United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture 

Veterinary Services 

Centers for Epidemiology and Animal Health August 2011 

_________________________________________________________________________________________________________________________ 

Algae Management and 

Off-flavor in U.S. Catfish 

Operations, 2009 

The term off-flavor refers to undesirable flavors in fish. 

Off-flavor is caused by some algae and is a critical 

problem for the catfish industry because it can lead to 

delayed fish harvests. By forcing producers to keep fish 

in ponds longer, harvest delays may cause economic 

losses by creating an increased risk of fish loss due to 

disease problems, reduced feed efficiency, over-sized 

fish and delays in stocking the next crop of catfish. At 

the producer level, off-flavor adds an estimated $15 to 

$23 million annually to catfish production costs in 

Mississippi alone. In addition, flavor quality is important 

to the catfish industry’s market share; if flavor is 

distasteful, consumers may purchase substitutes for 

catfish. 

 In January 2010, the USDA’s National Animal Health 

Monitoring System conducted its third national study of 

the U.S. catfish industry, Catfish 2010. A total of 424 

producers from 4 States participated in the study. These 

four States were divided into two regions. The East 

region was defined as Alabama and Eastern Mississippi; 

the West region was defined as Arkansas, Louisiana, 

and the Mississippi Delta. Results from the Catfish 2010 

study related to off-flavor in catfish are presented here. 

Algae management 

 Overall, 40.9 percent of foodsize fish producers 

attempted to prevent algae overgrowth with a control 

program; 24.8 percent controlled blooms only in 

response to problems such as off-flavor; and 

34.3 percent implemented no algae control treatments. A 

higher percentage of producers in the East region 

(54.8 percent) used an algae preventive program than 

producers in the West region (19.1 percent). Similarly, a 

higher percentage of operations in the West region 

(55.7 percent) had no algae control treatments 

compared with operations in the East region 

(20.7 percent). 

 The most common off-flavors are caused by 

metabolites produced by blue-green algae. These offflavors are referred to as “musty”—from 

2-methylisoborneol (MIB), and “muddy”— from geosmin. 

Blue-green algae generally do not grow or produce MIB 

in water temperatures below 60°F, and off-flavor is 

mostly likely to occur from June through September. Of 

producers that used preventive algae treatment 

programs, 89.8 percent began their program from March 

through June and 84.3 percent ended it from September 

through November. 

 Of producers that typically implemented an algae 

control program, most used a chemical or biological 

method to prevent and manage the growth of algae 

(fig. 1). Two algicides are currently approved for use in 

catfish ponds: copper sulfate or other copper-based 

products, and Diuron®, an herbicide known to have 

algicidal properties at low concentrations. While these 

algicides are used widely by the catfish industry, some 

producers also have tried a biological method (i.e., 

plankton-feeding fish) to reduce algae. 

0

20

40

60

80

100

Copper

sulfate/

other copper

sulfate

formulation

Biological 

control 

(e.g., threadfin

or gizzard shad)

Control Method

Percent

Figure 1. Percentage of operations that used

algae control programs in 2009, by control

method

Diuron

82.0 78.1

39.7

4.4

Other
United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture 

Off-flavor occurrence and duration

 In 2009, a higher percentage of catfish operations 

(80.7 percent) experienced delayed harvests due to offflavor than operations in 2002 (69.6 percent) [table 1]. 

With the exception of operations in 2002, the 

percentages of operations and ponds that experienced 

harvest delays due to off-flavor problems were higher in 

the East region than in the West region. 

Table 1. Percentage of operations and percentage of 

ponds from which foodsize fish were harvested that 

experienced harvest delays because of off-flavor 

problems in 2002 and 2009, by region 

Harvest 

delay on... 

East 

region 

West 

region 

All 

operations 

Operations 

(2002) 68.7 70.6 69.6 

Operations 

(2009) 86.8 72.0 80.7 

Ponds (2002) 59.2 50.8 53.3 

Ponds (2009) 56.6 43.9 48.1 

While larger operations are still more likely to 

experience delayed harvests due to off-flavor, the 

percentage of smaller operations (fewer than 20 acres) 

that had any harvest delays because of off-flavor 

problems more than doubled from 2002 to 2009 

(table 2). 

Table 2. Percentage of operations and percentage of 

ponds from which foodsize fish were harvested that 

experienced any harvest delays because of off-flavor 

problems in 2002 and 2009, by size of operation 

Size of Operation (surface acres)

Harvest 

delay on... 1–19 20–49 50–149 

150 or 

more 

Operations 

(2002) 21.7 61.9 78.0 86.3 

Operations 

(2009) 55.3 75.2 83.0 88.6 

Ponds (2002) 26.8 48.2 55.7 53.9 

Ponds (2009) 34.9 68.9 54.4 45.8 

 Catfish absorb MIB and geosmin from the water 

almost immediately. Absorption continues as long as 

MIB and geosmin are in the water. On operations with 

harvest delays due to off-flavor in 2009, the average 

length of off-flavor episodes was 15 to 30 days on 

37.9 percent of operations. Ongoing problems with offflavor in ponds were reported by 21.1 percent of 

operations. More than 9 of 10 operations (96.2 percent) 

reported that their shortest off-flavor episode was at 

least 7 days. 

Off-flavor treatment

 Purging off-flavor from fish does not occur until the 

water is free of MIB and geosmin, which may take 

anywhere from a few days to several weeks. Purging 

rate is affected by the initial level of MIB and geosmin in 

the fish, water temperature, and the size and fat content 

of the fish. 

 Producers have several options when trying to purge 

fish of off-flavor. Some do nothing, knowing that blooms 

of blue-green algae are transient and will eventually 

disappear. Some move fish to another pond with fresh, 

clean water. 

 As in 2002, Diuron or a combination of Diuron and 

copper sulfate were the chemical treatments of choice 

for affected ponds in 2009 (table 3). The percentage of 

ponds that were treated only with copper sulfate 

decreased by half from 2002 to 2009. This decrease 

could be due to the substantial increase in the cost of 

copper sulfate from 2002 to 2009. 

Table 3. Percentage of ponds that were treated with 

the following chemicals due to harvest delays 

because of off-flavor problems in 2002 and 2009

Percent of Ponds* 

Chemical 2002 2009 

Diuron only 27.2 23.7 

Copper sulfate only 12.1 6.7 

Both Diuron and 

copper sulfate 32.6 36.5 

No treatment 28.1 33.1 

Total 100.0 100.0 

*For ponds with delayed harvests because of off-flavor problems. 

 
United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture 

From 2002 to 2009, there was a shift from doing no 

treatment to treating with Diuron alone or Diuron and 

copper sulfate combined in operations with fewer than 

20 acres (table 4). For operations with 20 to 49 acres 

there was a shift from treating with both Diuron and 

copper sulfate combined to doing nothing. 

 From 2002 to 2009 there was a shift from doing no 

treatment or treating with Diuron alone to treating with 

Diuron and copper sulfate combined in operations with 

50 to 149 acres. For operations with 150 or more acres 

there was a tendency to not treat instead of treating with 

copper sulfate alone. 

Table 4. Percentage of operations that used the 

following treatment options for off-flavor 

Percent Operations 

Size of Operation (surface acres)

Treatment option 1–19 20–49 50–149

150 or 

more 

2002 

Diuron 1.9 6.9 17.9 31.0 

Copper sulfate 14.8 17.2 15.8 10.9 

Diuron and copper 

sulfate 19.0 64.8 39.8 29.1 

None 64.3 11.1 26.5 29.0 

2009 

Diuron 23.0 3.3 9.9 28.7 

Copper sulfate 11.9 14.8 15.9 3.8 

Diuron and copper 

sulfate 57.9 48.4 57.9 30.2 

None 7.2 33.5 16.3 37.3 

Conclusions

 Off-flavor is an economically important problem for 

the catfish industry. Off-flavor occurs throughout the 

industry and continues to appear in a lower percentage 

of small operations. Chemically treating ponds is still the 

most widely practiced method for prevention and control 

of algae associated with off-flavor. 

______________________________ 

For more information, contact: 

USDA–APHIS–VS–CEAH 

NRRC Building B, M.S. 2E7 

2150 Centre Avenue 

Fort Collins, CO 80526-8117 

970.494.7000 

Email: NAHMS@aphis.usda.gov 

http://nahms.aphis.usda.gov

#622.0811 

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