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Title: Managing summertime off-flavors in catfish

item Tucker, Craig
item Schrader, Kevin

Submitted to: NWAC (National Warmwater Aquaculture Center) Aquaculture Newsletter
Publication Type: Other
Publication Acceptance Date: 10/20/2013
Publication Date: 4/25/2014
Citation: Tucker, C.S., Schrader, K.K. 2014. Managing summertime off-flavors in catfish. NWAC (National Warmwater Aquaculture Center) Aquaculture Newsletter. P. 4-5.

Interpretive Summary:

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