1

MEMORANDUM

Tetra Tech, Inc.

10306 Eaton Place, Suite 340

Fairfax, VA 22030

phone 703-385-6000

fax 703-385-6007

DATE: March 15, 2012; revised February 26, 2014

TO: Paul Shriner and Lisa Biddle, EPA

FROM: Blaine Snyder, Henry Latimer, and Kelly Meadows, Tetra Tech

SUBJECT: Fragile Fish Species

EPA is reviewing approaches for including or excluding certain fish species from some 

impingement mortality (IM) requirements based on fragility, a concept suggested in 

comments. In this analysis, we characterize those fish identified in IM studies used in 

developing the IM performance standards based on their relative fragility. Specifically, 

EPA reviewed the species-specific mortality data from the studies that were used to 

develop the IM performance standard. (See the Technical Development Document for 

more details.) From that data, any species that exhibited a survival rate of less than 30% 

was deemed to be fragile and were then excluded from calculations for the performance 

standard. These species are: alewife, American shad, Atlantic herring, Atlantic longfinned squid, Atlantic menhaden, bay anchovy, blueback herring, bluefish, butterfish, 

gizzard shad, grey snapper, hickory shad, menhaden, rainbow smelt, round herring, and 

silver anchovy. These fragile species are specifically named in the regulatory language.

EPA corroborated this approach using two independent approaches; these approaches to 

defining the “fragility” of species subjected to impingement are summarized in the 

attached spreadsheet. The corroborating approaches are based on expert biologist

opinion, and compared to existing impingement survival information.

The first analysis assumes that all fish were late juveniles or adults. It’s difficult to craft a 

specific definition for “fragile” in this context, but one approach identifies those species 

that are considered particularly fragile because they are the least likely to survive stresses 

associated with impingement (e.g., the top third of the species on a theoretical

impingement mortality gradient). Even with a definition of “fragile” and/or a threshold 

for distinguishing fragile and non-fragile species, there would still be some intermediate 

species. This is reflected in the attached spreadsheet, which used a sampling of IM 

studies to develop a list of commonly impinged fish and then assessed their 
2

hardiness. We used red, yellow, and green highlights to tag species as particularly fragile, 

somewhat fragile, and rather hardy in an IM context, respectively.

The red category includes some species that should be universally recognized as fragile, 

such as the anchovies and the herrings. Others like mackerel and bluefish were included 

in the red category because they’re fast-swimming pelagic predator species that by nature 

are constantly moving and are unlikely to tolerate the “inactivity” of being impinged. The 

Salmonidae (trout and salmon) and Anguillidae (American eel) are highlighted in yellow 

because, depending on conditions and life stages, they can be somewhat fragile (e.g., 

American eel IM survival rates can vary widely).

Impingement survival can vary within species based on life stage, ambient conditions 

(e.g., water temperature or season, salinity, etc.), and intake conditions (e.g., amount of 

debris on screens, type of screen, screen wash frequency, screen travel time, etc.). 

However, late juvenile and adult species identified as fragile species are still categorized 

as fragile regardless of the other conditions, life stage, ambient conditions, and intake 

conditions. Therefore, other generalizations were not necessary to identify the most 

fragile (least hardiness categories) as highlighted in the spreadsheet.

After completing this analysis, we conducted a second analysis that compared EPRI’s 

estimates of impingement survival for fish families (as found in Impingement and 

Entrainment Survival Studies Technical Support Document. EPRI, Palo Alto, CA. 2005. 

Technical Report #1011278) to our species ratings. EPRI summarized impingement 

survival data by fish family, and separated families into three categories: high survival 

rate potential (71-100%); intermediate survival rate potential (31-70%); and low survival 

rate potential (0-30%). EPRI based their survival estimates on the apparent overall 

potential for surviving impingement in prior studies. They noted that these estimations 

should only be used as an approximation of survival. We re-classified all species based 

on the EPRI fish family ratings and inserted those classifications as a separate column in 

the attached spreadsheet.

Conclusion

There is good general agreement in fragile species ratings between the list of fragile 

species in the draft regulation and the two independent analyses, particularly for the low 

survival (or fragile) category. Nearly all of the species identified from the IM studies as 

fragile are also identified as fragile in the Tt analysis; the exceptions are the Atlantic 

long-finned squid, grey snapper, hickory shad, round herring, and silver anchovy. None 

of these species were identified in the Tt analysis but all belong to a family identified by

EPRI as being fragile. As a result, the analysis supports the draft definition and list of 

fragile species. 

Similarly, the EPRI data also supports the draft definition and resultant list of fragile 

species. The EPRI analysis identified families of fragile fish; this broader approach 

identified each of the species identified by EPA in reviewing studies for the IM 

performance standards, and resulted in a virtually identical list of species.
3

This revised analysis also generally agreed with the EPRI analysis. We highlighted 14 

fragile species, 12 of which were in families identified as having a low survival rate 

potential by EPRI; the other two (mackerel and Spanish mackerel) were in families 

identified as having an intermediate survival rate by EPRI.