McGuire Nuclear Station 

2006-2007 Impingement Study and Assessment of 

Adverse Environmental Impact 

Final Report, 2010 

Duke Energy 

P.O. Box 1006 

Charlotte, NC 28201-1006 
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 Power Corporation. 

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

following manner: 

McGuire Nuclear Station: 2006-2007 Impingement Study and Assessment of Adverse 

Environmental Impact, EPRI, Palo Alto, CA, 2010. 

v 

REPORT SUMMARY 

This report has been prepared for Duke Energy’s McGuire Nuclear Station to support Clean 

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

2000-2002 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Norman, from which the cooling water is withdrawn, and its fish community.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 

McGuire Nuclear Station (MNS) is located in Huntersville, Mecklenburg County, North 

Carolina on Lake Norman, an impoundment of the Catawba River formed by the Cowans Ford 

Dam. MNS has two nuclear reactor units, each with a net generation capacity of 1,129 MW for a 

total rating of 2,258 MW. MNS uses once-through cooling for its steam condensers. Unit 1 

began commercial operation in December 1981 and Unit 2 began in March 1984. Both units are 

operated as base-loaded and together had a capacity utilization of 94.5 percent for 2000 through 

2007. 

Estimates of numbers of fish impinged were obtained from monitoring the MNS traveling 

screens from 3 December 2000 through 29 November 2002. On 3 December 2000, all screens 

were washed of all fish and debris. Thereafter, the screens were allowed to accumulate impinged 

fish for one week before the screens were washed again. At the end of each week, all debris and 

fish from each screen were collected in coated-wire baskets (5-mm x 10-mm mesh openings) 

placed in the screen-wash sluiceway. These weekly collections would represent a complete 

census of impingement as long as there were no losses of impinged fish during the weekly 

intervals between screen rotations due to decay, predation, or other reasons. Fish losses to such 

sources would result in an underestimation of the impingement rate. 

To address this concern, a special study was conducted to estimate the loss rate of impinged fish 

during a weekly collection interval and to adjust impingement numbers to account for these 

losses. Over a seven-day period (21-27 July 2005), a total of 1,600 threadfin shad or alewives 

were fin-clipped and introduced onto all 16 freshly washed upper level intake screens at periodic 

intervals. Treatments consisted of the release of four lots of 400 marked fish each at intervals of 

seven, five, three, and one day prior to screen washing on 27 July. A set of four screens was 

randomly chosen for each of the four release dates (100 fish per screen), such that all 16 screens 

were tested once. At the end of this 7-day period, all screens were washed and all marked fish 

were collected. 
vi 

Impingement Sampling Results 

A total of 1,690 fish were impinged and collected in the first year of sampling, December 2000 – 

November 2001, and 698 fish were impinged and collected in the second year, December 2001 – 

November 2002. Over this two-year period, 23 fish species were identified in the impingement 

collections. Threadfin shad numerically dominated in 2000-2001, comprising 51.5 percent of the 

total, and alewife dominated in 2001-2002, comprising 23.8 percent. Other commonly impinged 

species (in decreasing order for both years combined) included white perch, bluegill, striped 

bass, white bass, channel catfish, and gizzard shad. Impingement was highest for all fish species 

combined in mid-winter (January) for the 2000-2001 and late summer (August) for the 2001-

2002. 

The impingement totals for 2000-2001 and 2001-2002 were adjusted for fish losses that might 

have occurred during the 1-week collection period, based on the results of the special study 

conducted in July 2005 using releases of marked fish in front of the traveling screens. A total of 

243 of the 1,600 fin-clipped fish released during the study were recovered at the screen washing 

on the seventh day. Releases of 400 marked fish on each of the four release dates resulted in 98 

being recovered after one day on the screens, 93 after three days, 36 after five days, and 16 after 

seven days. These data showed a steady decline in the number of recovered fish through time, 

indicating a cumulative loss of impinged fish due to decay, predation, or other reasons. 

Depending on the assumptions made, the adjusted total annual impingement would be 2,726 to 

2,996 fish for 2000-2001 and 1,126 to 1,238 fish for 2001-2002. These estimates would be 

conservative because the special study was conducted at warm temperatures conducive to rapid 

decomposition, the test species were comparatively fragile clupeids, and the adjustment factors 

were applied to all species (including durable ones) and all seasons, including cold months when 

clupeids were primarily impinged. 

AEI Assessment Results

The potential for the estimated levels of impingement at MNS 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 MNS cooling water intake structure 

on Lake Norman. 
vii 

There was no evidence that impingement at MNS 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: 

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 two species, removal rates 

amounted to less than 1 fish per day. 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 0.01 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 MNS 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 MNS cooling water intake and 

impingement losses. 

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

survived amount to only 111 to 168 lb (50.5 - 76.4 kg) per year. That amount translates 

to one-half pound per day or less. 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 MNS amounted 

to just over 111 - 152 lb (50.5 - 69.1 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 0.2 ounces per surface acre (37 g/ha) across 

the entire reservoir. 

7. The best estimate of the total economic value of fish impinged at MNS was $281 - 431 

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 
viii 

Fisheries 

Environmental impact assessment 
ix 

CONTENTS 

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

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

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

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

McGuire Nuclear Station ............................................................................................................ 2-1

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

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

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 ........................................................................................................................... 3-3

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

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

White Perch ........................................................................................................................... 3-6

Bluegill ................................................................................................................................... 3-7

Striped Bass .......................................................................................................................... 3-7

White Bass ............................................................................................................................ 3-8

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

Blue Catfish ........................................................................................................................... 3-9

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

Impingement Monitoring Program .............................................................................................. 4-1

Results ....................................................................................................................................... 4-2

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

VARIABLES AFFECTING IMPINGEMENT RATES .............................................................. 4-6
x 

Environmental Variables ................................................................................................... 4-6

Species-Specific Abundance and Spatial Distribution ...................................................... 4-8

Annual Losses ............................................................................................................................ 4-8

Annual Variability ................................................................................................................... 4-9

Intake Operation ............................................................................................................... 4-9

Environmental Variables ................................................................................................... 4-9

Fish Abundance ................................................................................................................ 4-10

Comparison of Annual Monitoring Programs .................................................................... 4-10

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

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

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

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

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

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

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

Trophic Structure .............................................................................................................. 5-7

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 ................................................................................................................................ 5-11

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-6
xi 

Weight at Beginning of Age ................................................................................................... A-7

Economic Valuation Methods ..................................................................................................... A-8

Direct Use Values .................................................................................................................. A-8

Market Values (Commercial Fishing) .................................................................................... A-8

Non-Market Direct Use Values (Recreational Fishing) .......................................................... A-9

Indirect Use Benefits ............................................................................................................. A-11

Non-Use Benefits .................................................................................................................. A-13

Adjustment for Other Species ................................................................................................ A-13

Uncertainty Analysis ................................................................................................................... A-14

Inputs to Direct and Indirect Use Benefits ............................................................................. A-15

Sensitivity Analysis ................................................................................................................ A-16

Monte Carlo Analysis ............................................................................................................. A-16
xii 

LIST OF FIGURES 

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

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

Figure 3-1 Sampling Zones and Locations for Lake Norman ................................................. 3-11

Figure 4-1 Plot of Number of Marked Fish Recovered After One to Seven Days of 

Traveling Screen Impingement at MNS, 21-27 July 2005 ................................................ 4-11

Figure 4-2 Seasonal Pattern of Impingement Rates for All Species Combined, 

December 2000 – November 2001 .................................................................................. 4-12

Figure 4-3 Seasonal Pattern of Impingement Rates for All Species Combined, 

December 2001 – November 2002 .................................................................................. 4-13

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

Shad, December 2000 – November 2001 ........................................................................ 4-14

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

December 2000 – November 2001 .................................................................................. 4-15

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

December 2001 – November 2002 .................................................................................. 4-16

Figure 4-7 Seasonal Impingement Pattern and Length Frequency of Impinged Threadfin 

Shad, December 2001 – November 2002 ........................................................................ 4-17

Figure 4-8 Seasonal Impingement Pattern and Length Frequency of Impinged White 

Perch, December 2000 – November 2001 ....................................................................... 4-18

Figure 4-9 Seasonal Impingement Pattern and Length Frequency of Impinged White 

Perch, December 2001 – November 2002 ....................................................................... 4-19

Figure 4-10 Seasonal Impingement Pattern and Length Frequency of Impinged Striped 

Bass, December 2000 – November 2001 ........................................................................ 4-20

Figure 4-11 Seasonal Impingement Pattern and Length Frequency of Impinged Striped 

Bass, December 2001 – November 2002 ........................................................................ 4-21

Figure 4-12 Seasonal Impingement Pattern and Length Frequency of Impinged Bluegill, 

December 2000 – November 2001 .................................................................................. 4-22

Figure 4-13 Seasonal Impingement Pattern and Length Frequency of Impinged Bluegill, 

December 2001 – November 2002 .................................................................................. 4-23

Figure 4-14 Seasonal Impingement Pattern and Length Frequency of Impinged White 

Bass, December 2000 – November 2001 ........................................................................ 4-24

Figure 4-15 Seasonal Impingement Pattern and Length Frequency of Impinged White 

Bass, December 2001 – November 2002 ........................................................................ 4-25

Figure 4-16 Seasonal Impingement Pattern and Length Frequency of Impinged Channel 

Catfish, December 2000 – November 2001 ..................................................................... 4-26

Figure 4-17 Seasonal Impingement Pattern and Length Frequency of Impinged Channel 

Catfish, December 2001 – November 2002 ..................................................................... 4-27

Figure 4-18 Seasonal Impingement Pattern and Length Frequency of Impinged Blue 

Catfish, December 2000 – November 2001 ..................................................................... 4-28
xiii 

Figure 4-19 Seasonal Impingement Pattern and Length Frequency of Impinged Channel 

Catfish, December 2001 – November 2002 ..................................................................... 4-29

Figure 4-20 Daily Lake Level and Intake Water Temperature vs. Number of Impinged 

Fish Collected for All Species Combined, December 2000-November 2001 ................... 4-30

Figure 4-21 Daily Lake Level and Intake Water Temperature vs. Number of Impinged 

Fish Collected for All Species Combined, December 2001-November 2002 ................... 4-31

Figure 4-22 Daily Lake Level and Intake Water Temperature vs. Number of Impinged 

Fish Collected for Young-of-Year (YOY) and Yearling and Older (YAO) Threadfin 

Shad, December 2000-November 2001 .......................................................................... 4-32

Figure 4-23 Daily Lake Level and Intake Water Temperature vs. Number of Impinged 

Fish Collected for Young-of-Year (YOY) and Yearling and Older (YAO) Threadfin 

Shad, December 2001-November 2002 .......................................................................... 4-33

Figure 4-24 Plots of Numbers of Impinged Young-of-Year (YOY) and Yearling and Older 

(YAO) Threadfin Shad Collected vs. Daily Mean Water Temperature at the MNS 

Intake, December 2000-November 2001 ......................................................................... 4-34

Figure 4-25 Plots of Numbers of Impinged Young-of-Year (YOY) and Yearling and Older 

(YAO) Threadfin Shad Collected vs. Daily Mean Water Temperature at the MNS 

Intake, December 2001-November 2002 ......................................................................... 4-35

Figure 4-26 Daily Lake Level and Intake Water Temperature vs. Number of Impinged 

Fish Collected for Young-of-Year (YOY) and Yearling and Older (YAO) Alewife, 

December 2000-November 2001 ..................................................................................... 4-36

Figure 4-27 Daily Lake Level and Intake Water Temperature vs. Number of Impinged 

Fish Collected for Young-of-Year (YOY) and Yearling and Older (YAO) Alewife, 

December 2001-November 2002 ..................................................................................... 4-37

Figure 4-28 Plots of Numbers of Impinged Young-of-Year (YOY) and Yearling and Older 

(YAO) Alewife Collected vs. Daily Mean Water Temperature at the MNS Intake, 

December 2000-November 2001 ..................................................................................... 4-38

Figure 4-29 Plots of Numbers of Impinged Young-of-Year (YOY) and Yearling and Older 

(YAO) Alewife Collected vs. Daily Mean Water Temperature at the MNS Intake, 

December 2001-November 2002 ..................................................................................... 4-39

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

Hydroacoustic Sampling, 1993-2006 ............................................................................... 5-13

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 ................................................................................ 5-14

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

Figure 5-4 Total Annual Electrofishing Catch for Bluegill, Channel Catfish and White 

Perch at 30 100-m Transects in Lake Norman, 1993-1997 and 1999-2007 .................... 5-16

Figure 5-5 Total Annual Electrofishing Catch for Striped Bass and White Bass at 30 

100-m Transects in Lake Norman, 1993-1997 and 1999-2007 ....................................... 5-17

Figure 5-6 Species Composition in the Littoral Zone of Lake Norman as Determined by 

Electrofishing in 1993-1997 and 1999-2007 (Shading Denotes Species Presence) ........ 5-18

Figure 5-7 Percent Composition of Fish Trophic Guilds in the Littoral Zone of Lake 

Norman Based on Electrofishing Results of 1993-1997 and 1999-2007 ......................... 5-20
xiv 

Figure 5-8 Estimates of the Range of Effects of Uncertainty in Each Input Parameter on 

Estimates of Annual Economic Benefit at MNS ............................................................... 5-21

Figure 5-9 Results of Monte Carlo Analysis of Uncertainty in Key Input Parameters on 

Estimates of Annual Economic Benefits at MNS ............................................................. 5-22
xv

LIST OF TABLES 

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

Flow Rate and Net Capacity Generation at MNS, 2000-2007 ............................................ 2-7

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

Duke 2004a, 2004b, 2006, 2007) ..................................................................................... 3-12

Table 4-1 Number of Impinged Fish Collected by Weekly Sampling at MNS, December 

2000-November 2002 (ranked according to abundance) ................................................. 4-40

Table 4-2 Estimated Total Number of Fish Impinged at MNS, December 2000 - 

November 2001 and December 2001-November 2002 (ranked according to 

abundance) ...................................................................................................................... 4-41

Table 4-3 Maximum Total Length (mm) Used to Distinguish Young-of-Year from 

Yearling Fish by Month .................................................................................................... 4-42

Table 4-4 Estimated Annual Impingement of Fish by Age Group at all Units of MNS, 

December 2000 - November 2001 ................................................................................... 4-43

Table 4-5 Estimated Annual Impingement of Fish by Age Group at all Units of MNS, 

December 2001 - November 2002 ................................................................................... 4-44

Table 4-6 Adjusted* Monthly Impinged Fish Collections at MNS, December 2000-

November 2001 at Actual Flows ...................................................................................... 4-45

Table 4-7 Adjusted* Monthly Impinged Fish Collections at MNS, December 2001-

November 2002 at Actual Flows ...................................................................................... 4-46

Table 4-8 Species Composition of the Purse Seine Catch in Lake Norman, 1993-2006 ....... 4-47

Table 4-9 Comparison of Relative Abundance of Fish Species in 2001and 2002 Littoral 

Zone Electrofishing Surveys and the December 2000- November 2002 MNS 

Impingement Collections (Unadjusted) ............................................................................ 4-48

Table 4-10 Adjusted* Monthly Impinged Fish Collections at MNS, December 2000-

November 2001 at Full Design Flows .............................................................................. 4-49

Table 4-11 Adjusted* Monthly Impinged Fish Collections at MNS, December 2001-

November 2002 at Full Design Flows .............................................................................. 4-50

Table 5-1 Trophic Guild Assignments for Fish Species in Lake Norman ................................ 5-23

Table 5-2 Estimates of Potential Total Annual Equivalent Fishery Yield (lb) from 

Individuals Impinged for Each Target Species at MNS .................................................... 5-24

Table 5-3 Estimates of Potential Total Annual Production Foregone (lb) from Individuals 

Impinged for Each Target Species at MNS ...................................................................... 5-25

Table 5-4 Estimates of Economic Value (2009$) of Fish Impinged at MNS ........................... 5-26
xvi 
1-1 

1 

INTRODUCTION 

Background 

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

Nuclear Station (MNS) 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. MNS 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, MNS 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. In December 2008, 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. In April 2009, the U.S. Supreme Court 
Introduction 

1-2

overturned the Second Circuit Court’s decision and determined that EPA had the authority to use 

a comparison of costs and benefits in making BTA determinations under §316(b). 

To comply with the regulations, Duke Energy intends to use the results of an impingement 

sampling program conducted from December 2000 through November 2002 as representative of 

current impingement rates and composition. In this report, the 2000-2002 study results are 

presented and interpreted as support for decision-making using a BPJ approach. Sampling 

results are interpreted in terms of the status of the fish community of Lake Norman, the 

impoundment from which the cooling water is withdrawn. 

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 MNS intake. Section 4 

presents the results of the 2000-2002 impingement sampling program, 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 MNS. 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 McGuire Nuclear Station, its 

operation, and its cooling water source, Lake Norman. 

McGuire Nuclear Station 

McGuire Nuclear Station (MNS) in located in Huntersville, Mecklenburg County, North 

Carolina on Lake Norman, an impoundment of the Catawba River formed by the Cowans Ford 

Dam (Figure 2-1). MNS has two nuclear reactor units, each with a net generation capacity of 

1,129 MW for a total rating of 2,258 MW. MNS uses once-through cooling for its steam 

condensers. Unit 1 began commercial operation in December 1981 and Unit 2 began in March 

1984. 

Cooling Water Intake Structure 

MNS has a shoreline intake structure and two lower level intakes located near the Cowans Ford 

Dam at a distance of approximately 550 m from the shoreline intake (Figure 2-2). The lower 

level intakes are designed to withdraw cooler water from a depth of approximately 30 m when 

the lake is stratified during summer, in order to meet NPDES permit-imposed thermal criteria 

and to increase generation efficiency. The Unit 2 lower level intake has been retired, and the 

Unit 1 lower level intake is only used sparingly (e.g., two weeks per year) due to concern for 

lowering the lake’s thermocline and depleting the hypolimnion. If the thermocline would be 

lowered, it would affect the operation of Duke’s Marshall Steam Station (MSS), which draws its 

cooling water from a depth greater than 18.3 m. 

The shoreline intake structure has eight 7.9-m wide bays, four for each unit. Each bay splits to 

house two trash rack structures with 11.4-cm on-center bar spacing and two conventional vertical 

traveling water screens. The traveling screens are 30.5 m wide and have 3/8-in. (9.5-mm) square 

woven-wire, stainless steel mesh. Downstream from the screens the bays converge for a single 

circulating pump (four per unit, or eight total pumps for the facility). Each of the eight 

circulating pumps is rated at 254,000 gpm (566 cfs) for a total design flow rate of 2,032,000 gpm 

(4,528 cfs) at 7 m of head. Generally, two circulating pumps per unit are operated during late 

fall to early spring, and three pumps per unit are operated during the remainder of the year. 

Cooling water withdrawn from the lower level intake near Cowens Ford Dam for Unit 1 is 

pumped by three 150,000-gpm pumps up to the shoreline intake and mixes with the shorelinedrawn water in the forebays in front of the traveling screens. The lower level intakes have trash 
Plant and Source Waterbody Descriptions 

2-2

racks similar to those on the shoreline intake. During late summer and early fall, hypolimnetic 

water is withdrawn through the lower level intake as necessary to maintain lower containment 

temperatures and discharge temperatures within permit specifications (Barwick et al. 2003). 

The approach velocity in front of the traveling screens is 0.9 ft/s (0.3 m/s), assuming a flow of 

283 cfs per screen at low water level (Alden 2004). The screens are rotated weekly (or more 

frequently if necessary) for cleaning by a high-pressure (85-psi) spray wash system. Debris and 

fish are washed into a trough leading to trash bins on either side of the intake structure. Heated 

water is discharged into a 91-m long discharge canal 168 m to the east of the intake (Figure 2-2). 

Plant Operation and Water Withdrawal 

Total generation and condenser cooling water pumping data are presented in Table 2-1 for 2000 

through 2007. Over this period, the average annual capacity factor, based on Maximum Net 

Dependable Capacity, was 94.5 percent. The average total annual intake flow rate for 2000-2007 

was 89.7 percent of the maximum design capacity, 2,264cfs or 1,463 MGD for the upper level 

cooling water pumps (Table 2-1). Annual flows ranged from 85.8 percent (2005) to 91.2 percent 

(2004) of the maximum design capacity for the upper level cooling water pumps. 

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 
2-3

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). MNS is at the southern 

tip of the lake. 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 crop land. The primary use of Lake Norman is for cooling 

water at MNS and MSS 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). Based on longterm 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 o

C (44.6 oF) in January to 30-32 oC (86-

89.6 o

F) in August (Duke 2004a). The lake begins to stratify thermally 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. 

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 
Plant and Source Waterbody Descriptions 

2-4

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). 

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. Divison 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. 
 Figure 2-1 Location of MNS
Plant and Source Waterbody Descriptions 

2-6

 Figure 2-2 Aerial View of MNS 
Table 2-1 Monthly Mean Cooling Water Flow Rate (MGD) and Percentage of Maximum Flow Rate and Net Capacity 

2000-2007 

Capacity Factors 

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

2000 102.3 96.9 105.5 105.2 100.4 102.7 100.8 92.1 52.0 80.4 101.2 2001 94.2 104.4 65.7 73.2 104.2 102.8 96.8 101.3 102.0 103.7 104.3 2002 105.1 92.9 50.7 104.7 104.0 102.6 101.0 97.3 72.0 83.6 104.4 2003 102.5 105.2 104.4 103.9 102.3 102.8 101.4 100.3 58.2 89.6 103.8 2004 105.1 105 60.8 82.7 103.8 102.3 101.3 101.2 101.9 79.5 84.4 2005 105.2 105 53.2 67.6 104.1 103.3 100.7 101.3 77.7 70.6 101.1 2006 105.5 105.5 105.2 105 103.2 102.8 101.5 101.4 76.2 51.6 82.9 2007 105.4 105.3 64.5 52.4 53.1 100 101.9 101.4 101.6 102.9 104.5 

Average 103.2 102.5 76.3 86.8 96.9 102.4 100.7 99.5 80.2 82.7 98.3 

Actual Plant Flow Rate (MGD) 

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

2000 2,500 2,500 2,503 2,653 2,900 2,900 2,900 2,900 1,683 2,694 2,857 2001 2,500 2,500 1,687 2,273 2,829 2,900 2,900 2,900 2,900 2,894 2,900 2002 2,500 2,361 1,800 2,627 2,900 2,900 2,900 2,900 2,187 2,858 2,900 2003 2,500 2,514 2,500 2,500 2,845 2,900 2,900 2,900 1,927 2,877 2,900 2004 2,506 2,500 1,635 2,500 2,881 2,900 2,900 2,900 2,900 2,900 2,837 2005 2,500 2,500 1,326 2,280 2,865 2,900 2,900 2,900 2,327 2,326 2,797 2006 2,500 2,500 2,503 2,783 2,900 2,900 2,900 2,900 2,263 2,181 2,700 2007 2,500 2,500 1,639 1,837 2,900 2,900 2,900 2,900 2,900 2,900 2,827 

Average 2,501 2,484 1,949 2,432 2,877 2,900 2,900 2,900 2,386 2,704 2,840 

Percentage of Maximum Flow Rate (4,528 CFS or 2,926 MGD) 

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

2000 85.4 85.4 85.6 90.7 99.1 99.1 99.1 99.1 57.5 92.1 97.6 2001 85.4 85.4 57.7 77.7 96.7 99.1 99.1 99.1 99.1 98.9 99.1 2002 85.4 80.7 61.5 89.8 99.1 99.1 99.1 99.1 74.7 97.7 99.1 2003 85.4 85.9 85.4 85.4 97.2 99.1 99.1 99.1 65.8 98.3 99.1 2004 85.7 85.4 55.9 85.4 98.4 99.1 99.1 99.1 99.1 99.1 96.9 2005 85.4 85.4 45.3 77.9 97.9 99.1 99.1 99.1 79.5 79.5 95.6 2006 85.4 85.4 85.6 95.1 99.1 99.1 99.1 99.1 77.4 74.5 92.3 2007 85.4 85.4 56.0 62.8 99.1 99.1 99.1 99.1 99.1 99.1 96.6 

Average 85.5 84.9 66.6 83.1 98.3 99.1 99.1 99.1 81.5 92.4 97.0 

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 MNS are 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. Ichthyoplankton sampling was either 

biweekly or weekly at up to five locations. A special study was conducted on 21 February 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. MNS is located in 

Zone 1 (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 
Fish Community Description 

3-2

with North Carolina Wildlife Resources Commission (NCWRC), age, 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 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 

15-20o

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 up-lake 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, three fish species listed by the North Carolina 

Natural Heritage Program (NCNHP) for Mecklenburg County as rare animal species for North 

Carolina (LeGrand et al. 2008). These are Atlantic highfin carpsucker (Carpoides sp. cf. velifer), 

the Santee chub, (Cyprinella zanema), and the Central Piedmont population of the Carolina 

darter (Etheostoma collis pop. 1). The Atlantic highfin carpsucker is currently listed by NCNHP 

as a state species of concern with a state ranking of S12

. The Santee chub is listed as a 

significantly rare species with a state ranking of S33. The Central Piedmont population of the 

Carolina darter is listed as a species of concern with a state ranking of S3. It is unlikely that 

these three species would occur in Lake Norman since they prefer lotic environments (Rohde et 

al. 1994). 

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 MNS cooling water intake. 

 

2

 S1=Critically impaired in North Carolina due to extreme rarity or some factor(s) making it especially vulnerable to 

extirpation from the state. 

3

 S3=Vulnerable to extinction in North Carolina because of rare or uncommon, or found only in a restricted range 

(even if abundant in some locations), or due to other factors making it vulnerable to extirpation. 
Fish Community Description 

3-4

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

of their economic value or value to the ecosystem. The selected target species include threadfin 

shad, alewife, white perch, bluegill, striped bass, white bass, channel catfish, and blue catfish. 

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

impingement collections at the MNS 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. The two catfish species—channel 

catfish and blue catfish— are traditionally important game species in Lake Norman. White 

perch, a newcomer to the fish community, has become abundant in the sportfishing catch in the 

lake and ranked third in abundance in the 2000-2002 impingement samples (Section 4). The 

other two temperate bass species, striped bass and white bass, are among the most popular game 

fish in the lake. In particular, striped bass is the primary game species managed in the lake 

through annual stocking and programs that monitor the health of the striped bass population and 

its forage. Bluegill was the fourth-most frequently impinged species in 2000-2002 and is a 

popular panfish species and a forage species for some gamefish such as largemouth bass. 

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

Threadfin Shad 

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

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 65-70o

F (18.3-21.1oC) and can have 

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

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

during ichthyoplankton sampling 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 northern portions of the lake rather than the southern portions nearer MNS, 

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 oC (Ruelle et al. 1977, Griffith 1978), in 
3-5

Lake Norman may be dependent upon the thermal input from MNS and MSS, 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, as 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 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 MNS intake in 2000-2002, 

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 2o

C to 27oC 

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

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

White Perch 

White perch was the third-most frequently impinged fish species at MNS during 2000-2002 

(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, and 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 oC (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 20oC, or may take as long as 96-108 hours at 

15oC. 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 Danie 1983). 

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 Danie 

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. 
3-7

Bluegill 

The bluegill, along with redbreast sunfish, is numerically the most abundant species in the Lake 

Norman fish community. While it may be too small to attract most fishermen in Lake Norman 

(Siler et al. 1981), it is an important prey species, particularly largemouth bass. The bluegill is 

native to much of the eastern and central U.S. and occurs in warm, slow or still water, usually 

associated with aquatic vegetation and woody debris (Rohde et al. 1994). Individual movements 

tend to be highly localized. Its diet consists mostly of invertebrates, but larger bluegills may 

become piscivorous. 

In the Middle Atlantic region, bluegill spawn during May and June when water temperatures 

reach 70o

F (21oC) (Rohde et al. 1994). The spawning season can be protracted over several 

months. Bluegill are colonial breeders in the sense that spawning fish build nests that are usually 

in close proximity to each other. Males construct nests that are about 0.3 m in diameter in 

shallow water (depths of 0.3-0.9 m). The eggs from several females can be fertilized and 

deposited in the nest, which is then defended by the male until the eggs have hatched. Because 

the nests are located in shallow depths, water level fluctuations can severely impact successful 

reproduction. Nests and young can be stranded by a lowering water level or disrupted by severe 

wave action (Heman et al. 1969), or survival of the young may be reduced by increased 

ultraviolet light radiation in highly transparent water (Olson et al. 2008). 

Bluegill in the southeastern U.S. commonly begin to mature by at age 1 or 75-125 mm for both 

sexes (Spotte 2007) and can live up to 11 years. Most adults do not exceed 0.5-0.75 lb (227-340 

g) (Rohde et al 1994). 

Striped Bass 

The striped bass fishery in Lake Norman is a “put-grow-and-take” fishery maintained by annual 

stocking of 162,500 fingerling fish in late spring. Striped bass were first introduced into the lake 

in the late 1960s or early 1970s with annual stockings of approximately 100,000 fingerling fish 

(Van Horn et al. 1999). While this has created a very popular recreational fishery, there are 

indications that growth and condition of the fish are limited by the forage base in the lake (Van 

Horn et al. 1999, Van Horn 2001, Thompson et al. 2005, Davias 2006), as well as summer water 

temperatures and dissolved oxygen depletion (Lewis 1983, Van Horn et al. 1996, Thompson et 

al. 2005, Davias 2006). As a result, growth rates for striped bass in Lake Norman are slower 

than for several other North Carolina reservoirs (Davias 2006). The NCWRC very actively 

manages the lake’s striped bass fishery through size and creel limits, forage fish density 

monitoring, and experimentation with stocking rates to increase the growth, condition, and 

standing crop of striped bass in the lake. Striped bass is largely a pelagic species, feeding on 

pelagic clupeids including threadfin shad, alewife, and blueback herring and gizzard shad when 

present. 

Thompson et al. (2005) observed striped bass in Lake Norman to average 269 mm TL at age 1, 

424 mm TL at age 2, 499 mm at age 3, 535 mm at age 4, 553 mm at age 5, 561 mm at age 6, 566 

mm at age 7, and 568 mm at age 8. Earlier, Van Horn et al. (1999) had reported age-specific 
Fish Community Description 

3-8

mean lengths at time of capture during 1994-1998 to be 413-533 mm TL at age 2, 487-569 mm 

at age 3, 533-588 at age 4, 541-641 mm at age 5, and 604-690 mm at age 6. 

White Bass 

The white bass is rated by anglers as a fair to good food fish and an excellent sport fish. In the 

mid-Atlantic region, white bass is most successful in large reservoirs in the piedmont and 

mountains (Rohde et al. 1994). It is a schooling species that prefers open, deeper areas in 

streams, rivers, and lakes (Rohde et al. 1994, Pflieger 1997). The primary prey species for white 

bass is gizzard shad or threadfin shad (Rohde et al. 1994, Guy et al. 2002). The abundance of 

white bass in Lake Norman recently has been declining, possibly as the result of interspecific 

competition with or predation by introduced species such as the white perch. However, it 

remains a popular sport fish in Lake Norman. 

White bass begin to spawn in April or May when water temperatures reach 60o

F (15.6oC) and 

they migrate from open waters into tributaries streams (Rohde et al. 1994). Spawning occurs 

over rocky or gravelly areas (Pflieger 1997). White bass broadcast their eggs, which then adhere 

to rocks or debris. They provide no parental care. The free-swimming fry and young begin to 

school and gradually add small fish to their diet. For the first 2 months post-hatch, growth rates 

can average 0.08 mm/day, then increase to 0.56 mm/day thereafter (Kindschi 1979, in Quist et al. 

2002a). In Glen Elder Reservoir, Kansas the growth rates for larvae 3 to 35 days old were 0.32 

mm/day (Quist et al. 2002). Annual survival rates and recruitment have been observed to be 

highly variable (Schultz and Robinson 2002, Lovell and Maceina 2002). 

As adults, white bass typically are fast growing and short-lived, often experiencing relatively 

high fishery exploitation rates (Guy et al. 2002, Lovell and Maceina 2002, Colvin 2002, Schulz 

and Robinson 2002). In Missouri reservoirs, most age-0 white bass were 150-230 mm long by 

the fall. Colvin (2002) found similar age structure for white bass in four large Missouri 

reservoirs, where the oldest fish were age 7, but there was a sharp decline in abundance and 

growth after age 4 when they reach 370-400 mm in length. They mature at ages 2-3 at a length 

of approximately 8-10 in. (203-254 mm) (Sallee et al. 2004). White bass in the Upper 

Mississippi River system can reach a maximum length of 18 in. (457 mm) and a maximum 

weight of 3 lb (1.4 kg). Females produce more ova than they shed during spawning, leading 

Ruelle (1977) to conclude that the average calculated “effective fecundity” is 124,000-134,000 

eggs per 300-mm female and 429,000-608,000 eggs per 390-mm female. 

Channel Catfish 

A total of 73 channel catfish were collected during impingement sampling at MNS in 2000-2002, 

making it the seventh-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 
3-9

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 65o

F 

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

over a range from 21o

C to 29 oC, with an optimal temperature of 27 oC (Hubert 1999). Sustained 

temperatures in this range (21o

C to 29 oC) 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 75-82oF (23.9-27.8 oC) 

(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). 

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. 

Blue Catfish 

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; the record fish was 51.25 in. (1,302 mm) 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. 
Fish Community Description 

3-10

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. 

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 68o

F to 86 oF (20-30oC), with 

optimum temperatures of 72-82 oF (22.2-27.8 oC) (Simon and Wallus 2003). In Lake Norman, 

water temperatures of 72-82oF 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 

79oF (23.9-26.1 oC) in the Ohio River (Simon and Wallus 2003). Eggs hatch in 7-8 days at 

temperatures between 21 to 24oC (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 

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). 
3-11

Figure 3-1 Sampling Zones and Locations for Lake Norman 

.
Fish Community Description 

3-12Table 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 
Table 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 two-year impingement monitoring program 

conducted at MNS during 2000-2002. The results of this program are used to estimate 

annual impingement rates at the station and are applied in an evalation of the potential for 

adverse environmental impact, presented in Section 5. 

Impingement Monitoring Program 

Weekly numbers impinged were obtained from impingement monitoring conducted by 

Duke from 3 December 2000 through 29 November 2002 (Barwick et al. 2003). On 3 

December 2000, all screens were washed of all fish and debris. Thereafter, the screens 

were allowed to accumulate impinged fish for one week before the screens were washed 

again. At the end of each week, all debris and fish from each screen were collected in 

coated-wire baskets (5-mm x 10-mm mesh openings) placed in the screen-wash 

sluiceway. Fish were sorted from the debris and identified to the lowest practical taxon. 

Fish identified to species were measured for total length (mm). In addition, surface water 

temperatures near the intake structure were recorded during each sampling period. 

The numbers of fish impinged weekly on each screen were summed for all screens and 

reported in Barwick et al. (2003) as monthly totals. On occasion, mechanical problems 

regarding rotation and washing of screens were encountered, and fish impingement could 

not be determined for the affected screen during a particular time period. In such cases, 

fish impingement for the missed sample was estimated to be that observed during the 

same period from the other paired screen for the specific circulating pump or from a 

similar time period immediately before the missed sample (in the event that it was not 

possible to wash both paired screens). During plant outages and at other times when the 

pumps were not operational during the weekly sampling period, the screens associated 

with those pumps were not washed and no fish impingement was assumed to have 

occurred. 

The weekly impingement collections during 2000-2002 would represent a complete 

census of impingement, rather than a sampling, as long as there were no losses of 

impinged fish during the weekly intervals between screen rotations. These losses could 

be caused by predators or scavengers taking impinged fish off the screens, decay, or fish 

bypassing the screens or being trapped in locations where they are not recoverable. Fish 

losses to such sources would result in an underestimation of the impingement rate at 

MNS. 
Impingement Mortality 

4-2

To address this concern, Duke conducted a special study to estimate the loss rate of 

impinged fish due to decay or other reasons during a weekly collection interval and to 

adjust impingement numbers to account for these losses (Barwick et al. 2005). Over a 

seven-day period (21-27 July 2005), a total of 1,600 fish were fin-clipped and introduced 

onto all 16 freshly washed upper level intake screens at MNS at periodic intervals. 

Treatments consisted of the release of four lots of 400 marked fish each at intervals of 

seven, five, three, and one day prior to screen washing on 27 July. A set of four screens 

was randomly chosen for each of the four release dates (100 fish per screen), such that all 

16 screens were tested once. At the end of this seven-day period, all screens were 

washed and all marked fish were collected. 

The test fish (threadfin shad and a few alewives) were collected from Lake Norman using 

a boat-mounted electrofisher during the morning on the day of introduction. Once 

sufficient numbers of fish were collected by electrofishing, they were transported to the 

Environmental Center fishery lab near MNS. In the lab, 400 fish were selected for use in 

each treatment and a subsample of 100 fish was randomly selected and measured for total 

length (mean 92-95 mm TL, range 77-116 mm TL). All fish were fin-clipped (complete 

removal of caudal fin) and bagged in groups of 100 and placed on ice. 

The marked fish were introduced between 1030 and 1130 h onto individual intake 

screens by placing them in paper bags, attaching the bags to a weighted rope, and 

lowering the bags to the center of the trash rack associated with individual intake screens. 

After a 3-minute waiting period, a sharp tug on the rope ruptured the wet paper bags and 

released the fish directly into the flow entering the intake bay. Condenser cooling water 

pump flows transported a portion of these fish onto the intake screens where they were 

impinged. 

Results 

A total of 1,690 fish were collected in the first year of sampling, December 2000 – 

November 2001, and 698 fish in the second year, December 2001 – November 2002 

(Table 4-1). Over this two-year period, 23 fish species were identified in the 

impingement collections. Threadfin shad dominated the 2000-2001 collections, 

comprising 51.5 percent of the total, and alewife dominated the 2001-2002 collections, 

comprising 23.8 percent (Table 4-1). Other commonly impinged species (in decreasing 

order for both years combined) included white perch (7.0 percent), bluegill (6.6 percent), 

striped bass (3.9 percent), white bass (3.2 percent), channel catfish (3.1 percent), and 

gizzard shad (2.6 percent). Ninety-four fish could not be identified to species, or 

approximately 4 percent of all fish collected during the two-year monitoring program. 

The special study to assess impinged fish losses during a collection week was conducted 

at mid-summer surface water temperatures ranging from 31.7 °C to 32.5 °C near the 

intake. Therefore, adjustments to impingement estimates to account for these losses 

would be conservative (overestimate losses) relative to months of cooler water 

temperatures and thus slower decay rates. Also, the test fish were threadfin shad or 

alewife, two species known to be fragile and susceptible to rapid decay. 
4-3

Of the 1,600 fin-clipped fish released during the study, a total of 243 were recovered at 

the screen washing on the seventh day. Releases of 400 marked fish on each of the four 

release dates resulted in 98 being recovered after one day on the screens, 93 after three 

days, 36 after five days, and 16 after seven days. These data showed a steady decline in 

the number of recovered fish through time, indicating a cumulative loss of impinged fish 

due to decay, predation, or other reasons. Barwick et al. (2005) presented photographic 

evidence of the effects of decay on the integrity of the impinged fish, where fish 

impinged for three days exhibited some decay (e.g., belly missing), fish impinged for five 

days were badly decayed (e.g., head and belly missing), and fish impinged for seven days 

were “decayed sufficiently that marked fish were not readily apparent and species 

identification was at times difficult.” 

Barwick et al. (2005) derived an adjustment factor for impingement estimates made 

during the 200-2002 monitoring program. They considered the proportion of released 

fish that became impinged to be the “introduction efficiency.” Introduction efficiency 

(%) was calculated as the number of marked fish released one day prior to the screen 

washing and recovered divided by the number introduced, multiplied by 100, i.e, 98/400 

or 24.5 percent. The overall recovery rate (%) for the entire seven-day test period was 

calculated as the number of marked fish recovered from all screens (243 fish) divided by 

the number of fish estimated to have been originally impinged on the screens (98 x 4, or 

392 fish) and multiplied by 100, yielding a recovery rate of 62 percent. This method 

assumed that (1) the introduction efficiency (24.5 percent) was the same for all four 

releases, (2) impingement rates were constant during the seven-day interval, and (3) no 

fish were lost during the first 24-h interval of impingement. 

An alternative method for estimating fish loss over the seven-day test would be to apply 

regression analysis on the number of recovered fish over time. When plotted, these data 

showed a clear linear relationship (Figure 4-1), which was statistically significant 

(R2

=0.910, 3 d.f., P=0.046) and described by the formula: 

Nr = 121.35-15.1(T) 

Where 

Nr = number of fish recovered 

T = number of days prior to screen washing 

The intercept value of 121.35 would represent the average number of impinged fish at 

time 0 for each release. This method therefore would produce an alternative estimate of 

the initial number impinged out of 400 fish released, i.e., 121 fish or an introduction 

efficiency of 30.0 percent, and remove the assumption that no fish were lost during the 

first 24 hours. Using this method and applying a constant of 121 fish initially impinged 

for each release, an overall recovery rate of 50.2 percent (243/484 fish) would result. The 

mean recovery over the entire seven-day test period (equivalent to 3.5 days at large) 

using the linear relationship would be 56.4 percent. This method assumes impingement 
Impingement Mortality 

4-4

rates were constant during the seven-day interval, but relies upon the empirical linear 

relationship to estimate the initial number of released fish impinged. 

The number of impinged fish collected weekly at MNS during the 2000-2001 and 2001-

2002 monitoring programs can be adjusted by either the 60 percent recovery rate or the 

56.4 percent recovery rate, depending on the assumptions found to be acceptable. The 

results of these adjustments do not differ greatly from one another (Table 4-2). Using the 

60 percent recovery rate, the total annual impingement would be 2,726 fish for 2000-

2001 and 1,126 fish for 2001-2002. Using the 56.4 percent recovery rate, the total annual 

impingement would be 2,996 fish for 2000-2001 and 1,238 fish for 2001-2002. 

Since these adjustment factors were applied equally for all species, there was no change 

in the relative abundance of species from the original, unadjusted collections shown in 

Table 4-1. Either set of estimates would be conservative because the special study was 

conducted at warm temperatures conducive to rapid decomposition, the test species were 

comparatively fragile clupeids, and the adjustment factors were applied to all species 

(including durable ones) and all seasons, including cold months when clupeids were 

primarily impinged. 

The most conservative adjustment to impingement numbers simply would be to use the 

ratio of the number of marked fish released to the number recovered, i.e., 243/1600 or 

15.2 percent mark recovery. Non-recovered fish would include those lost to decay or 

predation, as well as those that were not transported to the screens by the current or those 

otherwise not found in the intake structure. If this most conservative approach is used, 

the estimated annual total impingement would be 11,118 fish in 2000-2001 and 4,592 

fish in 2001-2002. This approach may be too conservative. Nevertheless, the estimated 

impingement totals remain relatively low. 

Seasonal Patterns 

Impingement was lowest for all fish species combined during the spring, March – June 

for both years, and highest in mid-winter (January) for the 2000-2001 period and late 

summer (August) for the 2001-2002 period (Figures 4-2 and 4-3). There were two peaks 

in number of fish impinged for all fish species combined from December 2000 to 

November 2001 (Figure 4-2); the highest peak occurred in January 2001 due to a large 

number of threadfin shad (Figure 4-4). A secondary, smaller peak occurred in July 2001 

and was comprised mainly of alewife (Figure 4-5). During the December 2001 to 

November 2002 sampling period, there was a large peak in impingement during August 

mainly due to alewife (Figure 4-6). 

Threadfin shad impingement clearly was a winter phenomenon, as illustrated by the 

increased impingement counts in both 2000-2001 (Figure 4-4) and 2001-2002 (Figure 4-

7), particularly in late December and January. Impinged threadfin shad ranged from 40 

to 109 mmTL and consisted of young-of-the-year (YOY) as well as yearling and older 

fish during this winter impingement period (Figures 4-4 and 4-7). Based on their 

observed length frequency distribution and literature values on age-length relationships 
4-5

for threadfin shad (Table 4-3), approximately 45 percent of the threadfin shad were YOY 

(Tables 4-4 and 4-5) . Impingement counts were an order of magnitude higher in the 

winter of 2000-2001 than in the winter of 2001-2002. 

Alewife impingement was greatest during summer and fall (July – October) (Figures 4-5 

and 4-6). Impinged alewives ranged from 27 to 157 mmTL. Yearling and older alewives 

were more prevalent in the 2001-2002 collections than in 2000-2001 (Tables 4-4 and 4-

5), probably reflecting the aging of the population following its initial appearance in 

1999. 

The majority of white perch were impinged during the summer and fall, July-November 

(Figures 4-8 and 4-9). Most were yearling or older fish (Tables 4-4 and 4-5), based on 

their observed length frequency distribution and literature values on age-length 

relationships (Table 4-3). Their lengths ranged from 51 to 307 mm TL. 

Striped bass are stocked as fingerlings each spring. However, few newly stocked fish 

were impinged at MNS. Almost all of the impinged striped bass were 300 mmTL or 

larger, and most were impinged during the summer and fall (Figures 4-10 and 4-11). The 

largest fish was 612 mm long. 

Bluegill were impinged throughout the year, with no obvious seasonality (Figures 4-12 

and 4-13). YOY and yearling or older fish were impinged, based on their observed 

length and region-specific literature values on length at age (Table 4-3). Lengths of 

impinged bluegill ranged from 37 to 254 mmTL, but most fish were less than 100 mm 

long. 

White bass were impinged throughout the year in relatively low numbers (Figures 4-14 

and 4-15). Impinged white bass were yearling or older fish (Tables 4-4 and 4-5), based 

on their lengths (Table 4-3); the smallest was 153 mmTL in August and the largest was 

373 mm. 

Channel catfish impingement was more common in 2000-2001 than in 2001-2002 

(Figures 4-16 and 4-17), but it occurred throughout the year during both annual surveys. 

Most impinged channel catfish were yearling or older fish (Tables 4-4 and 4-5). 

Impinged channel catfish ranged in length from 44 to 550 mmTL. 

Only 14 impinged blue catfish were collected in 2000-2001 and only 11 in 2001-2002. 

They ranged in length from 79 to 418 mmTL (Figures 4-18 and 4-19). Impingement was 

restricted to the months of August and November in 2002, but occurred in all four 

seasons in 2000-2001. 

The seasonality of impingement for species other than the four species discussed above 

can be summarized as follows (Tables 4-6 and 4-7): 

Gizzard shad—year-round 

Eastern silvery minnow—year-round 
Impingement Mortality 

4-6

Golden shiner—year-round 

Flathead catfish—primarily May-August 

Rainbow trout—no clear seasonal pattern 

Black crappie—no clear seasonal pattern 

Largemouth bass—primarily April-August 

Redbreast sunfish—year-round 

Redear sunfish—year-round 

Spotted bass—August 

Warmouth—primarily April-May 

White crappie—December-July 

Yellow perch—June-August 

VARIABLES AFFECTING IMPINGEMENT RATES 

Three categories of variables potentially affecting impingement rates are discussed: 

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

Environmental Variables 

Two potentially important environmental variables that could affect impingement rates 

are water temperature and lake level or elevation. Mean daily water temperature records 

from the MNS intake and daily lake level data4

 were available for the sampling period 

from December 2000 through November 2002. 

During the two-year study period, the mean daily intake temperature varied seasonally 

from a low of 46.0oF (7.8oC) on 12 January 2001 (Figure 4-20) to a high of 90.2oF 

(32.2oC) on 7 August 2002 (Figure 4-21). Water temperatures generally were warmer 

during the second year of study (2001-2002; Figure 4-21) than in the first year (2000-

2001; Figure 4-20). This was particularly true in the winter months (DecemberFebruary) when mean monthly temperatures in 2001-2002 were to 2.3 oF (February) to 

8.0 oF (December) warmer than in the winter of 2000-2001, and during August when the 

mean monthly temperature in 2002 was to 2.4 oF warmer than in 2001. 

The lake level varied 9.9 ft (3.0 m) during the 24-month sampling interval from 1 

December 2000 to 30 November 2002 (Figures 4-20 and 4-21). Lake levels were highest 

in April-May of each year, and lower lake levels occurred from November-March. On 22 

September 2002, the lake level was abruptly lowered to its minimum values for the twoyear study period, approximately 749 ft. above mean sea level, where it remained until 

the conclusion of the study. 

 

4

 The data presented were recorded at MNS and have since been calibrated to correspond to lake levels 

recorded at the Cowans Ford Dam. While the absolute values have changed, the range and incremental 

daily changes in lake level have not changed. 
4-7

Peak impingement collections for all species combined for 2000-2001 occurred on 12 

January 2001, when the lake’s surface elevation and the mean daily water temperature 

(46.0oF) were at or near their minimum level for the year (Figure 4-20). Impingement on 

this date consisted primarily of threadfin shad (448 of 459 fish collected or 97.6 percent). 

The peak impingement collection for all species combined was much lower in 2001-2002 

than in 2000-2001 (53 fish vs. 459 fish) and occurred on 16 August 2002. On this date, 

the lake’s surface elevation was declining but the daily mean water temperature (84.7 oF) 

was near maximum for the year (Figure 4-21). Thirty-three of the 50 impinged fish 

collected (66 percent) were alewives. 

The peak impingement event for threadfin shad on 12 January 2001 consisted of both 

YOY and yearling or older fish (Figure 4-22). YOY and yearling or older threadfin shad 

also experienced peak impingement events in January and February of 2002, but to a 

much lesser degree (Figure 4-23). 

The greater impingement levels for threadfin shad in the winter of 2000-2001 may have 

been related to the colder temperatures then, compared to the winter of 2001-2002. The 

two largest impingement collections for threadfin shad in 2000-2001 occurred on 12 

January and 5 January 2001 when water temperatures were 46 oF and 49 oF, respectively 

(Figure 4-24). Threadfin shad have been shown to be stressed at water temperatures less 

than 10oC (50 oF) and can suffer complete mortality at 4oC (Griffith 1978). Although 

there were two other sampling dates in the winter of 2000-2001 when water temperatures 

were less 50 oF (Figure 4-24), they occurred in late January or early February, possibly 

after the threadfin shad population had been reduced by the low temperatures (<50 oF) of 

late December 2000 and January 2001. In the winter of 2001-2002, there were no 

sampling events when water temperatures were <50 oF, and threadfin shad collections 

were lower, although there did appear to be a slight increase in the number of impinged 

yearling or older threadfin as temperatures approached 50 oF (Figure 4-25). 

In contrast, impingement of alewife peaked at warmer water temperatures. The peak 

impingement event for alewife on 27 July 2001 consisted exclusively of YOY fish 

(Figure 4-26). The peak impingement events on 9 and 16 August 2002 consisted of YOY 

alewives, although the 16 August event had nearly as many yearling or older fish (Figure 

4-27). The daily mean water temperatures on 27 July 2001, 9 August 2002, and 16 

August 2002 were 82 oF, 87 oF, and 85 oF, respectively (Figures 4-28 and 4-29). These 

temperatures should not have been stressful to alewives, being well below the upper 

incipient lethal temperature (89.8 oF) for fish acclimated to 75.2-78.8 oF (Wismer and 

Christie 1987). Dissolved oxygen (DO) concentrations near the intake also were 

adequate; DO measurements on 9 July and 4 August 2001, and on 5 August 2002, were 

6-7 mg/L (Duke 2003, 2004a). The observed peak impingement events, while not severe, 

may have been related to the relative abundance of alewife schools near the MNS intake 

on these dates. 
Impingement Mortality 

4-8

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 2000-2002 impingement 

collections provides insight into the role of species-specific abundance and spatial 

distribution on the susceptibility of species to impingement at MNS. 

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 2000-2002, 

threadfin shad appeared to be the dominant species in the pelagic zone, comprising 

approximately 75 to 87 percent of the purse seine catch (Table 4-8). Alewife comprised 

approximately 12 to 25 percent. Gizzard shad, once more common than now, were 

relatively rare in the purse seine catch. Blueback herring were not reported until 2004 

(Duke 2004c). 

Threadfin shad and alewife, the principal components of the pelagic fish community, also 

dominated the 2000-2002 impingement collections at MNS, comprising 42.1 percent and 

16.0 percent of the combined December 2000-November 2002 impingement collections 

(Table 4-9). Their numerical dominance of the impingement collections suggested that 

pelagic species are more vulnerable to impingement than littoral species. White perch, 

striped bass, and white bass also may be considered to be a pelagic species. They were 

relatively uncommon in the electrofishing catches from the littoral zone during the 2001-

2002 annual electrofishing surveys, but were the third, fifth, and sixth-most frequently 

impinged species (Table 4-9). 

Several abundant littoral zone species either did not appear in the impingement 

collections or were relatively minor components (Table 4-9). Examples included shiners 

(whitefin, spottail, and greenfin) and several centrachids (e.g., redbreast sunfish, redear 

sunfish, hybrid sunfish, largemouth bass). Their distribution, and possibly their size in 

terms of the shiners, appears to have made them less vulnerable to impingement. 

Annual Losses 

Impingement sampling during 2000-2001 and 2001-2002 has provided two years of 

estimated seasonal and annual impingement rates occurring at the MNS intake, and the 

best available estimate of current impingement for several reasons. Data collected by the 

2000-2002 monitoring programs are representative of the recent relative abundance of 
4-9

fish species in the vicinity of the intake and their susceptibility to impingement. In 2000-

2002, station operation was consistent with its use as a baseload facility, with cooling 

water flow rates of approximately 90 percent capacity (Table 2-1). 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 estimated adjusted annual 

totals of 2,996 impinged fish in 2000-2001 and 1,238 fish in 2001-2002 (Table 4-2). An 

adjusted estimate for Full Operations can be derived by substituting the design capacity 

for cooling water flow rates (2,926 MGD) rather than actual mean daily flow rates 

recorded during the 12-month sampling interval. Impingement losses under full 

operation are estimated to total 3,639 fish in 2000-2001 (Table 4-10) and 1,395 fish in 

2001-2002 (Table 4-11). These totals represent a 21 percent and 13 percent increase, 

respectively, in the numbers and biomass of impinged fish over Normal Operations. 

Annual Variability 

Data collected by the 2000-2002 monitoring programs are representative of relatively 

current conditions, especially the 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, or fish abundance varied to a significant 

degree. 

Intake Operation 

MNS is operated as a baseload facility, with an average annual capacity factor of 94.5 

percent during 2000-2007 (Section 2). The average annual intake flow rate for this 

period (2000-2007) was 89.7 percent of the maximum design capacity. MNS 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 for threadfin shad during 2000-2002 appeared to be influenced by low 

water temperatures. Peak impingement of threadfin shad occurred when daily mean 

water temperatures were <50o

F. The incidence of stressful water temperatures <50oF 

might influence impingement rates for threadfin shad at MNS in future years. 
Impingement Mortality 

4-10

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. 

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 based on 

fish abundance alone. Whole lake populations of threadfin shad and alewife, which 

contribute the greatest number of fish to impingement counts at MNS, 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. 

Other species such as Morone spp. also appear to be susceptible to impingement at MNS 

because of relatively pelagic distribution. 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). 

Comparison of Annual Monitoring Programs 

The results of two or more annual monitoring programs can be compared to evaluate 

inter-annual variability impingement at MNS. Impingement sampling during two 

consecutive years revealed an over two-fold variation in the total numbers of fish 

impinged and collected (1,690 in 2000-2001 vs. 698 in 2001-2002). Species composition 

also varied between the two years, with threadfin shad dominating impingement 

collections in 2000-2001, the year of greater impingement, and alewife dominating the 

collections in 2001-2002. This shift in dominant species appeared to be related to the 

unusual abundance of alewives during 2001-2002, when they comprised approximately 

25 percent of the forage species (alewife, threadfin shad, and gizzard shad) caught in 

purse seines. Alewives first appeared in purse seine catches in 1999 and their abundance 

relative to the other two clupeid species peaked in 2001 and 2002 (Table 4-9). 
4-11

0

20

40

60

80

100

120

012345678

Days since release

# recovered

Figure 4-1 Plot of Number of Marked Fish Recovered After One to Seven Days of 

Traveling Screen Impingement at MNS, 21-27 July 2005 
Impingement Mortality 

4-12

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

0

100

200

300

400

500

Impingement Numbers for All Species Combined

2000 - 2001

Impingement Numbers

Figure 4-2 Seasonal Pattern of Impingement Rates for All Species Combined, December 

2000 – November 2001 
4-13

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

0

20

40

60

80

100

Impingement Numbers for All Species Combined

2001 - 2002

Impingement Numbers

Figure 4-3 Seasonal Pattern of Impingement Rates for All Species Combined, December 

2001 – November 2002 
Impingement Mortality 

4-14

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

0

5

10

15

20

25

30

35

40

45

50

55

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

0

20

40

60

80

100

120

140

160

180

200

 Young-of-Year

 Yearling & Older

 Undetermined

Threadfin Shad

2000 - 2001

Impingement Numbers by Age

Length Frequency

 Percent in Length Interval

 Midpoint of Length Interval (cm)

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

Shad, December 2000 – November 2001 
4-15

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

0

10

20

30

40

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

0

5

10

15

20

25

30

35

40

45

 Young-of-Year

 Yearling & Older

Alewife

2000 - 2001

Impingement Numbers by Age

Length Frequency

 Percent in Length Interval

 Midpoint of Length Interval (cm)

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

December 2000 – November 2001 
Impingement Mortality 

4-16

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

0

10

20

30

40

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

0

5

10

15

20

25

 Young-of-Year

 Yearling & Older

Alewife

2001 - 2002

Impingement Numbers by Age

Length Frequency

 Percent in Length Interval

 Midpoint of Length Interval (cm)

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

December 2001 – November 2002 
4-17

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

0

5

10

15

20

25

30

35

40

45

50

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

0

2

4

6

8

10

12

14

 Young-of-Year

 Yearling & Older

 Undetermined

Threadfin Shad

2001 - 2002

Impingement Numbers by Age

Length Frequency

 Percent in Length Interval

 Midpoint of Length Interval (cm)

Figure 4-7 Seasonal Impingement Pattern and Length Frequency of Impinged Threadfin 

Shad, December 2001 – November 2002 
Impingement Mortality 

4-18

0 10 20 30 40 50 60 70

0

5

10

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

0

2

4

6

8

10

 Young-of-Year

 Yearling & Older

 Undetermined

White perch

2000 - 2001

Impingement Numbers by Age

Length Frequency

 Percent in Length Interval

 Midpoint of Length Interval (cm)

Figure 4-8 Seasonal Impingement Pattern and Length Frequency of Impinged White 

Perch, December 2000 – November 2001 
4-19

0 10 20 30 40 50 60 70

0

5

10

15

20

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

0

2

4

6

8

10

12

14

16

18

20

 Young-of-Year

 Yearling & Older

 Undetermined

White perch

2001 - 2002

Impingement Numbers by Age

Length Frequency

 Percent in Length Interval

 Midpoint of Length Interval (cm)

Figure 4-9 Seasonal Impingement Pattern and Length Frequency of Impinged White 

Perch, December 2001 – November 2002 
Impingement Mortality 

4-20

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

0

2

4

6

8

10

12

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

0

1

2

3

4

5

 Young-of-Year

 Yearling & Older

 Undetermined

Striped bass

2000 - 2001

Impingement Numbers by Age

Length Frequency

 Percent in Length Interval

 Midpoint of Length Interval (cm)

Figure 4-10 Seasonal Impingement Pattern and Length Frequency of Impinged Striped 

Bass, December 2000 – November 2001 
4-21

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

0

2

4

6

8

10

12

14

16

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

0

1

2

3

4

5

 Young-of-Year

 Yearling & Older

 Undetermined

Striped bass

2001 - 2002

Impingement Numbers by Age

Length Frequency

 Percent in Length Interval

 Midpoint of Length Interval (cm)

Figure 4-11 Seasonal Impingement Pattern and Length Frequency of Impinged Striped 

Bass, December 2001 – November 2002 
Impingement Mortality 

4-22

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

0

5

10

15

20

25

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

0

1

2

3

4

5

6

7

 Young-of-Year

 Yearling & Older

 Undetermined

Bluegill

2000 - 2001

Impingement Numbers by Age

Length Frequency

 Percent in Length Interval

 Midpoint of Length Interval (cm)

Figure 4-12 Seasonal Impingement Pattern and Length Frequency of Impinged Bluegill, 

December 2000 – November 2001 
4-23

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

0

5

10

15

20

25

30

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

0

1

2

3

4

5

6

7

 Young-of-Year

 Yearling & Older

 Undetermined

Bluegill

2001 - 2002

Impingement Numbers by Age

Length Frequency

 Percent in Length Interval

 Midpoint of Length Interval (cm)

Figure 4-13 Seasonal Impingement Pattern and Length Frequency of Impinged Bluegill, 

December 2001 – November 2002 
Impingement Mortality 

4-24

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

0

2

4

6

8

10

12

14

16

18

20

22

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

0

1

2

3

4

5

6

7 Young-of-Year

 Yearling & Older

 Undetermined

White bass

2000 - 2001

Impingement Numbers by Age

Length Frequency

 Percent in Length Interval

 Midpoint of Length Interval (cm)

Figure 4-14 Seasonal Impingement Pattern and Length Frequency of Impinged White 

Bass, December 2000 – November 2001 
4-25

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

0

2

4

6

8

10

12

14

16

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

0

1

2

3

4

5

6

7 Young-of-Year

 Yearling & Older

 Undetermined

White bass

2001 - 2002

Impingement Numbers by Age

Length Frequency

 Percent in Length Interval

 Midpoint of Length Interval (cm)

Figure 4-15 Seasonal Impingement Pattern and Length Frequency of Impinged White 

Bass, December 2001 – November 2002 
Impingement Mortality 

4-26

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

0

5

10

15

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

0

1

2

3

4

5

6

7

 Young-of-Year

 Yearling & Older

 Undetermined

Channel catfish

2000 - 2001

Impingement Numbers by Age

Length Frequency

 Percent in Length Interval

 Midpoint of Length Interval (cm)

Figure 4-16 Seasonal Impingement Pattern and Length Frequency of Impinged Channel 

Catfish, December 2000 – November 2001 
4-27

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

0

2

4

6

8

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

0

1

2

3

4

5

6

 Young-of-Year

 Yearling & Older

 Undetermined

Channel catfish

2001 - 2002

Impingement Numbers by Age

Length Frequency

 Percent in Length Interval

 Midpoint of Length Interval (cm)

Figure 4-17 Seasonal Impingement Pattern and Length Frequency of Impinged Channel 

Catfish, December 2001 – November 2002 
Impingement Mortality 

4-28

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

0

5

10

15

20

25

30

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

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

 Young-of-Year

 Yearling & Older

 Undetermined

Blue catfish

2000 - 2001

Impingement Numbers by Age

Length Frequency

 Percent in Length Interval

 Midpoint of Length Interval (cm)

Figure 4-18 Seasonal Impingement Pattern and Length Frequency of Impinged Blue 

Catfish, December 2000 – November 2001 
4-29

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

0

5

10

15

20

25

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

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

 Young-of-Year

 Yearling & Older

 Undetermined

Blue catfish

2001 - 2002

Impingement Numbers by Age

Length Frequency

 Percent in Length Interval

 Midpoint of Length Interval (cm)

Figure 4-19 Seasonal Impingement Pattern and Length Frequency of Impinged Channel 

Catfish, December 2001 – November 2002 
Impingement Mortality 

4-30

744

746

748

750

752

754

756

758

760

12/1/2000 2/1/2001 4/1/2001 6/1/2001 8/1/2001 10/1/2001

0

20

40

60

80

100

120

400

440

480

Date

All Taxa #'s Impinged 2000-2001

40

50

60

70

80

90

 Intake Temp.

Mean Intake Temp. (F)

 Elevation

 Elevation (ft)

Figure 4-20 Daily Lake Level and Intake Water Temperature vs. Number of Impinged Fish 

Collected for All Species Combined, December 2000-November 2001 
4-31

744

746

748

750

752

754

756

758

760

12/1/2001 2/1/2002 4/1/2002 6/1/2002 8/1/2002 10/1/2002

0

20

40

60

Date

All Taxa #'s Impinged 2001-2002

40

50

60

70

80

90

 Intake Temp.

Mean Intake Temp. (F)

 Elevation

 Elevation (ft)

Figure 4-21 Daily Lake Level and Intake Water Temperature vs. Number of Impinged Fish 

Collected for All Species Combined, December 2001-November 2002 
Impingement Mortality 

4-32

744

746

748

750

752

754

756

758

760

12/1/2000 2/1/2001 4/1/2001 6/1/2001 8/1/2001 10/1/2001

0

20

40

60

80

100

120

Date

YOY #'s Impinged 2000-2001

0

10

20

30

140

160

180

YAO #'s Impinged 2000-2001

40

50

60

70

80

90

 Intake Temp.

Mean Intake Temp. (F)

 Elevation

 Elevation (ft)

Figure 4-22 Daily Lake Level and Intake Water Temperature vs. Number of Impinged Fish 

Collected for Young-of-Year (YOY) and Yearling and Older (YAO) Threadfin 

Shad, December 2000-November 2001 
4-33

744

746

748

750

752

754

756

758

760

12/1/2001 2/1/2002 4/1/2002 6/1/2002 8/1/2002 10/1/2002

0

2

4

6

8

10

Date

YOY #'s Impinged 2001-2002

0

2

4

6

8

10

12

14

YAO #'s Impinged 2001-2002

40

50

60

70

80

90

 Intake Temp.

Mean Intake Temp. (F)

 Elevation

 Elevation (ft)

Figure 4-23 Daily Lake Level and Intake Water Temperature vs. Number of Impinged Fish 

Collected for Young-of-Year (YOY) and Yearling and Older (YAO) Threadfin 

Shad, December 2001-November 2002 
Impingement Mortality 

4-34

0

20

40

60

80

100

120

40.00 50.00 60.00 70.00 80.00 90.00

Average Intake Temperature (°F)

Total Numbers Impinged 

Threadfin Shad YOY (2000-2001)

0

20

40

60

80

100

120

140

160

180

40.00 50.00 60.00 70.00 80.00 90.00

Average Intake Temperature (°F)

Total Numbers Impinged

Threadfin Shad YAO (2000-2001)

Figure 4-24 Plots of Numbers of Impinged Young-of-Year (YOY) and Yearling and Older 

(YAO) Threadfin Shad Collected vs. Daily Mean Water Temperature at the 

MNS Intake, December 2000-November 2001 
4-35

0

1

2

3

4

5

6

7

8

9

40.00 50.00 60.00 70.00 80.00 90.00

Average Intake Temperature (°F)

Total Numbers Impinged 

Threadfin Shad YOY (2001-2002)

0

2

4

6

8

10

12

40.00 50.00 60.00 70.00 80.00 90.00

Average Intake Temperature (°F)

Total Numbers Impinged

Threadfin Shad YAO (2001-2002)

Figure 4-25 Plots of Numbers of Impinged Young-of-Year (YOY) and Yearling and Older 

(YAO) Threadfin Shad Collected vs. Daily Mean Water Temperature at the 

MNS Intake, December 2001-November 2002 
Impingement Mortality 

4-36

744

746

748

750

752

754

756

758

760

12/1/2000 2/1/2001 4/1/2001 6/1/2001 8/1/2001 10/1/2001

0

10

20

30

40

50

Date

YOY #'s Impinged 2000-2001

0

2

4

6

8

10

YAO #'s Impinged 2000-2001

40

50

60

70

80

90

 Intake Temp.

Mean Intake Temp. (F)

 Elevation

 Elevation (ft)

Figure 4-26 Daily Lake Level and Intake Water Temperature vs. Number of Impinged Fish 

Collected for Young-of-Year (YOY) and Yearling and Older (YAO) Alewife, 

December 2000-November 2001 
4-37

744

746

748

750

752

754

756

758

760

12/1/2001 2/1/2002 4/1/2002 6/1/2002 8/1/2002 10/1/2002

0

5

10

15

20

Date

YOY #'s Impinged 2001-2002

0

5

10

15

20

YAO #'s Impinged 2001-2002

40

50

60

70

80

90

 Intake Temp.

Mean Intake Temp. (F)

 Elevation

Elevation (ft)

Figure 4-27 Daily Lake Level and Intake Water Temperature vs. Number of Impinged Fish 

Collected for Young-of-Year (YOY) and Yearling and Older (YAO) Alewife, 

December 2001-November 2002 
Impingement Mortality 

4-38

0

5

10

15

20

25

30

35

40

45

40.00 50.00 60.00 70.00 80.00 90.00

Average Intake Temperature (°F)

Total Numbers Impinged 

Alewife YOY (2000-2001)

0

1

2

3

4

5

6

7

8

40.00 50.00 60.00 70.00 80.00 90.00

Average Intake Temperature (°F)

Total Numbers Impinged 

Alewife YAO (2000-2001)

Figure 4-28 Plots of Numbers of Impinged Young-of-Year (YOY) and Yearling and Older 

(YAO) Alewife Collected vs. Daily Mean Water Temperature at the MNS Intake, 

December 2000-November 2001 
4-39

0

2

4

6

8

10

12

14

16

18

20

40.00 50.00 60.00 70.00 80.00 90.00

Average Intake Temperature (°F)

Total Numbers Impinged 

Alewife YOY (2001-2002)

0

2

4

6

8

10

12

14

16

40.00 50.00 60.00 70.00 80.00 90.00

Average Intake Temperature (°F)

Total Numbers Impinged 

Alewife YAO (2001-2002)

Figure 4-29 Plots of Numbers of Impinged Young-of-Year (YOY) and Yearling and Older 

(YAO) Alewife Collected vs. Daily Mean Water Temperature at the MNS Intake, 

December 2001-November 2002 
Impingement Mortality 

4-40

Table 4-1 Number of Impinged Fish Collected by Weekly Sampling at MNS, December 

2000-November 2002 (ranked according to abundance) 

Species 12/00-11/01 % of Total 12/01-11/02 % of Total

Threadfin shad 871 51.54 134 19.20 

Alewife 215 12.72 166 23.78 

Bluegill 97 5.74 60 8.60 

White perch 61 3.61 107 15.33 

Channel catfish 54 3.20 19 2.72 

White bass 47 2.78 29 4.15 

Golden shiner 44 2.60 1 0.14 

Striped bass 44 2.60 50 7.16 

Gizzard shad 37 2.19 24 3.44 

Unidentified 31 1.83 29 4.15 

Flathead catfish 30 1.78 13 1.86 

Redbreast sunfish 29 1.72 14 2.01 

Yellow perch 23 1.36 2 0.29 

Warmouth 19 1.12 1 0.14 

Blue catfish 14 0.83 11 1.58 

Largemouth bass 13 0.77 6 0.86 

White crappie 12 0.71 0 0.00 

Unidentified catfish sp. 9 0.53 7 1.00 

Redear sunfish 9 0.53 12 1.72 

Black crappie 8 0.47 0 0.00 

Unidentified herring sp. 7 0.41 0 0.00 

Unidentified sunfish sp. 5 0.30 2 0.29 

Unidentified temperate bass 4 0.24 0 0.00 

Sunfish hybrid 3 0.18 0 0.00 

Rainbow trout 2 0.12 0 0.00 

Eastern silvery minnow 1 0.06 0 0.00 

Spotted bass 1 0.06 8 1.15 

Common carp 0 0.00 2 0.29 

Quillback 0 0.00 1 0.14 

Total 1690 100.00 698 100.00 
Table 4-2 Estimated Total Number of Fish Impinged at MNS, December 2000 - November 2001 and December 2001

(ranked according to abundance) 

2000-2001 2001-2002 

Species Unadjusted Adjusted A1 Adjusted B2 Unadjusted Adjusted A1 AdjusThreadfin shad 871 1405 1544 134 216 Alewife 215 347 381 166 268 Bluegill 97 156 172 60 97 White perch 61 98 108 107 173 Channel catfish 54 87 96 19 31 White bass 47 76 83 29 47 Golden shiner 44 71 78 1 2 Striped bass 44 71 78 50 81 Gizzard shad 37 60 66 24 39 Unidentified 31 50 55 29 47 Flathead catfish 30 48 53 13 21 Redbreast sunfish 29 47 51 14 23 Yellow perch 23 37 41 2 3 Warmouth 19 31 34 1 2 Blue catfish 14 23 25 11 18 Largemouth bass 13 21 23 6 10 White crappie 12 19 21 0 0 Unidentified catfish 9 15 16 7 11 Redear sunfish 9 15 16 12 19 Black crappie 8 13 14 0 0 Unidentified herring 7 11 12 0 0 Unidentified sunfish 5 8 9 2 3 Unidentified temperate bass 4 6 7 0 0 Sunfish hybrid 3 5 5 0 0 Rainbow trout 2 3 4 0 0 Eastern silvery minnow 1 2 2 0 0 Spotted bass 1 2 2 8 13 Common carp 0 0 0 2 3 Quillback 0 0 0 1 2 Total 1690 2726 2996 698 1126 1 62 % recovery rate (Barwick et al. 2005) 2 56.4% recovery rate based on linear relationship of test fish recoveries 
Impingement Mortality 

4-42Table 4-3 Maximum Total Length (mm) Used to Distinguish Young-of-Year from Yearling Fish by Month

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 0 110 110 110 115 120 35 65 85 95 110  1 155 155 155 160 165 130 140 150 155 155 Bluegill 0 70 70 70 75 80 85 30 45 60 70  1 125 125 125 130 135 145 95 110 120 125 Striped bass 0 190 190 195 215 240 270 100 130 160 190  1 385 385 390 405 430 460 300 330 360 385 White bass 0 145 145 150 160 170 190 80 100 120 140  1 240 240 245 255 265 280 210 220 230 240 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: Grist (2002), Nigro and Ney (1982), Pierce et al. (2003), van Horn et al. (1999), Carlander (1997), Hales and Belk (1992) 
4-43

Table 4-4 Estimated Annual Impingement of Fish by Age Group at all Units of MNS, 

December 2000 - November 2001 

Young-of-Year Yearling & Older Undetermined 

Species Count Percent Count Percent Count Percent 

Alewife 198 93.40 14 6.60 0 0.00 

Gizzard shad 0 0.00 0 0.00 32 100.00 

Threadfin shad 313 45.43 376 54.57 0 0.00 

Unidentified clupeid 0 0.00 0 0.00 3 100.00 

Eastern silvery minnow 0 0.00 0 0.00 1 100.00 

Golden shiner 0 0.00 0 0.00 43 100.00 

Blue catfish 0 0.00 17 100.00 0 0.00 

Channel catfish 4 8.00 46 92.00 0 0.00 

Flathead catfish 0 0.00 0 0.00 33 100.00 

Unidentified catfish 0 0.00 0 0.00 8 100.00 

Rainbow trout 0 0.00 0 0.00 1 100.00 

Striped bass 0 0.00 41 95.35 2 4.65 

White bass 0 0.00 45 100.00 0 0.00 

White perch 12 19.35 50 80.65 0 0.00 

Hybrid sunfish 0 0.00 0 0.00 3 100.00 

Lepomis spp. 0 0.00 0 0.00 1 100.00 

Redbreast sunfish 0 0.00 0 0.00 32 100.00 

Bluegill 44 41.51 62 58.49 0 0.00 

Redear sunfish 0 0.00 0 0.00 9 100.00 

Unidentified sunfish 0 0.00 0 0.00 3 100.00 

Warmouth 0 0.00 0 0.00 18 100.00 

Largemouth bass 0 0.00 0 0.00 13 100.00 

Spotted bass 0 0.00 0 0.00 2 100.00 

Black crappie 0 0.00 0 0.00 8 100.00 

White crappie 0 0.00 0 0.00 12 100.00 

Yellow perch 0 0.00 0 0.00 23 100.00 

Unidentified remains 0 0.00 0 0.00 13 100.00 

Total 571 38.53 651 43.93 260 17.54 
Impingement Mortality 

4-44

Table 4-5 Estimated Annual Impingement of Fish by Age Group at all Units of MNS, 

December 2001 - November 2002 

Young-of-Year Yearling & Older Undetermined 

Species Count Percent Count Percent Count Percent 

Alewife 126 75.45 41 24.55 0 0.00 

Gizzard shad 0 0.00 0 0.00 21 100.00 

Threadfin shad 59 44.03 75 55.97 0 0.00 

Unidentified shad 0 0.00 0 0.00 1 100.00 

Common carp 0 0.00 0 0.00 2 100.00 

Golden shiner 0 0.00 0 0.00 1 100.00 

Quillback 0 0.00 0 0.00 1 100.00 

Blue catfish 0 0.00 10 100.00 0 0.00 

Channel catfish 3 16.67 15 83.33 0 0.00 

Flathead catfish 0 0.00 0 0.00 15 100.00 

Unidentified catfish 0 0.00 0 0.00 5 100.00 

Striped bass 1 2.08 45 93.75 2 4.17 

White bass 0 0.00 28 100.00 0 0.00 

White perch 31 30.10 71 68.93 1 0.97 

Largemouth bass 0 0.00 0 0.00 6 100.00 

Redbreast sunfish 0 0.00 0 0.00 15 100.00 

Bluegill 21 35.00 39 65.00 0 0.00 

Redear sunfish 0 0.00 0 0.00 12 100.00 

Unidentified sunfish 0 0.00 0 0.00 2 100.00 

Warmouth 0 0.00 0 0.00 1 100.00 

Yellow perch 0 0.00 0 0.00 2 100.00 

Unidentified 0 0.00 0 0.00 7 100.00 

Total 241 36.57 324 49.17 94 14.26 
Table 4-6 Adjusted* Monthly Impinged Fish Collections at MNS, December 2000-November 2001 at Actual Flows 

Taxon 

Dec00 Jan- 01 Feb- 01 Mar- 01 Apr- 01 May- 01 Jun- 01 Jul- 01 Aug- 01 Sep- 01 Oct- 01 Nov- 01 

Alewife 9 5 4 0 4 0 30 94 46 43 92 55 Gizzard shad 2 7 4 4 4 4 2 18 4 7 7 5 Threadfin shad 183 1188 41 27 4 4 4 12 2 4 16 62 Unidentified herring 0 0 0 0 0 0 2 0 0 7 4 0 Eastern silvery minnow 2 0 0 0 0 0 0 0 0 0 0 0 Golden shiner 0 7 9 2 4 2 35 12 2 0 2 4 Blue catfish 2 0 7 2 0 2 0 5 4 2 2 0 Channel catfish 2 4 9 2 12 7 2 28 7 7 5 11 Flathead catfish 0 0 2 0 0 4 21 14 11 2 0 0 Unidentified catfish 0 0 0 0 0 0 2 9 4 0 0 2 Rainbow trout 0 0 4 0 0 0 0 0 0 0 0 0 Striped bass 2 2 0 0 4 5 5 16 11 20 9 5 Unidentified temperate bass 0 0 0 0 0 0 4 4 0 0 0 0 White bass 11 2 0 0 2 20 20 7 11 4 4 5 White perch 4 0 0 4 0 0 4 18 12 21 27 20 Hybrid sunfish 0 0 0 0 0 2 2 2 0 0 0 0 Redbreast sunfish 4 0 4 4 0 5 12 12 4 5 0 2 Bluegill 12 4 20 16 32 11 9 14 18 11 16 11 Redear sunfish 0 2 0 2 2 2 0 0 2 2 2 4 Unidentified sunfish 0 0 0 0 0 0 5 0 4 0 0 0 Largemouth bass 0 2 0 0 2 4 11 4 2 0 0 0 Spotted bass 0 0 0 0 0 0 0 0 2 0 0 0 Warmouth 0 0 2 2 16 11 0 2 0 0 0 2 Black crappie 0 0 2 4 0 0 5 0 0 0 0 4 White crappie 2 2 5 5 0 4 2 2 0 0 0 0 Yellow perch 0 0 0 0 0 0 7 28 5 0 0 0 Unidentified fish 0 0 0 4 0 7 12 7 11 9 0 5 TOTAL 232 1223 110 74 83 90 195 309 158 142 184 195 

*Based on 56.4 percent recovery rate of released marked fish in July 2005 special study 
Impingement Mortality 

4-46Table 4-7 Adjusted* Monthly Impinged Fish Collections at MNS, December 2001-November 2002 at Actual Flows 

Taxon 

Dec01 Jan- 02 Feb- 02 Mar- 02 Apr- 02 May- 02 Jun- 02 Jul- 02 Aug- 02 Sep- 02 Oct- 02 Nov

02 

Alewife 11 5 0 7 0 2 16 25 156 25 25 2Gizzard shad 4 5 0 2 2 2 4 5 0 7 7 Threadfin shad 32 67 60 11 4 2 4 5 5 7 12 2Common carp 0 0 0 0 0 2 0 0 2 0 0 Golden shiner 0 0 0 0 0 0 0 0 0 0 0 Quillback 0 0 0 0 2 0 0 0 0 0 0 Blue catfish 2 0 0 0 0 0 0 0 12 0 0 Channel catfish 0 7 2 0 2 5 0 0 4 4 9 Flathead catfish 0 0 0 0 0 2 5 11 2 0 4 Unidentified catfish 0 0 0 0 0 0 4 4 4 0 2 Striped bass 11 4 2 0 0 2 4 4 20 12 12 2White perch 5 2 7 0 2 7 0 27 55 14 35 3White bass 16 9 0 2 2 11 4 0 5 2 0 Redbreast sunfish 0 2 4 4 0 4 0 4 5 2 0 Bluegill 9 12 11 7 4 20 5 2 21 0 12 Redear sunfish 0 4 5 4 4 2 0 0 0 4 0 Unidentified sunfish 2 0 0 0 0 0 0 2 0 0 0 Largemouth bass 0 0 2 0 0 4 4 0 0 2 0 Spotted bass 0 0 0 0 0 2 7 2 0 2 2 Warmouth 0 0 0 2 0 0 0 0 0 0 0 Yellow perch 0 0 0 0 0 0 4 0 0 0 0 Unidentified fish 2 0 0 0 2 2 0 12 27 5 0 TOTAL 92 117 92 37 21 66 59 101 317 85 121 12

*Based on 56.4 percent recovery rate of released marked fish in July 2005 special study 
Table 4-8 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 Threadfin shad 100 99.94 99.95 100 99.99 99.95 99.26 87.4 76.47 74.96 82.59 86.55 Gizzard shad 0 0.06 0.05 0 0.01 0.05 0.26 0.22 0.01 0 0.14 0.24 Alewife 0 0 0 0 0 0 0.48 12.37 23.52 25.04 17.27 13.2 
Impingement Mortality 

4-48

Table 4-9 Comparison of Relative Abundance of Fish Species in 2001and 2002 Littoral 

Zone Electrofishing Surveys and the December 2000- November 2002 MNS 

Impingement Collections (Unadjusted) 

2001-2002 Electrofishing 2001-2002 Impingement 

Species Total No. % Rank Total No. % Rank

Bluegill 2949 31.41 1 157 6.57 4 

Whitefin shiner 2263 24.10 2 - - NA 

Redbreast sunfish 1080 11.50 3 43 1.80 10 tie

Redear sunfish 773 8.23 4 21 0.88 14 

Spottail shiner 705 7.51 5 - - NA 

Largemouth bass 515 5.49 6 19 0.80 16 

Blue catfish 275 2.93 7 25 1.05 12 tie

Hybrid sunfish 189 2.01 8 3 0.13 20 

Threadfin shad 128 1.36 9 1,005 42.09 1 

Greenfin shiner 97 1.03 10 - - NA 

Common carp 85 0.91 11 2 0.08 21 tie

Gizzard shad 81 0.86 12 tie 61 2.55 8 

Warmouth 81 0.86 12 tie 20 0.84 15 

Alewife 48 0.51 14 381 15.95 2 

Channel catfish 30 0.32 15 tie 73 3.06 7 

Spotted bass 30 0.32 15 tie 9 0.38 18 

Black crappie 12 0.13 17 8 0.34 19 

Flathead catfish 9 0.10 18 43 1.80 10 tie

Green sunfish 7 0.07 19 - - NA 

Yellow perch 6 0.06 20 25 1.05 12 tie

Pumpkinseed 4 0.04 21 - - NA 

Longnose gar 3 0.03 22 tie - - NA 

Quillback 3 0.03 22 tie 1 0.04 23 tie

Shorthead redhorse 3 0.03 22 tie - - NA 

Tessellated darter 3 0.03 22 tie - - NA 

White bass 3 0.03 22 tie 76 3.18 6 

White perch 3 0.03 22 tie 168 7.04 3 

Bowfin 1 0.01 28 tie - - NA 

Rainbow trout 1 0.01 28 tie 2 0.08 21 tie

Striped bass 1 0.01 28 tie 94 3.94 5 

Walleye 1 0.01 28 tie 0 - NA 

Golden shiner 0 0 NA 45 1.88 9 

White crappie 0 0 NA 12 0.50 17 

Eastern silvery minnow 0 0 NA 1 0.04 23 tie

Total 9389 100.00 2,294 96.07 
Table 4-10 Adjusted* Monthly Impinged Fish Collections at MNS, December 2000-November 2001 at Full Design F

Taxon 

Dec00 Jan- 01 Feb- 01 Mar- 01 Apr- 01 May- 01 Jun- 01 Jul- 01 Aug- 01 Sep- 01 Oct- 01 Nov

01 

Alewife 12 7 5 0 5 0 30 94 46 43 92 5Gizzard shad 2 9 5 7 5 4 2 18 4 7 7 Threadfin shad 243 1584 55 51 5 4 4 12 2 4 16 6Unidentified herring 0 0 0 0 0 0 2 0 0 7 4 Eastern silvery minnow 2 0 0 0 0 0 0 0 0 0 0 Golden shiner 0 9 12 3 5 2 35 12 2 0 2 Blue catfish 2 0 10 3 0 2 0 5 4 2 2 Channel catfish 2 5 12 3 18 7 2 28 7 7 5 1Flathead catfish 0 0 2 0 0 4 21 14 11 2 0 Unidentified catfish 0 0 0 0 0 0 2 9 4 0 0 Rainbow trout 0 0 5 0 0 0 0 0 0 0 0 Striped bass 2 2 0 0 5 6 5 16 11 20 9 Unidentified temperate bass 0 0 0 0 0 0 4 4 0 0 0 White bass 14 2 0 0 3 20 20 7 11 4 4 White perch 5 0 0 7 0 0 4 18 12 21 27 2Hybrid sunfish 0 0 0 0 0 2 2 2 0 0 0 Redbreast sunfish 5 0 5 7 0 6 12 12 4 5 0 Bluegill 17 5 26 31 47 11 9 14 18 11 16 1Redear sunfish 0 2 0 3 3 2 0 0 2 2 2 Unidentified sunfish 0 0 0 0 0 0 5 0 4 0 0 Warmouth 0 0 2 3 23 11 0 2 0 0 0 Largemouth bass 0 2 0 0 3 4 11 4 2 0 0 Spotted bass 0 0 0 0 0 0 0 0 2 0 0 Black crappie 0 0 2 7 0 0 5 0 0 0 0 White crappie 2 2 7 10 0 4 2 2 0 0 0 Yellow perch 0 0 0 0 0 0 7 28 5 0 0 Unidentified fish 0 0 0 7 0 7 12 7 11 9 0 TOTAL 310 1631 148 144 122 95 195 309 158 142 184 20

*Based on 56.4 percent recovery rate of released marked fish in July 2005 special study
Impingement Mortality 

4-50Table 4-11 Adjusted* Monthly Impinged Fish Collections at MNS, December 2001-November 2002 at Full Design F

Taxon 

Dec01 Jan- 02 Feb- 02 Mar- 02 Apr- 02 May- 02 Jun- 02 Jul- 02 Aug- 02 Sep- 02 Oct- 02 Nov

02 

Alewife 12 7 0 13 0 2 16 25 156 33 25 2Gizzard shad 4 7 0 3 2 2 4 5 0 10 7 Threadfin shad 35 90 85 20 4 2 4 5 5 10 13 2Common carp 0 0 0 0 0 2 0 0 2 0 0 Golden shiner 0 0 0 0 0 0 0 0 0 0 0 Quillback 0 0 0 0 2 0 0 0 0 0 0 Blue catfish 2 0 0 0 0 0 0 0 12 0 0 Channel catfish 0 9 2 0 2 5 0 0 4 5 9 Flathead catfish 0 0 0 0 0 2 5 11 2 0 4 Unidentified catfish 0 0 0 0 0 0 4 4 4 0 2 Striped bass 12 5 2 0 0 2 4 4 20 17 13 2White perch 6 2 10 0 2 7 0 27 55 19 36 3White bass 17 12 0 3 2 11 4 0 5 2 0 Redbreast sunfish 0 2 5 7 0 4 0 4 5 2 0 Bluegill 10 17 15 13 4 20 5 2 21 0 13 Redear sunfish 0 5 7 7 4 2 0 0 0 5 0 Unidentified sunfish 2 0 0 0 0 0 0 2 0 0 0 Largemouth bass 0 0 2 0 0 4 4 0 0 2 0 Spotted bass 0 0 0 0 0 2 7 2 0 2 2 Warmouth 0 0 0 3 0 0 0 0 0 0 0 Yellow perch 0 0 0 0 0 0 4 0 0 0 0 Unidentified fish 2 0 0 0 2 2 0 12 27 7 0 TOTAL 100 156 130 69 26 66 59 101 317 115 123 13

*Based on 56.4 percent recovery rate of released marked fish in July 2005 special study
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 

MNS 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 MNS? 

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 MNS 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 MNS’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 MNS. 

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 MNS 

on the fish community composition and function and on recreational fishing in Lake 

Norman. 

As described in Section 4, impingement monitoring studies were conducted at MNS over 

the two-year period December 2000 – November 2002. The results of these studies were 

used to estimate the expected annual impingement at MNS for each year under normal 

operations: 

Taxon 

 Dec 00 

- Nov 01 

 Dec 01 - 

Nov 02 Combined 

 Threadfin shad 1,544 238 1,782 

 Alewife 381 294 675 

 White perch 108 190 298 

 Bluegill 172 106 278 

 Striped bass 78 89 167 

 White bass 83 51 134 

 Channel catfish 96 34 130 

 Gizzard shad 66 43 109 

 Unidentified fish 55 51 106 

 Golden shiner 78 2 80 

 Flathead catfish 53 23 76 
Assessment of Adverse Environmental Impact 

5-4

 Redbreast sunfish 51 25 76 

 Blue catfish 25 20 45 

 Yellow perch 41 4 45 

 Redear sunfish 16 21 37 

 Warmouth 34 2 36 

 Largemouth bass 23 11 34 

 Unidentified catfish 16 12 28 

 White crappie 21 21 

 Spotted bass 2 14 16 

 Black crappie 14 14 

 Unidentified sunfish 9 4 13 

 Unidentified herring 12 12 

 Unidentified temperate bass 7 7 

 Hybrid sunfish 5 5 

 Common carp 4 4 

 Rainbow trout 4 4 

 Eastern silvery minnow 2 2 

 Quillback 2 2 

 TOTAL 2,996 1,240 4,236 

Length frequency distributions of the most abundance species collected presented in 

Section 4 demonstrate most were small and most likely either age 0 or age 1 individuals. 

Two pelagic forage species, threadfin shad and alewife, accounted for 43 to 64 percent of 

the fish impinged annually based on the results of this study. 

All but two of the species collected had estimates of annual impingement that averaged 

less than one fish per day in each year. 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. 

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 less than 2,000 individuals. 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 since 1997 has ranged from 47 million fish in 2004 to 91 million 

fish in 1998.. At these levels, the removal of these species by impingement at McGuire 

amounts to < 0.01 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 
5-5

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 MNS 

intake. The retrospective approach evaluated whether effects on individual species 

populations or the community as a whole are apparent by examining trends in the species 

richness and relative abundance of individual species during the time that MNS 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 MNS 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 
Assessment of Adverse Environmental Impact 

5-6

(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 (Section 4, Table 4-8). After first appearing in 

Lake 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 also 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). 

Threadfin shad and bluegill were the two most abundant species of the eight target 

species (threadfin shad, alewife, channel catfish, blue catfish, white perch, bluegill, white 

bass, and striped bass) in the 1993-2007 electrofishing catches (Figures 5-3 through 5-5). 

Since threadfin shad and alewife are pelagic species, the hydroacoustic data presented in 

Figure 5-1 may be a more accurate reflection of their annual abundance than the littoral 

zone electrofishing data (Figure 5-3). Channel catfish, blue catfish, white bass, striped 

bass, and white perch were caught less frequently 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 5-6). 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). 
5-7

Trophic Structure 

Species caught by annual spring electrofishing surveys were divided into trophic guilds 

consisting of insectivores, omnivores, herbivores, and piscivores (Table 5-1) 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 5-7). Omnivores and piscivores varied annually but each had a 14-

year mean of 11 percent. The eastern silvery minnow was the only herbivorous species 

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 5-7). 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-

7). 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. 

Retrospective Summary 

The fish community of Lake Norman has changed since the commencement of operation 

of MNS 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 
Assessment of Adverse Environmental Impact 

5-8

community changes are related to operation of the MNS cooling water intake and 

impingement losses. 

Effects on Human Uses 

Lake Norman, from which MNS 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 MNS 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 eight 

target species: threadfin shad, alewife, blue catfish, channel catfish, white perch, bluegill, 

striped bass, and white bass. Together, these eight species comprised approximately 83 

percent of annual impingement at MNS in each year. 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 MNS 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 MNS. 

 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 MNS. 

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 MNS. The monthly estimates were then summed to calculate total annual 

values. 

Only six of the eight target species evaluated in this assessment directly support fisheries 

in Lake Norman. Annual fishery yield that could result from the fish impinged at MNS 

totaled 111 lb (50.5 kg) under actual cooling water flows based on sampling from 
5-9

December 2000 through November 2001 and 168 lb (76.4 kg) based on sampling from 

December 2001 through November 2002 (Table 5-2). Under maximum (design) cooling 

water flows, annual fishery yield totaled 215 lb (97.7 kg) based on sampling from 

December 2000 through November 2001 and 183 lb (83.2 kg) based on sampling from 

December 2001 through November 2002. Most (>90 percent) of this equivalent yield 

was through striped and white bass. 

Annual production from the target species impinged at MNS totaled 111 lb (50.5 kg) 

under actual cooling water flows based on sampling from December 2000 through 

November 2001 and 152 lb (69.1 kg) based on sampling from December 2001 through 

November 2002 (Table 5-3). Under maximum (design) cooling water flows, annual 

production foregone totaled 249 lb (113.2 kg) based on sampling from December 2000 

through November 2001 and 166 lb (75.5 kg) based on sampling from December 2001 

through November 2002. Three species, bluegill, channel catfish and striped bass, 

accounted for just over 80 percent of the total annual production foregone. In the absence 

of impingement at MNS, 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 
Assessment of Adverse Environmental Impact 

5-10

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 MNS totaled $281 under actual 

cooling water flows based on sampling from December 2000 through November 2001 

and $431 based on sampling from December 2001 through November 2002 (Table 5-4). 

Under maximum (design) cooling water flows, annual production foregone totaled $546 

based on sampling from December 2000 through November 2001 and $470 based on 

sampling from December 2001 through November 2002. Most of this economic value 

(~74 percent) was attributable to the loss of striped bass or white bass. 

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 5-8. Uncertainty in recreational 

fishing price yielded the greatest range in estimates of annual economic value, -60 to 

+164 percent based December 2000 through November 2001 sampling and -61 to 173 

percent based on December 2001 through November 2002 sampling. Uncertainty in the 

fishing vulnerability yielded the second greatest range, -90 to -4 percent based December 

2000 through November 2001 sampling and -92 to 6 percent based on December 2001 

through November 2002 sampling. 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 

mortality rate, weight, trophic conversion factor, and equivalent predatory exploitation 

rate) had relatively minor effects on estimates of annual economic value (< + 30 percent). 
5-11

The results of the Monte Carlo analysis are illustrated in Figure 5-9. 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 49 percent higher for sampling from December 2000 through November 2001 and 62 

percent higher for sampling from December 2001 through November 2002 than the most 

probable estimate. The 25th and 75th percentiles were 30 percent and 70 percent higher, 

respectively, for sampling from December 2000 through November 2001 and 40 percent 

and 84 percent higher, respectively, for sampling from December 2001 through 

November 2002. There was less than a 10 percent chance that the true annual economic 

benefit was more than 2.1 times the most probable estimate reported in Table 5-4. 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 

recreational price per pound and the fish vulnerability rate, described above. 

Conclusions 

The results of this assessment provide no evidence that impingement at MNS is causing 

effects within Lake Norman that rise to a level that could be considered an “adverse 

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 two species, 

removal rates amounted to less than 1 fish per day. It is improbable 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 0.01 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 MNS 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 MNS cooling 

water intake and impingement losses. 

5. Estimates of total fishery catch that could result from the fish impinged at MNS 

had they survived amount to only 111 to 168 lb (50.5 - 76.4 kg) per year. That 
Assessment of Adverse Environmental Impact 

5-12

amount translates to one-half pound per day or less. 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 MNS 

amounted to just over 111 - 152 lb (50.5 - 69.1 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 0.2 

ounces per surface acre (37 g/ha) across the entire reservoir. 

7. The best estimate of the total economic value of fish impinged at MNS was $281 

- 431 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 insignificant compared to the likely total economic value of the 

recreational fishery in Lake Norman. 
5-13

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 
Assessment of Adverse Environmental Impact 

5-14

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 
5-15

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 
Assessment of Adverse Environmental Impact 

5-16

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

1994 1996 1998 2000 2002 2004 2006

0

40

80

120

160

White perch

Total Catch

0

10

20

30

40

50

60

Bluegill Channel catfish

Figure 5-4 Total Annual Electrofishing Catch for Bluegill, Channel Catfish and White 

Perch at 30 100-m Transects in Lake Norman, 1993-1997 and 1999-2007 
5-17

1994 1996 1998 2000 2002 2004 2006

0

2

4

6

8

10

Total Catch

White bass

0

2

4

6

8

10

Striped bass

Figure 5-5 Total Annual Electrofishing Catch for Striped Bass and White Bass at 30 100-

m Transects in Lake Norman, 1993-1997 and 1999-2007 
Assessment of Adverse Environmental Impact 

5-18

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 5-6 Species Composition in the Littoral Zone of Lake Norman as Determined by Electrofishing in 1993-1997 

(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 

Redbreast sunfish 

Green sunfish Pumpkinseed 

Bluegill 

Redear sunfish Hybrid sunfish Warmouth Black crappie White crappie 

Spotted bass 

Largemouth bass Hybrid black bass 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 5-6 continued 
Assessment of Adverse Environmental Impact 

5-20

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 5-7 Percent Composition of Fish Trophic Guilds in the Littoral Zone of Lake 

Norman Based on Electrofishing Results of 1993-1997 and 1999-2007
5-21

Figure 5-8 Estimates of the Range of Effects of Uncertainty in Each Input Parameter on 

Estimates of Annual Economic Benefit at MNS 
Assessment of Adverse Environmental Impact 

5-22

Figure 5-9 Results of Monte Carlo Analysis of Uncertainty in Key Input Parameters on 

Estimates of Annual Economic Benefits at MNS
5-23

Table 5-1 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 
Assessment of Adverse Environmental Impact 

5-24

Table 5-2 Estimates of Potential Total Annual Equivalent Fishery Yield (lb) from 

Individuals Impinged for Each Target Species at MNS 

Species 

Dec 2000 - Nov 2001 Dec 2001 - Nov 2002

Actual 

Flow 

Maximum 

Flow 

Actual 

Flow 

Maximum 

Flow 

Alewife 0 0 0 0 

Blue catfish 2 3 3 3 

Bluegill 3 6 3 3 

Channel catfish 4 7 3 3 

Striped bass 24 44 52 57 

Threadfin shad 0 0 0 0 

White bass 78 153 104 114 

White perch 1 2 3 4 

Total 111 215 168 183 
5-25

Table 5-3 Estimates of Potential Total Annual Production Foregone (lb) from Individuals 

Impinged for Each Target Species at MNS 

Species 

Dec 2000 - Nov 2001 Dec 2001 - Nov 2002

Actual 

Flow 

Maximum 

Flow 

Actual 

Flow 

Maximum 

Flow 

Alewife 2 4 5 5 

Blue catfish 7 15 12 12 

Bluegill 24 51 28 30 

Channel catfish 24 48 16 18 

Striped bass 53 98 79 85 

Threadfin shad 9 21 2 3 

White bass 4 7 3 4 

White perch 2 5 7 8 

Total 125 249 152 166 
Assessment of Adverse Environmental Impact 

5-26

Table 5-4 Estimates of Economic Value (2009$) of Fish Impinged at MNS 

Species 

Dec 2000 - Nov 2001 Dec 2001 - Nov 2002

Actual 

Flow 

Maximum 

Flow 

Actual 

Flow 

Maximum 

Flow 

Alewifea 0 0 0 0 

Blue catfish 1 1 1 1 

Bluegill 14 30 16 17 

Channel catfish 2 5 2 2 

Striped bass 53 96 112 122 

Threadfin shada 0 0 0 0 

White bass 154 304 208 227 

White perch 3 6 11 12 

Equivalent predatorb 6 11 7 8 

Non-target speciesc 48 93 82 82 

Total 281 546 431 470 

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. 

c

 Economic value associated with all impinged fish species that were not selected as target species.
6-1 

6 

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changing pelagia of Lake Ontario, 1978-1992. Transactions of the American Fisheries Society 

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herring. U.S. Fish and Wildlife Service, Washington, D.C. FWS/OBS-82/10.58. 

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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 McGuire Nuclear Station (MNS). For this 

assessment, annual impingement estimates were developed using recent impingement 

monitoring data collected at MNS over the period 10 December 2000 – 29 November 

2002 (Section 4). 

Estimates of Target Species Equivalent Losses 

In order to calculate economic value, estimates of annual impingement loss at MNS for 

the eight target species (threadfin shad, alewife, blue catfish, channel catfish, bluegill, 

striped bass, white bass 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 MNS 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 MNS 

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 MNS 

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 

MNS 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 MNS 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 MNS 

Sj→i = Total survival from age category (j) to age (i) 

nj = Number of age categories (j) impinged at MNS 

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 MNS 

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 MNS 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 

Bluegill Jun Aug 

Striped bass Mar May 

White bass May Jul 

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. 
A-5 

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. 

Bluegill – The maximum, most probable and minimum estimates of the daily 

instantaneous natural mortality rates for all ages were obtained from EPRI (2005) Table 

4-41. 

Striped bass – The maximum, most probable and minimum estimates of the daily 

instantaneous natural mortality rates for all ages were obtained from Moore et al. (1991), 

Hightower et al. (2001), and Thompson et al. (2005). 

White bass – The maximum, most probable and minimum estimates of the daily 

instantaneous natural mortality rates for all ages were obtained from EPRI (2005) Table 

4-31. 

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. 

Bluegill – The maximum, most probable, and minimum estimates of annual fishing 

mortality were set to 0.5, 0.13, and 0.1, respectively. These annual exploitation rates 

were then converted to daily instantaneous fishing mortality rates required for this 

analysis. 

Striped bass – The maximum, most probable, and minimum estimates of fishing mortality 

were set to 0.87, 0.66, and 0.16, respectively. 
Appendix A: Methods to estimate equivalent loss and economic value of fish impinged at Marshall 

A-6 

White bass – The most probable estimate of instaneous fishing mortality was calculated 

from Z value in Quist et al. (2002) and M value in Carlander (1997). The maximum 

estimate was set to 20% higher than the most probable and the minimum estimate was 20 

% less than the most probable. 

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: 

Species 

Youngest Age of 

Vulnerability 

Oldest Age of 

Vulnerability 

Blue catfish 4 5 

Channel catfish 5 6 

Bluegill 3 7 

Striped bass 3 7 

White bass 2 7 

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. 
A-7 

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 

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. 

Bluegill – 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 

Hales and Belk (1992). The maximum and minimum weights were assumed to be 20 

percent higher and 20 percent lower than the most probable value, respectively. 

Striped bass – 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 Van Horn et al. (1999) 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. 
Appendix A: Methods to estimate equivalent loss and economic value of fish impinged at Marshall 

A-8 

White bass – 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 Carlander (1997) The maximum and minimum weights were assumed to be 20 

percent higher and 20 percent lower than the most probable value, respectively. 

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 Carlander 

(1997). 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 MNS 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. 
A-9 

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 

MNS (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 MNS 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 III Rule (EPA 2006) which were then adjusted to reflect current (2009$). All 

of the recreationally-caught target species in this assessment except striped bass fall into 
Appendix A: Methods to estimate equivalent loss and economic value of fish impinged at Marshall 

A-10 

EPA’s "Panfish" category. Striped bass assigned values from the "Small game" category. 

Values used in this assessment were: 

 

Species Group

Recreational Value (2009 $/fish) 

Maximum Most Probable5 Minimum 

Panfish 1.82 0.99 0.54

Small game 12.97 5.39 0.54 

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 based on best professional judgment as follows: 

 

Species 

Average Weight per Fish Harvested (lb/fish) 

Maximum Most Probable Minimum 

Blue catfish 1.82 0.99 0.54 

Channel catfish 1.82 0.99 0.54 

Bluegill 1.82 0.99 0.54 

White bass 1.82 0.99 0.54 

Striped bass 12.97 5.39 2.15 

White perch 1.82 0.99 0.54 

The resulting values ($) per pound of fish for each target species are as follows: 

 

Species 

Recreational Value (2009$/lb) 

Maximum Most Probable Minimum 

Blue catfish 0.91 0.33 0.13 

Channel catfish 1.82 0.66 0.27 

Bluegill 18.22 5.37 1.79 

White bass 4.55 1.99 0.89 

Striped bass 7.41 2.16 0.61 

White perch 9.11 3.32 1.34 

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. 

 

5

 Used EPA’s mean value as most probable. 
A-11 

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 

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 MNS 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 MNS 

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 MNS. A trophic transfer coefficient of 10 
Appendix A: Methods to estimate equivalent loss and economic value of fish impinged at Marshall 

A-12 

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. 

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 (2006). These were then 

converted to a value per pound assuming an average weight per harvest striped bass of 5 

lb each as follows: 
A-13 

 Maximum Most Probable Minimum 

Value per fish 

(EPA 2006) 18.80 5.04 1.33 

Value per pound 3.76 1.01 0.27 

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 MNS 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 MNS but only 83.02 (2000-2001 

sampling) and 82.52 (2001-2002 sampling) 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 MNS: 



  



 



   4

1

1

0.996

1

TS

Bennon TS BenTS

where: 

Bennon-TS = Economic benefits accruing to non-target species 
Appendix A: Methods to estimate equivalent loss and economic value of fish impinged at Marshall 

A-14 

BenTS = Economic benefits accruing to four target species and the 

equivalent predator. 

The total annual economic value of fish impinged at MNS is the sum of the benefits to 

the target species plus the benefit to the equivalent predator and the benefit to non-target 

species. 

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 MNS. 

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 
A-15 

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 

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 
Appendix A: Methods to estimate equivalent loss and economic value of fish impinged at Marshall 

A-16 

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. 

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.