Martin Marietta Environmental Systems 

c 

THE EFFECTS OF SCREEN SLOT SIZE, SCREEN DIAMETER, 

AND THROUGH-SLOT VELOCITY ON ENTRAINMENT OF 

ESTUARINE ICHTHYOPLANKTON THROUGH 

WEDGE-WIRE SCREENS 

Prepared by: 

Stephen B. Weisberg

William H. Burton 

Eric A. Ross 

Fred Jacobs 

Martin Marietta Environmental Systems

9200 Rumsey Road 

Columbia, Maryland 21045 

Prepared for: 

Maryland Department of Natural Resources 

Power Plant Siting Program

Tawes State Office Building

Annapolis, Maryland 21401 

August 1984 
Martin Marietta Environmental Systems 

FOREWORD 

This final report, "The effects of screen slot size, screen 

diameter, and through-slot velocity on entrainment of estuarine 

ichthyoplankton through wedge-wire screens," was prepared by

Stephen R. Weisberg, William H. Rurton, Eric A. Ross, and Fred 

Jacobs of Martin Marietta Environmental Systems for Dr. Paul 

Miller of the State of Maryland Power Plant Siting Program.

The study was conducted.from August 1982 to July 1983 as part

of a program to test alternative-intake screening devices in 

the State of Maryland. The work was conducted in its entirety

by Martin Marietta Environmental Systems under Contract Numbers 

P1-83-02 and P1-84-02. 

ii 
Martin Marietta Environmental Systems 

Milnin Marietta Environmental Systems 

I. 1-1 

B, SAMPLING SCHEDULE,.,.....,................. 11-6 

111. 111-1 

A. COMPARISON OF ENTRAINMENT RATES THROUGH 

SCREENS OF DIFFERENT SLOT WIDTH AND AN 

OPEN PORT.o...*....*......r.......u.......... 111-1 

B. COMPARISON OF ENTRAINMENT RATES THROUGH 

SCREENS OF DIFFERENT SLOT WIDTH AND A 

BONGO NET.**.*....*..***o.o..............., 111-9 

C. COMPARISON OF ENTRAINMENT RATES THROUGH 

2-MM SCREENS OF DIFFERENT DIAMETER AND 

THROUGH-SLOT VELOCITY................,,.,.. 111-11 

IV. DISCUSSION...,.................................. IV-l 

V. CONCLUSIONS..............................,....,, v- 1 

APPENDIX A 

TEST CONDITIONS FOR EACH NIGHT DURING 

THE 1983 ENTRAINMENT STUDY..................,. A- 1 

APPENDIX B 

ANALYSIS OF VARIANCE TABLES FOR THE EFFECT OF 

SCREEN SLOT SIZE ON BAY ANCHOVY ENTRAINMENT... B- z 

APPENDIX C 

ANALYSIS OF VARIANCE TABLES FOR THE EFFECT OF 

SCREEN SLOT SIZE ON NAKED GORY ENTRAINMENT.... c-1 

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Martin Marietta Enuironmenta( Systems 

TABLE OF CONTENTS (Cont'df 

APPENDIX D 

ANALYSIS OF VARIANCE TABLES FOR THE EFFECT OF 

THROUGH-SLOT VELOCITY AND SCREEN DIAMETER ON 

BAY ANCHOVY ENTRAINMENT......,............... 

APPENDIX E 

ANALYSIS OF VARIANCE TABLES FOR THE EFFECT OF 

THROUGH-SLOT VELOCITY AND SCREEN DIAMETER ON 

NAKED GOBY ENTRAINMENT.,.......,............. 

RP-200/ 

RP-193 

PAGE 

D- 1 

E-1 

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LIST OF TABLES 

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PAGE

11-1 

11-2 

111-1 

111-2 

111-3 

111-4 

111-5 

111-6 

1x1-a 

111-9 

IV-1 

IV-2 

Specifications of the types of cylindrical

wedge-wire screens tested 11-5 

Treatment possibi.litLes for seven screen size/ 

pump speed combinations tested in 1983 11-7 

Salinity, dissolved oxygen concentrations, and 

surface water temperature during the studies in 

1982 and 1983 111-2 

Mean density (per 1000 m3) of bay anchovy

collected with each device by size class in 

1982 

Mean density (per 1000 m3) of bay anchovy 

collected with each device by size c1a.ss in 

1983 

Mean density (per 1000 m3) of naked goby

collected with eisch device by size class in 

1982 

Mean density (per 1000 m3) of naked goby

collected with each device by size class in 

1983 

Mean density (per 1000 m3) of bay anchovies 

collected in paired sets of bongo nets and 

screened samples 

Mean density (per 1000 m3) of naked gobies

collected in paired sets of bongo nets and 

screened samples 

111-3 

111-4 

111-7 

111-8 

111-LO 

I t I .-12 

Mean density (per 1000 m3) of each bay anchovy

size-class collected through screens with 

different through-slot velocity and screen 

diameter 111-13 

Mean density (per 1000 m3) of each naked goby 

size-class collected through screens with 

different through-slot velocity and screen 

d iamete1: I II--25 

Exclusion efficiencies of bay anchovies for 

three screen sizes, compared to an open port

and to canal density IV-2 

Exclusion efficiencies of naked gobies for three 

screen sizes, compared to an open port and 

compared to canal density IV-3 

V 
LIST OF FIGURES 

PAGE 

1-1 Schematic of a bulkhead-mounted screen, with 

cutaway of wedge-wire configuration 1-2 

11-1 Location of Chalk Point power plant 11-2 

11-2 Location of the test facility in the intake 

canal 11-3 

11-3 Schematic of test facility used in this study 11-4 

IV-1 Relationship of standard length to head width 

for bay anchovy IV-4 

IV-2 Relationship of standard length to head width ~ 

for naked goby IV-5 

c 
Martin Marietta Environmental Systems 

ABSTRACT 

Tests were conducted in the Patuxent estuary from August 

1982 to July 1983 to determine how certain design characteristics 

of wedge-wire screens affect entrainment of ichthyoplankton 

through the screens. The study examined the effect of a) 

altering screen slot sizeL (lmm, 2mm, and 3mm screens) while 

maintaining a constant thlrough-slot velocity (20 cm/sec), b) 

altering through-slot velocity (9.5 cm/sec, 20 cm/sec and 40 

cm/sec) while maintaining a constant screen slot size (2 mm), 

and c) altering screen diameter while maintaining constant 

through-slot velocity (-20 cm/sec) and constant slot size 

(2mm). Size specific responses of two species, bay anchovy 

(Anchoa mitchilli) and naked goby (Gobiosoma bosci), were examined. 

In comparison with an open port, screens of all slot 

sizes reduced entrainment, the effect being highly dependent 

on fish size. Entrainment through a l-mm screen was reduced 

for both species when fish reached 5 mm in length. For smaller 

fish, neither species was excluded by.screens. Virtually 

total exclusion by l-mm screens occurred for both species when 

fish were greater than 10 mm in length. Evidence for two 

mechanisms of exclusion, physical exclusion and escape by 

fish from quickly dissipated flow fields, were apparent. 

Screen slot size haid a measurable, but small, effect on 

the number of fish entrained. For small fish, no effect related 

to slot size was found. For large fish, the effect of slot 

size was the difference between total exclusion produced by 

the physical barrier of a l-mm screen, and an J 80% reduction 

in entrainment found with screens of larger slot size. For 

fish of intermediate size, the difference in exclusion eEtI.sii2n.:y 

between a l-mm and 3-mm screen was approximately 25%. 

The effect of altering through-slot velocity over a fourfold range was found to be significant for naked gobies, but 

not for bay anchovies. Higher through-slot velocity resulted 

in greater rates of entrainment for naked gobies. For small 

gobies, this was apparent over the whole range of through-slot 

velocities tested. For larger gobies, only the highest thrcughslot velocity increased entrainment. It is suggested that 

species such as naked gobies, which inhabit areas near the 

screen, will be more affected by these changes in flow than 

will open water species, such as anchovies, which typically 

remain outside the influence of the withdrawal field. 

. The effect of screen diameter was found not to be very important, particularly for larger fish. For small individuals, 

a reduction of the screen diameter was accompanied by a slight 

reduction in entrainment. 

vii 
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Martin Marietta Environmental Systems 

I. INTRODUCTION 

The loss of biota by impingement and entrainment is a 

potentially major impact which large volume water users can 

have on aquatic communities. In an effort to reduce such 

impacts, much research has been conducted to identify costeffective and ecologically sound water-intake modifications 

(American Society of Civil Engineers, 1982). One promising

technology designed to eliminate impingement and reduce entrainment is the cylindrical wedge-wire screen (also referred to 

as profile wire screens, or Johnson screens). 

Wedge-wire screens are a passive system, designed to reduce 

entrainment by physical exclusion and by exploiting hydrodynamics.

Physical exclusion occurs when the mesh size of the screen 

(e.g., 1 mm) is smaller than the organism susceptible to entrainment. Hydrodynamic exclusion results from maintenance of a 

low through-slot velocity which, because of the screen's cylindrical configuration (Fig. I-I), is quickly dissipated, thereby

allowing organisms to escape the flow field. When located 

where ambient water velocity across the screen exceeds velocity 

through the screen, their effectiveness in reducing impingement 

and entrainment is enhanced (Hanson et al., 1978). 

In situ observations have shown that impingement is virtually

eliminated when.wedge-wire screens are used.(Hanson et al., 

1978; Lifton, 1979; Rrowne et al., 1981). Furthermore, laboratory

studies (Hanson, 1981; Heuer and Tomljanovich, 1978) and field 

studies (Zeitoun et al., 1981; Lifton, 1979; Rrowne et al., 

1981; Delmarva Power and Light, 1982; Weisberg et al., 1983),

have shown that fine mesh wedge-wire screens also reduce entrainment. ; 

Despite the apparent success of wedge-wire screens in 

reducing entrainment, there has been no attempt to distinguish

the relative importance of low through-slot velocity and small 

slot 

the effect 

width on 

of 

exclusion. 

varying screen 

Although 

slot size 

some 

(Browne 

studies 

et 

have 

al., 

examined' 

1981; i 

i:

Zeitoun et al., 1981; Lifton, 1979), and others have examined 

the effect of altering the hydrodynamic field under laboratory i 

conditions, (Hanson, et al., 19781, no study has partitioned

these two factors under field conditions. Further, it can-be 4

assumed that the effectiveness of both mechanisms for exclusion P 

are influenced by fish size, but only Delmarva Power and Light 

(1982) has examined the effect of fish size in relation to ; c either mechanism of entrainment reduction. If wedge-wire 

screens are to be used for entrainment reduction, then an 

understanding of these mechanisms would seem to be a necessary t 

condition for choosing an optimal deployment strategy. 

1-1 
Martin Marietta Environmental Systems 

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1-2 
Martin Marietta EnvironmentalSystems 

In 'this study, collections were made under field conditions 

to determine: 

The effect of Screen slot width on entrainment rate when 

through-slot velocity is held constant. 

The effect of through-slot velocity on entrainment rate 

when slot-width size is held constant. 

The effect of screen surface area on entrainment rate 

when both slot-width size and through-slot velocity are 

held constant. 

The influence of these factors on different size groups

of two common estuarine ichthyoplankton species. 
Martin hhrietts Environrnentei Systems 

11. METHODS 

A. STUDY SITE 

Tests were conducted at the Chalk Point Steam Electric 

Station, located on the Patuxent River in Southeastern Prince 

Georges County, Maryland (Fig. 11-114 The test device was 

moored midchannel in the Chalk Point intake canal approximately 25 m downstream frolm the mouth (Fig. 11-21. The intake 

canal is 62 m long, 25 m wlide, and about 3 m deep, and draws 

its water from the Patuxent River via a canal dredged through 

Swanson Creek. During the tests, a unidirectional current of 

about 15 cm/sec was maintained in the canal by the operation 

of the power plant. 

Test Facility 

The model intake test. facility used in this study was a 

7.0-m x 5.2-m barge (Fig. 11-31 equipped with two identical 

25-horsepower turbine pumps that could be operated at two speeds. 

In 1982, each pump had a capacity of approximately 7.7 m3/min 

at high speed and 3.7 m3/min at low speed. Refurbishment of 

the sumps, prior to 1983 studies', increased withdrawal rate to 

12 m /min at high speed arid 5.5 m3/min at low speed. Intake 

orifices were 2-m apart, 35 cm in diameter and 1--m below the 

water surface. Water exited from the barge through identical 

25-cm diameter pipes cissociated with each pump. The barge was 

situated in the canal so that one port was upstream of the 

other and the axi.s of the screens was perpendicular to the 

current. The upstream port and the downstream port are referred 

to as the left and right ports, respectively. 

Screens 

Four types of cylindrical screens: 1-mm, 2-m, and 3-mm 

screens of nominal diameter, and a 2-mm screen of reduced 

diameter-were tested. The dimensions of each are shown in 

Table 11-1. Each screen was mounted on backing plates that 

inserted into vertical tracks to guide the screen over the 

intake orifices (Fig. 1-11. The average ve1ocit;y through the 

open slots of the 1-mm, the larger diameter 2-m, and the 3-mm 

screens was 20 cm/sec arrd 9.5 cm/sec for high and low pump

speeds, respectively, in 1483. Flow diffusers, intended to 

equalize flow over the screen surface, were present in all 

screens. 

11-1 
Martin Marietta EnvironmentalSystems 

.-.. 

I 
Martin Marietta Environmental Systms 

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c 

0 

m 

c "I Q)

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Martin Marietta Environmental Systems 

Right port 

4 

Flow 

Left port 

f 

Upper d& \

Sample port 

Lower deck 

Main port 

Figure 11-3. Schematic of test facility used in this study 

11-4 
~~ 

Martin Marietta Environmental Systems 

Table 11-1. Specifications of the types of cylindrical

wedge-wire screens tested. 

Screen 

Slot Size Diameter 

(mm) (cm) 

1 76 

2 (small) 54 

2 (large) 76 

3 76 

Wire Percent 

Length Width Open 

(cm) (m) Area 

86 2 50 

46 2 66 

61 2 66 

55 2 75 

11-5 
Martin Marietta Environmental Systems 

B. SAMPLING SCHEDULE 

Auqust 1982 

Two samplings were conducted in 1982. All tests were 

done at night to maximize the number of organisms present and 

to reduce gear avoidance by visual cues. In tests conducted 

on 19 August, entrainment through a large diameter 2-mm 

screen was compared with entrainment through an open p0r.t.

Six replicate pairs of samples .were taken. Each pair consisted 

of a'screened and an unscreened sample. Each condition was 

tested three times on each port in random order. Towed bongo net 

samples were taken in the intake canal simultaneously with 

each sample. In similar tests conducted on 22 August, entrainment through a l-mm screen was compared with entrainment I_ 

through an open port. The pumps were run at high speed on 

both dates. 

Summer 1983 

Sampling was conducted 11 times (from 12 July to 28 July)

in 1983. As in 1982, all samples were coLlected at night.

Seven different treatments (Table 11-21 were tested on each 

port each night. The treatment order was random (the order of 

test conditions for each night is shown in Appendix A). Towed 

bongo net'samples were taken in the intake canal simultaneously

with each sample. 

C. PROCEDURES 

Ichthyoplankton entrained through the test facility were 

collected at the two main exit ports (Fig. 11-31 using

505-urn mesh plankton nets 1 m in diameter by 4.2 m long.

In 1982, approximately 100 m3 of water was pumped for each 

collection. In 1983, 360 m3 and 165 m3 of water were filtered 

at high and low pump speeds, respectively. 

The stepped oblique bongo tows (0.5 m diameter,

505um mesh) were taken at the surface and at 1 m and 2 m 

depths approximately 5-15 m upstream of the test facility. 

11-6 
&rtin firietta Environmental Systems 

Table 11-2. Treatment possibilLties for seven screen size/ 

pump speed combinations tested in 1983, 

Screen Type 

Open port 

l-mm screen 

2-mm screen 

2-mm screen 

2-mm screen 

2-mm screen 

3-mm screen 

* Velocity 

Pump Speed Average ThroughScreen Velocity 

(cm/sec) 

High * 

High 20 

small 1 High 40 

small) Low 19 

large 1 Nigh 20 

(large) Low 9.5 

High 20 

at the orifice of the open port was 400 an/ 

sec at high pump speed and 190 cm/sec at low pump 

speed 

11-7 
Martin Marietta Environmental Systems 

Samples from both halves of the bongo nets were combined for 

processing. Tows took approximately 5 minutes and when combined 

filtered about SO m3 of water. The actual volume of water 

filtered for each canal sample was calculated using a General 

Oceanics flowmeter installed in the mouth of one side of the 

bongo net. 

For each sample, salinity, temperature, and dissolved 

oxygen were measured at the surface and near the bottom of the 

intake canal. Salinity and temperature were measured with a 

Reckman salinometer. Dissolved oxygen was measured with a YSI 

model 57 meter. 

All samples were preserved in the field with 5% formalin, 

in the laboratory, all fishes were sorted by species. Because 

fin damage precluded measurement of total length, the standard 

length (SL) of each was recorded to the nearest millimeter. 

Collections in which eggs were too numerous to count were 

first sorted for larvae: eggs were then counted by subsampling with a Folsom plankton splitter (McEwen et al., 1954). 

D. STATISTICAL METHODS 

Preliminary Analysis 

For most analyses in this report, variation in sample density was partitioned using either an analysis of variance 

(ANOVA) or an analysis of covariance (ANCOVA). A major assumption

underlying these linear modeling methods is that homogeneity

(equality) of variances exists across different treatment 

groups. The log transformation [lOge(x+l)I was used for all I 

tests in this report to meet this assumption. 

In order to examine size-specific effects of test conditions within species, we analyzed bay anchovy and naked goby

data by size category. The categories were selected after 

determining which size increments would not result in large

numbers of empty cells and unequal treatment variances. For 

bay anchovies, the following size classes were used: C4 mm, 

5-7 mm, 8-10 mm, 11-14 mm, and >15 mm. For naked gobies,

the following size classes were used: <4 mm, 5-6 mm, 7-8 iiirn, 

and >9 mm. 

Comparison of Entrainment Rates Through Screens of Different 

Slot Width and an Open Port 

For data collected in 1982, the hypothesis that no difference existed in the densities of each size class between ports 
Martin Marietta Environmontol Syitamr 

(left and right) or under varying screen conditions (open port, 

1-mm screen, 2-mm screen) was tested with a two-way analysis 

of covariance, using canal density as the covariate. 

The model used in this analysis was: 

where 

Yijm = mth (m=1, 2...6) observation of pump

density taken at the ith port (i=l, 21, the jth 

screen condition (j=l, 2, 3) 

D = overall mean 

ai = effect of the ith port 

= effect of the :jth screen condition 

(ayIij = combined effect of the ith port and the jth 

screen condition 

= combined effect of the jth screen condition and Y jB canal density , 

ai8 = combined effect of the ith port and the . 

canal density 

(ayIij6 = combined effect of the ith port and the jth 

screen condition and canal density 

Xjm = mth observation of bongo net density taken at the 

same times as the jth screen condition 

X = overall mean canal density 

6 = pooled regression slope of pump density on canal 

density 

Eijm = error associated with the mth pump density 

observed at the ith port and the jth screen 

condition. Thie errors are assumed normally 

distributed with common mean 0 and variance u2. 

For data collected in 1983, the hypothesis that no difference 

existed in the densities of each size class under varying

conditions (open port, lmm screen, large diameter 2mm screen 

11-9 
Martin Marietta EnvironmentalSystems 

at high pump speed, and 3-mm screen) was tested with a blocked 

one-way analysis of covariance, using canal density as the 

covariate, and the 22 date-port combinations as blocks. 

The model used in this analysis was: 

where 

'ij = observation of pump densit taken at the ith 

block (i=l, 2. .22) the jTh screen condition 

(j=l, 2, 3, 4) 

p = overall mean 

ai = effect of block i 

= effect of the ith screen condition 

yi B = combined effect of the jth screen condition 

and the canal density 

Xij = observation of net density taken at the ith 

block and the jth screen condition 

X = overall mean canal density 

B = pooled regression slope of pump density on canal 

density 

Eij = error associated with pump density

observed at the ith port and the jth screen 

condition. The errors are assumed normally

distributed with common mean 0 and variance 02. 

When interaction terms were found to be insignificant,

they were excluded from the model and a new fit was made that 

did not include the insignificant terms. When a significant

screen-condition effect was found, pairwise comparisons of the 

adjusted treatment means for the various screen conditions 

were made (Snedecor and Cochran, 1980). 

If the covariate was found to be insignificant (i.e.,

slope not significantly different from zero) or if interaction 

terms involving the covariate were found to be significant

(implying unequal slopes across treatment groups), ANOVA models, 

similar to the model above without the covariate terms, replaced

the ANCOVA model. If the screen effect was significant in the 

ANOVA model, Duncan's new multiple range test (Ott, 1977) was 

used for comparisons among treatments. 

11-10 
Martin Marietta Environmental Systems 

Due to the high degrlee of exclusion caused by screens, or to 

the low numbers of fish in certain size categories present in the 

intake canal, Cochran's t'est for equality of variance showed 

that in some cases unequal variances across treatment groups

remained even after transformation. When the assumption of 

homogeneity of variance could not be met with transformation, 

the Friedman rank sum statistic (Conover, 1971) was used. In 

this test, the the observations are ranked within blocks and 

ranks are then summed over treatments. For the 1982 data, 

port (left, right) was used as the blocking factor; for 1983, 

date-port combinations were used. When treatment differences 

were significant, pairwise comparisons of the treatment mean 

ranks were made (Conover, 1971). The critical values for each 

pairwise comparison were determined using a type I error of 

alpha/2m (where m is the number of comparisons to be made) to 

account for increases in the Type I error from multiple hypothesis

testing. The overall alpha was set at 0.05, and most individual 

comparisons were conducted at a'=0.009. 

Comparisons of Intake Facility Collections and Net Collections 

The null hypothesis of no difference in density between 

samples collected in the canal with bongo nets and samples

collected through screens on the intake facility was tested 

using paired t-tests. Th.ehypothesis was tested separately

for each screen condition (1-mm, 2-mm, and 3-mm screens). Comparisons were made using data from both years. 

Comparison of Entrainment. Rates Throuqh Screens of Different 

Diameter/ThroUah-Slot Velocitv 

The hypothesis of no difference in the densities of each 

size class when using screens of different diameter and through.-

slot velocities (small diameter 2-nun screen at high and low 

pump speeds, large diameter 2-mm screen at high and low pump

speeds) was tasted using a blocked one way analysis of covariance. 

Port-day combinations were treated as a blocking factor and 

canal density as the covariate. The model used in the analysis 

was identical to the one used to test for the effect of different 

slot widths, except that screen diameter/through-slot velocity 

was substituted for screen slot size. 

As with the earlier analysis, when interaction terms were 

insignificant, they were excluded from the model, and a new ti4 

was made which did not include the insignificant terms. When a 

significant screen condition effect was found, pairwise comparisons of the adjusted treatment means under varying test condjLions 

were made. 

When the covariate was found to be insignificant, or the 

covariate by screen condition interactions was found to be significant (indicating unequal slopes across treatment groups), 

11-11 
Martin Marietta Environmental Systems 

the ANCOVA model was replaced by a blocked one-way ANOVA. 

Significant differences found using the ANOVA model were tested 

using Duncan's new multiple range test. 

When low numbers resulted in unequal variances across 

treatment groups that could not be corrected for by transformation, 

Friedman's rank sum statistic, described earlier, was used. In 

this case, date-port was used as the blocking factor and screen 

diameter/through-slot velocity condition was the main effect. 

When treatment differences were Significant, pairwise comparisons

of the treatment mean ranks were made (Conover, 1971). The 

critical values for each pairwise comparison were determined 

using a Type I error of alpha/2m with alpha set at 0.05. 
-- 

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Martin Marietta Environmental Systems 

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ILII. RESULTS

The mean values for salinity, dissolved oxygen, and temperature during,the 1982 and 1983 study periods are presented in 

Table 111-1. In 1982, salinity ranged from 7.3 .- 11.3 ppt 

with a mean value of 9.0 ppt. Lower salinities were recorded in 

1983 (x= 7.2 ppt). Mean water temperature was higher in 1983 

than in 1982 (29.1 vs 27.13OC). Dissolved oxygen concentrations 

were close to saturation in both years. 

The species of organisms captured by the model intake 

facility and by nets towed in the discharge canal were similar 

in both 1982 and 1983. Ni3ked goby (Gobiosoma bosci) and bay

anchovy (Anchoa mitchilli) were the most abundant larval and 

juvenile fish during the study, with hogchoker (Trinectes

maculatus) and silverside (Menidia menidia) larvae captured

occasionally. Grass shrimp (Palaemonetes spp.), mysid shrimp

(Neomysis americana), and an unidentified isopod of the family

Cymothoidae dominated the macroplankton. Amphipods (mostly

Leptocheirus) and crab zoea were collected occasionally. 

A. COMPARISON OF ENTRAINMENT RATES THROUGH SCREENS OF , 

DIFFERENT SLOT WIDTH AND AN OPEN PORT 

Screens had the effect of lowering entrainment rate, 

relative to an open port, for all but the smallest size category

of both species and their eggs. The degree of entrainment 

reduction was dependent on fish size, with larger ichthyoplankton 

more successfully excluded. Screen slot size had some effect 

on entrainment rates, but the effect was not large. 

Anchovies 

Tables 111-2 and 111-3 show the mean density of anchovies, 

captured by size class under each screen condition in 1982 and 

1983, respectively. The size distribution of anchovies present

in the ambient waters (as measured by the bongo net collections 

and the open port) varied between years. Eggs and smaller 

larvae were prevalent in July 1983, whereas in August 1982 no 

eggs were found and the larger size classes predominated. 

The data for bay anchovy met the assumptions of parametric

statistics for most size classes. However, it was necessary to 

use nonparametric methods for size class 2 (5-7 mm) in 1982, 

111-1 
Martin Marietta Environmental Systems 

rable 111-1. Salinity, dissolved oxygen concentration, and 

surface water temperature during the studies in 

1982 and 1983. 

Salinity 

(ppt) 

9.0 

7.3-11.3 

7.2 

4.6-9.9 

Dissolved 

Oxygen Temperature 

(ppm) ("C) 

7.6 27.8 

4.3-10.5 25.9-28.9 

6.9 29.1 

5.5-9.6 27.1-31.5 

-982 Mean 

Range 

1983 Mean 

Range 

111-2 
Martiri Marietta Environmental Systems 

Table 111-2. Mean density (per 1000 m3) of bay anchovy

collected with each device by size class 

in 1982. 

l-lNn 

Screen 

0.0 1 

OoO I 0.0 0.0 

0.0 

0.0 

-

8-10 mm 1.5 0.0 

-

11-14 IWI 152.9 31.1 10.5 0.0 

-

> 15 mm I 57.3 15.0 1.5 - 2f469*4 

111-3 
Martin Marietta Environmental Systems 

Table 111-3. Mean density (per 1000 m3) of bay anchovy

size class 

Size 

Class 

Eggs 

-<4m 

5-7 mm 

8-10 mm 

11-14 mm 

-> 15 mrn 

collected with each device by

in 1983. 

Bongo 

Net I 

Open 

Port 

3-mm 

Screen 

2-mm 

Screen 

1-mm 

Screen 

19610 2341 1707 18435 10966 

6.0 9.6 13.6 21.0 9.2 

37.6 20.1 11.3 9.2 10.8 

11.2 7.7 2.6 1.6 1.0 

3.5 1.3 0.3 0.0 0.0 

9.3 3.3 0.5 0.4 0.0 

111-4 
Martin IMarietta EnvironmentalSystems 

for size class 4 (11-14 mi) in both years, and for size class 5 

(> 15 mm) in 1983. Except for class 3 (8-10 mm) in 1983, 

use of canal density as a covariate was ineffective for bay 

anchovies. 

Table 8-1 presents the results of ANOVA for anchovy eggs.

No significant screen effect was found. Although there was a 

large difference in the mean number of eggs entrained through

the open port and the 1 and 2-mm screens, however this difference was small relative to the large variability associated 

with capture of eggs. 

There was no observed effect of screen size on entrainment 

of anchovies in size class 1 (< 4 mm). ANOVA performed on 

1983 data showed screen effectto be insignificant for this 

size class (Table B-2). 'In 1982, no anchovies of this size 

class were captured through the intake facility (Table 111-2). 

There was a statistically Significant effect of screen 

size on entrainment of size class 2 (5-7 mm) bay anchovies in 

1983, but not in 1982, In 1983 the open port collected approximately twice as many.orgalnisms as any of the screens (Table 

111-2). Entrainment through the open port was significantly

different from entrainment through all screens (Table B-3),

but no difference could bs detected among the different screens. 

In 1982, no size class 2 (5-7 mm) anchovies were captured

through either the 1 or 2-mm screens (Table 111-2). However 

Friedman's Test did not show these screens to reduce entrainment 

significantly (x2 = 3.6, p = .17), as only 5 individuals of 

this size class dere collected through the open port in 1982. 

The effect Df screen size on size class 3 (8-10 mm) bay

anchovies was significant in 1983, but not in 1982. In 1982, 

no size class 3 anchovies were captured through the 1 mm screen, 

and the few captured through the open port and 2 mm screen 

were insufficient to allow detection of a screen effect (Table 

B-4). In 1983, significantly more anchovies of this size 

class were collected through the open port than through any of 

the screens (Table B-5). There were no significant differences 

in entrainment for the different screen sizes (Table B-5). 

In both years of the study, there was a significant screen 

effect for the 11-14 mm size class of anchovies. Friedman's 

test conducted with 1982 data showed that the number of anchovies 

collected through the open port was significantly greater than 

that through the l-mm screen ( x2 = 12.2, p < .01). However, 

entrainment through the open port was not significantly different 

from entrainment through the 2-mm screen, nor was the l-mm 

screen different from the 2-mm screen. Friedman's test on 

1983 data showed significant differences between the open port

and all screen sizes (x2 = 22.4, p < .Ol), but no differences 

among screens (Table B-6). Anchovies of this si-ze class were 

III-s 
Martin Marietta Environmental Systems 

not captured through a 1-mm screen in either year, and in 1983 

none were captured through the 2-mm screen. 

The open port captured many times more size class 5 (> 15m) 

hay anchovies in 1982 than the 1 or 2-mm screens (Table 111-2). 

Differences in numbers collected under the different test conditions 

were significant (Table B-7). In 1983, no anchovies of this 

size class were collected through the 1-mm screen (Table 111-3). 

Friedman’s test showed a significant screen effect (x2 = 

29.97, p < .01). The open port collected a significantly 

greater number of anchovies than were entrained through any 

screen, but no significant difference in entrainment among 

screens of different sizes was found. 

Naked Gobies 

Tables 111-4 and 111-5 present the mean number collected 

through screens of each size for the four naked goby size 

classes in 1982 and 1983, respectively. As with bay anchovies, 

more individuals of small size were present in ambient 

waters in July 1983 than in August 1982. More individuals of 

large size were present in the 1982 study (as measured by 

bongo net and open port collections) than in 1983. 

Parametric methods were found to be appropriate for all 

naked goby analyses except that for the 1983 size class 4 

(> 9 mm) data. Canal density as a covariate was found to be 

appropriate for size classes 1 ((4 mm) and 2 (5-6 mm) in 

both years, but not for size classes 3 (7-8 mm) and 4 ( >- 9 mm) 

in either year. 

No significant screen effect was found for size class 1 

(C 4 mm) in either 1982 (Table C-1) or 1983 (Table C-2). 

In 1982, the mean number of individuals captured through the 

1-mm screen was less than 10% of that collected through the open 

port (Table 111-4): however, this was found to be statistically 

insignificant using ANOVA (p = .63, Table C-1). In 1983, the 

mean number of individuals captured through the open port varied 

by less than 5% from the mean value of fish captured through any 

of the screens (Table 111-S): these differences were also 

statistically insignificant (Table C-2). 

The effect of screen slot size was found to be significant 

for 5-6 mm naked gobies in 1983 (Table C-3), but not in 1982. 

rn 1983, the 1-mm screen captured significantly fewer individuals 

of this size class (250%) than the open port (Table C-3). The 

1-mm screen collected significantly fewer individuals than the 

2-mm or 3-mm screens. The numbers collected through the larger 

mesh screens did not differ significantly from those collected 

through the open port (Table C-3). In 1982, the 1-mm screen 

111-6 
Martin Marietta Environmental Syttenk 

'able 111-4. 

Size Class 

(mm3 

< 4 -

5-6 

7-8 

> 9

-

Mean density (per 1000 m3) of naked goby 

collected with each device by size class 

in 1982. 

Bongo

Net 

Open

Port 

2-mm 

Screen 

l-mm 

Screen 

95.3 17.2 13.5 1.5 

117.6 22.9 19.5 6.0 

95.5 38.5 16.5 5.8 

342.3 201.5 64.6 35.8 

111-7 
Martin Marietta EnvironmentalSystems 

1-mm 

Screen 

562.5 

66.5 

3.9 

~ 

111-8 
Martin Marietta Environmental Systems 

also collected less than half the number of 5-6 mm individuals 

collected through the open port (Table 111-41, but the screen 

effect was not found to be significant (Table C-4). 

Tables C-5 and C-6 show results of ANOVA �or the 7-8 mm 

size class in 1982 and 19183, respectively. The screen effect 

was significant, and the open port collected a significantly 

greater number of individuals than all screens in both years. 

The 1-mm screen collected significantly fewer individuals than 

the 2-mm screen in 1982 (Tables 111-4 and C-51, and significantly 

fewer than the 2 or 3-mm screen in 1983 (Tables 111-5 and 

C-6). 

The screen effect waLs found to be significant for the > 9 

mm size class in both yeairs of the study. In 1982, both the 1 

and 2-mm screened samples contained significantlty fewer gobies 

than the open port, but no difference between the screened 

samples was found (Table C-7). In 1983, the open port also 

collected significantly more gobies of this size than each of 

the screens according to results of the Friedman's test (x2 = 39.50, p < .01). As in 1982, the effect of the 1-mm screen 

did not differ significantly from the effect of the 2-mm screen 

in 1983 tests; however, significantly fewer gobies were entrained 

through the 3-mm screen. 

B. COMPARISON OF ENTRAINMENT RATES THROUGH SCREENS 

OF DIFFERENT SLOT WIDTH AND A BONGO NET 

The pattern of exclusion by screens was very similar when 

compared to bongo nets or when compared with open ports. 

Screens had the effect of reducing entrainment, relative to 

density measured with a bongo net, for all but the smallest 

size category of each species and for eggs. The degree of 

entrainment reduction increased with fish size, and was affezted 

only to a minor degree by screen slot size. 

Bay Anchovies 

Canal density and densities of concurrent screened samples 

of each bay anchovy size class are presented in Table 111-6. 

There was no significant difference in density of eggs for 

screens of any slot size relative to canal density. For the 

1-mm and 2-mm screens, mean density in the canal exceeded the 

screened density three to four fold, but these differences 

were insignificant due to the high variability associated with 

ambient egg density. 

111-9 
Martin Marietta Environmentai Systems 

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111-1c 
Martin Marietta Environmental Systems 

Although the density of bay anchovy size class 1 (< 4 mm)

in screened samples always exceeded canal density, none-of the 

differences werl? statisti-cally significant. For all other 

size classes, tile density in the canal was always higher than 

that of screene(l samples,, All the comparisons were statistically 

significant, exzept for that between 3-mm screen der,sity and 

canal density for size class 4 (11-14 mm) anchovies. In this 

comparison, the density through the screen was only about 10% 

of the density estimated from the bongo net collections, but 

low numbers of fish were taken with both devices, which probably 

accounts for the lack of significance. 

Naked Gobies 

Table 111-7 shows the mean density of naked gobies for 

each size class collected in screened samples compared with 

canal densities. All cornparisons of bongo net samples and 

screened samples for naked gobies were statistically significant, but the direction of the difference was dependent on fish 

size. The density of gobies in size class 1 (< 4 mm) was 

always greater in screened samples than in the-canal samples. 

However, for all other size categories, the canal samples had 

a considerably higher mean density than screened samples. 

C. COMPARISON OF ENTRAINMENT RATES THROUGH 

2-MM SCREENS OF DIFFERENT DIAMETER AND 

THROUGH-SLOT VELOCITY 

The four combinations of screen diameter 

velocity were not found to affect entrainment 

but did affect that of naked gobies. Density 

in our samples generally increased with higher 

velocity, but was dependent on fish size. 

Bay Anchovies 

and through-slot

of bay anchovies, 

of naked goby 

through-slot 

Table 111-8 shows the mean density for each bay anchovy 

size category collected through 2-mm screens of different 

diameter and through-slot velocity. The assumptions of 

parametric statistics were met for all size classes except 

size class S.(> 15 mm). The canal density was a significant 

covariate with homogeneous slope only for size class 3 (8-lorn). 

Tables D-1 through D-5 show ANOVAS for eggs and fish of size 

classes 1-4. In none of these cases was the effect of throughslot velocity or screen diameter significant. For size class 5 

(> 15 mm), Friedman's test showed that effects of through-slot 

111-11 
Martin Marietta Environmental Systems 

I-+- i 

i I I 

I I 

I I I 

111-12 

I 
___~ 

Martin Marietta EnvironmentalSystems 

rable 111-8. Mean density (per 1000 m3) of each bay anchovy

size class collected through screens with 

different through-slot velocity and screen 

d iame ter . 

screen Diameter 

Pump Speed 

rhrough Slot 

Jeloci ty ( cm/sec) 

Fish 

Size 

(mm) 

8-10 

11-14 

-

>15

Low 

9.5 

22229 

23.7 

15.7 

1.9 

0.0 

Small Small 

20 19 40 

18435 24752 3358 

21 .o 32.3 8.8 

9.2 16.3 13 .O 

1.6 3.3 3.3 

0.0 0.3 0.5 

0.4 1 0.8 1 0.8 

I 

L 
Martin Marietta EnvironmentalSystems 

velocity and screen diameter were insignificant ( ~2 = 

3.0, p = .39). These findings are supported by the data in 

Table 111-8, which show no consistent pattern of large differences 

in entrainment rates for different test conditions. It appears 

that for bay anchovies, neither screen diameter nor through-slot 

velocity, within the range tested, affected entrainment through 

the screens. 

Naked Gobies 

Table 111-9 shows the mean density for each naked goby

size class collected through 2-mm screens of different diameter 

and through-slot velocity. For all size categories, the assumptions necessary for use of parametric statistics were met. 

The covariate was found to be significant only for size classes 

1 ( <- 4 mm) and 2 (5-6 mm). 

Unlike results obtained for bay anchovies, the through-slot

velocity and screen diameter effect was significant for every

naked goby size class (Tables E-1 to E-4). There were some 

minor differences in rank order for different test conditions, 

but the small screen at high pump speed always captured the 

greatest density of gobies. For size categories 1 ( < 4-mm)

and 2 (< 5-6-mm) gobies, the magnitude of difference betweer? 

this and other screen conditions was relatively small (C 50%). 

For goby size categories 3 (> 7-8-mm) and 4 (> 9-mm), the 

difference increased to more than 100% (Table 111-9). 

ANOVA done on size category 1 (< 4 mm) showed that screens 

of both large and small diameter collected significantly mor2 

individuals per volume filtered when operated at high pump

speed than at low pump speed (Table E-11, while collections 

from screens of different sizes, used with the same pump speed, 

did not differ. For size class 2 (5-6 mm), both high purne

speed conditions entrained a greater density of gobies cha~ 

the large screen at low pump speed (Table E-2 and Table 111-9).

No other significant differences were found within this size 

group. For size classes 3 (7-8 mm) and 4 (2 9 mm), the density

of fish collected through the small screen operated at high

speed was significantly greater than the density under all 

other conditions, and no other significant differences were 

found (Tables E-3 and E-4). 

111-14 
Martin Marietta EnvironmentalSystems 

Table 111-9. Mean density (per 1000 m3) of each naked goby 

size class collected through screens with 

different through-slot velocity and screen 

d iame t er . 

Screen Diameter 

I Large I Larqe Sma 11 Srna 1.1 

Pump Speed Low High Low High 

Through-Slot 

Velocity (cm/sec) 9.5 20 19 40 

<4 400.5 513.4 424.8 598.6 I I 

->9 4.7 4.4 3.6 11.4 
-- 

Martin Marietta Environmental Systems 

IV. DISCUSSION -

Do Screens Reduce Entrainment Relative to an Open Port? 

Entrainment rates of bay anchovy eggs and of larvae 4 ITUII 

in length or less, were not significantly affected by screens. 

However, approximately 50% of the 5 to 7 mm bay anchovies were 

excluded in comparison with an open port. Individuals greater 

than 7 mm long were increasingly excluded with size. For 

anchovies greater than 10 mm in length, almost 100% exclusion 

by a l-mm screen was noted. In the case of naked gobies, fish 

4 mm in length or less were not excluded by screens when entrainment rates were compared to those for an open port. Exclusion 

began to occur for fish of 5 to 6 mm, and exceeded 90% for 

gobies 7 mm or longer when using a 1-mm screen. Tables IV-1 

and IV-2 show screening efficiency for each size class of bay 

anchovy and naked goby, respectively. 

The degree of physical exclusion should be expected to 

increase as fish grow larger. Figures IV-1 and IV-2 show the 

relationship between body length and head width for bay anchovies 

and naked gobies, respectively. Physical exclusion by screens 

might not be expected for fish of both species 4 mm in length 

or less, since their average head width is less than 0.5 mm. 

On the other hand, fish greater than 8 mm long are close to, 

or exceed, 1 mm in width and should not be entrained through 1 

mm slot width screens. Data from the study support this. For 

bay anchovies, only a single individual above 8 mm in length 

was ever caught through a l-mm screen, and the exclusion efficiency 

of a l-mm screen was 95% for naked gobies larger than 7 mm. 

Swimming performance and ability to avoid flow fields also 

increase with fish size, and probably contributed to exclusion 

of larger fish. Hanson (1981) 'observed that striped bass under 

7 mm in total length (TL) do not possess sufficient swimming 

ability to avoid screens under laboratory conditions, whereas 

larger fish do. In our study, individuals of either species 

4 mm (SL) in length or less were not excluded by screens. Bay 

anchovy eggs possess no swimming ability and were not excluded 

by l-mm screens according to both this study and the study of 

Browne et al, (19811, even though the eggs exceed 1 mm in 

diameter. In cont.rast, individuals larger than 4 mm were 

excluded by all screens in our study, even though they could 

easily pass through screens with 2 and 3-mm slot width. 

IV- 1 
Martin Marietta Environmental Systems 

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IV-2 
Martin Marietta EnvironmentalSystems 

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Martin Marietta Environmental Systems 

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IV-4 
Martin Marietta Environmental Systems 

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IV- 5 
Martin Marietta Environmental Systems 

Other studies have also shown that fish size is an important determinant of screening efficiency. In flume studies 

conducted by Hanson (19811, yellow perch of less than 8 mm (TL) 

were not excluded by a l-mm screen. Exclusion efficiency increased with increasing fish size and became 100% when yellow 

perch reached 13 mm (TL). A similar pattern, with total exclusion at 11 mm (TL), occurred for striped bass. In field studies 

with a l-mm screen, exclusion of striped bass was insignificant 

for individuals less than 10 mm (TL), but was greater than 90% 

for individuals larger than 11 mm, Delmarva Ecollogical laboratory 

(1980). They also found similar size related patterns for 

other fish species. This was the only other study in which 

the effect of fish size on screening efficiency in the field 

has been examined. However, in several field studies it has 

. been noted that the maximum size of fish entrained through

l-mm screens is less than that collected through open ports or 

towed nets (Dames and Moore, 1979; Otto et al., 1981; Zeitoun 

et al., 1981). 

In the present study, fish size was found to be important 

in determining the effectiveness of wedge-wire screens. In many 

other studies of reduction in entrainment through wedge-wire 

screens, the effect of fish size has not been examined, which 

may help to ex?lain why apparent inconsistencies in conclusions 

were reached by previous researchers. Dames and Moore (1979) 

and Delmarva Power and Light (1982) both found close to 100% 

exclusion of bay anchovies in comparison to Browne et al., 

(1981) who found only 61% exclusion. The mean size of anchovies 

was not given in the first two studies, and may have been 

large. In contrast, the mean size of anchovies collected 

through the open port in the study of Browne et al., was only 

4.2 nun (TL). In the case of naked gobies, Dames and Moore 

(1979) found 56% exclusion efficiency through a l-mm screen, 

while Browne et al., (1981) found no significant difference 

between collection through a l-mm screen and through an open 

port. Again the small mean size of gobies entrained through 

the open port in the later study (4.8 mm) may have led to 

their lesser estimate of exclusion. 

Do Screens Reduce Entrainment Relative to Canal Density? 

Tables IV-1 and IV-2 show screening efficiency compared 

to canal density for 1, 2 and 3-mm screens. The pattern of 

exclusion is very similar to that observed when screens were 

compared with an open port, i.e., screens effectively reduced 

entrainment, though the magnitude was dependent on fish size. 

For bay anchovies, entrainment of eggs or individuals less 

than 5 mm in length was not significantly affected by screens. 

Individuals larger than 5 mm were excluded in comparison to 

either open port or canal density, though the magnitude of 

exclusion was greater in relation to canal density. For anchovies 

IV-6 
Martin Marietta Environmental Systems 

laryer'than 14 mm, the exclusion efficiency of all screen 

sizes tested exceeded 99% .in relation to canal density. 

Naked gobies of 4 mm in length or less were captured in 

significantly greater numbers in screened samples than in 

bongo net samples. The attraction through the screens indicated 

by these data could be due to this species' habitat preference 

for structures. Naked gobies were often observed during the 

study swimming in or near the biofouling community that developed 

on the barge. This behavior would tend to cause underestimates 

of ambient density when measured by the bongo net, which was 

towed upstream of the barge. For gobies greater than 4 mm in 

length, screens significantly reduced entrainment in relation 

to canal density. For gobies larger than 6 mm, exclusion was 

always greater than 90%. 

In our study, as well as in others, towed net samples have 

almost always been shown to contain more ichthyoplankton than 

unscreened port samples. This is not surprising, since an 

unscreened port creates a flow field which ichthyoplankton can . 

detect and avoid (Hocutt and Edinger, 1980). Bongo nets, on 

the other hand, are an active collection method and are generally 

towed at speeds which make avoidance by smaller ichthyoplankton 

unlikely. When calculating screening efficiency, the use of 

towed net samples as a control will therefore result in exclusion 

efficiences higher than those found when using unscreened ports. 

We have presented exclusion efficiency using both methods 

as reference points. For most fish size classes, the conclusion 

reached in our study would be the same using either method, 

though the magnitude of exclusion would vary. The open port 

method is the more conservative approach and is our recommended method for estimating entrainment reduction. However, 

the towed net method should not be discounted. Several authors 

have shown that ichthyoplankton estimates increase with increasing 

boat speed up to about 8 m/sec (Thayer et al. 1983; Colton et 

al. 1980; Clutter and Anraku, 1968). Since our tows were conducted at a boat speed of approximately 0.5 m/sec., they may 

still produce underestimates of actual numbers present in the 

ambient waters. 

Did Screen Slot Size Affect the Numbers Collected? 

In this study, fewer individuals of both species were 

entrained through the l-mm screen than through the 2 or 3-mm 

screens. For most size classes of naked gobies these differences 

were significant. However, for several size classes of bay 

anchovies, the difference in entrainment rates among different 

screens was not significant. Examination of the data indicates 

that the lack of statistical significance may be an artifact 

of low numbers. For example, not a single anchovy in the 11 

to 14 mm size class was entrained through a l-mm screen in our 

IV-7 
Martin Marietta Environmental Systems 

tests. Total exclusion by 2 or 3-mm screens. was not observed. 

However, because of low ambient anchovy.abundance, and the 

70-80% exclusion offered b;y 2 and 3-mm screens, sufficient 

samples containing no anchovies were collected through the 

larger screens so that these results could not be statistically 

distinguished from those for the l-mm screen. 

In other studies that have compared entrainment through 

screens of different slot size, the mean number olf individuals 

collected through the larger screen has always been greater. 

In the study by Dames and Moore (19791, the difference in fish 

collected when using a 1 and 2-mm screen was only 8%. However, 

Browne et al. (1981) found, on the average, 80% greater entrainment of naked gobies and bay anchovies through a 2-mm screen 

than through a l-mm screen. Zeitoun et al. (1981) found, on 

the average, that 40% more individuals were entrained through 

a 2-mm screen than through a 9.5-mm screen. 

Thus, a consistent trend has been found in a11 studies; 

larger mesh screens entrain more organisms but the differences 

are not statistically significant. There are two possible 

explanations for the lack of statistical significance. First, 

this may be due to low numbers captured, as was the case with 

bay anchovies in our study. With low numbers variance tends 

to increase and small differences become difficult to detect. 

Secondly, in all of the studies mentioned above, fish of different 

size classes were grouped together for analysis. However, 

different size groups behave differently with respect to screen 

size. Small individuals may not be affected by changes in 

slot size if they can pass through the smallest slot sizes 

(e.g., the fish < 4 mm size in our study). Similarly, large 

i'ndividuals will-not be a�fected by changes in screen slot size 

if they are so large as to be physically excluded by all scrzens 

or possess sufficient swimming ability to avoid entrainment 

through screens of all sizes. When analysis is conducted on 

data that is pooled over several size groups, the increased 

variance may obscure significant patterns that occur only 

within a limited size range. 

Did Screen Diameter Affect the Numbers Captured? 

Screen diameter appeared to have little effect on the number 

of ichthyoplankton entrained. The effect of screen diameter 

on the number of individuals captured was determined by c.omparing the large diameter 2-mm screen at high pump speed with results 

for the small diameter 2-mm screen at low pump speed. Both 

screens had the same through-slot velocity. For bay anchovies 

there was no statistical difference. For naked gobies, only 

for the smallest category was there a significant difference. 

In the latter case, the large screen at high pump speed collected 

approximately 20% more fish. 

IV-8 
Martin Marietta Environmental Systems 

Did Throuqh-Slot Velocity Affect the Numbers Captured? 

The effect of through-slot velocity on the density of 

individuals collected can be discerned by comparing effects of 

screens of identical diameter and slot width size, but of 

different withdrawal rates. In our study, such a comparison 

was made twice: between the effects of high and low pumping 

rate when the large diameter 2-mm screen was used (20 cm/sec 

and 9.5 cm/sec through-slot velocity, respectively), and the 

effects of high and low pumping rate when the small diameter 

2-mm screen rate was used (40 cm/sec and 19 cm/sec, respectively). 

The density of bay anchovies did not differ when collected 

through screens with different through-slot velocity. The 

number of bay anchovies collected through 2-mm screens, particularly 

for the larger size classes, was low. No consistent differences 

in the mean density of individuals collected under different 

conditions were apparent (Table 111-81, though the low numbers 

may have hampered our ability to detect significant differences. 

For every naked goby size class, the greatest density was 

found in samples taken at the highest through-slot velocity 

(Table 111-9). Significant differences were observed for the 

smallest size class of gobies between 9.5 cm/sec and 20 cm/sec., 

as well as between 19 cm/sec and 40 cm/sec. For the two largest 

goby size classes, significant differences were found only 

between 19 cm/sec and 40 cm/sec. This pattern suggests that 

the swimming ability of smaller gobies is sufficient to overcome 

through-slot velocities of 9.5 cm/sec, but not of greater 

withdrawal rates, and that larger gobies can overcome velocities 

as great as 20 cm/sec but not as great as 40 cm/sec. 

In tests of screen slot size, swimming ability was undoubt5,5iy 

important in reducing entrainment of both species through 

screens with slots too large to exclude them physically (i.e.! 

2, and 3-mm screens). Therefore, it is surprising that entrainment of bay anchovies was not affected by changes in the flow 

field. The through-slot velocities we chose to test may havs 

Seen too low to affect anchovies, but high enough to affect 

naked gobies. Hanson et al. (1978) used anchovies in tests 

with through-slot velocities that were higher than those we 

used, and found little effect on the fish. However, anchovies 

used in tests by Hanson et al. were larger than those in our 

studies and it is unlikely that the anchovies in our study

could escape currents of 40 cm/sec. Further, maximum swimming 

speed of most larval fish, including anchovy (Webb and Corolla, 

1981) and goby (Logachev and Mordvinov, 1979) species, is 

generally less than 10 cm/sec (e.g., Blaxter, 1969; Hettler, 

1978; Hartwell and Otto, 1978; Turnpenny and Bamber, 1983). 

Even 15 mm anchovies would not possess enough swimming ability 

to escape the 40 cm/sec through-slot velocity used in our 

tests. 

IV-9 
.. 

Martin Marietta EnvironmentalSystems 

A better explanation for differences in response to throughslot velocity may include differences in behavior: of the two 

species, along with the differences in swimming ability.

Naked gobies tend to prefer structures and were observed swimming 

very close to the screen, as if feeding on detritus drawn 

toward it. Anchovies were not observed to move as close to 

the screen on a regular basis. The through-slot velocities 

used in this test were qui.ckly dissipated by the screen's circular 

configuration. In our attempts at measuring flow, water withdrawal 

was indistinguishable from ambient flow at a distzance of less 

than 10 cm from the screen surface. Gobies, because they swim 

close to the screen, should be more affected by khis change in 

their local environment than anchovies, which generally remain 

outside this flow field. 

IV-10 
Martin Marietta Environmental Systems 

V. CONCLUSIONS 

In this study we have shown that the effectiveness of 

wedge-wire screens in reducing entrainment is dependent on fish 

size. Evidence for exclusion by maintenance of low through-slot

velocities as well as by physical exclusion, were-apparent. A 

l-mm screen reduced entrainment of both species when they

reached 5 mm in length. Below that size, no exclusion by 

screens was apparent. Above 10 mm in length, virtually total 

exlusion by l-mm screens occurred for both species. 

Screen slot size had an effect on the number of fish 

entrained, but the effect was not large. A l-mm screen was 

found to represent a barrier for 100% of the larger fish, while 

Screens with larger slots excluded 5 80% of the ichthyoplankton

relative to an open port. 

Altering through-slot velocity over a four-fold range

affected entrainment of naked gobies but not of bay anchovies. 

Due to the circular configuration of the screens, inward flow 

vectors quickly dissipate. We suggest that species such as 

naked gobies, which inhabit areas near the screen, will be 

affected more by these changes in flow than will open water 

species such as anchovies, which typically remain outside the 

influence of the withdrawal field. 

The effect of screen diameter was found to be rather 

unimportant, particularly for larger fish. For small individuals 

there was a slight reduction in entrainment with a lower 

withdrawal rate. 

V-l 
Martin Marietta Environmental Systems 

VI. REFERENCES -

American Society of Civil Engineers. 1982. Design of Water 

Intake Structures for Fish Protection. 163 pp. ASCE, NY, NY. 

Blaxter, J.H.S. 1969. Swimming Speeds of Fish. FA0 Fisheries 

Report 62:69-100. 

Browne, M.E., L.B. Glover,, D.W. Moore and D.W. Ballengee, 1981. 

In-situ biological and engineering evaluation of fine-mesh 

profile-wire cylinder as power plant intake screens. pp.

36-46. In: Advanced Intake Technology for Power Plant 

Cooling Water Systems, Proceedings of the Workshop of 

Advanced Intake. Technology. P.B. Dorn, and J.T. Johnson, 

eds., San Diego, CA. 

Clutter, R.I. and M. Anraku. 1968. Avoidance of samplers. 

pp. 57-76. In: Zooplankton Sampling. Monographs on 

OceanographicMethodology 2. D.J. Tranter, ed. 

Colton, J.B., J.R. Green, R.R. Byron and J.L. Frisella. 1980. 

Bongo net retention rates as effected by towing speed and 

mesh size. Can. J. Fisheries Aquat. Sci. 37:606-623. 

Conover, W.J. 1971, Practical Nonparametric Statistics. John 

Wiley and Sons. New York, New York. 

Dames and Moore, 1979. Seminole Plant Units No. 1.and No. 2 

316b Study and Report. Report to Seminole Electric 

Cooperative, Inc, 

Delmarva Ecological Laboratory. 1980. Ecological Studies of 

the Nanticoke River and Nearby Area. Vol. 11. Profile 

Wire Studies. Report to Delmarva Power and, Light Company. 

Delmarva Power and Light. 1982. Vienna Power Station. 

Prediction of aquatic impacts of the proposled cooling 

water intake. A 316b demonstration. 

Hanson, B.N., W.H. Bason, B.E. Beitz and K.E. Charles. 1978. 

A practical intake screen which substantially reduces the 

entrainment of early life stages of fish. pp. 392-407. 

-In: Fourth National. Workshop on Entrainment and Impingement. 

L.D. Jensen, ed., Ecological Analysts, Inc.,, Melville, NY. 

VI-1 
-- --- 

-- 

Martin Marietta Environmental Systems 

Hansori, R.N. 1981. Studies of larval Striped bass (Morone 

- _l_l_ saxatilis) and yellow perch (Perca flavcscens) exposed t o ___---- - a Imm slot proEile-wire screen model intake, pp. 22-35. In: 

P.B. Dorn, and J.T. Johnson, eds. Advanced Intake Technology for Power Plant Cooling Water Systems, Proceedings 

of the Workshop of Advanced Intake Technology. San Diego, 

CA. 

Hartwell, S.I. and R.G. Otto. 1978. Swimming performance of 

juvenile menhaden (Brevoortia tyrannus). Trans. Am. 

Fish. SOC. 107:793-798. 

Hettler, W.H. 1978. Swimming speeds of juvenile estuarine 

fish in a circular flume. Proc. S..E. Assoc. Game Fish 

Corn. 31 :392-398. 

Heuer, J.H. and D.A. Tomljanovich. 1978. A study on the 

protection of fish larvae at water intakes using wedgewire screening, pp. 169-194. In: Larval Exclusion 

Systems For Power Plant CoolingWater Intakes. R. K. 

Sharmer and J.B. Palmer, eds., Argonne National Lab., Argonne, 

IL. Publ. no. ANL/ES-66. 

Hocutt, C.H. and J.E. Edinger. 1980. Fish behavior in flowfields. pp. 143-181. -

In: Power Plant Effects on Fish

and Shellfish Behavior. C.H. Hocutt, J,R. Stauffer, Jr., 

J.E. Edinger, L.W. Hall, Jr. and R.P. Morgan 11, eds., 

Academic Press, NY. 

Lifton, W.S. 1979. Biological aspects of screen testing on the 

St. Johns River, Palatka, Fla. pp. 87-96. In: Proceedings 

of Passive Screen Intake Workshop, Johnson Division UOP 

Inc., St. Paul, MN. 

Logachev, V.S. and Y.E. Mordvinov. 1979. Swimming speed and 

activity of larvae of round goby and some predatory crustacea.ns of the Black Sea. Sov. J. Mar. Biol, 5:227-229. 

McEwen, G.F., M.W. Johnson and T.R. Folsom. 1954. A statistical 

analysis of the performance of the Folsom plankton sample 

splitter based upon test observations. Arch. Meteorol. 

Geophys. Bioklimotol., Ser. A. 7:502-527. 

Ott, L. 1977. An Introduction to Statistical 'Methods and 

Data Analysis. Duxbury Press. North Scituate, MA, 

Otto, R.G., T.I. Hiebert and V.R. Kranz. 1981. The effectiveness 

of a remote profile-wire screen intake module in reducing 

the entrainment of fish eggs and larvae. pp. 47-56. In: 

Advanced Intake Technology for Power Plant Cooling Water 

Systems, Proceedings of the Workshop of Advanced Intake 

Technology. P.B. Dorn and J.T. Johnson, eds., San Diego, 

CA. 

VI-2 
Martin Marietta Environmental Systems 

Snedecor, G.W. and W.G. Cochran. 1980. Statistical Methods. 

Iowa State University Press. Ames, IA. 

Thayer, G.W., D.R. Colby, M.A. Kjelson and M.P. Weinstein. 

1983. Estimates of larval-fish abundance: Diurnal 

variation and influences of sampling gear and towing speed. 

Trans. Am. Fish. SOC. 112:272-279. 

Turnpenny, A.W.H. and R.N. Bamber. 1983. The critical swimming

speed of the sand smelt (Atherina presbyter Cuvier) in 

relation to capture at a power station cooling water 

intake. J. Fish Biol. 23:65-73. 

Webb, P.W. and R.T. Corolla. 1981. Burst swimming performance 

of Northern Anchovy, Enqraulis mordax, larvae. Fish. Bull. 

79:143-150. 

Weisberg, S.B., F. Jacobs, W.H. Burton, and R.N. Ross. 1983. 

Report on preliminary studies using the wedge wire screen 

model intake facility. Report NO. PPSP-CP-83-1. Prepared

for State of Maryland, Power Plant Siting Program. ?re?c;&:Ci 

by Martin Marietta Environmental Center, Baltimore, MD. 

Zeitoun, I.H., J.A. Gulvas and J.Z. Reynolds. 1981. Effectiveness 

of small mesh cylindrical wedge-wire screens in reducing

fish larvae entrainment at an offshore and an onshore 

location of Lake Michigan, pp. 57-64. In: Advanced 

Intake Technology for Power Plant CoolingWater Systems,

Proceedings of the Workshop of Advanced Intake Technology.

P.B. Dorn, and J.T. Johnson, eds.I San Diego, CA. 

VI-3 
Martin Marietta Environmental Systems 

APPENDIX A 

TEST CONDITIONS, SURFACE SALINITY, TEMPERATURE, AND 

DISSOLVED OXYGEN FOR EACH NIGHT DURING 

THE 1983 ENTRAINMENT STUDY 

A-1 
Martin Marietta Environmental Systems 

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Martin Marietta Environmental Systems 

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A- 8 
Martin Marietta Environmental Systems 

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A-10 
Martin Marietta Environmental Systems 

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Martin Marietta Environmental Systems 

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Martin Marietta Environmental Systems 

Tah1.e H-1. Two-way analysis oE variance for- 1983 data 

an bay anchovy eggs using date-port as a hlockinc 

factor and screen size (open, l-mm, 2-m, and 

3-mml as the main effect 

~~ 

Effect DF ss F P 

Blocking Factor 21 209.29 0.99 .49 

Screen 3 37.15 1.03 .30 

Error 63 636.49 

-
Martin Marietta Environmental Systems 

Ta ble B-2. Two-way analysis of variance for 1983 data on 

size class 1 (2-4 mm) bay anchovy using date-port 

as a blocking factor and screen size (open, 1-mm, 

2-mm, and 3-mm) as the main effect 

~~ 

Effect DF ss F P 

Blocking Factor 21 42.801 2.99 c.01 

Screen 3 2.789 1.37 .26 

Error 63 42.89 

B-3 
Martin Marietta EnvironmentalSystems 

Table E-3. Two-way analysis of variance for 1B83 data on 

size class 2 (5-7 mm) bay anchovy using date-port 

as a blocking factor and screen size (open, 1-mm, 

2-mm, and 3-mm) as the main effect 

Effect DF ss F P 

Blocking Factor 21 38.432 4.02 <.01 

..

Screen 3 6.308 4.62 .os 

Error 63 28.666 

Duncan's New Multiple Range Test For Screen Effect: 

open l-ItUn 3-mm 2-mm 

port screen screen screen 

i 
Martin Marietta Environmental Systems 

Table B-4. Two-way analysis of variance for 1982 data on 

size class 3 (8-10 mm) bay anchovy using port 

(left, right) and screen size (open, 1-mm, and 

2-mm) as main effects 

-

Effect DF ss F P 

Port 1 0.00 0.03 .86 

Screen 2 0.06 0.38 .69 

Error 19 1.50 

B-5 

I 
Martin Marietta Environmental Systems 

Table 8-5. Two-way analysis of covariance for 1983 data on 

size class 3 (8-10 mm) bay anchovy using dateport as a blocking factor, screen size (open 

port, 1-mm, 2-mm and 3-mm) as a main effect, and 

canal density as the covariate 

Effect DF ss F P 

Port 21 7.152 1.37 .17 

Screen 3 9.963 13.32 c.01 

Covar iate 1 1.040 4.17 .05 

1 

Error 62 15.45 

Multiple Comparisons For Screen Effects: 

Open 1-mm 2-mm 3-mm 

Open Port X <.01 <.01 <.01 

1-mm X .70 .18 

2-mm X .33 

3 -mm X 

B- 6 
Martin Marietta Environmental Systems 

Table 6-6. Two-way analysis of variance for 1983 bay anchovy 

data on size class 4 (11-14 mm) using date-port

a5 a blocking factor and screen size (open port, 

1-mm, 2-mm and 3-mm) as the main effect 

Effect DF ss F P 

Blocking Factor 21 1.28 1.28 .22 

1.43 10.00 <.01 

Screen 3 

ErrOK 63 3.00 

Duncan's New Multiple Range Test For Screen Effect: 

Open 3-mm 2-mm l-lWtl 

Port Screen Screen Screen 

I 

B- 7 
Martin Marietta Environmental Systems < 

Table B-7. Two-way analysis of variance for 1982 data on 

size class 5 (> 15 mm) bay anchovy using port 

(left, right) and screen size (open, 1-mm and 

2-mm) as main effects 

Effect DF ss F P 

Port 1 1.15 3 .lo7 0.10 

Screen .- 2 11.32 15.17 < .Ol 

Error 19 7.09 

Duncan's New Multiple Range Test For Screen Effect: 

Opeh 2-mm l-IIUll 

Port Screen Screen 

B- 8 
Martin Marietta Environmental Systems 

APPENDIX C 

ANALYSIS OF VARIANCE TABLES FOR 

THE EFFECT OF SCREEN SLOT SIZE ON 

NAKED GOBY ENTRAINMENT 

c-1 
Martin Marietta Environmental Systems 

Table C-1. Two-way analysis of covariance for 1982 data on 

size class 1 (< 4 mm) naked gobies using port

(left, right) and screen size (open, 1-nun and 

2-mm) as main effects and canal density as the 

covariate 

Effect DF ss F P 

i 

Port 1 2.09 9.79 .01 

Screen 2 0.20 0.48 .63 

Covaria te 1 2.98 14.01 <.01 

Error 18 3.83 

c-2 
Martin Marietta Environmental Systems 

Table C-2. Twci-way analysis of covariance far 1983 data on 

size class 1 (< 4 mm) naked goby using date-port 

as a blocking Tactor, screen size (open port, 

l-mm, 2-mm and 3-mm) as the main effect, and 

canal density as the covariate 

Effect 

Blocking Factor 

Screen 

Cavariate 

Errar 

DF ss F P 

21 34.082 4.59 < .01 

3 1.095 1.03 .38 

1 2.771 7 -83 < .01 

62 21.935 
Martin Marietta Environmental Systems 

Table C-3. Two-way analysis of covariance for 1983 data on 

size class 2 (5-6 mm) naked goby using date-port 

as a blocking factor, screen size (open port, 

l-mm, 2-mm and 3-mm) as the main effect and 

canal density as the covariate 

Effect DF ss F P 

Blocking Factor 21 29.717 2.71 <.01 

Screen 3 7.829 4.99 <.01 

Covariate 1 3 8474 6.65 .01 

Error 62 32.405 

Multiple Comparisons For Screen Effect: 

Open l-mm 2-mm 3-mm 

Open X < .01 .ll .13 

1-mm X 003 .03 

2-mm X .98 

3-mm X 
Martin Marietta Environmental Systems 

*rable C-4. Two-way analysis of covariance for 1982 data on 

size class 2 (5-6 mm) naked gobies using port

(left, right) and screen (open, l-mm, and 2-mm) 

as main effects 

Effect DF ss F P 

Fort 1 0.90 2 .,74 .12 

Screen 2 0.19 0 "29 .75 

Covar ia te 1 5.26 16.02 

Error 18 5.91 

c-5 
Martin Marietta Environmental Systems 

Table C-5. Two-way analysis of variance for 1982 data on 

size class 3 (7-8 mm) naked gobies using port 

(leEt, right) and screen size (open, 1-mm, and 

2-mm) as main effects 

Effect DF ss F P 

Port 1 1.30 5.20 .03 

Screen 2 5.25 10.53 C.01 

Error 19 4.74 

Duncan's New Multiple Range Test For Screen Effect: 

Open Port 2-mm 1-REll 

C-6 
Martin Marietta EnvironmentalSystems 

*r;lt)ic C-6. Twc~wayanalysis OF variance for 19133 data On 

size c3.ass 3 (7-8 mrn) naked qohies using date-port. 

as a blocking factor and screen size (open port, 

1-mm, 2-mm alnd 3-mm) as the main efjfect 

Effect DF ss F P 

Blocking Factor 21 25.980 2.53 <.01 

Screen 3 44.658 30.46 <. 01 

Error 63 30.788 

Duncan's New Multiple Range Test For Screen Effect: 

Open Port 3-mm 2-mm -

1-mm 

c-7 
Martin Marietta Environmental Systems 

Table c-7. Two-way analysis of variance for 1982 data on 

size class 4 0 9 rnm) naked gobies using port 

(left, right) and screen size (open port, 1-mm, 

and 2-mm) as main effects 

Effect DF ss F P 

Port 1 1.41 3.46 .08 

Screen 2 13.21 16.25 < .01 

Error 19 7.72 

Duncan's New Multiple Range Test For Screen Effect: 

Open Port 2-mm l-llllll 

C-8 
Martin Marietta Environmeptal Systems 

APPENDIX D 

ANALYSIS OF VARIANCE TABLES FOR 

THE EFFECT OF THROUGH-SLOT VELOCITY/SCREEN 

DIAMETER ON BAY ANCHOVY ENTRAINMENT 

D- 1 
Martin MariettaEnvironmentalSystems 

Tab le D-1. Two-way analysis of variance for bay anchovy 

eggs using date-port as a blocking factor and 

screen diameter and through-slot velocity as 

the main effect 

Effect DF ss F P 

Blocking Factor 21 308.07 1.20 .28 

Velocity 3 44 -44 1.21 .31 

Error 63 770.15 

D- 2 
. . .. 

Martin Marietta Environmental Systems 

-~ 

Table D-4. Twg-way analysis of covariance for bay anchovy

size class 3 (8-10 mm) using date-port as a 

blocking factor and through-slot velocity and 

screen diameter as the main effect 

Effect 

Blocking Factor 

Ve loci ty 

Csvariate 

Error 

DF ss F P 

21 10.59 1.70 .OS 

3 1.06 1.19 .32 

1 1.14 3.83 .OS 

62 18.44 

D-5 
Martin .Allariette.EnvironmentdSystems 

Table D-5, Two-way analysis of variance for bay anchovy 

size class 4 (11-4 mm) using date-port as a 

blocking factor and through-slot velocity and 

screen diameter as the main effect 

Effect DF ss F P 

Blocking Factor 21 0.64 0.87 .63 

Ve 1oci ty 3 0.23 2.23 .09 

Errar 63 2.21 

D-6 
Martin Marietta Environmental Systems 

APPENDIX E 

ANALYSIS OF VARIANCE TARLES FOR 

THE E'FFECT OF THROUGH-SLOT VELOCITY AND1 SCREEN 

DIAMETER ON NAKED GORY ENTRAINMENT 

E-1 
Martin Marietta Environmental Systems 

Table E-1. Two-way analysis of covariance for size class 1 

((4 nun) naked goby using date-port as a blocking

factor, screen diameter and through-slot velocity 

as the main effect, and canal density as the 

covariate 

DF ss F -P 

Factor 21 23.88 3.41 <.01 

3 3.35 3.35 .02 

1 6.87 20.56 <.01 

62 20.08 

Comparisons For Main Effect: 

Effect 

Blocking

I Velocity 

Covaria te 

Error 

I

1 Multiple 

1

i Log 

SL SH 

Means 

SL - 1.19 X .03 

ISH - 0.79 X 

LL - 1.15 

LH - 0.78 

SL - Small Screen/Low Pump Speed 

SH - Small Screen/High Pump Speed 

LL - Large Screen/Low Pump Speed 

LH - Large Screen/High Pump Speed 

LL LH 

.83 .02 

.04 .93 

X .03 

X 

E-2 
Martin Marietta Environmental Systems 

Table E-2, Two-way analysis of covariance for naked goby

size class 2 (5-6 mm) data using date-port as a 

blocking factor, through-screen slot velocity 

and screen diameter as the main effect, and canal 

density as the covariate 

Effect DF ss - F P 

Blocking Factor 21 32.54 1.76 .04 

Velocity 3 7.63 2.89 .04 

Covar ia te 1 7.76 8-81 <.01 

Error 62 54.63 

Multiple Compa'risons For Main Effect: 

SL SH LL LH 

Log

Means 

SL - 2.92 X .12 23 .46 

SH - 2.47 X <.01 .39 

LL - 3.27 X .05 

LH - 2.71 X 

SL - Small Screen/Low Pump Speed 

SH - Small Screen/High Pump Speed 

LL - Large Screen/Low Pump Speed 

LH - Large Screen/High Pump Speed 

E-3 A 
Martin Marietta Environmental Systems 

Table E-3. Two-way analysis of variance for size class 3 

(7-8 mm) naked goby data using date-port as a 

blocking factor and screen diameter and 

through-slot velocity as the main effect 

Effect DF ss F P 

Blocking Factor 21 21.36 1.30 .21 

Velocity 3 21-00 8 -93 < .01 

Error 63 49.41 

Duncan's New Multiple Range Test For Main Effect: 

Small Screen Large Screen Large Screen Small Screen 

High Speed High Speed Low Speed Low Speed 

E-4 
-- 

Martin Marietta Environmental Systems 

Table E-4. Two-way analysis oE variance �a~-size class 

4 (> 9 mm) naked goby data using date-port as 

a biocking factor, and screen diameter and 

through-slot velocity as the main effects 

Effect 

Rlocking Factor 

Velocitry 

Errar 

DF ss F P 

21 16.53 1.58 e08 

3 6.84 4.58 < '01 

63 31.35 

Duncan's New Multiple Range Test For Main Effect: 

- Small Screen Large Screen Large Screen Small Screen 

High Speed High Speed Low Speed Law Speed