Pond Boss Forums INTRO Questions & Observations Fish Kill,,,Assessing the damage and moving on

Fish kills can easily occur in unfertilized ponds. Two main ways.

First way is from oxygen loss due to killing or death of too many weed/plants at one time and massive decay of the dead material consumes dissolved oxygen down to concentrations too low for fish to "breathe"/respire. Also involved in first way is plants not producing enough dissolved oxygen during the daylight (cloudy periods) to supply the fish with adequate DO during the dark period. Respiration exceeds oxygen production.

Second way. When a pond stratifies or forms two layers with warm water on top and colder water below, the oxygen will be depleated in the colder bottom layer. When the oxygen is depleated then hydrogen sulfide gas (HS) begins to build up in the cold layer. HS concentrations become higher as the stratified conditions persist. HS is very poisionous to fish because the HS readily moves through a fish' gills and very easily into the blood stream. In fact HS is much more soluable in water & blood than oxygen.

When a pond goes thru a fairly quick mid-summer inversion or turnover (usually due to weather conditons) then everything in the deep cold layer is "brought" up into the upper warm layer. If the water quality in the upper warm layer is degraded enough then -- FISH KILL.

The fish kill "usually" does not happen during the autumn turnover because the autumn turnover is a more gradual event and the two layers are slowly blended over several days or a week+. Also during autumn, the water is cooler and is able to "hold" more dissolved oxygen than during the warmer water of mid-summer. The upper warm layer can absorb or cleanse a certain amount of "bad" water from an inversion. Note --- many mid-summer turnovers are not complete turnovers and only a small percentage of the deep water is inverted and no fish kill occurs.

The number of fish that die due to a turnover is dependant on: 1. the degree or percentage of inversion, 2. the severity of the HS buildup, 3. the dissolved oxygen concentraion in the surface water, 4. the water temperature, 5. the volume ratio of deep cold water vs warm surface water, 6. the biologial oxygen demand (BOD) of the warm water layer, 7. species and sizes of fish present, 8..etc...

Partial fish kills due to mid-summer turnovers are common occurrence.