U.S. Department of the Interior

U.S. Geological Survey

USGS Open-File Report 2006-1106

April 2006

Acute Toxicity of the Lampricides TFM and Niclosamide 

to Three Species of Unionid Mussels

By Michael A. Boogaard

The sea lamprey (Petromyzon 

marinus), a jawless parasitic eel-like 

fish native to the Atlantic Ocean 

(fig. 1), was accidentally introduced 

into the Great Lakes in the early 20th

century through the construction of 

shipping canals. A member of the 

Petromyzonidae family, the primitive parasite has been identified as 

a major cause of the collapse of the 

Great Lakes fishery in the 1940s and 

1950s. The lampricides 3–trifluoromethyl–4–nitrophenol (TFM) and 

2’5–dichloro–4’–nitrosalicylanilide 

(niclosamide) have been used to 

control larval sea lampreys in tributaries of the Great Lakes since the 

early 1960s. The lampricide TFM is 

the main compound used to keep sea 

lamprey populations in check while 

niclosamide is used primarily in 

combination with TFM as a cost-saving measure. The addition of niclosamide at a ratio of 1% to TFM 

will reduce the amount of TFM required for effective treatment 

by about 40%.

Concern has been expressed in recent years over the risk of 

lampricide applications to nontarget fauna. Of particular concern 

are several fish, mussel, amphibian, and other rare aquatic species 

listed as threatened, endangered, or of special concern by state 

and Federal agencies. As part of a long standing commitment 

to the Great Lakes Fishery Commission, Ann Arbor, Michigan, 

scientists at the Upper Midwest Environmental Sciences Center, 

La Crosse, Wisconsin, conduct toxicological risk assessments of 

the lampricides to nontarget species of interest. 

Freshwater mussels are an important part of the aquatic 

community in many waters, often comprising a significant portion of the benthic biomass. In the past century, the diversity and 

abundance of freshwater mussels have dramatically declined. 

Freshwater mussels are now recognized as one of the most imperiled faunal groups in North America. Little information exists on 

the relative toxicity of the lampricides to unionid mussels. This 

Open-File Report highlights the results of acute toxicity tests 

conducted on three species of unionid mussels—giant floater 

(Pyganodon grandis), fragile papershell (Leptodea fragilis), and 

pink heelsplitter (Potamilus alatus; fig. 2)—exposed to TFM and 

a 99% TFM:1% niclosamide mixture.

Six laboratory exposures were conducted in a continuous-flow dilution system to assess the levels where mussels are 

sensitive to the lampricides 

(fig.3). Exposure duration was 

12 hours and was designed to 

match the duration of a typical 

larval sea lamprey control 

treatment. Each exposure set 

consisted of nine exposure 

concentrations and one control 

with dilution water only 

with 10 mussels and 10 sea 

lampreys per concentration. 

Mussels were held for 2 weeks 

after exposure to assess 

delayed mortality resulting 

from the lampricides. Mortality of the three mussel species 

was compared to the observed 

sea lamprey minimum lethal 

concentration (MLC), defined 

as the lowest concentration 

resulting in 100% mortality 

among larval sea lampreys.

Table 1 represents acute 

toxicity results for the three 

unionid mussels compared 

to sea lamprey larvae after a 

12-hour exposure to TFM. 

No mortality was observed 

among mussels at the observed 

MLC for sea lamprey of 2.6–

Figure 3. Photograph of the continuous-flow dilution 

system used to assess the toxicity of the lampricides on 

the three mussel species in relation to larval sea lampreys 

(Petromyzon marinus).

Figure 1. Sea lampreys 

(Petromyzon marinus). Note 

oral disk with concentric 

rows of teeth.

A

B

C

Figure 2. Three species of 

unionid mussels: (A) giant floater 

(Pyganodon grandis), (B) pink 

heelsplitter (Potamilus alatus),

and (C) fragile papershell 

(Leptodea fragilis).
USGS Open-File Report 2006-1106

2.7 mg/L TFM. Occasionally, lampricide concentrations were 

applied at 1.5 × the MLC for sea lamprey larvae at selected points 

within the stream to compensate for attenuation of the chemical 

bank as it moves downstream. This level represents the highest 

lampricide concentration organisms would encounter during sea 

lamprey control operations. Mortality at 1.5 × (3.8–4.1 mg/L 

TFM) for the observed sea lamprey MLC was 20% for the giant 

floater and 0% for the fragile papershell and pink heelsplitter. 

Exposure of the mussels to the niclosamide mixture yielded 

slightly higher mortalities (Table 2). Mussel mortalities associated with the mixture were 10% for the giant floater and 0% for 

the fragile papershell and pink heelsplitter at the observed 

sea lamprey MLC (1.3–1.4 mg/L as TFM) and were 0% for 

the giant floater and 10% for the fragile papershell and pink 

heelsplitter at 1.5 × (2.0–2.1 mg/L as TFM) the observed sea 

lamprey MLC. However, 20% of giant floaters and fragile papershells were killed at 1.2 × MLC (1.6–1.7 mg/L as 

TFM). 

In the TFM exposures, mortality was minimal among 

the three unionid species at concentrations typically applied 

to streams to control sea lamprey larvae (1.0 –1.5 × sea lamprey MLC) and indicates control operations with TFM would 

have a negligible effect on populations of these unionid 

mussels. However, the slight increase in unionid mortality 

observed when exposed to the niclosamide mixture indicates 

mussels may be at risk when the mixture is used in control 

operations. Niclosamide was originally developed as a molluscicide to eliminate snails, the intermediate host of parasites causing schistosomiasis in humans. Therefore, it is not 

surprising that molluscs are more sensitive to this compound. 

Treatment managers should use caution when using the 

combination of TFM and niclosamide in streams with known 

mussel populations and every effort should be made to maintain lampricide concentrations at or near the MLC for sea 

lamprey to minimize the risk to this important faunal group. 

Table 2. Acute toxicity (2-week post-exposure percent mortality) 

of a 99% 3–trifluoromethyl–4–nitrophenol (TFM):1% 2’5–dichloro–

4’–nitrosalicylanilide (niclosamide) mixture to the giant floater 

(Pyganodon grandis), fragile papershell (Leptodea fragilis), and 

pink heelsplitter (Potamilus alatus) exposed for 12 hours with direct 

comparison to larval sea lamprey (Petromyzon marinus). 

99% TFM:1% 

niclosamide 

concentration

(mg/L as TFM)

Percent mortality

Giant

floater 

(n = 10)

Fragile

 papershell

(n = 10)

Pink

heelsplitter

(n = 10)

Sea

lamprey

(n = 10)

3.9–4.1 30 30 –a 100

3.0–3.3 50 20 60 100

2.4–2.6 40 20 40 100

2.0–2.1 0 10 10 100

1.6–1.7 20 20 0 100

1.3–1.4 10 0 0 100

1.0–1.1 –a 0 0 50

0.8–0.8 –a –a 0 6.7

0.6–0.7 –a –a 0 0

Exposure control 0 0 0 0

a

Not exposed to these concentrations.

For more information contact

Michael A. Boogaard

U.S. Geological Survey

Upper Midwest Environmental Sciences Center

2630 Fanta Reed Road

La Crosse, Wisconsin 54503

Telephone (608) 781-6238

E-mail: mboogard@usgs.gov

http://www.umesc.usgs.gov

Table 1. Acute toxicity (2-week post-exposure percent mortality) of 3–

trifluoromethyl–4–nitrophenol (TFM) to the giant floater (Pyganodon 

grandis), fragile papershell (Leptodea fragilis), and pink heelsplitter 

(Potamilus alatus) exposed for 12 hours with direct comparison to larval 

sea lamprey (Petromyzon marinus). 

 TFM

concentration

(mg/L)

Percent mortality

Giant

floater 

(n = 10)

Fragile

 papershell

(n = 10)

Pink

heelsplitter

(n = 10)

Sea

lamprey

(n = 10)

6.3–6.3 60 –a –a 100

4.9–5.1 30 30 –a 100

3.8–4.1 20 0 0 100

3.1–3.3 10 10 0 100

2.6–2.7 0 0 0 100

2.0–2.2 20 0 0 77

1.7–1.8 –a 0 0 20

1.4–1.4 –a 0 0 3

1.0–1.1 –a –a 0 0

Exposure control 0 0 0 0

a

Not exposed to these concentrations.