Water Quality Management For Fish Farmers

By Conrad Kleinholz

Successful aquaculture depends on healthy fish and proper water quality management. Poor water quality reduces growth and affects health of fish. Fish diseases usually occur after stress from impaired water quality. water quality problems may develop suddenly from environmental phenomena (heavy rains, pond overturn), or gradually through mismanagement. Water quality management is very important if water is limited and runoff or other surface water is the principal source of water for aquaculture.

Three principal water quality problems occur in fish culture ponds:

1. excess phytoplankton production
2. low dissolved oxygen
3. toxic metabolite accumulation

EXCESSIVE PHYTOPLANKTON PRODUCTION
Phytoplankton are free-floating microscopic algae. Photosynthetic activity by large plankton populations can produce enough oxygen to cause oxygen supersaturation of water during mid-afternoon on bright sunlit days.

Phytoplankton growth is stimulated by addition of nitrogen, phosphorous and potassium. Populations may "bloom" 7 to 10 days after large inputs of nutrients, or "crash" when nutrients are depleted, or if toxic chemicals are added to the water. Phytoplankton respiration may be nearly 80% of oxygen consumption in water, and respiration by large phytoplankton populations may deplete oxygen in ponds during sustained periods of cloudy weather or at night.

NUTRIENT SOURCES
Fish feed is the principal source of phytoplankton nutrients in culture ponds. Feeding rates should not exceed 25-30 lb/acre/day unless dissolved oxygen (D.O.) is monitored daily and aeration equipment is available. Feeding more than 50 lb/acre/day will probably require supplemental aeration to avoid oxygen depletion.

Runoff from cultivated land may contain excessive amounts of nutrients from fertilizer or crop residues.

Nutrients in livestock manure may enter ponds directly if animals are allowed to water in the ponds, or indirectly in runoff if culture ponds are located downhill from livestock facilities.

ASPECTS OF EXCESSIVE PHYTOPLANKTON POPULATIONS
Phytoplankton often tint the water green, but may also cause the water to appear blue-green, red or brown. If plankton populations are the principal source of turbidity in ponds, population densities may be determined by several methods:
1. Secchi disk visibility A secchi disk is a flat, weighted disk, 8" (200 mm) in diameter, with alternate black and white quadrants. Water clarity is determined by lowering the disk into the water until no longer visible. Secchi disk visibility should be 15-20 inches (380-500 mm). Lower visibility indicates excessive plankton and greater light penetration promotes growth of rooted aquatic vegetation. Excessive growth of rooted aquatic vegetation can also cause oxygen depletion.

2. Dissolved oxygen fluctuation Dissolved oxygen may be consistently less than 25% of saturation before dawn in ponds with excessive phytoplankton populations, and mid-afternoon levels of D.O. may be 125-200% of saturation. Percent saturation may be determined by comparison of the D.O. content with saturation values listed in Appendix 1

3. Water appearance Excessive phytoplankton populations are indicated if pond water becomes green, blue-green, red or brown within a 7-10 day period. Surface scums of algae also indicate excessive populations and often occur 1-3 days before an algal crash.

Control of Phytoplankton Population
Phytoplankton populations may be controlled by several methods:

1. Flush Pond Ponds can be flushed of algae by partially draining and refilling with clean water. If water is removed from the pond, bottom, some of the nutrients causing the algal bloom will be removed during draining.
2. Harvest Some or all of the fish can be removed, but make sure fish are not "off-flavor" from the algae before you harvest, if the fish are to be sold for food. Plankton populations are reduced when smaller amounts of feed are added to ponds.

3 . Chemical Treatment Phytoplankton populations may be reduced by treating with copper sulfate (CUS04) or Aquathol to kill algae. No more than 25 % of a pond should be treated at one time. Apply chemicals near the leeward shore during mid-afternoon. Dissolved oxygen should be monitored carefully for 3-5 days before more chemical is added. Decay of the dead algae may cause oxygen depletion, and kill fish and other algae. Extreme caution should be exercised when treating with CUSO4. The recommended treatment rate for CUS04 is:

PPM CUS04 = total alkalinity (ppm).
100
Alkalinity should be measured before each treatment with CUS04 If total alkalinity is below 60 ppm, a fish bioassay should be conducted before treatment with CUS04.

Low Dissolved Oxygen
Low dissolved oxygen is the most common stress for cultured fish. Infrequent exposure to low oxygen causes a temporary reduction in food consumption by fish. Chronic low levels of D.O. cause reductions in feeding, feed conversion and growth. Low D. O. in culture ponds is often associated with elevated levels of carbon dioxide (CO2) and unionized ammonia (NH3), both of which are toxic to fish. Combination of low D.O. with high C02 and NH3 dramatically increases the susceptibility of fish to diseases.
Dissolved oxygen requirements vary between species of fish. Most warmwater fish can withstand levels of oxygen as low as I ppm for short periods of time, but levels necessary for adequate growth are at least 5 ppm for warmwater fish (channel catfish, largemouth bass, most minnows), 6 ppm for coolwater fish (smallmouth bass, walleye, striped bass) and 7 ppm for Coldwater fish (trout and salmon).

Causes of Low Oxygen
Chronic levels of low oxygen are most common during July-September when fish weights and feed rates are rapidly increasing. High nutrient levels promote phytoplankton growth and oxygen is often nearly depleted by dawn from high rates of respiration.
Low dissolved oxygen may occur suddenly during late summer or early fall when the first cold fronts occur. Strong, cold winds may cause mixing of surface water with oxygen-poor or anoxic bottom water. If large amounts of partially decayed fish waste, fish feed or plant material are present on the pond bottom, an additional oxygen demand will occur when the water layers mix, and oxygen can be totally depleted within 15-30 minutes. Oxygen may also be depleted if silt-laden runoff from heavy storms causes die-offs of algae or rooted aquatic vegetation.

Measurement
Dissolved oxygen should be monitored regularly in culture ponds. Measurements can be made with electronic probes connected to meters or with titration kits.
Several methods are available to predict the occurrence of low D.O. One method compares D.O. at dawn and during mid-afternoon. If oxygen is at least 5 ppm at dawn and does not decrease during the day, oxygen may not need to be checked at night. A second method graphically compares D.O. at dusk and 2-3 hours after dark. Oxygen decrease is linear, so a straight line drawn between the points can be used to estimate if D.O. will be depleted before dawn (Figure 1). Boyd (1982) discusses these methods and offers a mathematical model to predict oxygen decline in culture ponds.

Control
Emergency aeration can be used to increase D.O. Paddlewheel aerators are the most efficient systems currently used. Water exchange and reduction of feeding rates may also be used to increase D.O. Ponds with low oxygen due to phytoplankton mortality can be fertilized with phosphorus to encourage regrowth of the plankton population. Be careful not to over-fertilize. Add no more than 3 ppm (8 lbs/acre-ft) of superphosphate (P205).

Toxic Metabolite Accumulation
Toxic metabolites are excreted by fish, bacte- ria and plankton in fish culture systems. Ammonia, nitrite, carbon dioxide and hydrogen sulfide are the principal metabolites.

Ammonia
Ammonia (NH3) is passively excreted through fish gills, but toxic NH3 concentrations may occur in the body when environmental levels of NH3 are high. Ammonia reacts with water to form the ammonium ion (NH4), which is relatively non-toxic to fish. Measurements of ammonia in water are the sum of NH3 and NH4 and are called total ammonia nitrogen (TAN). Levels of 1-2 ppm NH3 frequently occur in productive fish ponds. The amount of toxic NH3 in TAN increases with increases in pH and temperature (Appendix 2). Ammonia toxicity usually occurs during warm clear afternoons when photosynthesis has removed C02 from the water and raised the pH. Ammonia toxicity is most frequently encountered in hauling or holding facilities. Additionally, ammonia may be very toxic to catfish in cold water. When water temperatures are less than 400F, catfish may be unable to withstand exposure to ammonia. The toxicity may be the result of prior stress from handling or crowding.
Ammonia toxicity may cause fish to congregate around inflowing water or at the pond edge. Fish may become lethargic and lie on the bottom or swim slowly near the surface or along pond banks. Gills may be red, swollen and flared. Affected fish do not respond to aeration. Ammonia toxicity may be suspected if fish regularly appear ill in the afternoon and apparently recover by dawn. Ammonia toxicity is most common in ponds with less than 50 ppm total alkalinity or less than 50 ppm calcium hardness. Ammonia may be partially removed from water with paddlewheel aerators during warm, sunny weather. Successful methods of ammonia reduction are pond flushing in small ponds or reduction of feeding rates.

Nitrite
Nitrite (NO2) is a product of bacterial reduction of ammonia. Nitrite replaces oxygen in the blood to form methemoglobin. When concentrations of methemoglobin in catfish blood reach 20-30 %, the gill filaments become a chocolate brown color and the fish are diagnosed as having "brown blood" disease. Brown blood often occurs in the fall, even if ponds are not heavily stocked. Behavioral signs of fish with "brown blood" disease are the same as ammonia toxicity. Affected fish do not respond to pond aeration. Catfish ponds sometimes contain 0.5-5.0 ppm or more or N02, and nitrite is especially dangerous when hauling fish.
Channel catfish may die when water contains 4.5-13 ppm N02. Tilapia are more tolerant of N02 than catfish, and centrarchids (bluegill, largemouth bass) are very resistant to N02 toxicity.

Nitrite toxicity can be controlled by adding calcium (agricultural limestone) to soft water or by adding salt to both soft and hard waters. Chloride (CI) concentrations between 15-30 ppm (I10-220 lbs sodium chloride/acre-ft) usually prevent brown blood problems.

Carbon Dioxide
Carbon dioxide (CO2) is a byproduct of aerobic respiration. Carbon dioxide levels in fish culture ponds cycle daily, with highest levels near dawn and lowest levels in mid-afternoon. Fish affected by C02 become listless, then lie motionless on the pond bottom. Affected fish usually recover rapidly after aeration.

Hydrogen Sulfide
Hydrogen sulfide (H2S) is excreted by bacteria during anaerobic decomposition of waste products on the pond bottom. Any detectable level should be considered detrimental to fish production. Hydrogen sulfide smells like rotten eggs, and hence may be easily detected. Hydrogen sulfide may cause water to appear gray, then suddenly clear within a few hours, with a black film on the pond bottom and vegetation.
Control of H2S is achieved primarily by maintaining oxygen at all depths in culture ponds. Water should be aerated, circulated, or maintained at depths no greater than 6 feet. If hydrogen sulfide is detected, water should be carefully removed from the pond bottom. Replace water slowly, and keep temperature differences less than 50F (30C). Do not attempt to remove fish by seining. Stop feeding until plankton growth is observed.

Water Quality Monitoring
Water quality parameters may be monitored by various methods. Dissolved oxygen (D.O.) is commonly measured with a probe, attached to a battery-powered meter. Most D.O. meters
must be calibrated to obtain accurate data, but some are advertised as self-calibrating. Dissolved oxygen is measured by suspending the probe in the culture pond, at least 12 inches below the surface and 36 inches from shore. Dissolved oxygen may also be determined by titration of water samples from culture ponds. Prepared chemicals are added to a sample, and chemical reactions indicate the amount of oxygen in the sample. Titration is slower than metered measurements, but results are comparable, and titration kits are less expensive than meters. Do not store kits in vehicles because heat degrades the chemicals. Chemicals should be replaced twice a year to ensure proper results.

pH can be measured by calorimetric analysis or with battery-powered probes. Colorimetric analyses are not acceptable if the culture ponds are turbid, or if a high degree of accuracy is necessary. Colorimetric analyses should be conducted in a laboratory or fish house, using the same light source at all times. Pocket-size pH probes are now available and should soon replace calorimetric analyses.

Total ammonia nitrogen (TAN) and nitrite are determined by calorimetric analyses.

Water quality data should be obtained from several sites in each pond. Water currents from water inlets or wind-driven wave action may provide variation in water quality. Data from water quality analyses should be recorded and stored for future reference to aid in estimating timing of annual dissolved oxygen problems, maximum safe fish density in each pond or other information.

Conclusion
Most water quality problems can be avoided by maintaining fish stocks less than 2,000 lbs/acre and feeding rates less than 30 lbs/acre. At higher stocking and feeding rates, farmers must be prepared to monitor water quality and to manage any water quality problems encountered.

Sources of Instruments and Chemicals

Air-O-Lator1
Dept. AB-65 8100 Paseo
Kansas City, MO 64131
(800) 821-3177

AREA, Inc.1,2
P.O. Box 1303
Hornestead, FL 33090
(305) 248-4205

Aquatic Ecosystemsl,2
P.O. Box 1446
Apopka, FL 32704-1446
(305) 886-3939

Argent Chemical Laboratories2,1
8702 152nd Ave. N.E.
Redmond, WA 98052
(800) 426-6258

Crescent Research Chemicals2,3
4331 East Western Star Blvd.
Phoenix, AZ 85044
(602) 943-4733

Fritz Aquaculturel,2,3
P.O. Drawer 17040
DaRas, TX 75217
(800) 527-1323

Horizon Ecology Company2,3
7435 North Oak Park Ave.
Chicago, IL 60648
(312) 647-7644

La Motte Chemical Co. 1,2
P.O. Box 329
Chestertown, MD 21620
(800) 344-3100

Wildlife Supply Company2,3
301 Cass St.
Saginaw, MI 48602
(517) 799-8100

Forestry Supplies, Inc.2,3
P.O. Box 8397
Jackson, MS 39204-0397
(800) 647-5368

Hach Company2,3
P.O. Box 389
Loveland, CO 80539
(800) 227-4224