By David Beasley, Lead Fisheries Biologist
Proper pond
management will greatly reduce the chances of a fish kill due to low dissolved oxygen
levels, but in some cases, it does not eliminate the scenario. Keeping aquatic vegetation, algae and
microscopic plants (phytoplankton) within acceptable densities while properly
aerating is the best way to reduce the chances of having a fish kill. Many stormwater ponds require
management due to the high level of nutrients that accumulate from surface run
off. These nutrients feed algae and
plants that grow at a very rapid rate when water temperatures are warm. These vulnerable ponds require different
management tools and techniques to control plant matter and limit the chances
of having a fish kill.
The oxygen
requirement in ponds is highest when water temperatures reach the 80’s. During
these warm temperatures, the water’s ability to hold oxygen is at its
lowest. These two factors team together
to reduce the stability of the pond, making the scenario of a fish kill more
probable. When a pond’s water quality
becomes unbalanced, something as simple as a few cloudy days are enough to
induce a fish kill. It may come as an
interesting fact to most, but fish kills almost always occur after several days
of cloudy weather. The reason for this
is because when sunlight is not present, plants actually use oxygen
(respire). This scenario is natural and
more common than many realize. One thing
that many don’t realize is that when a fish kill is occurring or is about to
occur, it will often happen over a series of days. When oxygen levels are low the fish in the
pond will hang out near the surface of the pond. The fish will be very skittish and stressed
out. One big give away is when dozens of
fish all get startled at once as you approach the pond. Once oxygen levels reach into the lethal
range it will likely take several days to kill the entire fish population. This gives pond owners a chance to aerate
with surface pond aerators and add water from a neighboring pond using a siphon or a
pump. Although it is not always
practical, sometimes with enough notice the severity of a fish kill can be
reduced by notifying a pond expert and taking the corrective actions
necessary.
Avoiding the
situation where a fish kill is possible is the best approach to preventing a
fish kill. The two best practices that
will keep the water in your pond healthy and your fish safe are to properly
aerate while maintaining an acceptable level of vegetation.
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