Prepared in Cooperation with the 

Pennsylvania Department of Transportation

Phase 1 Freshwater Mussel Survey and Comparison to 

Historical Surveys at the Pond Eddy Bridge, Delaware River, 

New York and Pennsylvania

Open-File Report 2012–1224

U.S. Department of the Interior

U.S. Geological Survey

Phase 1 Freshwater Mussel Survey and 

Comparison to Historical Surveys at the 

Pond Eddy Bridge, Delaware River, 

New York and Pennsylvania

By Heather S. Galbraith 

Prepared in cooperation with the 

Pennsylvania Department of Transportation

Open-File Report 2012–1224

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Galbraith, H.S., 2012, Phase 1 freshwater mussel survey and comparison to historical surveys at the Pond Eddy bridge, 

Delaware River, New York and Pennsylvania: U.S. Geological Survey Open-File Report 2012–1224, 17 p.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................2

Description of Study Area ...................................................................................................................2

Methods of Survey.........................................................................................................................................2

Historical Surveys.................................................................................................................................2

Statistical Assessment of Sampling Effort .......................................................................................6

Mussel Species at the Pond Eddy Bridge Site .........................................................................................6

Historical Data.......................................................................................................................................6

Data from Resampling..........................................................................................................................7

Conclusions and Limitations of the Study..................................................................................................7

Summary........................................................................................................................................................15

References Cited .........................................................................................................................................16
iv

Figures

1. Map showing Pond Eddy Bridge site located at the main stem upper Delaware River 

between New York and Pennsylvania ......................................................................................3

2. Maps showing catch per unit effort of A, A. implicata, B, S. undulatus, C, E. complanata, and D, total mussel count for each 25-meter x 25-meter cell at the Pond Eddy 

Bridge site, New York and Pennsylvania, 2011........................................................................4

3. Blueprint showing proposed construction plans for the new Pond Eddy Bridge 

between New York and Pennsylvania ......................................................................................5

4. Maps showing statistically significant clusters of high and low catch per unit effort 

values, and outlier catch per unit effort values in which high values are surrounded 

by low values and low values are surrounded by high values for selected mussel 

species, Pond Eddy Bridge site, New York and Pennsylvania, 2011....................................8

5. Boxplots of selected mussel species CPUE and community metrics obtained by 

statistically resampling 400-meter sections of the upper Delaware River sampled 

in 2001 in relation to data collected at the Pond Eddy Bridge site, New York and 

Pennsylvania, in 2011 ...................................................................................................................9

6. Boxplots of selected mussel species CPUE and community metrics obtained by 

statistically resampling data from cells at the Pond Eddy Bridge site, New York 

and Pennsylvania, 2011..............................................................................................................10

7. Graphs showing relation of percent of cells to the mean values for six community 

metrics obtained from statistical resampling at the Pond Eddy Bridge site, 

New York and Pennsylvania, 2011 ...........................................................................................11

8. Graphs showing variance of mussel species and selected community metrics 

obtained from statistical resampling in relation to the percent of cells at the 

Pond Eddy Bridge site, New York and Pennsylvania, 2011..................................................12

9. Map showing estimated zone of impact of new bridge construction and mussel 

Anodonta implicata, in catch per unit effort, at the Pond Eddy Bridge site, 

New York and Pennsylvania, 2011 ...........................................................................................14
v

Tables

1. Catch per unit effort (CPUE) and percentage of all individuals for three species of 

mussels from the Delaware River at the Pond Eddy Bridge site, New York and 

Pennsylvania .................................................................................................................................7

2. Results of linear and segmented regression analysis on community metrics 

generated from statistical resampling and calculated at different sampling efforts 

for the 2011 survey at Pond Eddy Bridge site, New York and Pennsylvania ....................13

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

meter (m) 3.281 foot (ft)

kilometer (km) 0.621 mile (mi)

Phase 1 Freshwater Mussel Survey and Comparison to 

Historical Surveys at the Pond Eddy Bridge, Delaware 

River, New York and Pennsylvania

By Heather S. Galbraith

Abstract

A qualitative freshwater mussel survey was conducted in 

a section of the main stem Delaware River near the Pond Eddy 

Bridge site, New York and Pennsylvania, during summer 2011 

to assess population levels of state and Federal threatened and 

endangered species. Historical data that were collected at this 

site were compared to data from the 2011 survey to assess 

changes in mussel community composition and differences in 

survey methodology. A total of 4,080 mussels of three species—Elliptio complanata, Anodonta implicata, and Strophitus undulatus—were sampled at the Pond Eddy Bridge site in 

2011. No mussel species (Alasmidonta heterodon or Alasmidonta varicosa) that are on Federal or state lists of threatened 

or endangered species or their shells were collected in this 

survey. These results are comparable to historical surveys 

at the site, with some differences in estimated catch per unit 

effort (CPUE) and species detection, depending upon survey 

methodology. The CPUE of the three species and species richness and diversity were evaluated. The percentages of species 

composition for A. implicata, E. complanata, and S. undulatus

were 0.02, 0.97, and 0.002, respectively, in 2011.

Introduction

Freshwater mussels are a highly imperiled fauna with 

greater than 70 percent of species in North America considered vulnerable (Williams and others, 1993; Lydeard and 

others, 2004). Mussels perform a variety of important ecological services, and there has been a surge of interest in their 

conservation (Vaughn, 2010). Mussels are threatened by a 

variety of anthropogenic disturbances, including contaminants, 

invasive species (for example, zebra mussels), and habitat loss 

and modification (Strayer, 1999; Watters, 2000). Construction 

activities in or near water bodies can have substantial effects 

on freshwater mussels through hydrologic alterations and 

siltation (Neves and others, 1997; Watters, 2000). Incidental 

loss of mussels can result directly from construction efforts 

or indirectly by habitat alteration in the immediate vicinity of 

construction sites (Smith and others, 2001). Therefore, prior to 

major construction activities, mussel surveys for rare or threatened species are conducted to determine where minimization 

and mitigation efforts can reduce the impact on species listed 

as threatened or endangered. 

Survey techniques for freshwater mussels vary considerably depending upon system, spatial scale, and community 

metric of interest. Qualitative surveys are often used to assess 

species richness and diversity because typically more individuals are sampled (and thus more species); however, qualitative surveys can be biased towards finding large, sculptured, 

or surface-dwelling species (Miller and Payne, 1993; Vaughn 

and others, 1997; Dorazio, 1999). Alternatively, quantitative sampling is often used for estimates of mussel density, 

recruitment, and population demographics but can be time 

consuming (therefore, expensive) and an inefficient means 

of detecting rare species (Miller and Payne, 1993; Dorazio, 

1999 ). Rare species detection can be particularly important in 

assessing potential impacts of anthropogenic disturbance on 

mussel communities; however, studies of basic mussel ecology or long-term monitoring of broader community characteristics, such as richness or density, may be more relevant. 

Therefore, there is a tradeoff in sampling design depending 

on the primary focus of the study. Standardized sampling 

methods are often considered important for quality control and 

comparison among survey locations (Smith and others, 2001; 

Huang and others, 2011). Many management agencies are 

developing standardized survey protocols for freshwater mussels; however, few studies have assessed the repeatability of 

standardized survey methods (Huang and others, 2011). This 

study was conducted to assess changes in the mussel communities and evaluate survey methods. 

In September 2001, a Phase 1 survey for dwarf wedgemussel (Alasmidonta heterodon) was conducted near the Pond 

Eddy Bridge at which time no individuals of this species 

were found (Normandeau Associates, 2002). Only the eastern 

elliptio (Elliptio complanata) and alewife floater (Anodonta 

implicata) individuals were identified at this site. Additionally, 

Lellis (2001, 2002) completed qualitative surveys in this area 

starting in the summer of 2000 while conducting an independent survey of the Delaware River main stem. During this 
2 Phase 1 Freshwater Mussel Survey and Comparison to Historical Surveys at Pond Eddy Bridge, Delaware River, N.Y. and Pa.

qualitative survey no listed mussel species were found at the 

Pond Eddy Bridge; however, a single brook floater (Alasmidonta varicosa) was detected approximately 450 meters (m) 

upstream from the bridge. 

Surveys of freshwater mussel are considered valid by the 

U.S. Fish and Wildlife Service for 3 to 5 years, after which 

species distributions and abundance may change (Robert 

Anderson, U.S. Fish and Wildlife Service, written commun., 

2011). Therefore, U.S. Fish and Wildlife Service recommended that the 2000/2001 mussel surveys be repeated to validate the assumption that the dwarf wedgemussel is still absent 

from this site and to confirm that no other Federal or state 

listed species are present at the Pond Eddy Bridge location. 

The U.S. Geological Survey, in cooperation with the Pennsylvania Department of Transportation (PennDOT), conducted a 

study in 2011 to resurvey the Pond Eddy bridge site to document threatened or endangered species. Given that repeated, 

high-resolution freshwater mussel data are sparse, the survey 

results were compared to the results from the early 2000s, and 

the sampling effort necessary to adequately estimate mussel 

community structure at a typical reach within the upper Delaware River was evaluated. 

Purpose and Scope

This report presents the results of a repeat survey of 

the mussel community in the Delaware River at Pond Eddy 

Bridge site conducted in 2011. Mussel data from 2011 are 

compared with data from the early 2000s. The sampling effort 

necessary for estimating community characteristics for Delaware River mussel beds is discussed. 

Description of Study Area

The Delaware River is the longest undammed river east 

of the Mississippi. It flows through and serves as a boundary 

between Delaware, New York, New Jersey, and Pennsylvania 

(fig. 1). Historically, the Delaware River contained 12 to 14 

species of freshwater mussels including several populations 

of federally listed, endangered A. heterodon (Ortmann, 1919; 

Lellis, 2001). Populations of A. varicosa, listed by the State 

of New York as threatened and endangered, and the critically 

imperiled (but unlisted) A. implicata have also been found in 

the Delaware River (Lellis 2001, 2002). PennDOT is planning 

to replace the bridge crossing the upper Delaware River from 

Pond Eddy, N.Y. (Sullivan County) to Pond Eddy, Pa., at an 

upstream site (Pike County; fig. 1). 

Methods of Survey

Phase 1 of the Pond Eddy mussel survey was conducted 

from August 15 to September 23 of 2011. Survey crews 

consisted of four to six people. Qualitative sampling in the 

main stem Delaware River extended bank to bank 200 m 

downstream from the existing Pond Eddy Bridge to 50 m 

upstream from the confluence of Mill Brook for a total stretch 

of river just under 400 m in length (fig. 2). An additional 50 m 

was surveyed upstream in Mill Brook starting at the confluence with the main stem. Using methods described by Smith 

and others (2001), the river was divided into a series of 25-m 

by 25-m cells, and each cell was searched for a total of 2 

working hours (a working hour is equivalent to one person 

working for 1 hour; fig. 2). In some cases, cells were searched 

for less than 2 working hours if they included dry bank or if 

they were primarily bedrock (substrate generally uninhabitable for freshwater mussels). In these instances, the percentage of the cell that was surveyable was approximated, and 

survey time was adjusted accordingly (for example, if only 

75% of the cell was underwater, it was searched for a total 

of 90 minutes). In each cell, the substrate surface was intensively searched by persons using snorkeling gear, SCUBA 

gear, or a combination of both. All mussels were identified by 

species, counted, and recorded. In addition, mussel middens 

along the banks on both sides of the river were searched for 

shells of listed species. After completing the survey, total mussel catch per unit effort (CPUE) was calculated, as was CPUE 

for each species, each species as a percentage of the total 

count, richness, and diversity (inverse Simpson’s index, 1/D 

(Hill, 1973)). 

ArcGIS was used to map the mussel community composition of each cell at the Pond Eddy Bridge site. Cluster and 

outlier analysis (Anselin Local Morans I analysis (Anselin, 

1995)) was used to determine where clusters of each species 

were significantly higher (or lower) than predicted on the basis 

of populations in the neighboring cells. 

The predicted impact the impending bridge replacement 

would have on the freshwater mussel community was assessed 

using blueprints provided by Skelly and Loy, Inc. (fig.3). The 

current plans for bridge construction will locate the new, 9-m 

(30-ft) wide bridge approximately 13 m (43 ft) upstream from 

the standing Pond Eddy Bridge. A temporary causeway will be 

constructed upstream from the new bridge, although the exact 

location of the causeway has not yet been determined. Skelly 

and Loy, Inc., estimated that the zone of temporary impact 

would constitute a section of river approximately 46 m (150 ft) 

long upstream to downstream, across the entire width of the 

river (about 162 m or 530 ft). For the purposes of this report, 

the zone was assumed to center on the new bridge. Given that 

the causeway location and the number and location of the support piers are not yet precisely defined, the 46-m width of the 

estimated impact zone includes a substantial buffer. However, 

all direct effects of the bridge replacement project will occur 

within this zone, which is approximately 46 m by 162 m. 

Historical Surveys

Lellis (2001, 2002) conducted mussel surveys along 

230 kilometers (km) of the upper Delaware River from 

Hancock, N.Y., to Portland, Pa., in 2000 and 2001. Five to 

eight surveyors snorkeled approximately 200-m straight line 
Methods of Survey 

3

72°30' 75°

42°

30'

40°

NEW YORK

PENNSYLVANIA

NEW JERSEY

DELAWARE

e D

wal

revi

R

era

0 50 100 25 KILOMETERS

0 40 80 20 MILES

CONNECTICUT

MARYLAND

MASSACHUSETTS

Figure 1. Pond Eddy Bridge site located at the main stem upper Delaware River between New York and Pennsylvania. Arrow indicates direction of flow.
4 Phase 1 Freshwater Mussel Survey and Comparison to Historical Surveys at Pond Eddy Bridge, Delaware River, N.Y. and Pa. CPUE 0.00 - 0.60 0.61 - 1.20 1.21 - 1.80 1.81 - 2.40 2.41 - 3.00 3.01 - 3.60 3.61 - 4.20 4.21 - 4.80 4.81 - 5.40 5.41 - 6.00 6.01 - 6.60 EXPLANATION EXPLANATION EXPLANATION CPUE EXPLANATION 0.00 - 10.00 10.01 - 20.00 20.01 - 30.00 30.01 - 40.00 40.01 - 50.00 50.01 - 60.00 60.01 - 70.00 70.01 - 80.00 80.01 - 90.00 90.01 - 100.00 100.01 - 110.00 CPUE 0.00 - 0.10 0.11 - 0.20 0.21 - 0.30 0.31 - 0.40 0.41 - 0.50 0.51 - 0.60 0.61 - 0.70 0.71 - 0.80 0.81 - 0.90 0.91 - 1.00 1.01 - 1.10 CPUE 0.00 - 10.00 10.01 - 20.00 20.01 - 30.00 30.01 - 40.00 40.01 - 50.00 50.01 - 60.00 60.01 - 70.00 70.01 - 80.00 80.01 - 90.00 90.01 - 100.00 100.01 - 110.00

A. Anodonta implicata

B. Strophitus undulatus

C. Elliptio complanata

D. Total

Figure 2. Catch per unit effort of A, A. implicata, B, S. undulatus, C, E. complanata, and D, total mussel count for each 25-meter x 25-meter cell at the Pond Eddy Bridge site, New York and Pennsylvania, 2011. (Scales differ for each illustration. CPUE, catch per unit effort)
Methods of Survey 5Figure 3. Proposed construction plans for the new Pond Eddy Bridge between New York and Pennsylvania. (Provided by Skelly and Loy, Inc.)
6 Phase 1 Freshwater Mussel Survey and Comparison to Historical Surveys at Pond Eddy Bridge, Delaware River, N.Y. and Pa.

transects (hereafter referred to as sections) from upstream to 

downstream for target coverage of 10 to 15 search hours per 

1.6 km of river. Unique habitats, backwaters, eddies, and pools 

less than 10 m deep were searched. At the end of each section, 

all mussels were identified, counted and recorded (Lellis 2001, 

2002). During the 2000 sampling, the area from the main 

stem Delaware River/Mill Brook confluence to the Pond Eddy 

Bridge was surveyed, as was the area from the bridge 200 m 

downstream. This roughly corresponds to the section of river 

assessed in the 2011 survey. Community metrics (described 

above) were calculated for the 2000/2001 surveys and compared to metrics from the present study. 

Normandeau Associates, Inc., completed a survey at the 

Pond Eddy Bridge site in 2001 as part of the bridge replacement project (Normandeau Associates 2002). The methods 

used were identical to those described for the present study; 

however, the extent of this survey included qualitatively 

surveying only 48 cells, about half of the cells in the current 

survey, for 2 working hours each. Water levels in 2001 were 

substantially lower than in 2011. At the USGS streamgaging 

station in Callicoon, N.Y. (site number 01427510), mean discharge (± standard deviation (SD)) in 2001 during surveying 

was 32.6 (±1.0) cubic meters per second (m3/s); 2011 discharge was 76.0 m3

/s (±17.4). This may account for the lower 

number of cells surveyed in 2001. Additionally, the 2001 

survey extended 100 m upstream from the bridge, whereas the 

2011 survey extended approximately 175 m upstream from the 

bridge. The report by Normandeau Associates, Inc., does not 

explicitly present catch per unit effort (CPUE) data but does 

present raw numbers of individuals. Therefore, CPUE was 

calculated by assuming that all 48 cells were searched for the 

entire 2 working hours to compare the 2001 survey to the 2011 

survey (Normandeau Associates, Inc., 2002). Species richness 

and diversity (inverse Simpson’s index) were calculated to 

compare the Lellis (2001) survey and the 2011 survey. 

Statistical Assessment of Sampling Effort

All statistical procedures were completed using the 

statistical software R (R Development Core Team, 2011). 

To assess the sampling effort necessary to estimate community metrics of a typical site in the upper Delaware River, it 

was first determined whether the Pond Eddy Bridge site was 

representative of a randomly selected 400 m stretch of the 

Delaware River. To do this, random samples of two 200-m 

sections of the upper Delaware River were obtained from the 

early 2000’s data (Lellis, 2001, 2002) using statistical resampling procedures (Efron, 1979). For each historical sample, 

a suite of community metrics was calculated: total mussel 

CPUE, CPUE of individual species, species richness, and 

diversity. This process was iterated 1,000 times to generate 

statistical sampling distributions from the historical record and 

define the 95-percent confidence limit for each metric. Each 

community metric at the Pond Eddy Bridge site was compared 

to the sampling distribution from the entire river. If the Pond 

Eddy metrics fell within the 95-percent confidence limits for 

the upper Delaware, the Pond Eddy site was assumed to be 

not significantly different from any other 400-m stretch in the 

river. 

Random samples of 25-m x 25-m cells were drawn (with 

replacement) from within the Pond Eddy data, ranging from a 

single cell to 100 percent of all surveyed cells to simulate the 

entire range of possible sampling efforts at the site. The same 

community metrics described above were calculated for each 

random sample, and this process was iterated 1,000 times. 

This generated a series of statistical distributions for each level 

of sampling effort (from 1 to 100% of the cells) and community metric. Sampling effort (as percent of cells surveyed) was 

plotted in relation to the mean and variance of each metric, 

and segmented regression was used to determine breakpoints 

within the distribution. These breakpoints, which indicate 

the point at which there is a non-constant regression parameter, were used to indicate the sampling effort that approximated the true (that is, population) community metric. It was 

assumed that a baseline amount of at least 10 percent of the 

site would be surveyed (because of high variance); therefore, 

these data points were eliminated. Segmented regression was 

computed only on samples of greater than 10 percent of the 

site. Therefore, estimates of the sampling effort needed were 

calculated above a baseline of 10 percent of the sampling 

effort. Segmented regression was completed only if a significant linear relationship was first identified. 

Mussel Species at the Pond Eddy 

Bridge Site

A total of 4,080 mussels of three species—E. complanata, A. implicata, and Strophitus undulatus (table 1)—were 

collected at the Pond Eddy Bridge site in 2011. No Federal 

or state listed mussel species (A. heterodon or A. varicosa) or 

their shells were obtained during this survey, and all mussels were found in the main stem Delaware River (that is, no 

mussels in Mill Brook). More than 97 percent of all mussels were E. complanata. A. implicata and S. undulatus were 

rare, accounting for approximately 2 percent and less than 

1 percent of the mussels, respectively. Mean (±SD) CPUE for 

all mussels combined was approximately 32.44 individuals per 

hour (±25.32), but the mean CPUE varied considerably from 

cell to cell, according to species and location within the river 

(table 1; fig. 2). 

Significant spatial clustering was observed for all species 

on the Pennsylvania side of the river and specifically the Pennsylvania side upstream from the bridge for A. implicata. All S. 

undulatus, except for a single individual, were found upstream 

from the bridge (fig. 4).
Conclusions and Limitations of the Study 7

Table 1. Catch per unit effort (CPUE) and percentage of all individuals for three species of mussels from the Delaware River at 

the Pond Eddy Bridge site, New York and Pennsylvania. Comparison of community metrics among three qualitative mussel surveys 

completed at the Pond Eddy Bridge. 

[CPUE, catch per unit effort; USGS, U.S. Geological Survey; %, percent; SD, standard deviation; N, number of species; Diversity, inverse Simpson’s index]

Species Community metric

USGS,

2011

(SD)

Lellis, 

2001

Normandeau 

Associates, 

Inc., 2002

A. implicata

CPUE 0.66 (1.19) 5.85 0.24

% composition 0.02 0.05 0.03

E. complanata

CPUE 31.73 (24.64) 101.95 8.10

% composition 0.97 0.94 0.97

S. undulatus

CPUE 0.05 (0.17) 0.98 0.00

% composition 0.002 0.01 0.00

Total CPUE 32.44 (25.32) 108.78 8.34

Richness (N) 3 3 2

Diversity 1.05 1.13 1.06

Historical Data

Data on mussels, by transect, from Lellis (2001) were 

similar in species richness and percent species composition to 

the data for the 2011 survey (table 1). However, CPUE values 

were 2 to 20 times higher than those for 2011, depending on 

the species, and their estimate of diversity was slightly larger. 

Normandeau Associates, Inc., found only two (E. complanata

and A. implicata) of the three species detected in the other two 

surveys, and the estimated CPUEs were substantially lower 

(3-4 times lower than those observed in the present study and 

12 to 25 times lower than those observed by Lellis (2001)) 

(Normandeau Associates, Inc., 2002). The percentage of each 

species (excluding S. undulatus) determined by Normandeau 

Associates, Inc., however, was similar to that of this study and 

that of Lellis (2001) (table 1). 

Data from Resampling

Data on species richness and percent species composition at the Pond Eddy Bridge site during 2011 fell on or within 

the 95-percent confidence limits of the entire upper Delaware 

River for all community metrics (fig. 5), which indicates 

that the Pond Eddy Bridge site is a typical site in this river; 

therefore, the bridge site was further assessed for the sampling effort necessary to attain an accurate estimate of mussel 

community composition. As expected, community metrics 

closely approximated those of the “true” community (that is, 

the entire population at Pond Eddy) with increasing sampling 

effort (fig. 6). Overall, there was little effect of sampling effort 

on the mean community metrics (figs. 6 and 7); however, the 

median of all metrics increased towards the population value 

with increased sampling (fig. 6). Variance decreased with 

increasing sampling effort for each community metric (fig. 8). 

Segmented regression analyses identified breakpoints for 

community metric variance generally near 25 cells (approximately 30% of the surveyed area), but the percentage of cells 

varied slightly according to the metric of interest (table 2; 

figs. 7 and 8).

Conclusions and Limitations of the 

Study

The 2011 survey results from the Pond Eddy site are 

consistent with results of previous studies conducted at this 

location (Lellis, 2001; Normandeau Associates, Inc., 2002). 

No threatened or endangered mussel species were found. E. 

complanata was the most abundant and dominant species 

present (table 1). Because the entire section of river was qualitatively searched only on the surface, it cannot be said with 

100 percent certainty that no threatened or endangered species 

are present; it is possible (although unlikely) that subsurface 

individuals could have been missed. For example, Smith 

and others (2001) found that several mussel species had low 

detectability in surface surveys. Although qualitative sampling 

generally offers better detection of rare species than quantitative surveys, qualitative surveys are biased towards finding 

surface-dwelling, large, and sculptured species (Miller and 

Payne, 1993; Vaughn and others, 1997). Unfortunately, both 

A. heterodon and A. varicosa are extremely small, decreasing 

their detection probability in qualitative surveys.
8 Phase 1 Freshwater Mussel Survey and Comparison to Historical Surveys at Pond Eddy Bridge, Delaware River, N.Y. and Pa. NSLHLLHL HH Statistically significant cluster Anodonta implicata Strophitus undulatus Elliptio complanata Total EXPLANATION Figure 4. Statistically significant clusters of high and low catch per unit effort values, and outlier catch per unit effort values in which high values are surrounded by low values and low values are surrounded by high values for selected mussel species, Pond Eddy Bridge site, New York and Pennsylvania, 2011. (HH, high value; LH, low values surrounded by high values; LL, low values; HL, high values surrounded by low values; NS, not significant)
Conclusions and Limitations of the Study 

9

EXPLANATION

Median community metric for the Upper Delaware River95-percent confidence limitsMean community metric for the Upper Delaware RiverMean community metric for the Pond Eddy Bridge site

Alasmidonta heterodon Alasmidonta undulata

Anodonta implicata Elliptio complanata

Pyganodon cataracta Strophitus undulatus

All mussels Richness

Alasmidonta varicosa

Lampsilis cariosa

Margaritifera margaritifera

Diversity

0.80.60.40.20.0

CPUE

Number of species

Inverse Simpson’s Index

0

850425

1,7001,275

0.80.60.40.20.0

0.40.30.20.10.0

9630

12 4

3210

0

850425

1,7001,275

86420

2.01.51.00.50.0

1.20.90.60.30.0 0.0

0.600.450.300.15

0

40302010

CPUE CPUE CPUE

Figure 5. Boxplots of selected mussel species CPUE and community metrics obtained by statistically resampling 400-meter sections of the upper Delaware River sampled in 2001 in relation to data collected at the Pond Eddy Bridge site, New York and Pennsylvania, in 2011. (Upper Delaware River was surveyed by Lellis (2001). Note differences in y-axis scales. CPUE, catch per unit effort)
10 Phase 1 Freshwater Mussel Survey and Comparison to Historical Surveys at Pond Eddy Bridge, Delaware River, N.Y. and Pa. EXPLANATION Median value Mean value Anodonta implicata Richness Elliptio complanata All mussels Strophitus undulatus Diversity 01 02 03 04 05 06 07 08 09 0 100 01 02 03 04 05 06 07 08 09 0 100

0

6.004.503.001.50

906030

120

1.000.750.500.250.00 0

906030

120

3.002.251.500.750.00 1.0

1.401.301.201.10

0

CPUE Number of species

Inverse Simpson’s Index

Percent of cells Percent of cells

CPUE

Figure 6. Boxplots of selected mussel species CPUE and community metrics obtained by statistically resampling data from cells at the Pond Eddy Bridge site, New York and Pennsylvania, 2011. (Note differences in y-axis scale. CPUE, catch per unit effort)
Conclusions and Limitations of the Study 11

EXPLANATION

Data eliminated from regression analysisBreakpoint

Anodonta implicataRichness

Elliptio complanataAll mussels Strophitus undulatusDiversity

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100

1.000.750.500.250.00

9.0

36.027.018.0

1.000.750.500.250.00 0

9.0

36.027.018.0

3.002.251.500.750.00 1.0

1.401.301.201.10

0

Mean number of species

 Simpson’s index

Percent of cells Percent of cells

Mean CPUE Mean CPUE (x10)

Mean CPUE

Figure 7. Relation of percent of cells to the mean values for six community metrics obtained from statistical resampling at the Pond Eddy Bridge site, New York and Pennsylvania, 2011. (Note differences in y-axis scales. CPUE, catch per unit effort)
12 Phase 1 Freshwater Mussel Survey and Comparison to Historical Surveys at Pond Eddy Bridge, Delaware River, N.Y. and Pa. Figure 8.

Variance of mussel species and selected community metrics obtained from statistical resampling in relation to the percent of cells at the Pond Eddy Bridge site, 

New York and Pennsylvania, 2011. (Note differences in y-axis scale. CPUE, catch per unit effort.) 

EXPLANATION

Data eliminated from regression analysisBreakpoint

Anodonta implicataRichness

Elliptio complanataAll mussels Strophitus undulatusDiversity

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100

2.001.501.000.500.00

600450300150

3.002.251.500.750.00 0

600450300150

0.440.330.220.110.00

1.20.90.60.30.0

0

Variance in number of species

Variance in Inverse Simpson’s 

Index (x100)

Percent of cells Percent of cells

CPUE variance CPUE variance (x100)

CPUE variance
Conclusions and Limitations of the Study 13

Table 2. Results of linear and segmented regression analysis on community metrics generated from statistical resampling and 

calculated at different sampling efforts for the 2011 survey at Pond Eddy Bridge site, New York and Pennsylvania. 

[SE, standard error; Seg. 1 slope, regression slope prior to breakpoint; Seg. 2 slope, regression slope after breakpoint; lin., linear; seg. segmented; CPUE, catch 

per unit effort; *, statistical significance at α less than or equal to 0.05; NA, not applicable]

Metric R2 lin. R2 seg. Breakpoint 

(SE) Seg. 1 slope Seg. 2 slope

A. implicata CPUE Mean 0.003 N/A N/A N/A N/A

Variance 0.699* 0.961* 25.12 (0.65) -5.55x10-03 -5.52x10-04

E. complanata CPUE Mean -0.007 N/A N/A N/A N/A

Variance 0.711* 0.978* 25.19 (0.50) -2.37x10+00 -2.36x10-01

S. undulatus CPUE Mean 0.036 N/A N/A N/A N/A

Variance 0.701* 0.975* 25.21 (0.52) -1.14x10-04 -1.09x10-05

Total CPUE Mean 0.007 N/A N/A N/A N/A

Variance 0.711* 0.978* 25.21 (0.49) -2.50x10+00 -2.49x10-01

Richness Mean -0.012 N/A N/A N/A N/A

Variance 0.711* 0.972* 25.29 (0.57) -2.05x10-03 -2.08x10-04

Diversity Mean 0.076* 0.202* 23.9 (3.18) 7.59x10-05 9.57x10-07

Variance 0.700* 0.963* 25.10 (0.63) -2.52x10-05 -2.50x10-06

The likelihood of not detecting threatened or endangered 

species at Pond Eddy is small based on several factors. First, 

this site was surveyed by three separate crews on three different occasions with no A. heterodon or A. varicosa collected. 

In extensive qualitative surveys of the upper Delaware River 

by Lellis (2001), both of these and other rare species were 

detected with substantially less search time (mean ± standard 

error (SE) = 1.67 hr ±0.04 in a 200-m section of river). If 

threatened or endangered species were present at the Pond 

Eddy Bridge site, the greater than 125 hours of survey effort 

invested in the 2011 study should have been enough to detect 

them. Second, anecdotal evidence (Lellis and others, 2008) 

indicates that habitat at the Pond Eddy Bridge site is suboptimal for at least A. heterodon. In the historical Delaware River 

surveys, A. heterodon was most often found in sandy shoals, 

habitat that was essentially nonexistent at Pond Eddy: the 

predominant substrate type was large boulders. Less is known 

about A. varicosa’s habitat preference. Third, given the long 

life span and limited colonization opportunities for freshwater 

mussels in general, chances are low that a rare mussel species would have colonized this site since the early 2000s and 

established a stable, detectable population. If any new species 

outside of those identified during the 2000s were found at the 

Pond Eddy Bridge site, they likely would have been transients 

that were deposited during a spate (Hastie and others, 2001). 

Conditions overall at the Pond Eddy site did not appear to 

be ideal even for common mussel species. Little substrate was 

present that is suitable for burrowing or moving in general, 

and mussels often were found trapped between or under large 

boulders where they probably were deposited. That said, in 

comparison to the entire upper Delaware River, Pond Eddy’s 

mussel community did fall between the 95 percent confidence 

limits for all community metrics (fig. 5); however, the metrics were on the low end of the distribution, falling either on 

or below the mean. The only metric in which the Pond Eddy 

Bridge site fell above the average for the upper Delaware 

River was species richness.

A. implicata, although not on Federal or state lists of 

endangered or threatened species in Pennsylvania or New 

York, nor a species of special concern (having regulatory 

protections) in New York, is considered critically imperiled 

in New York. Analysis of spatial structuring indicated that the 

highest clustering of A. implicata can be found on the Pennsylvania side of the river upstream from the current Pond Eddy 

Bridge (fig. 4). On the basis of the tentative construction plans, 

approximately 14 percent of the A. implicata at this site are at 

risk (fig. 9), along with 15 percent and 18 percent of E. complanata and S. undulatus, respectively. On the other hand, the 

area upstream from the bridge, extending 50 m from the bank 

along the Pennsylvania side contained almost 65 percent of all 

A. implicata found (fig. 2). Although only a small portion of 

this stretch is included in the “direct-effects area,” the entire 

upstream reach along the Pennsylvania bank would likely be 

considered a target for minimization and conservation efforts 

to protect A. implicata.

Similarities among the three surveys conducted at 

the Pond Eddy site varied according to community metric 

(table 1). Results of all three studies are similar for the percent 

species composition and to some extent diversity and richness. 

The largest discrepancies were evident in the estimates of 
14 Phase 1 Freshwater Mussel Survey and Comparison to Historical Surveys at Pond Eddy Bridge, Delaware River, N.Y. and Pa.

New bridge

Zone of impact

Anodonta implicata, CPUE

0.00 - 0.60

0.61 - 1.20

1.21 - 1.80

1.81 - 2.40

2.41 - 3.00

3.01 - 3.60

3.61 - 4.20

4.21 - 4.80

4.81 - 5.40

5.41 - 6.00

6.01 - 6.60

EXPLANATION

Figure 9. Estimated zone of impact of new bridge construction and mussel Anodonta implicata, in catch per unit effort, at the Pond 

Eddy Bridge site, New York and Pennsylvania, 2011. (CPUE, catch per unit effort)
Summary 15

CPUE. Lellis (2001) had consistently higher CPUE estimates, 

and Normandeau Associates, Inc., (2002) had consistently 

lower CPUE estimates than those reported for the present 

study.

Given the differences in the survey techniques used by 

Lellis (2001) and the present survey, differences in estimated 

CPUEs are not surprising. The transect method allows surveyors to maximize the time spent surveying suitable mussel 

habitat and, therefore, is biased towards finding more individuals in a shorter period of time. In the 2011 survey, surveyors 

were constrained to equally search all habitats, even marginal 

habitats, which decreased the number of individuals collected 

per hour of search effort. Differences in CPUEs may also be 

attributable to mussel community changes over time which 

can happen relatively quickly (Galbraith and others, 2010), 

but given the different survey methods, these factors cannot be 

teased apart.

The differences between the 2011 survey results and 

those of Normandeau (Normandeau Associates, Inc., 2002) 

are somewhat surprising, given the overlap in survey methods and location. Normandeau surveyed a slightly smaller 

area (approximately 5% less than the area surveyed in 2011); 

however, even after accounting for this difference there is still 

a discrepancy in the number of mussels collected, the CPUE, 

and the lack of S. undulatus detected. Recruitment by reproduction is an unlikely explanation (see above). Perhaps these 

differences could be explained by a large colonization event: 

upstream populations may have been washed downstream and 

deposited at the Pond Eddy site during a flood. High flows, 

such as those experienced in the Delaware River in the mid 

2000s, are known to affect mussel distribution (Hastie and 

others, 2001). Although Hastie and others (2001) observed 

primarily mortality and mussel loss under catastrophic 

flood events, there were sites in which mussel communities 

remained unchanged or in which densities increased. Nonetheless, these results imply that standardizing survey methodology may not necessarily lead to comparable survey results. 

Caution is needed when interpreting these data because the 

surveys were completed 10 years apart.

Results of resampling analysis indicate that in order 

to estimate the community metrics of a typical site (that is, 

Pond Eddy) in the upper Delaware River, a minimum of 

approximately 25 cells need to be surveyed (fig. 8). Although 

surveying fewer cells may approximate the mean community 

metric, the variance is substantially higher (there is a higher 

probability of over- or underestimating the metric of interest). 

Results of segmented regression indicate that for a survey size 

of greater than 25 cells (an area corresponding to 15,625 m2

or approximately 27% of the site), the variance levels off such 

that there are no further dramatic drops in variance (fig. 8). 

Although increasing the survey effort further decreases variance (as indicated by the negative slopes of the segmented 

regression lines; table 2), the slopes for most of the metrics 

are negligible (table 2; fig. 8). In other words, further sampling efforts result in only a marginal decrease in variance. 

An a priori notion of the minimum sampling effort necessary 

to assess community composition provides efficient, costeffective sampling. This approach is less applicable, although 

perhaps adaptable, to construction surveys for endangered species when the goal is to assess the entire river stretch to assure 

no incidental take occurs. This strategy, however, may be 

transferrable to basic studies of mussels in the Delaware River. 

The methods of the present study have several shortcomings to be considered before designing a sampling methodology for the upper Delaware River. First, this kind of high-resolution (25 m x 25 m) data are available only for a single site. 

Additional studies using similar sampling methods to those 

presented here for several sites in the Delaware River would 

be useful to confirm the results of this study. Second, estimates 

of relative abundance (percent species composition here) using 

qualitative surveys alone have been shown to be biased in the 

Allegheny River (Smith and others, 2001). Whether or not 

this is true in the relatively species poor upper Delaware River 

remains to be assessed. Third, resampling was not conducted 

in a spatially explicit manner (spatial structuring in the mussel 

distribution was not considered). Thus, the point in the river 

where samples are collected could affect the number of cells 

necessary to assess the community metric of interest. Smith 

and others (2003) allude to the potential importance of adaptive cluster sampling as an efficient mussel survey technique 

but found problems using this method in low-density mussel 

communities. Therefore, further studies that include spatial 

clustering are needed before the technique is incorporated into 

mussel survey methods.

Despite its flaws, this study provides preliminary data 

on the qualitative sampling methodologies necessary for 

assessing mussel community structure in the Delaware River. 

Although other studies have examined different sampling 

methodologies for mussels, many have focused on rivers with 

high species richness and high evenness (Miller and Payne, 

1993; Vaughn and others, 1997; Smith and others, 2001), 

characteristics unlike those in the upper Delaware River. 

Whether or not these sampling schemes are relevant in such a 

system warrants further investigation. This report also alludes 

to potential discrepancies between and within different survey 

methodologies. These differences need to be further assessed 

before standardized methods in mussel survey techniques are 

adopted. Finally, this study confirms the absence of threatened 

or endangered mussels in the vicinity of the Pond Eddy Bridge 

site.

Summary

This report documents results of a qualitative mussel survey completed in 2011 at the Pond Eddy Bridge spanning New 

York and Pennsylvania. Mussel data from the 2011 survey are 

compared to data from two similar surveys completed in the 

early 2000s. In general, the results from 2011 are comparable 

to those from the early 2000s in that no mussel species on 

Federal or state threatened or endangered species lists were 
16 Phase 1 Freshwater Mussel Survey and Comparison to Historical Surveys at Pond Eddy Bridge, Delaware River, N.Y. and Pa.

found at the site. The relative abundance of each of three 

species—E. complanata, A. implicata, and S. undulatus—was 

similar among studies, but catch per unit effort (CPUE) varied 

considerably depending on survey methodology. Discrepancies in CPUE and species richness were found between 2001 

and 2011 surveys despite the use of identical survey methodologies. These differences may be attributable to changes in 

mussel communities over the last 10 years.

A statistical resampling analysis was completed on the 

2011 survey data to determine the sampling effort necessary 

to quantify key community metrics (CPUE of each species, 

richness, and diversity) at a “typical” mussel location in the 

Delaware River. Segmented regression output indicates that 

surveys of approximately 30 percent of a site in the Delaware River may provide a reasonable estimate of community 

metrics within a site; however, increased sampling effort 

decreased variance in all of the metrics. These results likely 

will be useful in designing cost-effective studies of the river in 

the future, but caution is warranted given that these results are 

based on a single site in the river.

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Prepared by the West Trenton Publishing Service Center

For additional information, write to:

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Galbraith—Phase 1 Freshwater Mussel Survey and Comparison to Historical Surveys at Pond Eddy Bridge, Delaware River, N.Y. and Pa.—Open-File Report 2012–1224