.9 percent of 

the purse seine catch (Duke 2007). Alewife comprised the remaining 5.1 percent. 

Gizzard shad, once more common than now, did not occur in the 2006 purse seine catch. 

Blueback herring are rare and were reported in the gill net survey of 2004 (Duke 2004c). 
4-7

Threadfin shad and alewife, the principal components of the pelagic fish community, also 

dominated the 2006-2007 impingement collections at MSS, indicating that pelagic 

species are more vulnerable to impingement than littoral species. Together, they 

comprised 92.0 percent of the estimated annual impingement. In the collections, 

threadfin shad outnumbered alewife by over six to one, which was a lower ratio than the 

approximately 19:1 ratio in the purse seine catch of 2006. Thus, it would appear that 

alewife were more susceptible to impingement than threadfin shad, possibly reflective of 

the spatial distribution of the two species and increased impingement of alewife in late 

summer, August-September 2006 (Figure 4-2). While not caught by the 2006 purse seine 

surveys, gizzard shad appeared in limited numbers (two fish) in the 2006-2007 

impingement collections. 

Seven additional species were impinged at MSS during 2006-2007: blue catfish, white 

perch, channel catfish, bluegill, redbreast sunfish, pumpkinseed, and white crappie (Table 

4-10). Together, these seven species comprised only 6.6 percent of the total annual 

impingement collections. Two of the seven species, bluegill and redbreast sunfish, were 

the two most frequently caught species during the March-April 2006 electrofishing 

surveys of the littoral zone of Lake Norman (Table 4-10). Bluegill and redbreast sunfish 

are residents of the littoral zone, along with pumpkinseed; however pumpkinseed, while 

impinged (two fish collected), were not caught during the 2006 electrofishing surveys. 

Four impinged species—blue catfish, channel catfish, white perch, and white crappie—

are often associated with open water, and thus might be expected to be less common in 

the electrofishing survey results. These four species were not abundant in the 

impingement collections in terms of numbers (Table 4-10), but their presence in the 

impingement collections further indicates that pelagic or open water species could be 

more susceptible to impingement than strictly littoral zone species. It is possible that 

pelagic species would more likely encounter and enter the intake cove, while littoral zone 

species would be excluded by the 60-ft deep skimmer wall at its entrance unless they 

already resided in the intake cove. 

Annual Losses 

Impingement sampling during 2006-2007 has provided a single year’s estimate of 

seasonal and annual impingement rates occurring at the MSS intake, and the best 

available estimate of current impingement for several reasons. Data collected by the 

2006-2007 monitoring program are representative of the current relative abundance of 

fish species in the vicinity of the intake and their susceptibility to impingement. In 2006-

2007, station operation was consistent with its forecasted use as a baseload facility, with 

cooling water flow rates of approximately 73 percent capacity. The monitoring program 

design encompassed complete diel and seasonal cycles of impingement, and the program 

was conducted with strict adherence to Standard Operating Procedures. 

Two alternative approaches to estimating the current levels of impingement are: (1) 

“Normal Operations” representing recent historical and projected operation of the 

facility, and (2) “Full Operations” representing the full design capacity for water 

withdrawals by the facility. Normal Operations would be an estimated annual total of 
Impingement Mortality 

4-8

188,419 impinged fish weighing 1,036 kg (Table 4-2). An estimate for Full Operations 

can be derived by substituting the design capacity for cooling water flow rates (2,264 

CFS or 1,463 MGD) rather than actual mean daily flow rates recorded during the 12-

month sampling interval. Impingement losses under full operation are estimated to total 

246,549 fish weighing 1,168 kg (Table 4-11). These totals represent a 31 percent 

increase in the numbers and a 13 percent increase in biomass of impinged fish over 

Normal Operations. 

Annual Variability 

Data collected by the 2006-2007 monitoring program are representative of current 

conditions, especially the relative abundance of fish species and their susceptibility to 

impingement. The data must be interpreted in the context of the existing conditions. 

Seasonal and annual variability in impingement mortality could occur if cooling water 

intake operation, lake level, water temperatures, DO concentration, or fish abundance 

varied to a significant degree. 

Intake Operation 

MSS is operated generally as a baseload facility, with an average annual capacity factor 

of 79 percent during 2002-2007 (Section 2). The average annual intake flow rate for 

most of this period (2002-2006) was 73 percent of the maximum design capacity. MSS is 

expected to continue to operate as a baseload facility, so the influence of cooling water 

flow rates on the variability of annual impingement rates should be minimal. 

Environmental Variables 

Impingement rates during 2006-2007 appeared to be influenced by water temperatures, 

lake levels, or DO concentrations. Peak impingement of threadfin shad occurred when 

daily mean water temperatures were approximately 44-46°F (6.7-7.8°C) in February and 

early March, and the lake level was low. Peak alewife impingement occurred in August 

when DO concentrations were well below 4 mg/L. Stress related to cold temperatures or 

low DO may have increased impingement rates for these two species. These variables 

might influence impingement rates in future years, as also indicated by higher 

impingement rates during winter in 1974-1975. 

Fish Abundance 

The abundance, species composition, and size composition of fishes in Lake Norman 

could vary annually as the result of changing habitat suitability, trophic dynamics, 

reproductive success, interspecific competition, or several other factors. These changes 

also could affect impingement rates at the intake. Some annual variability would appear 

random while some may reflect long-term trends in the abundance of individual species. 
4-9

Future changes in annual impingement totals might be expected if impingement is a 

function of fish abundance. If the results of annual hydroacoustic and electrofishing 

surveys are indicative of the annual abundance of fish in Lake Norman, up to a two or 

three-fold variation in total impingement reasonably might be expected annually. Whole 

lake populations of threadfin shad and alewife, which contribute the greatest number of 

fish to impingement counts at MSS, combined have varied from a low of 41 to 91 million 

fish since 1997 (see Section 5), an approximately two-fold difference. While not 

contributing as greatly to impingement totals as pelagic populations, the total 

electrofishing catch for littoral zone fish varied approximately two to three-fold since 

2000, from 2,753 fish in 2006 to 8,266 fish in 2003. 

Threadfin shad and alewife dominate impingement collections, so annual variations in 

their relative abundance would have the greatest influence on annual impingement totals. 

A third species, white perch, also appears to be especially susceptible to impingement at 

MSS because of its open water distribution, as also shown elsewhere, particularly in 

estuarine environments (Klauda et al. 1988, Jacobsen et al. 2004). White perch is a 

newcomer to the fish community of Lake Norman (Section 3) and has the potential to 

expand its population rapidly in the reservoir, as it has in the Kerr Reservoir, 

Virginia/North Carolina and other southeastern reservoirs (Harris 2006). In time, white 

perch may be added to impingement collections as a dominant species, thus possibly 

increasing annual impingement totals at MSS. 

Comparison of Annual Monitoring Programs 

The results of two or more annual monitoring programs can be compared to evaluate 

inter-annual variability impingement at MSS. The impingement monitoring program in 

1974-1975 collected an estimated 1,050,505 impinged fish (Table 4-5). This estimate 

was based on biweekly collections at one screen per unit (four screens total). The 

sampled screens were allowed to accumulate fish over the 14-day period between 

collections. Decomposition and predation/scavenging may have reduced the number of 

impinged fish recovered. While only representing four of the 16 fixed screens at MSS 

and not including fish lost to decomposition or predation or scavenging, the 1974-1975 

total of 1,050,505 fish was more than five times greater than the 2006-2007 impingement 

estimate (188,419 fish) for all 16 screens combined. 

In 1974-1975, a high percentage (38 percent) of fish could not be identified to species 

due to decomposition. Of those identified, threadfin shad were the most numerous, 

comprising 61.9 percent of the total collections. When combined with gizzard shad, shad 

unidentifiable to species, and unidentified clupeid species, these four taxa comprised 99.8 

percent of the total impingement. This dominance by clupeid species was similar to the 

2006-2007 results, where threadfin shad and alewife comprised 92 percent of the annual 

impingement. Seasonal patterns in impingement rates also were similar for the two 

monitoring programs. In 1974-1975, impingement was greatest during the late fall and 

winter months of November through February, while in 2006-2007 impingement rates 

were greatest in late summer and mid-winter, especially February. 
Impingement Mortality 

4-10

Reductions in impingement of clupeid species was the primary reason why impingement 

totals declined dramatically between 1974-1975 and 2006-2007. In 1974-1975 over 1 

million impinged clupeids were collected from four screens, compared to an estimated 

173,462 clupeids (threadfin shad, alewife, and gizzard shad; Table 4-2) impinged on all 

16 screens in 2006-2007. Thus, there appeared to be a reduction of more than an order of 

magnitude in clupeid impingement between the early and current studies. 

Sampling methods differed between the 1974-1975 and 2006-2007 monitoring programs 

in terms of the number of screens sampled per event, the selection of screens for 

sampling, and the period of time the screens were allowed to accumulate fish before 

screen impinged fish recovery (14 days in 1974-1975 and one day in 2006-2007). The 

degree to which the difference in sampling methods is responsible for the difference in 

results is not clear. 
4-11

Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar

0

100

200

300

400

500

600

700

800

900

1,000

1,100

1,200

1,300

1,400

1,500

Weekly Impingement Rates for All Species Combined

2006 - 2007

Impingement Rate per 106 m3 Figure 4-1 Seasonal Pattern of Impingement Rates for All Species Combined 
Impingement Mortality 

4-12

0 5 10 15 20 25 30 35 40 45

0

10

20

30

40

Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar

0

50

100

150

200

250

300

Impingement Rates by Age Young-of-Year

 Yearling & Older

 Undetermined

Alewife

2006 - 2007

Impingement Rate per 106 m3

Length Frequency

 Percent in Length Interval

 Midpoint of Length Interval (cm)

Figure 4-2 Seasonal Impingement Pattern and Length Frequency of Impinged Alewife 
4-13

0 5 10 15 20 25 30 35 40 45

0

5

10

15

20

25

30

35

40

45

50

Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar

0

500

1000

1500

Impingement Rates by Age

 Young-of-Year

 Yearling & Older

 Undetermined

Threadfin Shad

2006 - 2007

Impingement Rate per 106 m3

Length Frequency

 Percent in Length Interval

 Midpoint of Length Interval (cm)

Figure 4-3 Seasonal Impingement Pattern and Length Frequency of Impinged Threadfin 

Shad 
Impingement Mortality 

4-14

0 5 10 15 20 25 30 35 40 45

0

5

10

15

20

25

30

35

40

Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar

0

50

100

150

Impingement Rates by Age Young-of-Year

 Yearling & Older

 Undetermined

Blue catfish

2006 - 2007

Impingement Rate per 106 m3

Length Frequency

 Percent in Length Interval

 Midpoint of Length Interval (cm)

Figure 4-4 Seasonal Impingement Pattern and Length Frequency of Impinged Blue 

Catfish 
4-15

0 5 10 15 20 25 30 35 40 45

0

5

10

15

20

25

30

Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar

0

5

10

15

20

Impingement Rates by Age Young-of-Year

 Yearling & Older

 Undetermined

Channel catfish

2006 - 2007

Impingement Rate per 106 m3

Length Frequency

 Percent in Length Interval

 Midpoint of Length Interval (cm)

Figure 4-5 Seasonal Impingement Pattern and Length Frequency of Impinged Channel 

Catfish 
Impingement Mortality 

4-16

0 10 20 30 40 50 60 70 80 90

0

5

10

15

20

25

30

35

40

45

50

Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar

0

5

10

15

20

25

Impingement Rates by Age

 Young-of-Year

 Yearling & Older

 Undetermined

White perch

2006 - 2007

Impingement Rate per 106 m3

Length Frequency

 Percent in Length Interval

 Midpoint of Length Interval (cm)

Figure 4-6 Seasonal Impingement Pattern and Length Frequency of Impinged White 

Perch 
4-17

754

755

756

757

758

759

760

4/1/2006 6/1/2006 8/1/2006 10/1/2006 12/1/2006 2/1/2007

0

200

400

600

800

1,000

1,200

1,400

Date

Impingement Rate per 106

m3

40

50

60

70

80

90

Mean Intake Temp. (

OF)

0

1

2

3

4

5

6

7

8

9

10

11

12

 Dissolved Oxygen

Dissolved Oxygen (mgL-1)

 Elevation

 Elevation (ft)

Figure 4-7 Daily Lake Level, Dissolved Oxygen Concentration, and Intake Water 

Temperature vs. Impingement Rate for All Species Combined, April 2006-March 2007 
Impingement Mortality 

4-18

754

755

756

757

758

759

760

0

200

400

600

800

1,000

1,200

1,400

4/1/2006 6/1/2006 8/1/2006 10/1/2006 12/1/2006 2/1/2007

0

100

200

300

400

500

 YOY

Date

YOY Impingement Rate per 106

m3

40

50

60

70

80

90

Mean Intake Temp. (

OF)

0

1

2

3

4

5

6

7

8

9

10

11

12

 Dissolved Oxygen

Dissolved Oxygen (mgL-1

)

 Elevation

 Elevation (ft) YAO Impingement Rate per 106

m3

 YAO

Figure 4-8 Daily Lake Level, Dissolved Oxygen Concentration, and Intake Water 

Temperature vs. Impingement Rate for Young-of-Year (YOY) and Yearling and Older (YAO) 

Threadfin Shad, April 2006-March 2007 
4-19

754

755

756

757

758

759

760

0

50

100

150

200

250

300

350

400

4/1/2006 6/1/2006 8/1/2006 10/1/2006 12/1/2006 2/1/2007

0

5

10

15

20

25

30

35

40

 YOY

Date

YOY Impingement Rate per 106

m3

40

50

60

70

80

90

Mean Intake Temp. (

OF)

0

1

2

3

4

5

6

7

8

9

10

11

12

 Dissolved Oxygen

Dissolved Oxygen (mgL-1

)

 Elevation

 Elevation (ft) YAO Impingement Rate per 106

m3

 YAO

Figure 4-9 Daily Lake Level, Dissolved Oxygen Concentration, and Intake Water 

Temperature vs. Impingement Rate for Young-of-Year (YOY) and Yearling and Older (YAO) 

Alewife, April 2006-March 2007
Impingement Mortality 

4-20

Table 4-1 Fixed Screens Sampled for Impingement (Marked as “1”) by Unit and Sampling 

Date, 2006-2007 

 Unit 1 Unit 2 Unit 3 Unit 4 

Screen A AB B A AB B A AB B BC C A AB B BC C 

Date Sampled 

4/6/2006 1 1 1 1 

4/12/2006 1 1 1 1 

4/20/2006 1 1 1 1 

4/27/2006 1 1 1 1 

5/4/2006 1 1 1 1 

5/11/2006 1 1 1 1 

5/18/2006 1 1 1 1 

5/25/2006 1 1 1 1 

6/1/2006 1 1 1 1 

6/8/2006 1 1 1 1 

6/15/2006 1 1 1 1 

6/22/2006 1 1 1 1 

6/29/2006 1 1 1 1 

7/6/2006 1 1 1 1 

7/13/2006 1 1 1 1 

7/20/2006 1 1 1 1 

7/27/2006 1 1 1 1 

8/3/2006 1 1 1 1 

8/10/2006 1 1 1 1 

8/17/2006 1 1 1 1 

8/24/2006 1 1 1 1 

8/31/2006 1 1 1 1 

9/7/2006 1 1 1 1 

9/14/2006 1 1 1 1 

9/21/2006 1 1 1 1 

9/28/2006 1 1 1 1 

10/5/2006 1 1 1 1 

10/12/2006 1 1 1 1 

10/19/2006 1 

10/26/2006 1 1 1 1 

11/2/2006 1 1 1 1 

11/9/2006 1 1 1 1 

11/16/2006 1 1 1 1 

11/21/2006 1 1 1 1 

11/30/2006 1 1 1 1 

12/7/2006 1 1 1 1 

12/14/2006 1 1 1 1 

12/21/2006 1 1 1 1 

12/28/2006 1 1 1 1 

1/4/2007 1 1 1 1 
4-21

Table 4-1 continued 

 Unit 1 Unit 2 Unit 3 Unit 4 

Screen A AB B A AB B A AB B BC C A AB B BC C 

1/11/2007 1 1 1 1 

1/18/2007 1 1 1 1 

1/25/2007 1 1 1 1 

2/8/2007 1 1 1 1 

2/15/2007 1 1 1 1 

2/22/2007 1 1 1 1 

3/1/2007 1 1 1 1 

3/8/2007 1 1 1 1 

3/15/2007 1 1 1 1 

3/22/2007 1 1 1 1 

3/29/2007 1 1 1 1 

Total Samples per 

Screen 10 17 12 12 19 12 15 14 13 11 5 11 13 7 19 11 

Total Samples per Unit 39 43 58 61 

All Unit Pumps Off 
Impingement Mortality 

4-22

Table 4-2 Estimated Total Number and Biomass of Fish Impinged at Marshall Steam Station, 1 April 2006 – 

Unit 1 Unit 2 Unit 3 Unit 4 

Species No. Wt. (g) No. Wt. (g) No. Wt. (g) No. Wt. (g) No

Alewife 1,023 20,674 1,317 21,294 7,129 140,671 18,681 372,896 28,1Gizzard shad 20 6,448 0 0 0 0 32 8,899 Threadfin shad 16,424 28,141 14,745 24,985 55,952 91,985 58,139 121,752 145,2Blue catfish 224 1,078 1,579 5,976 222 2,296 5,927 23,087 7,9Channel catfish 127 249 296 987 304 1,037 1,223 5,227 1,9White perch 321 14,291 320 12,766 591 20,544 2,490 91,424 3,7Bluegill 37 503 180 1,604 74 930 919 10,248 1,2Pumpkinseed 0 0 0 0 37 140 18 69 Redbreast sunfish 21 231 11 122 9 98 4 50 White crappie 0 0 0 0 6 1,357 18 4,071 Total 18,196 71,614 18,447 67,733 64,324 259,059 87,452 637,723 188,4
Table 4-3 Total Number and Percent Composition of Fish Collected at Marshall SS, 6 April 2006 – 30 M

Unit 1 Unit 2 Unit 3 Unit 4 Total 

Species No. % No. % No. % No. % No. 

Alewife 13 10.00 20 3.89 114 4.72 643 20.35 783 1Gizzard shad - - - - - - 1 0.03 2 Threadfin shad 98 75.38 467 90.86 2,262 93.66 2,199 69.59 5,026 8Blue catfish 8 6.15 13 2.53 22 0.91 178 5.63 212 Channel catfish 3 2.31 5 0.97 6 0.25 38 1.20 57 White perch 5 3.85 7 1.36 7 0.29 84 2.66 114 Bluegill 2 1.54 1 0.19 2 0.08 16 0.51 19 Redbreast sunfish 1 0.77 1 0.19 2 0.08 1 0.03 3 Pumpkinseed - - - - - - - - 2 White crappie - - - - - - - - 1 Total 130 100.00 514 100.00 2,415 100.00 3,160 100.00 6,219 10

Table 4-4 Total Biomass and Percent Composition of Fish Collected at Marshall SS, 6 April 2006 – 30 M

Unit 1 Unit 2 Unit 3 Unit 4 

Species 

Weight 

(g) % 

Weight 

(g) % 

Weight 

(g) % 

Weight 

(g) % 

Weig

(g) 

Alewife 321 29.63 73 5.53 1,853 28.60 12,221 57.26 14,6Gizzard shad - - - - - - 233 1.09 6Threadfin shad 180 16.64 769 58.13 3,676 56.74 4,517 21.16 9,1Blue catfish 19 1.74 257 19.45 802 12.37 690 3.23 8Channel catfish 208 19.15 185 14.00 111 1.72 164 0.77 2White perch 5 0.47 18 1.34 19 0.30 3,020 14.15 4,2Bluegill 22 2.03 9 0.68 10 0.16 222 1.04 2Pumpkinseed - - - - - - - - Redbreast sunfish 329 30.35 12 0.88 8 0.12 278 1.30 White crappie - - - - - - - - 2Total 1,084 100.00 1,322 100.00 6,479 100.00 21,344 100.00 30,2
Impingement Mortality 

4-24

Table 4-5 Number of Impinged Fish Collected* at Marshall Steam Station, 30 April 1974 – May 19

Species Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr Gizzard shad 3 281 824 5 21 23 34 6 29 20 19 1 50 

 

Threadfin shad 1,688 32 275 14 39 1,826 505 316 180,288 211,264 166,113 16,246 61,029 

 

Shad - 6,780 - 69 1 - - 49,032 328,387 - - - - 

 

Common carp - - - - - 1 - - - - - - 1 

 

Golden shiner - - - 1 - - - - - - - - - 

 

Shiner 4 2 1 - - - - - - - 1 - - 

 

Catfish/bullhead - - - 44 - 1 1 1 - - - - - 

 

Flat bullhead - - - 4 - - - - - - - - 1 

 

White catfish 2 - 2 37 10 3 9 11 - - - - - 

 

Striped bass - - - 1 - - - - - - - - - 

 

Black crappie 1 1 1 1 1 - 1 - - - - 1 - 

 

Bluegill - 1 - - - - 1 - - - - - 1 

 

Largemouth bass - 1 - - - - - - 1 - - - - 

 

Sunfish - - - 1 - - - - - - - - - 

 

White crappie 4 1 - - - - - - - - - - - 

 

Yellow perch 708 4 10 6 3 10 23 12 7 3 - 7 31 

 

Unidentifiable 1,273 708 2,433 6 8 8 10 4 3 - - 5,356 - 

 

Total* 3,683 7,811 3,546 189 83 1,872 584 49,382 508,715 211,287 166,133 21,611 61,113 

 

*based on collections from one screen from each of four units 
Table 4-6 Number and Percent Composition of Taxa Other Than Fish at MSS, 6 April 2006 – 30 Marc

Unit 1 Unit 2 Unit 3 Unit 4 To

Taxon No. % No. % No. % No. % No. 

Corbicula 10 76.92 55 98.21 4 100 1 9.09 70 Crayfish 3 23.08 1 1.79 - - 10 90.91 14  Total 13 100 56 100 4 100 11 100 84 
Impingement Mortality 

4-26Table 4-7 Estimated Total Number and Biomass of Fish Impinged Monthly at all Units of MSS, 1 April 2006 - 31 Ma

January February March April May 

Species Count 

Biomass 

(g) Count 

Biomass 

(g) Count 

Biomass 

(g) Count 

Biomass 

(g) Count 

Bioma

(g) 

Alewife 2,570 17,304 1,978 10,560 164 1,018 56 471 105 1,6Gizzard shad 0 0 0 0 0 0 0 0 0 Threadfin shad 13,714 30,119 72,941 118,133 11,977 21,811 2,079 3,606 439 7Blue catfish 33 108 23 28 90 148 0 0 18 Channel catfish 34 162 0 0 63 208 61 84 142 2White perch 54 5,830 8 997 0 0 0 0 0 Bluegill 615 6,057 222 1,914 139 2,196 0 0 35 1Pumpkinseed 0 0 0 0 0 0 0 0 0 Redbreast sunfish 0 0 0 0 0 0 25 275 5 White crappie 0 0 0 0 0 0 0 0 0 Total 17,021 59,580 75,173 131,631 12,434 25,382 2,221 4,436 744 2,9

July August September October November 

Species Count 

Biomass 

(g) Count 

Biomass 

(g) Count 

Biomass 

(g) Count 

Biomass 

(g) Count 

Bioma

(g) 

Alewife 521 12,799 17,394 425,432 4,609 73,721 525 9,496 19 1Gizzard shad 0 0 0 0 41 12,213 11 3,135 0 Threadfin shad 42 202 1,286 4,572 16,755 39,074 12,692 23,825 6,533 10,3Blue catfish 7,048 24,686 446 2,918 86 3,674 13 565 43 Channel catfish 729 2,808 874 3,863 0 0 0 0 0 White perch 0 0 1,126 39,307 2,179 78,773 283 9,811 21 8Bluegill 37 309 47 980 26 495 7 127 0 Pumpkinseed 0 0 0 0 0 0 0 0 0 Redbreast sunfish 0 0 0 0 0 0 14 168 0 White crappie 0 0 0 0 23 5,427 0 0 0 Total 8,377 40,803 21,174 477,073 23,719 213,377 13,545 47,127 6,616 11,4
Table 4-7 continued 

All 

Species Count 

Biomass 

(g) 

Alewife 28,150 555,535 Gizzard shad 52 15,348 Threadfin shad 145,260 266,862 Blue catfish 7,952 32,437 Channel catfish 1,950 7,501 White perch 3,722 139,025 Bluegill 1,210 13,286 Pumpkinseed 55 209 Redbreast sunfish 45 501 White crappie 23 5,427 Total 188,419 1,036,130 
Impingement Mortality 

4-28

Table 4-8 Maximum Total Length (mm) Used to Distinguish Young-of-Year from Yearling Fish by M

Species Age Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nthreadfin shad 0 65 68 72 75 78 45 50 55 60 65  1 89 89 90 94 99 80 82 84 86 87 alewife 0 130 130 130 140 150 50 70 90 115 130  1 200 200 200 205 210 160 175 190 200 200 blue catfish 0 100 100 100 110 120 130 50 60 80 100  1 180 180 180 185 190 195 145 160 175 180 white perch 0 105 105 105 110 115 35 65 85 90 105  1 150 150 150 155 160 125 135 145 150 150 channel catfish 0 80 80 80 90 100 110 50 60 70 80  1 180 180 180 185 190 195 125 145 165 180 Note: shaded month indicated appearance of new year class Sources: Klauda et al. (1988), Grist (2002), Graham (1999), Nigro and Ney (1982), Pierce (2003) 
Table 4-9 Estimated Annual Impingement of Fish by Age Group at all Units of MSS, 1 April 2006 - 31 M

Young-of-Year Yearling & Older Undetermined 

Species Count Percent Count Percent Count Percent Alewife 6,793 24.13 21,268 75.55 89 0.32Gizzard shad 0 0 0 0 52 100Threadfin shad 77,722 53.51 67,226 46.28 312 0.21Blue catfish 379 4.77 7,572 95.23 0 0Channel catfish 263 13.46 1,648 84.52 39 2.02White perch 28 0.75 3,694 99.25 0 0Bluegill 0 0 0 0 1,210 100Pumpkinseed 0 0 0 0 55 100Redbreast sunfish 0 0 0 0 45 100White crappie 0 0 0 0 23 100Total 85,186 45.21 101,408 53.82 1,825 0.97
Impingement Mortality 

4-30

Table 4-10 Comparison of Relative Abundance of Fish Species in the March-April 2006 

Littoral Zone Electrofishing Surveys and the 2006-2007 MSS Impingement Collections 

2006 Electrofishing 2006-2007 Impingement 

Species Total No. % Rank Total No. % Rank

Bluegill 1,166 42.35 1 19 0.31 6 

Redbreast sunfish 724 26.30 2 3 0.05 7 

Threadfin shad 218 7.92 3 5,026 80.82 1 

Redear sunfish 162 5.88 4 - 

Spotted bass 137 4.98 5 - 

Largemouth bass 97 3.52 6 - 

Hybrid sunfish 83 3.01 7 - 

Warmouth 53 1.93 8 - 

Whitefin shiner 28 1.02 9 - 

Commom carp 19 0.69 10 - 

Spottail shiner 17 0.62 11 - 

Green sunfish 13 0.47 12 - 

Channel catfish 10 0.36 13 57 0.92 5 

Flathead catfish 6 0.22 14 - 

Hybrid black bass 4 0.15 15 tie - 

Shorthead redhorse 4 0.15 15 tie - 

Gizzard shad 3 0.11 17 tie 2 0.03 8 tie 

Quillback 3 0.11 17 tie - 

Black crappie 2 0.07 19 - 

Golden shiner 1 0.04 20 tie - 

Striped bass 1 0.04 20 tie - 

Tesselated darter 1 0.04 20 tie - 

Yellow perch 1 0.04 20 tie - 

Alewife - - 783 12.59 2 

Blue catfish - - 212 3.41 3 

Pumpkinseed - - 2 0.03 8 tie 

White crappie - - 1 0.02 10 

White perch - - 114 1.83 4 

Total 2,753 100.00 6,219 100.00 
Table 4-11 Estimated Total Number and Biomass of Fish Impinged Monthly at all Units of MSS for Full Flow Oper

31 March 2007 

January February March April May 

Species Count 

Biomass 

(g) Count 

Biomass 

(g) Count 

Biomass 

(g) Count 

Biomass 

(g) Count 

Biom

(g

Alewife 3,313 22,278 2,558 13,646 242 1,510 79 695 160 2,Gizzard shad 0 0 0 0 0 0 0 0 0 Threadfin shad 18,509 40,655 94,959 153,801 17,198 31,313 3,340 5,791 621 1,Blue catfish 44 144 30 35 159 259 0 0 21 Channel catfish 43 203 0 0 89 293 155 198 229 White perch 80 8,685 12 1,502 0 0 0 0 0 Bluegill 1,087 10,587 288 2,476 199 3,150 0 0 48 Pumpkinseed 0 0 0 0 0 0 0 0 0 Redbreast sunfish 0 0 0 0 0 0 74 816 10 White crappie 0 0 0 0 0 0 0 0 0 Total 23,075 82,553 97,847 171,461 17,886 36,525 3,648 7,500 1,088 4,

July August September October November 

Species Count 

Biomass 

(g) Count 

Biomass 

(g) Count 

Biomass 

(g) Count 

Biomass 

(g) Count 

Biom

(g

Alewife 647 15,828 17,405 425,692 4,615 73,831 708 12,492 38 Gizzard shad 0 0 0 0 41 12,226 11 3,135 0 Threadfin shad 53 253 1,287 4,574 16,981 39,330 22,964 41,138 12,682 20,Blue catfish 8,819 30,755 447 2,919 86 3,680 13 566 131 Channel catfish 889 3,445 875 3,865 0 0 0 0 0 White perch 0 0 1,126 39,323 2,185 78,998 350 12,424 36 1,Bluegill 55 460 47 981 26 496 7 127 0 Pumpkinseed 0 0 0 0 0 0 0 0 0 Redbreast sunfish 0 0 0 0 0 0 28 319 1 White crappie 0 0 0 0 23 5,431 0 0 0 Total 10,462 50,742 21,186 477,355 23,957 213,991 24,079 70,201 12,886 22,
Impingement Mortality 

4-32Table 4-11 continued 

Total 

Species Count 

Biomass 

(g) 

Alewife 30,048 572,819 Gizzard shad 52 15,361 Threadfin shad 198,218 358,709 Blue catfish 9,939 38,844 Channel catfish 2,344 8,562 White perch 3,860 147,105 Bluegill 1,862 19,824 Pumpkinseed 91 346 Redbreast sunfish 113 1,256 White crappie 23 5,431 Total 246,549 1,168,258 
5-1 

5 

ASSESSMENT OF ADVERSE ENVIRONMENTAL 

IMPACT 

Introduction 

Section 316(b) of the Clean Water Act calls for use of the “...best technology available to 

minimize adverse environmental impact.” In 2004, EPA issued the Phase II Rule which 

addressed implementation of §316(b) for existing cooling water intake structures that 

have a design flow of more than 50 MGD. As a result of challenges to this Rule by 

several groups, the U.S. Court of Appeals for the Second Circuit reviewed the Rule and 

remanded several key provisions on 25 January 2007 (see Section 1). As a result of this 

remand, EPA issued a suspension of the Phase II Rule on 9 July 2007 and directed that 

“Permit requirements for cooling water intake structures at Phase II facilities should be 

established on a case-by-case best professional judgment (BPJ) basis”.

Prior to the Phase II Rule, BPJ compliance with §316(b) was based on a determination of 

whether or not the cooling water intake structure was resulting in adverse environmental 

impact (AEI). With the remand of the Phase II Rule, this approach continues to be 

appropriate in BPJ determinations. Unfortunately, neither the Act nor subsequent 

guidance provided by the EPA provides any definition of the term “adverse 

environmental impact” for use by permit writers in making their BPJ determinations. 

Consequently, it is left up to the permitting authority to determine what constitutes 

adverse environmental impact to be minimized under §316(b). 

In this analysis, Duke Energy will evaluate the potential for an adverse impact of the 

MSS cooling water intake structure as a result of an unacceptable reduction in: 

 biological integrity as measured in terms of aquatic community species composition, 

diversity, and functional organization; or 

 human use of the aquatic resources in the source waterbody, especially fishing 

opportunity or catch quantity or quality. 

These two evaluation criteria are consistent with the EPA’s original draft §316(b) 

guidance (EPA 1977) as well as historical practice (Dey et al. 2000). 

Duke proposes to use multiple lines of investigation to determine the potential for 

unacceptable adverse impacts by evaluating: 
Assessment of Adverse Environmental Impact 

5-2

 potential ecosystem impacts by: 

o conducting a prospective evaluation based on losses to impinged species 

o conducting a retrospective evaluation based on analysis of a long-term 

data base, and 

 impacts to human use by: 

o estimating economic losses from impingement. 

How do we go about assessing the risk of adverse environmental impact for MSS? 

Recognizing the need for a consistent and technically sound approach to assessing the 

potential effects of a wide variety of anthropogenic stresses on ecosystems, the EPA 

began an intensive effort to develop an approach for ecological risk assessment during 

the late 1980s. The intent of this effort was to develop an approach that would parallel 

that used for human health risk assessment but focus on the non-human ecosystem. 

These efforts, which were independent of those to re-promulgate §316(b) regulations, 

resulted in the publication of a “Framework for Ecological Risk Assessment” (ERA) in 

1992 (EPA 1992). Following considerable public review, input and debate, “Guidelines 

for Ecological Risk Assessment” (ERA Guidelines) were finalized in October 1998 (EPA 

1998). 

These Guidelines replace the earlier framework, describe general principles, and give 

examples of how ecological risk assessment can be applied to a wide variety of systems, 

stressors, and spatial and temporal scales. The general principles reflect an effort to 

promote consistency in assessing ecological risk among EPA’s offices and regions, while 

at the same time permitting development of site-specific methods to meet local needs. 

EPA’s ERA approach has been used to address a variety of chemical, physical, and 

biological stressors, as well as the combined effects of multiple stressors, across many 

agencies throughout the United States. 

EPA defines ecological risk assessment as “...a process that evaluates the likelihood that 

adverse ecological effects may occur or are occurring as a result of exposure to one or 

more stressors” (EPA 1992). Since entrainment and impingement result in the exposure 

of aquatic organisms to physical and chemical stresses, it seems that a logical approach to 

assessing the risk of adverse environmental impact of cooling water withdrawals as 

required by §316(b) would be through the use of an ecological risk assessment 

framework, such as that provided by EPA. 

As noted above, the risk of adverse environmental impact from the continued operation 

of the MSS cooling water intake system needs to be addressed from two perspectives: 

ecosystem (i.e., biological integrity) and human (i.e., fishing opportunity). Potential 

adverse effects from these two perspectives were addressed within the framework of an 

ecological risk assessment as described in EPRI (2002). The results of these two 

assessments are then used together as part of a “weight-of-evidence” approach to 
5-3

determine the overall risk of adverse environmental impact from MSS’s cooling water 

intake structure on Lake Norman. 

Assessment of Ecosystem Effects 

EPA recognized that it is unlikely that risk can be accurately determined through use of a 

single measure. Therefore, the ecological risk assessment framework calls for careful 

review of all information relevant to the issue under a “multiple lines of evidence” 

approach to risk assessment. These lines are independently assessed using two broad 

categories of methods: prospective and retrospective methods. Prospective methods 

attempt to predict the effects of a stressor, such as cooling water withdrawal, on the 

ecological endpoints selected for the assessment. Retrospective methods, on the other 

hand, evaluate the potential effects of an existing stressor, such as a cooling water intake 

structure, by measuring the actual effects on the ecological endpoints. In this assessment, 

we use a combination of prospective and retrospective methods to determine the potential 

ecosystem effects of cooling water withdrawals at MSS. 

Prospective Approach 

EPA defines use of prospective methods for ecological risk assessment as “an evaluation 

of the future risks of a stressor not yet released into the environment or of future 

conditions resulting from an existing stressor” (EPA 1998). For this assessment, 

prospective methods entail predicting the potential consequences of impingement at MSS 

on the fish community composition and function and on recreational fishing in Lake 

Norman. 

As described in Section 4, impingement monitoring was conducted at MSS over the 

period April 2006 – March 2007. The results of these studies were used to estimate the 

expected annual impingement at MSS under normal operations: 

Species Count 

Biomass 

(kg) 

Mean 

Weight per 

fish (g) 

Threadfin shad 145,260 266,862 1.8 

Alewife 28,150 555,535 19.7 

Blue catfish 7,952 32,437 4.1 

White perch 3,722 139,025 37.4 

Channel catfish 1,950 7,501 3.8 

Bluegill 1,210 13,286 11.0 

Pumpkinseed 55 209 3.8 

Gizzard shad 52 15,348 295.2 

Redbreast sunfish 45 501 11.1 

White crappie 23 5,427 236.0 

Total 188,419 1,036,130 5.5 
Assessment of Adverse Environmental Impact 

5-4

Mean weights per fish suggest that, with the exception of a small number of adult gizzard 

shad and white crappie impinged, most of the fish collected were small and most likely 

either age 0 or age 1 individuals. Two pelagic forage species, threadfin shad and alewife, 

accounted for 92 percent of the fish and 79 percent of the biomass impinged annually 

based on the results of this study. 

All but three of the species collected had estimates of annual impingement that averaged 

less than ten fish per day. Given that all of these species are common to Lake Norman, it 

is hard to image that the removal of individuals of each species daily, most likely 

juveniles, could be considered an adverse environmental impact from Lake Norman, a 

lake of more than 13,000 ha. 

Blue catfish, the species with the third highest estimated annual impingement, is one of 

two catfish species that are popular target of anglers. The estimated annual impingement 

of this species represents a removal rate of less than 22 individuals per day. At an 

average weight of only 4.1 grams each, most of these were likely only a few months old. 

Assuming a conservative survival rate of 5 percent from the time of impingement to the 

average age of harvest by anglers, these 22 juveniles result in approximately one averagesized harvestable adult (i.e., 5 years old and about 0.5 lb or 227 g). Hence, the current 

annual impingement of blue catfish is equivalent to the removal of less than one adult per 

day that could be caught by an angler. As with the other species, it is hard to imagine 

that such a removal rate would constitute an adverse environmental impact on Lake 

Norman. 

The two most abundant species impinged, threadfin shad and alewife, are abundant 

throughout the pelagic areas of Lake Norman. Together, estimated annual impingement 

of these two species amounted to slightly more than 170,000 individuals weighing 823 kg 

(1,811 lb). Both species are pelagic and serve as important forage for a variety of 

gamefish in Lake Norman. Owing to their importance as forage, pelagic species 

(predominantly threadfin shad and alewife) have been the subject of annual 

hydroacoustic studies designed to determine overall abundance in Lake Norman (Section 

3). Based on the results of these studies, the number of pelagic fish throughout the lake 

during 2000-2003 has varied from just over 64 million to 86 million individuals. At these 

levels, the removal of these species by impingement at Marshall amounts to 0.2 to 0.3 

percent of the population each year. Given the rapid growth, short life span, and high 

reproductive potential of both species, it is hard to imagine that such removal rates can 

have any detectable effects on either population persistence for either species or on the 

amount of food available to support higher trophic levels. 

Retrospective Approach 

Within the ecological risk assessment framework, a retrospective assessment of risk is an 

evaluation of the causal linkages between observed ecological effects and stressors 

present in the environment (EPA 1998). For this assessment, the stressor would be the 

loss of fish from the Lake Norman community caused by impingement at the CWIS. The 

retrospective approach evaluated whether effects on individual species populations or the 
5-5

community as a whole are apparent by examining trends in the species richness and 

relative abundance of individual species during the time that MSS has operated. 

Community metrics involving species richness and abundance would be appropriate 

because impingement is equivalent to harvesting fish, without adding pollutants or 

altering the physical habitat of the lake, for which other community metrics might be 

appropriate. 

Fortunately, there exists a database on the occurrence of fish species and their relative 

abundance in Lake Norman, which includes recent data from 1993 through 2007. Trends 

in species richness and relative abundance should be apparent if operation of MSS has 

had a discernable effect on the Lake Norman fish community. If there are discernable 

effects, are they consistent with the selective loss of individual species to impingement 

that could be interpreted as being causal in nature? 

Hypothetically, selective harvesting of fish species by impingement could result in 

species or trophic imbalance in the community, or reduced standing crops of fish. If true, 

metrics involving species richness, relative abundance, and standing crop (absolute 

abundance) should reveal significant, sustained changes in species occurrence and 

abundance, or a trophic imbalance. Also, a persistence or increase in top carnivores 

(piscivores) might indicate stability or improvement in the community, and vice versa. 

In the database, the lake is divided into two zones (pelagic and littoral, i.e., open water 

and near-shore, respectively). From 1993 until the present, the pelagic zone has been 

sampled by mobile hydroacoustics and purse seine surveys, resulting in annual estimates 

of absolute abundance of the primary pelagic species (combined): threadfin shad, alewife, 

and gizzard shad. The littoral zone was sampled annually by boat electrofishing from 

1993 through the present (see Section 3 for sampling method descriptions). 

The results of these surveys in terms of fish abundance, species richness, and trophic 

structure of the fish community are discussed as follows. 

Fish Abundance 

The abundance of pelagic clupeid species, according to annual hydroacoustic surveys in 

September, varied dramatically from 1993 to 1997, then appeared to stabilize after 1997 

(Figure 5-1). Acoustic surveys prior to 1996 used only a down-looking transducer. For 

the 1996-2006 surveys, a multiplexed side-scan transducer was added to include fish 

residing within 2 m of the water surface. From 1996 on, abundance data should be 

directly comparable and would reflect the most recent status of the pelagic forage fish 

populations. Since 1997, population estimates for threadfin shad, alewife, and gizzard 

shad combined have ranged from 47,172,559 fish in 2004 to 91,335,000 fish in 1998. 

Although gizzard shad were once a major component of the forage fish community, 

especially in littoral areas (Siler et al. 1981), in recent years they contribute very few fish 

compared to threadfin shad and alewife (Table 5-1). After its initial appearance in Lake 
Assessment of Adverse Environmental Impact 

5-6

Norman in 1999, alewife increased in abundance to make up 12 to 25 percent of the purse 

seine catch, but was reduced to 5 percent or less in 2005-2007. 

In the littoral zone of Lake Norman, electrofishing surveys during 1993-2007 (Figure 5-

2) have shown a nearly three-fold variation in the annual total catch for all species 

combined (2,849 in 1995 to 8,266 fish in 2003). However, there has been no sustained 

trend in the overall numerical abundance of fish. 

Bluegill and redbreast sunfish have been the dominant species in electrofishing surveys 

since 1993, and were abundant in the earlier 1978-1979 rotenone surveys (Siler et al. 

1981). The annual variation in total electrofishing catch from 1993 to 2007 to some 

degree reflected the variation in catch of bluegill in particular (Figure 5-2). The annual 

catch of redbreast sunfish and largemouth bass has shown less variation over the 15-year 

period (Figure 5-2). 

The five target species (threadfin shad, alewife, channel catfish, blue catfish, and white 

perch) chosen for detailed impingement analysis in Sections 3 and 4, with the exception 

of threadfin shad, were not as abundant in the 1993-2007 electrofishing catches as 

bluegill, redbreast sunfish, and largemouth bass (Figures 5-3 and 5-4). Threadfin shad 

and alewife are pelagic species; therefore, the hydroacoustic data presented in Figure 5-1 

should be a more accurate reflection of their annual abundance than the littoral zone 

electrofishing data (Figure 5-3). Channel catfish, blue catfish, and white perch are caught 

infrequently by electrofishing but show occasionally higher annual catch totals that do 

not indicate trends in their abundance. White perch had not been caught prior to 1998. 

Species Richness 

Since 1993 there has been an increase in the number of fish species found during 

electrofishing surveys, from 20-22 species annually prior to 1998 to as many as 29 

species after 1998 (Figure 5-5). Species added since 1997 include alewife, bluehead 

chub, bowfin, fathead minnow, goldfish, green sunfish, rainbow trout, spotted bass, 

striped mullet, swallowtail shiner, walleye, white catfish, white crappie, and white perch. 

Of these 14 species, bowfin, bluehead chub , white catfish, and white crappie previously 

had been reported as occurring in Lake Norman during 1974-1980 (Table 3-1 in Section 

3). 

Trophic Structure 

Species caught by annual spring electrofishing surveys were divided into trophic guilds 

consisting of insectivores, omnivores, herbivores, and piscivores (Table 5-2) according to 

classifications used by NCDENR (2006). Insectivores dominated the annual 

electrofishing catch, representing 51 to 93 percent of all fish caught and a 14-year mean 

of 78 percent (Figure 5-6). Omnivores and piscivores varied annually but each had a 14-

year mean of 11 percent. The eastern silvery minnow was the only herbivorous species 
5-7

and comprised only 01.-0.2 percent of all fish caught in the years when they occurred 

(1993 and 2000). 

Omnivores (primarily threadfin shad) were unusually abundant in 1993-1994 when they 

comprised 26-36 percent of the total electrofishing catches (Figure 5-6). The abundance 

of threadfin shad in 1993 and 1994 also was reflected in the pelagic fish population 

estimates (Figure 5-1). After 1994 omnivores comprised <15 percent of all fish caught 

annually. 

From 1993 to 1999, piscivores represented 13 to 20 percent of all fish caught (Figure 5-

6). After 1999, they decreased to 6-11 percent of the total catch. The number of 

piscivores caught annually decreased from a mean of 623 fish in 1993-1999 to 409 fish in 

2000-2007. The decrease in piscivores largely reflected a decrease in the abundance of 

largemouth bass, the dominant piscivore. Coincident with this decrease was the 

introduction and gradual proliferation of spotted bass, from six fish in 2001 to 194 fish in 

2007. 

Retrospective Summary 

The fish community of Lake Norman has changed since the commencement of operation 

of MSS in terms of fish abundance, species richness, species composition, and trophic 

structure. Initially, the changes were related to the evolution of the fish community from 

a riverine community to a lacustrine community following the impoundment of the 

Catawba River in the early 1960s. Typical riverine species were lost and lacustrine 

species became better established. Recent changes (since 1993) are readily tracked by 

examining the results of standardized annual sampling of the fish community using 

hydroacoustics, net gear, and electrofishing. 

Despite long-term changes in the community, there is evidence of stabilization, 

particularly in this past decade. The fall population size of pelagic forage species 

stabilized after 1997. Since 1993, there also has been no sustained trend in the littoral 

zone catch size. However, there has been an increase in species richness since 1993, with 

the addition of 14 new species in the past 10 years. The trophic structure has become 

more stable after 1994, with insectivores numerically representing approximately 70-90 

percent of the littoral zone catch, with the remainder nearly equally divided between 

piscivores and omnivores. 

Intensive recreational fishery management measures and species introductions likely will 

modify the fish community of Lake Norman in the future, and have been largely 

responsible for changes experienced in the past. However, there is no evidence that any 

community changes are related to operation of the MSS cooling water intake and 

impingement losses. 
Assessment of Adverse Environmental Impact 

5-8

Effects on Human Uses 

Lake Norman, from which MSS withdraws its cooling waters, provides important fishing 

opportunities to people from North Carolina and adjacent states. Popular targets of 

anglers include striped, largemouth and spotted bass; several species of catfish; and a 

variety of panfish species including bluegill, crappie, and perch. In addition, the lake 

supports a healthy community of other water-dependent species that provide passive use 

opportunities such as nature watching. Hence, in addition to potential effects on 

biological integrity, it is reasonable to address the potential effects of impingement of fish 

at MSS on human uses of Lake Norman, especially fishing. 

Two different types of measures of the potential effects of impingement on human uses 

of the aquatic resources in Lake Norman, equivalent loss and economic value, were used 

in this analysis. Estimates of the two types of measures were calculated for each of five 

target species: threadfin shad, alewife, blue catfish, channel catfish, and white perch. 

Together, these five species comprised more than 99 percent of total impingement at 

MSS. The methods and parameter values used to develop these two measures are 

discussed in Appendix A. The resulting measures are described below. 

Equivalent Loss Measures 

Since impingement at MSS involves several species of fish and a variety of sizes within 

each species, equivalent loss calculations provide a means to express these losses in 

biological terms that are more amenable to interpretation. In this assessment, two 

different equivalent loss measures were estimated for each target species: 

 Equivalent yield provides a measure of the additional weight of each species that 

could have been harvested by anglers in the absence of any impingement at MSS. 

 Production foregone provides a measure of the additional biomass production that 

could have resulted from the impinged individuals in the absence of any impingement 

at MSS. 

Using the methodology and the most probable estimates for each of the input parameters 

described in Appendix A, estimates of equivalent fishery yield and production foregone 

for each of the five target species were generated using the monthly impingement 

estimates at MSS. The monthly estimates were then summed to calculate total annual 

values. 

Only three of the five target species evaluated in this assessment, blue catfish, channel 

catfish and white perch, directly support fisheries in Lake Norman. Annual fishery yield 

that could result from the fish impinged at MSS totaled 254 lb (115.2 kg) under actual 

cooling water flows during the course of the impingement monitoring program and 304 

lb (137.9 kg) under maximum (design) cooling water flows (Table 5-3). Most (67 

percent) of this equivalent yield was through blue catfish. 
5-9

Annual production from the target species impinged at MSS totaled 5,134 lb (2,329 kg) 

under actual cooling water flows and 6,037 lb (2,738 kg) under maximum cooling water 

flow (Table 5-4). Three species, alewife, blue catfish and threadfin shad, accounted for 

just over 83 percent of the total annual production foregone. In the absence of 

impingement at MSS, this additional production could support additional biomass of fish 

targeted by anglers and provide energy to support other ecosystem functions. 

Economic Value 

Calculation of economic value takes the estimates of equivalent yield and production 

foregone described above and assigns economic value based on estimates of “willingness 

to pay” for additional resources by the users of that resource. As such, this measure 

provides another perspective on the issue of adverse environmental impact by weighting 

the estimates of equivalent loss by the relative “importance” of each species assigned by 

humans. Calculation of economic value has a long history of use including for the 

assignment of economic benefits of the Phase II rulemaking (EPA 2004a, 2004b, 2004c). 

In this assessment, economic values were assigned to each of the four categories of 

benefit values relevant to §316(b) regulations for existing facilities: 

 market direct use benefits; 

 nonmarket direct use benefits; 

 indirect use benefits; and, 

 nonuse benefits. 

The value of marketed goods is equivalent to the sum of predicted changes in “consumer 

and producer surplus.” Producer surplus is the difference between the price obtained for 

a good (e.g., fish) and the cost of producing that commodity. Consumer surplus is the 

difference between the perceived value of a good or service to the consumer and the cost 

of acquiring that good or service. Non-marketed goods, such as recreational fishing, 

normally require using indirect markets, such as travel, to infer their value. In addition, 

some positive changes in resources such as forage fish might increase the direct use 

values even though the resources themselves are not directly used. Finally, in addition to 

these direct and indirect use-related values, there is a potential for environmental changes 

to increase the welfare of individuals who do not use the resource at all. These latter 

benefits are considered nonuse benefits. Methods and input values used to calculate 

economic values for each of these benefits categories are described in Appendix A. 

The annual economic value for all fish impinged at MSS totaled only $577 under actual 

cooling water flow and only $680 under maximum cooling water flow (Table 5-5). Most 

of this economic value (~51 percent) was attributable to the loss of threadfin shad and 

alewife through their combined effects on the equivalent predator (striped bass). 
Assessment of Adverse Environmental Impact 

5-10

Uncertainty in Economic Value 

Given the large number of parameters needed to calculate economic values, coupled with 

the difficulty in obtaining precise estimates of the input values for each parameter, there 

can be considerable uncertainty in the estimates of economic value generated for any 

individual facility. It is for this reason that EPA guidance on conducting such economic 

evaluations recommends that an uncertainty analysis be conducted as part of the 

evaluation (EPA 2000). The purpose of such an uncertainty analysis is to make 

transparent all the underlying sources of uncertainty in the calculation of economic value 

such that the reader can independently determine whether the results have sufficient 

precision and accuracy to meet regulatory needs. 

In this report, uncertainty was addressed using two methods. First, sensitivity analysis 

was conducted on key input parameters to determine the relative importance of the 

uncertainty in the values for each parameter to the estimates of economic value. Second, 

Monte Carlo analysis was used to evaluate the cumulative uncertainty in the estimates in 

economic benefit, combining the uncertainty in each of the input parameters. For 

simplicity, this uncertainty analysis focused on the estimates of economic benefit based 

on actual cooling water flows. Uncertainty for the maximum cooling water flow case 

should be almost identical. Results for each of the two methods are discussed below. 

The results of sensitivity analysis are illustrated in Figure 5-7. Uncertainty in natural 

mortality rate yielded the greatest range in estimates of annual economic value (-48 to 

+281 percent) while uncertainty in the recreational fishing price yielded the second 

greatest range (-71 to +112 percent). Uncertainty in these two parameters was clearly 

most important in determining the total uncertainty in the estimate of annual economic 

value. Uncertainty in the remaining five input parameters (natural mortality rate, fishing 

vulnerability, weight, trophic conversion factor, and equivalent predatory exploitation 

rate) had relatively minor effects on estimates of annual economic value (< + 30 percent). 

The results of the Monte Carlo analysis are illustrated in Figure 5-8. The frequency 

distribution of the resulting annual estimates appeared generally symmetrical, although 

most of the estimates of economic value resulting from this uncertainty analysis were 

higher than the most probable estimate. The median value from the Monte Carlo analysis 

was 17 percent higher than the most probable estimate while the 25th and 75th 

percentiles were 0 percent and 34 percent higher, respectively. There was less than a 10 

percent chance that the true annual economic benefit was more than 50 percent greater 

than the most probable estimate reported in Table 5-5. The fact that most of the estimates 

of economic value were higher than the most probable estimate is likely related to the 

non-symmetrical effects of the two key parameters, the natural mortality rate and the 

recreational price per pound, described above. 

Conclusions 

The results of this assessment provide no evidence that impingement at MSS is causing 

effects within Lake Norman that rise to a level that could be considered an “adverse 
5-11

environmental impact.” This conclusion is based on a careful evaluation of all relevant 

lines of evidence, including the following observations: 

1. Numbers of fish lost as a result of impingement are exceedingly small compared 

to likely sizes of fish populations in Lake Norman. For all but three species, 

removal rates amounted to less than 10 fish per day. For blue catfish, removal 

rates were less than 22 young juveniles per day. These young juveniles could be 

expected to yield only a single harvestable adult, on average. As with the other 

species, it is hard to imagine that such a removal rate would constitute an adverse 

environmental impact on Lake Norman. 

2. Total numbers impinged each year for the two most abundant species, threadfin 

shad and alewife, amounted to less than 0.3 percent of the population in the lake. 

Both species are highly prolific spawners with high growth rates and short life 

spans. This high reproductive potential can easily compensate for the relatively 

small losses at MSS with no long-term effects. 

3. After evolving from a riverine to lacustrine community, the aquatic community of 

Lake Norman appears to have stabilized in terms of fish abundance and trophic 

structure, particularly in this past decade. Fish abundance in pelagic areas has 

stabilized since 1997. There also has been no sustained trend in the littoral zone 

catch size since 1993, while species richness has increased with the addition of 14 

new species in the past 10 years. 

4. Intensive recreational fishery management and species introductions likely have 

been responsible for changes experienced in the past. However, there is no 

evidence that any community changes are related to operation of the MSS cooling 

water intake and impingement losses. 

5. Estimates of total fishery catch that could result from the fish impinged at MSS 

had they survived amount to only 254 lb (115.2 kg) per year, most of which was 

blue catfish. That amount translates to less than 1 lb per day. This level of 

harvest is less than what would be expected from a small number of anglers. 

6. Estimates of the total production foregone as a result of fish impinged at MSS 

amounted to just over 5,100 lb (2,313 kg) per year of biomass. Almost all of this 

was a result of threadfin shad and alewife, both of which are highly abundant 

species in Lake Norman. That biomass translates to less than 3 ounces per 

surface acre (210 g/ha) across the entire reservoir. 

7. The best estimate of the total economic value of fish impinged at MSS was $577 

per year. Even considering the uncertainty in these estimates, it is highly unlikely 

that the economic value exceeds $800 per year. These values must be considered 

trivial compared to the likely total economic value of the recreational fishery in 

Lake Norman. 
Assessment of Adverse Environmental Impact 

5-12

1992 1994 1996 1998 2000 2002 2004 2006

0

1x108

2x108

3x108

4x108

5x108

6x108

7x108

Hydroacoustic Population Estimate

Figure 5-1 Annual Population Estimates of Pelagic Fish in Lake Norman Using 

Hydroacoustic Sampling, 1993-2006 
5-13

1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

Total

Bluegill

Redbreast sunfish

Largemouth bass

Electrofishing Catch

Figure 5-2 Total Annual Electrofishing Catch for Bluegill, Redbreast Sunfish, Largemouth 

Bass, and All Species Combined at 30 100-m Transects in Lake Norman, 1993-1997 and 

1999-2007 
Assessment of Adverse Environmental Impact 

5-14

1994 1996 1998 2000 2002 2004 2006

0

200

400

600

800

1,000

Threadfin shad

Total Catch

0

10

20

30

40

50

60

Alewife

0

2

4

6

8

10

Blue catfish

Figure 5-3 Total Annual Electrofishing Catch for Blue Catfish, Alewife, and Threadfin 

Shad at 30 100-m Transects in Lake Norman, 1993-1997 and 1999-2007 
5-15

1994 1996 1998 2000 2002 2004 2006

0

40

80

120

160

White perch

Total Catch

0

10

20

30

40

50

60

Channel catfish

Figure 5-4 Total Annual Electrofishing Catch for Channel Catfish and White Perch at 30 

100-m Transects in Lake Norman, 1993-1997 and 1999-2007 
Assessment of Adverse Environmental Impact 

5-16

Electrofishing 

Species 1993 1994 1995 1996 1997 1999 2000 2001 2002 2003 2004 2005 2

Longnose gar 

Bowfin 

Alewife 

Gizzard shad 

Threadfin shad Bluehead chub Commom carp Eastern silvery minnow 

Fathead minnow 

Golden shiner 

Goldfish 

Greenfin shiner 

Spottail shiner Swallowtail shiner 

Whitefin shiner 

Brassy jumprock Quillback Shorthead redhorse 

Silver redhorse Blue catfish 

Brown bullhead 

Channel catfish 

Flat bullhead 

Flathead catfish 

Snail bullhead 

Figure 5-5 Species Composition in the Littoral Zone of Lake Norman as Determined by Electrofishing in 1993-1

(Shading Denotes Species Presence) 
Species 1993 1994 1995 1996 1997 1999 2000 2001 2002 2003 2004 2005 2

White catfish 

Rainbow trout 

Striped mullet 

Mosquitofish White bass 

White perch 

Striped bass 

Black crappie 

Bluegill Green sunfish Hybrid black bass Hybrid sunfish Largemouth bass Pumpkinseed 

Redbreast sunfish 

Redear sunfish Spotted bass Warmouth White crappie 

Swamp darter 

Tessellated darter 

Unidentified darter Walleye 

Yellow perch 

Total No. of Species 20 20 20 21 22 23 24 28 23 29 25 24 

Figure 5-5 continued 
Assessment of Adverse Environmental Impact 

5-18

1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

0

10

20

30

40

50

60

70

80

90

100

 % Herbivore % Insectivore % Omnivore % Piscivore

 Percent Trophic Level Composition

Figure 5-6 Percent Composition of Fish Trophic Guilds in the Littoral Zone of Lake 

Norman Based on Electrofishing Results of 1993-1997 and 1999-2007
5-19

Vulnerability

Fishing Mortality

Exploitation Rate

Trophic Conversion

Weight

Recreational Price

Natural Mortality

-300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300

Percent Difference

Figure 5-7 Estimates of the Range of Effects of Uncertainty in Each Input Parameter on 

Estimates of Annual Economic Benefit at MSS 
Assessment of Adverse Environmental Impact 

5-20

0

20

40

60

80

100

-100 -80 -60 -40 -20 0 20 40 60 80 100

0

5

10

15

20

Percent Frequency

Percent Difference from Estimate

 Cumulative Percent

Figure 5-8 Results of Monte Carlo Analysis of Uncertainty in Key Input Parameters on 

Estimates of Annual Economic Benefits at MSS
Table 5-1 Species Composition of the Purse Seine Catch in Lake Norman, 1993-2006 

Species Composition (%) 

Species 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 Threadfin shad 100 99.94 99.95 100 99.99 99.95 99.26 87.4 76.47 74.96 82.59 86.55 98.1 Gizzard shad 0 0.06 0.05 0 0.01 0.05 0.26 0.22 0.01 0 0.14 0.24 0 Alewife 0 0 0 0 0 0 0.48 12.37 23.52 25.04 17.27 13.2 1.9 
Assessment of Adverse Environmental Impact 

5-22

Table 5-2 Trophic Guild Assignments for Fish Species in Lake Norman 

Taxa 

Trophic 

Guild Taxa 

Trophic 

Guild 

Longnose gar P White catfish I 

Bowfin P Rainbow trout P 

Alewife F Striped mullet O 

Gizzard shad O Mosquitofish I 

Threadfin shad O Striped bass P 

Bluehead chub O White bass P 

Commom carp O White perch P 

Eastern silvery minnow H Black crappie P 

Fathead minnow O Bluegill I 

Goldfish O Green sunfish I 

Golden shiner O Hybrid black bass P 

Greenfin shiner I Hybrid sunfish I 

Spottail shiner I Largemouth bass P 

Swallowtail shiner I Pumpkinseed I 

Whitefin shiner I Redbreast sunfish I 

Brassy jumprock I Redear sunfish I 

Quillback O Spotted bass P 

Shorthead redhorse I Warmouth P 

Silver redhorse I Swamp darter I 

Blue catfish P Tessellated darter I 

Brown bullhead I Unidentified darter I 

Channel catfish P Walleye P 

Flat bullhead O White crappie P 

Flathead catfish P Yellow perch I 

Snail bullhead I 

Source: NCDENR (2006) 

I = Insectivore 

P = Piscivore 

O = Omnivore 

H = Herbivore 
5-23

Table 5-3 Estimates of Potential Total Annual Equivalent Fishery Yield (lb) from 

Individuals Impinged for Each Target Species at MSS 

Species 

Annual Fishery Yield (lb) 

Actual Flow Maximum Flow 

Threadfin shad 0 0 

Alewife 0 0 

Blue catfish 170 211 

Channel catfish 44 51 

White perch 40 42 

Total 254 304 

Table 5-4 Estimates of Potential Total Annual Production Foregone (lb) from Individuals 

Impinged for Each Target Species at MSS 

Species 

Annual Production Foregone (lb) 

Actual Flow Maximum Flow 

Threadfin shad 1,068 1,433 

Alewife 1,654 1,709 

Blue catfish 1,561 1,933 

Channel catfish 669 775 

White perch 181 187 

Total 5,134 6,037 
Assessment of Adverse Environmental Impact 

5-24

Table 5-5 Estimates of Economic Value of Fish Impinged at MSS 

Species 

Total Impingement Value (2008$) 

Actual Flow Maximum Flow 

Threadfin shada 0 0 

Alewife a 0 0 

Blue catfish 145 179 

Channel catfish 75 87 

White perch 61 64 

Equivalent predatorb 295 347 

Non-target speciesc 2 3 

Total 577 680 

a Economic benefits for threadfin shad and alewife only occur through forage benefits to the equivalent 

predator. 

b

 Equivalent biomass production for all Target Species converted to an equivalent biomass of a recreationally 

important predator (striped bass). 

c

 Economic value associated with all impinged fish species that were not selected as target species.
6-1 

6 

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channel catfish in a navigation pool of the Upper Mississippi River. Transactions of the 

American Fisheries Society. 117:202-208. 

Hubert, W.A. 1999. Biology and management of channel catfish. Pages 3-22 in E.R. Irwin, W.A 

Hubert, C.F. Rabini, H.L. Schramm, Jr., and T.Coon. editors. Catfish 2000: proceedings of the 

international ictalurid symposium. American Fisheries Society Symposium 24, Bethesda, 

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Irons, K.S., T.M. O’Hara, M.A. McClelland, and M.A. Pegg. 2002. White perch occurrence, 

spread, and hybridization in the middle Illinois River, Upper Mississippi River System. 

Transactions of the Illinois State Academy of Science 95(3): 207-214. 

Jacobson, P.M., D.A. Dixon, W.C. Leggett, B.C. Marcy, Jr., and R.R. Massengill. 2004. The 

Connecticut River Ecological Study (1965-1973) revisited: ecology of the lower Connecticut 

River 1973-2003. American Fisheries Society Monograph 9. 

Klauda, R.J., J.B. McLaren, R.E. Schmidt, and W.P. Dey. 1988. Life history of white perch in 

the Hudson River estuary. In: Barnthouse, L.W., R.J. Klauda, D.S. Vaughan, and R.L. Kendall 

(eds.). Science, law, and Hudson River power plants: a case study in environmental impact 

assessment. American Fisheries Society Monograph 4: 69-88. 

Koel, T.M., R.M. Sparks, and R.E. Sparks. 1998. Channel catfish populations in the Upper 

Mississippi River System. Illinois Natural History Survey Reports, September-October 1998. 

Kohler, C.C. and J.J. Ney. 1981. Consequences of an alewife die-off to fish and zooplankton in 

a reservoir. Transactions of the American Fisheries Society 110:360-369. 

Lagler, K.F. 1961. Freshwater fishery biology. William and Brown Company, Dubuque, Iowa. 

Marcy, B.C. and F.P. Richards. 1974. Age and growth of the white perch Morone americana in 

the lower Connecticut River. Transactions of the American Fisheries Society 103:117-

120.McMahon, T.E. and J.W. Terrell. 1982. Habitat suitability index models: channel catfish. 

U.S.D.I. Fish and Wildlife Service. FWS/OBS-82/10.2 29 pp. 

McMahon, T.E. and J.W. Terrell. 1982. Habitat suitability index models: channel catfish. 

U.S.D.I. Fish and Wildlife Service. FWS/OBS-82/10.2 29 pp. 

Nigro, A.A. and J.J. Ney. 1982. Reproduction and early-life history accommodations of 

landlocked alewives in a southern range extension. Transactions of the American Fisheries 

Society 111:559-569. 

North Carolina Department of Environment and Natural Resources (NCDENR). 2003. 

Basinwide assessment report, Catawba River Basin. Division of Water Quality, Water Quality 

Section, Environmental Sciences Branch. June 2003. 
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North Carolina Department of Environment and Natural Resources (NCDENR). 2006. Standard 

operating procedure. Biological monitoring. Stream Fish Community Assessment Program. 

Division of Water Quality, Environmental Sciences Section, Biological Assessment Unit. 1 

August 2006. 

North Carolina Department of Environment and Natural Resources (NCDENR). 2008. Lake & 

reservoir assessments, Catawba River Basin. Intensive Survey Unit, Division of Water 

Quality. 5 June 2008. 

North Carolina Department of Environment and Natural Resources (NCDENR) and North 

Carolina Wildlife Resources Commission (NCWRC). 2001. Catawba River Basin Natural 

Resources Plan. October 2001. 

North Carolina Wildlife Federation (NCWF). 2008. 

http://www.ncwildlifefederation.org/home/lakenormanfishery.htm

O’Gorman, R., O.E. Johannsson, and C.P. Schneider. 1997. Age and growth of alewives in the 

changing pelagia of Lake Ontario, 1978-1992. Transactions of the American Fisheries Society 

126:112-126.Pardue, G.B. 1983. Habitat suitability index models: alewife and blueback 

herring. U.S. Fish and Wildlife Service, Washington, D.C. FWS/OBS-82/10.58. 

Pardue, G.B. 1983. Habitat suitability index models: alewife and blueback herring. Fish and 

Wildlife Seervice, U.S. Department of the Interior. Washington, D.C. FWS/OBS-82/10.58. 

September 1983. 

Pflieger, W.L. 1997. The Fishes of Missouri. Missouri Department of Conservation, Jefferson 

City, MO. 372 pp. 

Pierce, C.L., C.S. Guy, M.A. Pegg, and P.J. Braaten. 2003. Fish growth, mortality, recruitment, 

condition and size structure. Volume 4. Population structure and habitat use of benthic fishes 

along the Missouri and Yellowstone rivers. U.S. Geological Survey, Cooperative Research 

Units, Iowa State University. Science Hall II, Ames, Iowa. 

Pitlo, J., K. Brummet, and M. Endris. 2004. Channel catfish (Ictalurus punctatus). In: Pitlo, J.M. 

Jr. and J.L. Rasmussen (eds.). UMRRC Fisheries Compendium. 3rd Edition. Upper 

Mississippi River Conservation Committee. Rock Island, Illinois. January 2004. p. 151-155. 

Prout, M.W., E.L. Mills, and J.L. Forney. 1990. Diet, growth, and potential competitive 

interactions between age-0 white perch and yellow perch in Oneida Lake, New York. 

Transactions of the American Fisheries Society 119:966-975. 

Ricker, W.E., ed. 1975. Computation and Interpretation of Biological Statistics of Fish 

Populations. Bulletin 191. Department of the Environment, Fisheries and Marine Service, 

Ottawa. 382 pp. 
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Rohde, F.C., R.G. Arndt, D.L. Lindquist, and J.F. Parnell. 1994. Freshwater fishes of the 

Carolinas, Virginia, Maryland, & Delaware. The University of North Carolina Press. Chapel 

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Agriculture, Forest Service, Rocky Mountain Research Station. 59p. 

(www.fs.fed.us/rm/pubs/rmrs_gtr72.html). 

Ruelle, R., W. Lorenzen, and J. Oliver. 1977. Population dynamics of young-of-the-year fish in 

a reservoir receiving heated effluent. Proceedings of the Conference on Assessing the Effects 

of Power-Plant Mortality on Fish Populations. Reprinted in Duke 1977. Oconee Nuclear 

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requirements of coastal fishes and invertebrates (North Atlantic). U.S. Department of the 

Interior, Fish and Wildlife Service, Washington, DC FWS/OBS-82/11.7. October 1983. 

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United States Environmental Protection Agency (EPA). 1977. Guidance for Evaluating the 

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Wildlife Agencies 2001:316-323. 
A-1 

A 

APPENDIX A: METHODS TO ESTIMATE 

EQUIVALENT LOSS AND ECONOMIC VALUE OF 

FISH IMPINGED AT MARSHALL 

Introduction 

This appendix describes the methods used to develop estimates of annual equivalent loss 

and economic value of fish impinged at the Marshall Steam Station (MSS). For this 

assessment, annual impingement estimates were developed using recent impingement 

monitoring data collected at MSS over the period 6 April 2006 – 30 March 2007 (Section 

4). 

Estimates of Target Species Equivalent Losses 

In order to calculate economic value, estimates of annual impingement loss at MSS for 

the five target species (threadfin shad, alewife, blue catfish, channel catfish and white 

perch) must be converted to equivalent measures that can be assigned economic value for 

these species. For species of commercial and/or recreational importance, the equivalent 

measure selected was the yield to the fishery that would have been expected had the 

individuals not been impinged. For species that serve as forage for other, generally 

larger, aquatic organisms, the measure selected was the production of biomass available 

as food for higher trophic levels that would have been expected had the individuals not 

been impinged. Methods used in this report to calculate these two measures are 

consistent with those used by EPA in the Phase II rulemaking and are described below. 

The measure “yield to the fishery” is defined as the annual total yield (in weight) that 

could have occurred in the commercial or recreational fishery from those individuals lost 

to impingement in the absence of compensatory changes in total mortality. This yield is 

calculated using the Equivalent Yield Model (EYM), which integrates Baranov’s catch 

equation (Ricker 1975) with estimates of the mean weight by age (Dey 2002, EPRI 

2004). This method is conservative in that potential density-dependent changes in 

mortality or growth rates that often occur in natural populations were not included. 

Using the EYM, fishery yield equivalent to the annual impingement loss at MSS for each 

target species was estimated as follows: 

    

 



 



 

f l n

i i

i i

i i

n

j

j j i Z

V F EY NL S AW

1 1
Appendix A: Methods to estimate equivalent loss and economic value of fish impinged at Marshall 

A-2 

where: 

EY = Equivalent yield to fishery 

NLj = Number of each age or life stage category (j) lost annually to impingement 

at MSS 

Sj→i = Total survival from age or life stage category (j) to age (i) 

nj = Number of age or life stage categories (j) impinged at MSS 

Vi = Vulnerability of age (i) to fishing 

Fi = Instantaneous fishing mortality rate for age (i) 

Zi = Instantaneous total mortality rate for age (i) 

Ai = Total mortality rate for age (i) = 1-e-Zi 

Wi = Average weight for individual of age (i) captured in the fishery 

nf = Maximum number of ages (i) vulnerable to fishery. 

The EYM results in an estimate of yield defined in the same units used to describe the 

average weight of the individuals and integrates yield across all ages. In this assessment, 

the EYM was not applied to threadfin shad or alewife as neither supports any direct 

commercial or recreational fishing. 

The measure of biomass production that could have resulted from individuals impinged at 

MSS was calculated using the Production Foregone Model (PFM) (Dey 2002, EPRI 

2004). As with the EYM, this method is also conservative in that potential densitydependent changes in mortality or growth rates that often occur in natural populations 

were not included. Using the PFM, potential biomass production from individuals 

impinged was estimated for each of the target species as follows: 

 

     

i i

G Z

i i

n

j

j j i L

i

i G Z

NL S G W e

P

i i

l



















1

1

1

and the total production foregone (P) can be found by summing over all the age 

categories that are impinged: 

 



m

i

P Pi

1
A-3 

where: 

P = Total production foregone 

Pi = Production foregone for individuals impinged at MSS in age category (i) 

Gi = Instantaneous growth rate in weight for age category (i) 

NLj = Number of each age category (j) lost to impingement at MSS 

Sj→i = Total survival from age category (j) to age (i) 

nj = Number of age categories (j) impinged at MSS 

Wi = Average weight of individuals in age category (i) 

Zi = Average instantaneous mortality rate for age category (i) 

m = Total number of age categories impinged at MSS 

L = Final age category. 

The PFM was applied to all target species that as individuals serve as food for other 

aquatic organisms during at least part of their life cycle. 

Both the EYM and PFM are consistent with the benefits calculation procedures used by 

EPA in their regional analysis of the benefits of the Phase II Rule (EPA 2004b). 

Estimation of the biological input parameters for each of the target species is described 

below. 

Assignment of Age Categories for Impinged Individuals 

One of the necessities of equivalent loss calculation is that the direct measures of 

impingement mortality must be assigned to individual age categories as defined in the 

production foregone and equivalent yield models. In this assessment, age was assigned 

using length information for each target species obtained from the current impingement 

monitoring together with estimates of maximum length at age for these same species 

obtained from the scientific literature and from an analysis of the length-frequency 

patterns for each species. 

Life Table Information 

Biological input parameters for the Production Foregone and Equivalent Yield models 

include life stage durations, instantaneous natural and fishing mortality rates, and the 

fraction vulnerable to the fishery for each life stage and age, as well as mean weights at 
Appendix A: Methods to estimate equivalent loss and economic value of fish impinged at Marshall 

A-4 

the beginning of each life stage and age for each target species. Each of these model 

inputs were determined as described below. 

Life Stage Durations 

Estimates of impingement were developed for this assessment on a monthly basis. 

Consequently, the duration of each month was set as 30.4 days, the average monthly 

duration across the entire year. However, it is important to recognize that fish do not 

become vulnerable to impingement until they are approximately 25 mm long and 

typically 1 to 2 months of age. Hence, the number of monthly increments in the first year 

of life is less than 12. Spawning months and months at which each species first became 

vulnerable to impingement at MSS were assigned as follows: 

Species 

Peak Spawning 

Month 

Average Month 

When Individuals 

Reach 25 mm TL 

Threadfin shad Jun Jul 

Alewife Jun Jul 

Blue catfish Jul Aug 

Channel catfish Jul Aug 

White perch May Jul 

Natural Mortality Rates 

A range (maximum, most probable, and minimum) of instantaneous natural mortality 

rates for each target species were obtained from the following sources: 

Threadfin shad – Daily instantaneous natural mortality rates were obtained from EPRI 

(2005) Table 4-43. For age 0, the most probable value was assumed to be the midpoint 

between the maximum and minimum values listed. For ages 1 and 2, maximum and 

minimum values were assumed to be 25 percent higher and 25 percent lower than the 

most probable value, respectively. 

Alewife – Most probable daily instantaneous natural mortality rates were obtained from 

EPRI (2005) Table 4-24. For all ages, maximum and minimum values were assumed to 

be 25 percent higher and 25 percent lower than the most probable value, respectively. 

Blue catfish – The maximum, most probable and minimum estimates of the daily 

instantaneous natural mortality rates were assumed to be the same as that for the closely 

related channel catfish, discussed below. 

Channel catfish – The maximum, most probable and minimum estimates of the daily 

instantaneous natural mortality rates for all ages were obtained from EPRI (2005) Table 

4-25. 
A-5 

White perch – The maximum and most probable estimates of the daily instantaneous 

natural mortality rates for Age 0 were based on survival rates provided in EPRI (2005) 

Table 4-12. The minimum value was assumed to be the same distance less than the most 

probable value as the maximum was higher. The most probable estimates for all older 

ages were based on survival rates provided in EPRI (2005) Table 4-12. The maximum 

and minimum values were assumed to be 25 percent higher and lower than the most 

probable value, respectively. 

These most probable values were used to provide the best estimates of equivalent loss, 

while the maximum and minimum values were considered in the uncertainty analysis. 

Fishing Mortality Rates 

A range (maximum, most probable, and minimum) of instantaneous fishing mortality 

rates for the three species that directly support fishing were selected as follows: 

Blue and channel catfish – The maximum, most probable, and minimum estimates of 

annual fishing mortality were set to 0.15, 0.10, and 0.05, respectively. These annual 

exploitation rates were then converted to daily instantaneous fishing mortality rates 

required for this analysis. 

White perch –The maximum, most probable, and minimum estimates of annual fishing 

mortality were set to 0.20, 0.15, and 0.10, respectively. These annual exploitation rates 

were then converted to daily instantaneous fishing mortality rates required for this 

analysis. 

As with natural mortality, the most probable values were used to provide the best 

estimates of equivalent loss, while the maximum and minimum values were considered in 

the uncertainty analysis. Since there is little or no harvest of either threadfin shad or 

alewife in Lake Norman, fishing mortality rates for both of these species were set to 0. 

Fishing Vulnerability Rates 

Fishing vulnerability rates refer to the fraction of each age at a size vulnerable to be 

harvested by anglers. For the fishing vulnerability rates used in this assessment, 

individuals were assumed to be not vulnerable (rate = 0) up to a set age and completely 

vulnerable (rate = 1) above that age. The maximum and minimum ages of complete 

vulnerability were estimated using best professional judgment based on length at age 

information from the scientific literature. Resulting estimates are as follows: 
Appendix A: Methods to estimate equivalent loss and economic value of fish impinged at Marshall 

A-6 

Species 

Youngest Age of 

Vulnerability 

Oldest Age of 

Vulnerability 

Blue catfish 4 5 

Channel catfish 5 6 

White perch 3 4 

For the most probable vulnerability rates, half of the individuals were assumed to be 

vulnerable at the youngest age while the remaining half did not become vulnerable until 

the oldest age. 

The most probable values were used to provide the best estimates of equivalent loss, 

while the maximum and minimum values were considered in the uncertainty analysis. 

Since there is little or no harvest of either threadfin shad or alewife in Lake Norman, 

fishing vulnerability rates for both of these species were set to 0. 

Weight at Beginning of Age 

A range (maximum, most probable, and minimum) of estimated weights at the beginning 

of each age were obtained for each target species from the following sources: 

Threadfin shad – The most probable mean weight at the time of first impingement was 

assumed to be 1 g each while those for subsequent ages were obtained from Thompson et 

al. (2005). The maximum and minimum weights were assumed to be 20 percent higher 

and 20 percent lower than the most probable value, respectively. 

Alewife – The most probable mean weight at the time of first impingement was assumed 

to be 1 g each while those for subsequent ages were based on length-at-age data from 

O’Gorman et al. (1997) coupled with a length-weight relationship from Thompson et al. 

(2005). The maximum and minimum weights were assumed to be 20 percent higher and 

20 percent lower than the most probable value, respectively. 

Blue catfish – The most probable mean weight at the time of first impingement was 

assumed to be 1 g each and the maximum and minimum mean weights were assumed to 

be 20 percent higher and lower, respectively, than the most probable values. The 

maximum and minimum weights at age for all other ages were based on mean lengths at 

age for ages < 10 from Grist (2002) and for ages 11 – 18 from Graham (1999). 

Maximum mean weights were calculated from these lengths using a length-weight 

relationship from Graham (1999) while the minimum mean weights were calculated from 

these lengths using a length-weight relationship from unpublished data for the Missouri 

River. The most probable mean weights were assumed to be at the mid-point between 

the maximum and minimum values. 

Channel catfish – The most probable mean weight at the time of first impingement was 

assumed to be 1 g each while for subsequent ages mean weights were based on mean 

lengths at age provided by Pierce et al. (2003) for the Missouri River and a length-weight 
A-7 

relationship for Georgia rivers obtained from www.fishbase.org. For all ages, the 

maximum and minimum mean weights were assumed to be 20 percent higher and lower, 

respectively, than the most probable values. 

White perch – The most probable mean weight at the time of first impingement was 

assumed to be 1 g each while those for subsequent ages were obtained from Marcy and 

Richards (1974). The maximum and minimum weights were assumed to be 20 percent 

higher and 20 percent lower than the most probable value, respectively. 

For all species, weights at the beginning of each month within an age were interpolated 

using an instantaneous growth rate based on the weights at the beginning and end of that 

age. The most probable values were used to provide the best estimates of equivalent loss, 

while the maximum and minimum values were considered in the uncertainty analysis. 

Economic Valuation Methods 

The potential economic values of impingement at MSS were calculated for each of the 

benefit categories described using standard economic concepts outlined in EPA (2004b, 

Part A: Evaluation Methods, Chapter A9: Benefit Categories and Valuation). Methods 

used to estimate values associated with each of the four benefit categories (market direct 

use benefits, nonmarket direct use benefits, indirect use benefits, and nonuse benefits) are 

discussed below. 

Direct Use Values 

Direct use values accrue to individuals who directly use the aquatic resources affected, in 

other words, recreational anglers. The economic value of this benefit category is then 

equivalent to the economic value of the harvest by anglers that would have resulted from 

the fish that were impinged. The biological benefit values were defined in terms of 

pounds to be consistent with reported fishery economic statistics. Using these estimates 

of increased fishery harvest, the economic value of the direct use benefit category was 

estimated for each sub-component (market and nonmarket) as follows. 

Market Values (Commercial Fishing) 

Market benefits refer to economic benefits that can be directly measured from data 

obtained in the marketplace. Changes in the magnitude of commercial fish and shellfish 

harvests are the principal market benefits relevant to §316(b) regulations. Since 

impingement losses at cooling water intake structures had the potential to increase stock 

size, and hence commercial harvests, positive market benefits could potentially accrue 

had they not been impinged. At present, commercial fishing on Lake Norman appears to 

be non-existent. Hence for this assessment, the market value of any increase in 

commercial fishing was assumed to be essentially non-existent and no further estimation 

of this category was required. 
Appendix A: Methods to estimate equivalent loss and economic value of fish impinged at Marshall 

A-8 

Non-Market Direct Use Values (Recreational Fishing) 

This category includes benefits derived through use of the resource that are not reflected 

in the market for the resource. Relative to §316(b) regulations, the most common benefit 

that could accrue from impinged fish would be through increases in recreational fishing 

opportunities. Increased abundance of adult fish that could result had impingement 

losses not occurred can lead to increased catch rates for individual anglers as well as an 

increase in the number of fishing trips by anglers. 

Unfortunately, benefits from increased recreational use of the resource are not directly 

reflected in the primary market, i.e., the market for recreational fishing. However, EPA 

concluded that there is considerable literature to support valuing this benefit through 

estimation of an angler’s “willingness to pay” for recreational opportunities. Thus, the 

non-market direct use value for additional recreational catch can be defined as the 

increase in the total “willingness to pay” across all anglers resulting from any greater 

recreational opportunities due to reduction in impingement losses. The recreational 

values used represent the marginal benefit per unit change in recreational catch. 

For this assessment, total “willingness to pay” for each target species was calculated by 

multiplying the estimated biological benefits (pounds) of impinged fish by the expected 

value per pound recreational fishermen were willing to pay for that increased harvest: 

 RFBS  BBRS  RPS

where: 

RFBS = Economic benefits ($) to recreational fishing for each target species (S) 

BBRS = Biological benefits (lbs) to recreational fishing for each target species (S) 

RPS = Value per pound recreational anglers are willing to pay for the increase in 

harvest of the target species (S). 

The biological benefits to the recreational fishery for each target species estimated for 

MSS (Table 5-2) are likely to be a very small fraction of the estimated recreational catch 

of comparable species in Lake Norman in recent years. Hence, there is no reason to 

expect that changes in recreational fishing harvest that could result had the fish not been 

impinged at MSS would be sufficiently large so as to affect the price the anglers are 

willing to pay. Therefore, the values that recreational anglers are currently willing to pay 

for each species should be a reasonable measure of the values they would be willing to 

pay assuming impingement was eliminated. This is the same assumption used by EPA in 

calculating the national benefits of the Phase II Rule (EPA 2004b). 

For this assessment, we used the same recreational values per fish as used by the EPA in 

the Phase II Rule (EPA 2004c). The EPA values (2002$) were then adjusted to reflect 

current (2008$) yielding: 
A-9 

 

Species Group

Recreational Value (2008 $/fish) 

Maximum Most Probable10 Minimum 

Panfish 5.51 2.55 0.77

Perch 0.52 0.46 0.40

Following the EPA approach, values for species group “panfish” were assumed to apply 

to blue and channel catfish whereas the values for species group “perch” were assumed to 

apply to white perch. 

To convert these values to a weight basis to be consistent with the biological benefits 

calculation, each of these recreational values were divided by an estimated average 

weight per harvested fish using the life table information described in Section 3. The 

resulting average weights per fish that directly support recreational fishing are as follows: 

 

Species 

Average Weight per Fish Harvested (lb/fish) 

Maximum Most Probable Minimum 

Blue catfish 4.00 3.00 2.00 

Channel catfish 2.00 1.50 1.00 

White perch 0.40 0.30 0.20 

The resulting values ($) per pound of fish for each target species are as follows: 

 

Species 

Recreational Value (2008$/lb) 

Maximum Most Probable Minimum 

Blue catfish 2.76 0.85 0.19 

Channel catfish 5.51 1.70 0.39 

White perch 2.60 1.53 1.00 

The most probable values were used to provide the best estimates of equivalent loss while 

the maximum and minimum values were considered in the uncertainty analysis. 

Indirect Use Benefits 

This category includes benefits that accrue to humans from the use of the resource 

indirectly. Relative to §316(b) regulations, the indirect benefit that could result from fish 

that were not impinged would be through an increased consumption by higher trophic 

levels of production that result from these organisms. This increased consumption could 

result in greater growth and survival rates among fish in these higher trophic levels, and 

 

10 Used EPA’s mean value as most probable. 
Appendix A: Methods to estimate equivalent loss and economic value of fish impinged at Marshall 

A-10 

hence, increase fishing opportunities. Production consumed by higher trophic levels 

results from pelagic forage species, such as threadfin shad and alewife, as well as 

individuals of recreationally important species that die of natural causes. 

Unfortunately, there are no generally accepted methods to assign a value directly to this 

benefit. Instead, the value of this benefit is assigned indirectly by quantifying the amount 

of commercially and/or recreationally important species that could be supported by the 

production potentially generated by these impinged organisms, which were subsequently 

harvested by man. Hence, the value of the production equivalent to the fish impinged is 

equal to the value of commercial and/or recreational harvest that could be supported by 

that production. 

The value of the indirect benefits of impinged fish at MSS uses the following three-step 

process. 

Step 1 – Estimation of Total Biomass of Higher Trophic Levels Supportable by the 

Annual Productivity Equivalent to the Impingement. The total biomass of higher trophic 

levels supportable is calculated by using a trophic transfer method as follows: 

 HTB   PFB TTC

S

  S  

5

1

where: 

HTB = Total annual higher trophic level biomass (pounds) supported by production 

foregone attributable to impingement 

PFBS = Biomass production foregone for each target species (S) attributable to 

impingement 

TTC = Trophic transfer coefficient. 

This approach is identical to that used by EPA in the Section 316(b) Final Phase II Rule 

(EPA 2004b). 

For this assessment, production foregone estimates equivalent to impingement at MSS for 

each of the target species were calculated using the Production Foregone Model as 

described above and listed in Table 5-3. These estimates of production foregone were 

based on annual impingement estimates from MSS. A trophic transfer coefficient of 10 

percent, consistent with EPA (2004b), was then used to convert the total biomass forgone 

to an amount of higher trophic level biomass supportable by that production foregone. 

The coefficient means that an average of 10 percent of the production foregone would 

have ended up as predator biomass. For the uncertainty analysis, a range for the trophic 

transfer coefficient of 5 to 15 percent was assumed. 
A-11 

Step 2 – Estimation of Equivalent Predator Harvest Supported by Annual Productivity 

Equivalent to the Reduction in Impingement. As previously noted, the value of indirect 

benefits is determined by the value of the commercial and/or recreational harvest 

supported by the increased productivity. Hence, the total higher trophic level biomass 

estimated under Step 1 needs to be converted to an estimate of actual yield to the fishery 

expressed as equivalent predator harvest. This is accomplished by assuming that the total 

higher level biomass is converted to biomass of a popular commercial or recreational 

species and then multiplying the value by the annual fishery exploitation rate for that 

species as follows: 

EPY  HTB  EREP

where: 

EPY = Equivalent predatory yield (pounds) attributable to impingement compliance 

HTB = Higher trophic level biomass (pounds) attributable to impingement compliance 

EREP = Fishery exploitation rate for the selected equivalent predator. 

For this assessment, striped bass was selected as the equivalent predator endpoint upon 

which the estimate of indirect value is based. In reality, any potential production increase 

of course would be transferred among many species, including some with little or no 

recreational or commercial importance. However, this species is a popular target of 

recreational anglers in Lake Norman. Hence, this species should be a reasonably 

representative species for assigning recreational importance, and hence value, for this 

assessment. Using the life table information, the annual exploitation rate for striped bass 

was estimated to be 20 percent. For the uncertainty analysis, a range of 10 to 30 percent 

was assumed. We believe that the selection of the striped bass as the equivalent predator 

provides a reasonably conservative estimate of indirect-use value, since the “willingness 

to pay” (WTP) estimates for this species is at the upper boundary of the range of WTP 

values for other species addressed in this assessment. 

Step 3 – Estimation of Indirect Use Economic Benefit. Estimates of the economic value 

of the increased recreational harvest resulting from increases in forage biomass were 

calculated using the methods previously described for direct-use non-market benefits. 

For this assessment, the value per fish for striped bass (the equivalent predator) was 

based on the values per fish assigned to this species in EPA (2004c). These were then 

converted to a value per pound assuming an average weight per harvest striped bass of 5 

lb each as follows: 

 Maximum Most Probable Minimum 

Value per fish 

(EPA 2004c) 19.48 14.33 3.92 

Value per pound 3.90 2.87 0.78 
Appendix A: Methods to estimate equivalent loss and economic value of fish impinged at Marshall 

A-12 

Non-Use Benefits 

As previously discussed, this category, also known as passive use values, includes all 

benefits above and beyond any accrued through use of the resource. Most commonly 

cited non-use benefits include bequest and existence values. EPA acknowledges that 

these benefits can be estimated best by using contingent valuation methods on a sitespecific basis. However, they concluded that such studies are unlikely to be conducted 

for specific facilities and were clearly beyond the scope and budget of EPA for 

development of the §316(b) regulations (EPA 2004a). 

In the final rule, EPA provided that "In cases where an impingement mortality and 

entrainment characterization study does not identify substantial harm to a threatened or 

endangered species, to the sustainability of populations of important species of fish, 

shellfish, or wildlife, or to the maintenance of community structure and function in a 

facility's waterbody or watershed, monetization [of non-use benefits] is not necessary." 

[69 Fed. Reg. Vol. 131 (9 July 2004) page 41648]. However, they do require a 

qualitative discussion of these benefits if they are believed to exist. 

The impingement monitoring studies provide no evidence that impingement at MSS is 

causing “substantial harm” to threatened and endangered species or to aquatic 

populations and communities of the Lake Norman. Therefore, consistent with the EPA 

approach for the Section 316(b) Phase II Rule, non-use values were not included in this 

economic valuation. 

Adjustment for Other Species 

The total annual economic benefits for the target species is the sum of the annual 

economic benefits for all benefit categories described above. These target species, 

however, do not account for all fish impinged at MSS but only 99.6 percent. The 

following equation was used to account for the benefits resulting from reductions in 

impingement for the remaining 0.4 percent of fish impinged at MSS: 



  



 



   4

1

1

0.996

1

TS

Bennon TS BenTS

where: 

Bennon-TS = Economic benefits accruing to non-target species 

BenTS = Economic benefits accruing to four target species and the 

equivalent predator. 

The total annual economic value of fish impinged at MSS is the sum of the benefits to the 

target species plus the benefit to the equivalent predator and the benefit to non-target 

species. 
A-13 

Uncertainty Analysis 

EPA’s Phase II §316(b) Rule requires that a facility seeking a site-specific determination 

based on benefits valuation also submit “an analysis of the effects of significant sources 

of uncertainty on the results of the study” [§125.95(b)(6)(iii)(C)]. Uncertainty refers to 

the lack of knowledge about measures and components that go into each element of the 

benefits valuation procedures. Under Phase II, the purpose of the uncertainty analysis is 

to make transparent all the underlying sources of uncertainty in the calculation of 

economic value such that the appropriate regulatory authority can independently 

determine whether the results have sufficient precision and accuracy to meet regulatory 

needs (EPA 2004b) and form sufficient basis for sound regulatory decisions. The 

purpose of this section is to evaluate the effects of uncertainty in key input parameters on 

the estimates of economic value for MSS. 

Uncertainty is not unique to the calculation of economic value under Phase II §316(b). 

Some uncertainty exists in almost all predictions of future conditions based on past or 

existing information and this is especially true when dealing with environmental science 

and economic issues. In both environmental science and economics, inherent variability 

and limited understanding of underlying processes, coupled with difficulty in making 

accurate measurements of underlying parameters, makes consideration of uncertainty in 

these cases especially important. Uncertainty analysis is becoming an increasingly 

important part of cost-benefit assessments as they play a greater role in the environmental 

regulatory process. 

EPA (2000) identified the following minimum requirements applicable to most 

uncertainty analyses related to environmental regulations: 

 To present the outcomes or conclusions based on expected or most plausible values; 

 To provide descriptions of all known key assumptions, biases, and omissions; 

 To perform sensitivity analysis on key assumptions; and 

 To justify the assumptions used in the sensitivity analysis. 

Uncertainty arises in assessments from three general sources: natural variation, 

uncertainty in model structure, and uncertainty in model parameters. Natural variation 

results from natural differences across elements within a population or in a population 

across time. For example, not all members of the United States population are expected 

to value increased recreational fishing opportunities to the same degree. Alternatively, 

wide year-to-year differences in entrainment and impingement densities at the same 

station are common. These differences in abundance can result in large differences in the 

annual economic value. Both of these examples can yield uncertainty in the total 

economic value estimated under §316(b). 

Uncertainty related to model structure arises from the lack of knowledge as to what is the 

most appropriate form of the model to accurately describe the process being modeled. 

For example, the form of the relationship between the number of trips and travel cost in a 
Appendix A: Methods to estimate equivalent loss and economic value of fish impinged at Marshall 

A-14 

simple travel cost model has been assumed to be linear, semi-log, or log by various 

analysts (Rosenberger and Loomis 2001). However, the most appropriate form is still a 

matter of debate. Likewise, alternatives that reduce total cooling water flow can result in 

reduced impingement. Most commonly, an assumption of a linear relationship with 

cooling water volume is made. However, this assumption also remains an area of debate. 

Finally, uncertainty in model parameters can result from difficulty in measurement or in 

inherent variability in the model parameter. This is likely to be the most frequently 

encountered source of uncertainty in economic valuation under the Phase II Rule. 

Examples of this source of uncertainty include estimates of life stage-specific life table 

parameters for equivalent loss estimation and measurements of angler’s “willingness to 

pay” obtained from surveys. 

In this section we address three broad areas of uncertainty: parameters used to estimate 

direct and indirect use benefits, questions regarding the use and magnitude of nonuse 

benefits, and the economic factors used to estimate the lifetime benefits, focusing 

primarily on uncertainty in model parameters. Each of these areas is discussed below. 

Inputs to Direct and Indirect Use Benefits 

Key input parameters addressed in this section include: 

 Natural mortality rate, 

 Fishing mortality rate, 

 Age of recruitment to the fishery, 

 Mean weight at beginning of each stage, 

 Trophic conversion factor, 

 Exploitation rate for equivalent predator, and 

 Recreational value per pound. 

Two other key input parameters, impingement rates and age composition, were not 

addressed in this uncertainty analysis as reliable information on the uncertainty was 

unavailable from the historical studies. 

Uncertainty for this assessment was addressed by two means. First, a sensitivity analysis 

was conducted on individual input parameters. Second, a Monte Carlo analysis was 

conducted to determine the likely overall uncertainty in the estimates of annual economic 

value resulting from the current levels of uncertainty. The results of each of these 

analyses are provided below. 
A-15 

Sensitivity Analysis 

A sensitivity analysis was conducted individually on each of the seven input parameters 

listed above. Multiple calculations of annual equivalent loss and economic value were 

made using the extreme values (i.e., maximum and minimum) for each parameter while 

holding all other parameters constant at their most probable values. The purpose of this 

sensitivity analysis was to determine the parameters for which the current levels of 

uncertainty have the greatest effect on each of the estimates. 

Monte Carlo Analysis 

Monte Carlo analysis was used to assess the overall uncertainty in the estimates of total 

annual equivalent loss and economic benefits based on the current levels of uncertainty in 

each of the seven input parameters. For each of these parameters, random values were 

selected from a triangular distribution, wherein the maximum and minimum values for 

the distribution were set to the maximum and minimum values for each parameter 

described earlier and the mode of the distribution was set to the mid-point used as the 

best estimate for each parameter. Values for each parameter were randomly selected 

separately for each species and life stage and the Monte Carlo analysis was run using 

1,000 iterations to define the resulting frequency distribution in annual estimates of 

equivalent loss measures and economic benefit.