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Cause of fish kill
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Symptoms
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Problem
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Recommend Solutions
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Summerkill
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Cloudy, hot, still days and nights. Low oxygen levels in pond water. Fish found dead and/or gasping for air at the surface.
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High tempuratures (>85° F) in shallow ponds; very warm water does not hold enough oxygen. Cloudy skies prevent plants from producing oxygen & calm winds prevent mixing. Shallow, weedy ponds are most vulnerable.
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Deepen pond &/or install aeration system.
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Inversions
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Low oxygen levels in pond water. Dead or gasping fish found after heavy downpours and high winds.
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Sudden inflows of cool rainwater & strong winds cause bottom water (low oxygen) to upwell & mix with surface water resulting in critically low oxygen levels. Shallow, weedy ponds with steep drainage areas (high runoff) most vulnerable.
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Deepen pond &/or install aeration system to circulate and aerate bottom water that lacks oxygen.
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Phytoplankton die-off
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Low oxygen levels in pond water. Fish found dead or gasping for air. Green cast/green paint water prior to or during fish kill. (High Phytoplankton)
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Nutrient enriched ponds produce dense blooms of phytoplankton (algae) which can suddenly die-off and decompose causing an oxygen shortage.
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Reduce nutrient inputs by diverting overland runoff that is rich in nutrients (animal feed lots, crop fields, etc.).
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Dead Vegetation
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Low oxygen in pond water. Dead and/or gasping fish within a few days after large treatment of vegetation with herbicide.
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Mass die-off of aquatic vegetation from natural causes or herbicide use. Large amounts of rotting vegetation use up oxygen supply.
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Banks should have 3:1 slope to reduce vegetation. Treat no more than 25% of the pond with herbicide at one time.
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Winterkill
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Low oxygen levels. Dead fish seen after ice melts. Few, if any, fish caught in the spring compared to the previous season.
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Snow covered ice stays on pond for an extended time keeping sunlight from reaching plants to produce oxygen.
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Shovel snow greater than 2 inches deep, removing 30% &/or install aeration system to prevent complete ice cover.
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Organic Pollution
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Fish die from oxygen shortage. Look for large sources of organic matter that enter the pond, after hard rains.
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Excess animal wastes, leaves, decaying vegetation, and other matter consumes oxygen as it decays.
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Prevent organic matter from building up. Cut trees back away from pond. Divert animal waste runoff. Use aeration to speed up decay.
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Toxic Substances
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Fish die from direct exposure to toxins. Complete or partial kill depending on amount & dilution rate of runoff. Toxins often kills other aquatic life (insects, tadpoles), oxygen shortages will not.
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Pesticides, herbicides, petroleum products, fertilizers, & other toxic chemicals enter pond via surface from nearby land. Often occurs after heavy rains wash insecticides or fertilizer into the pond.
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Levee the pond or divert runoff from potentially toxic sources (crop fields, golf courses, etc.). Avoid using toxic substances within the watershed.
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Natural
Causes
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A few fish found dead along the shoreline in early spring.
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After a long winter, a fish’s natural resistance to disease is lowest in early spring. Spawning stress may also cause a few fish to die. Larger & older fish most likely to die of natural causes.
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None: let nature take its course.
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