Location: Warmwater Aquaculture Research Unit
Title: Managing summertime off-flavors in catfish Authors
Submitted to: NWAC (National Warmwater Aquaculture Center) Aquaculture Newsletter
Publication Type: Abstract Only
Publication Acceptance Date: October 20, 2013
Publication Date: April 25, 2014
Citation: Tucker, C.S., Schrader, K. 2014. Managing summertime off-flavors in catfish. NWAC (National Warmwater Aquaculture Center) Aquaculture Newsletter. P. 4-5. Technical Abstract: Summertime phytoplankton blooms in channel catfish ponds often contain blue-green algae that produce musty or earthy odors. The odorous compounds are absorbed by fish across their gills and deposited in fatty tissues, giving fish undesirable “off-flavors.” When fish are declared off-flavored by processing plant taste-testers, many farmers automatically assume that blue-green algae are responsible and treat ponds with an algicide hoping to kill the algae and allow fish to become “on-flavor.” Using algicides to combat off-flavors does not always work because not all off-flavors are caused by algae. Catfish become off-flavored when any odorous compound is deposited in edible tissues. For example, off-flavors can be caused by inadvertent pollution, such as small spills of diesel fuel. Off-flavors can also develop when fish eat foods containing odorous compounds. Diet-related off-flavors are rare in summertime because high-quality commercial feeds are formulated so that they do not cause flavor problems. In wintertime, however, catfish are not routinely fed and they may scavenge for food in the pond. Some of these food items, such as dead fish or decaying plant matter, may give fish undesirable “decay” or “fishy” flavors. The only off-flavors that are “treatable” are those caused by algae growing in warm water. The key to successful off-flavor management involves identifying and treating only those problems that will respond to algicides. This is analogous to proper disease management where you diagnose the disease before you treat it. Off-Flavor Ecology The following are key facts about off-flavors in catfish grown in northwest Mississippi and southeast Arkansas: 1) Most summertime flavor problems are of the musty-earthy type. Musty off-flavors are caused by 2-methylisoborneol (MIB) and earthy off-flavors are caused by geosmin. 2) Only a few blue-green algae species produce MIB and geosmin. MIB is produced by Planktothrix perornata. Geosmin is much less common and, when present, is usually produced by species of Anabaena. 3) Once the odor-producing blue-green algae disappear, MIB and geosmin production stops and odorous compounds are purged from fish. Purging rates for MIB and geosmin are highly temperature-dependant: they are purged within days in warm water but much more slowly (weeks to months) in cold water. 4) All other off-flavors are slowly purged from fish at all water temperatures. These four facts can be used to develop a management plan for pre-harvest off-flavors. If the plan is followed, off-flavor treatments can have a high probability of success. Also, by treating only problems that will respond, you will not waste time and money on treatments that have no hope of working. Water Temperature Odorous blue-green algae do not grow in cold water. If fish are off-flavored in cold water, the odorous compounds were either produced by algae during a previous period of warm water or the off-flavor was not produced by algae (such as off-flavor derived from foods consumed during scavenging). In either case, it is pointless to use algicides because there are no odor-producing algae to treat. If fish are off-flavored in cold water, the only option is resampling fish in a couple of weeks to see if flavor quality has improved. Sampling more frequently is seldom productive because off-flavors purge slowly in cold water. Taste-Testing When fish are off-flavored in warm water, the first step is to determine the type of off-flavor. This is critical because only musty-earthy off-flavors produced by blue-green algae are treatable using algicides. Determining the off-flavor type is simple. Cook a small, unseasoned piece of fish fillet in a microwave oven. Smell the fillet immediately after cooking and then taste a portion. MIB gives fish a musty-camphorous flavor that is difficult to describe, yet is very distinctive even at low concentrations. Geosmin gives fish an earthy-moldy flavor that is somewhat reminiscent of the odor of freshly plowed soil or a damp basement. If the off-flavor is not in the musty-earthy category, the proper decision is to wait a week or two and taste the fish again. But, if a musty-earthy off-flavor is present, the next step is to determine whether the odorous compound is actively being produced in the pond. This is done by examining the pond water for odor-producing algae. Microscopic Examination Fish pond phytoplankton blooms continuously change. Populations of odor-producing algae seem to appear from nowhere, persist for days or months, and then disappear. Blooms of odorous algae may never be found in some ponds while occurring several times every year in other ponds. Bloom phenomena are mysterious, and the only way to know what algae are present is to examine a drop of pond water under a microscope. Common odor-producing blue-green algae in northwest Mississippi and southeast Arkansas are easy to identify microscopically (Figure 1). The blue-green alga Planktothrix perornata is a free-floating, straight filament that is slightly bent and gradually tapering at one end. It is a relatively large plant compared with most other filamentous blue-green phytoplankton and contains many gas-filled vesicles that make the filament look dark and grainy when viewed under a microscope. Species of Anabaena are easy to recognize. Filaments are free-floating, straight or coiled, and consist of a series of spherical or barrel-shaped cells that look like a string of beads. If fish have musty-earthy off-flavors and either Planktothrix perornata or Anabaena are seen under the microscope—even in very low numbers—then musty-earthy compounds are being actively produced in the pond. Treating the pond with the proper algicide will kill the odorous algae and allow fish to purge the off-flavor. If fish have musty-earthy off-flavors but odor-producing algae are not present, that means that the algae that produced MIB or geosmin have recently disappeared from the pond as part of the natural cycle of the bloom. In this case, the farmer is fortunate because the off-flavor will rapidly disappear from fish without needing to treat the pond with algicides. Fish should be sampled for flavor quality daily because musty-earthy off-flavors usually disappear from fish rapidly in warm water. The bottom line is that algicides only work when odorous algae are present. If fish are declared off-flavor and odorous algae are not present, then the problem is essentially untreatable and the only recourse is to sample fish periodically to determine when the flavor has purged from fish naturally.