1

MEMORANDUM

Tetra Tech, Inc.

400 Red Brook Blvd., Suite 200

Owings Mills, MD 21117

phone 410-356-8993

fax 410-356-9005

DATE: February 24, 2009

TO: Paul Shriner and Jan Matuszko, EPA

FROM: Ann Roseberry Lincoln, Henry Latimer, Blaine Snyder, and Kelly Meadows, Tetra 

Tech

SUBJECT: Negative entrainment reductions

EPA requested that Tetra Tech review two data points in the entrainment data set that appear to be 

counterintuitive, implying that an intake technology increased entrainment rates over a control scenario. 

Tetra Tech reviewed Chapter 11 of the draft Technical Development Document (TDD) from December 

2008; specifically Table 11-8, titled “Total Organisms: Percent Reduction of Entrainment by Slot Width 

and Slot Velocity.” There were two studies cited in the table that showed negative percent entrainment 

reductions. Generally speaking, both study authors conclude that some or all of the results were not 

statistically significant or could not be used to draw conclusions. Specific discussion for each study is 

presented below.

Field Evaluation of Wedgewire Screens for Protecting Early Life Stage of Fish at Cooling Water Intake 

Structures: Chesapeake Bay Studies (EPRI 2006)

This study showed a -7.6% reduction of total organisms for a 1 mm slot width and 0.30 m/s slot velocity. 

Tetra Tech believes that Table 11-8 combines entrainment results from this study that were presented

separately for fish larvae (which were statistically significant) with those for bay anchovy eggs (which 

were not statistically significant).

Specifically, this study was conducted by EPRI, utilizing a test barge containing two pumps and three 

screens. One screen (9.5 mm square mesh) was a control screen, while the other two (0.5 mm and 1 mm) 

were cylindrical wedgewire test screens. The pumps operated at two different intake velocities (0.15 m/s 
2

or 0.30 m/s) during testing. Water and entrained organisms drawn through the test and control screens 

were discharged into separate 0.75 m diameter, 335 micron mesh plankton nets. Entrained organisms for 

each trial were collected, washed into a 1-L sample jar, and preserved for analysis. The test in question 

was the one comprising the 1 mm screen with 0.30 m/s slot velocity (compared to results from the control 

screen). The authors report the results for entrained larvae (multiple species) separately from entrained 

bay anchovy eggs, which were the overwhelming majority of entrained eggs (148,181 fish eggs total, of 

which all but seven were bay anchovy eggs). They reported a mean number of larvae entrained over 20 

trials (106.3 larvae per 100 m3entrained with the control screen, and only 49.4 larvae per 100 m3

entrained using the test screen) for a statistically significant reduction of 53.1%. However, for bay 

anchovy eggs, the authors reported a mean entrainment (over 20 trials) of 271.7 eggs per 100 m3(control) 

versus 356.9 eggs per 100 m3(test) for a “reduction” of -31.3%. The authors specifically state that this 

difference is not statistically significant (p>0.05): “The 1.0 mm screen did not provide a significant 

reduction in egg density compared to control samples for either slot velocity.”

Brown, M.E. 1979 (as cited in Fish Protection at Cooling Water Intake Structures: A Technical 

Reference Manual. EPRI, Palo Alto, CA: 2007. 1014934)

This study (regarding sampling conducted in the intake canal at Oyster Creek) showed a -24.7% reduction 

of total organisms for a 2 mm slot width and 0.152 m/s slot velocity. While Tetra Tech was not able to 

locate the original study results, a summary of the original field study was provided in EPRI (2007). This 

document specifically states “Organisms were not entrained in large enough numbers to draw any 

significant conclusions.” The citation for this study refers to an abstract published in “Passive Intake 

Screen Systems Workshop, Chicago, IL Dec 4-5, 1979 (pp. 17-38)” but no entrainment data are presented 

in that document, and the focus of the talk appeared to be on screen fouling.

The observation of small, statistically insignificant differences in ichthyoplankton densities at various 

locations within a sampling area is common. Therefore, it is not surprising that some studies identify 

slightly higher, but statistically similar, densities of ichthyoplankton in water that has passed through 

different screens designed to limit entrainment. Such a result should not be interpreted to mean that 

ichthyoplankton densities are enhanced by passage through the screen. Observation uncertainty exists as 

an inherent characteristic of any measurement, even in the best-designed scientific studies. This 

uncertainty results from a combination of causes including sampling error (the sample is an imperfect 

estimate of the true population because of natural variability) and measurement error (the sample is 

obtained using an imperfect measuring device). Smaller sample sizes typically result in larger 

uncertainties. Sampling error in ichthyoplankton studies results from spatial and temporal variability in 

ichthyoplankton densities. Snyder (1983) presents uneven horizontal ichthyoplankton distribution due to 

either patchiness or mass movement as a problem associated with surveying fish eggs and larvae. He 

identifies water temperature, sample size, predation, currents, and meteorological conditions as factors 

influencing horizontal patchiness. Measurement error results from a number of sources including 

extrusion from plankton nets, damage to eggs and larvae from passing through pumps, and passive 

avoidance. Other factors may contribute to measurement error, as well, depending upon the methods 

employed.

Conclusion

Based on Tetra Tech’s assessment of the available information and our understanding of uncertainty and 

ichthyoplankton variability, we agree with the authors of these two studies. Specifically, the reported 

differences were either not statistically significant (Chesapeake Bay Study) or based on too small a 

number of entrained organisms to be meaningful (Oyster Creek Study). As such, we do not believe that 

the either result should be used to quantify entrainment reductions and the results from these studies 

should not be included in the analysis. 
3

Cited Literature

Brown. M.E. 1979. Preliminary Engineering and Environmental Evaluation of Fine-Mesh Profile 

Wire as Powerplant Intake Screening. Pages 17-38. In: Passive Intake Screen Workshop, 

Chicago, Illinois, December 1979.

Field Evaluation of Wedgewire Screens for Protecting Early Life Stage of Fish at Cooling Water Intake 

Structures: Chesapeake Bay Studies. EPRI, Palo Alto, CA: 2006. 1012542.

Fish Protection at Cooling Water Intake Structures: A Technical Reference Manual. EPRI, Palo 

Alto, CA: 2007. 1014934

Snyder, D.E. 1983. Fish Eggs and Larvae. Pages 165–197 in L.A. Nielsen and D.L. Johnson, editors. 

Fisheries Techniques. American Fisheries Society, Bethesda, MD.