How Many Fish Can This Pond Hold?

The dangers of an overstocked aquatic habitat

carolyn_2 An overstocked pond is
an invitation to disaster
… a ticking time bomb.
Parasites proliferate and spread
like wildfire in an overcrowded
pond. Water quality is somewhere
between difficult and
impossible to maintain, making
the pond less attractive to
viewers. Looking closely at the
fish, one can see sores, missing
scales and torn fins, and viewers
can spot uneaten food on the
bottom. There may be an odor
connected with this pond, and
surely algae growth is unabated.
Eventually, an overstocked
pond leads to a call to a pond
professional for help.

Most overstocked ponds do
not happen overnight. In many
cases, a few nice fish are added to
the new pond, but the pond still
has room for plenty more fish.
Then they will grow. And then
they will spawn if you give them
good water quality and proper
nutrition. There is no miracle sterilizing
food or medication to add
to the water to prevent the annual which includes the fun of buying inexpensive,
cute, pretty fish when they are
young. Fish development can be fascinating.
Would you want to be deprived of
this stage of fishkeeping? Or should one
add only large, mature (and expensive)
fish that may lose color in one to three
years, but can be sexed, to prevent spawning
and thereby not overload the pond?
That is the dilemma. To do or not to
do? That is the question. And the answer
comes in the form of another question:
What is the goal of fishkeeping for the
hobbyist? Is it to enjoy all facets of the
fish, or is it to keep specific fish for the
duration of their lives, beginning to end?

The Joys of Fish-Raising

Practically every new pond owner
wants to experience fish-raising for
himself. He wants to see the excitement
of a spawn and spends hours watching
for the emergence of fry. He wants to see
what his fish will produce. It is almost
like that first childbirth: “Will he have
my mouth, your nose, blond hair like the
mother or brown eyes like the father?”

Unfortunately, it’s too easy to get
caught up with the development of the
fish and lose sight of the overall stocking.
If each fish grows 1/16”, and there are
20 to 50 fry growing up in the pond, the
biomass will quickly add up! If the pond
is not thinned regularly, this small growth
has the potential to throw the pond off
balance from one season to the next.

Exponential Growth

Imagine a 1,000-gallon pond with
three recently matured koi that are ready
to spawn. Figuring the standard (but not
written in stone) 10 gallons per inch of
fish, up to now the pond has been happily
stocked and perfectly balanced. Each koi
has grown to between 12 and 18 inches.
Then, the inevitable happened: The
fish matured and spawned, and fry
survived in the pond.

The following season there were 33 fish
in the pond, and the average size of the
progeny is 4 inches. Although as they age
their growth will slow down, the parent
fish grew another 1 to 2 inches. Before
spawning the collective length of the fish
was 45 inches, so they had ample room
in the 1,000-gallon pond. After the first
spawn, however, the total length of the 30
fry is 120 inches! All by themselves,
the fry now have exceeded
the maximum stocking for
the 1,000-gallon pond
by 20 gallons. And the
parents are still in there.
Going by the minimum
requirement, 10 gallons
per inch of fish, the
present population will
need 1,690 gallons or
more to maintain a healthy
environment. Of course, that
is dependent upon good filtration
and circulation in the pond.
By the end of this season, these fish will
have spawned again and the fry will likely
double their size. To compound the
problem, many of the fry have reverted
to their wild ancestry (magoi) in coloring,
and thus will be very difficult to find,
catch and remove from the pond. This is
how ponds easily become overloaded.

Feeding and Filtration

With proper flow rate and bioconversion,
a pond will sustain a very
large fish load. When water quality is
negatively impacted, the most common
cause is overfeeding. Owners see more
open mouths, so they will naturally feed
more. However, to underfeed an overstocked
pond is inhumane. These are
large growing fish that can potentially
reach lengths of 36 inches. A properly
designed pond includes bacterial and
phyto-filtration. To blame overfeeding
or overstocking, you can also say a pond
does not sustain a high enough level of
bioconversion. At what point is an imbalance
recognizable? What are the typical
symptoms of imbalance? Is it sick fish, fish
losses, declining water clarity or increased
algae issues? There usually will be a predictable
downhill slide when people
continually add fish (either
through purchase or
spawn), increase
the feed to match
the stock load, and
have not anticipated an
increase in filtration capacity.

“The inherent problem with the
inch-per-square-foot or inch-per-gallons
formula is in the size of the fish,” said
Meyer Jordan, owner of Ripples Inc. in
Pensacola, Florida. “10 10-inch fish have
considerably more total biomass than 50
2-inch fish. More biomass means more
ammonia production, requiring more
bioconversion. It is also necessary to take
into account the initial level of bioconversion
available in any pond. Although
this has been discussed in the past, it is important to mention that manufacturers
do not really help the situation. They
may claim a certain model biofilter will
meet the needs of a 1,000-gallon pond,
for instance, but is this a pond with fish
and, if so, what is the maximum limit of
fish in pounds/kilograms? Manufacturers’
filter ratings are, at best, vague and often
misleading. The only true determination
of fish load is in total weight of the fish
population.”

Often a manufacturer seems to quote
capacities of filter/pump without fish, or
with fish other than koi, which produce
more organic waste
than goldfish or
sunnies. Once
koi enter the equation
the figures don’t always
work out the way they were planned.
Other factors affecting the pond include
the climate. Northern ponds do not run
year-round, and biofiltration needs to
restart each spring. Does the biofilter
die off in winter? Do southern ponds
continue without interruption and
thereby support more livestock?

We know that surface area is
important to oxygen levels in the pond.
A fairly shallow pond with large surface
area will support more fish than a very
deep pond with limited surface area, even
if they are the same total volume. With
koi, the reasons for a deep pond are:

  • Predator protection
  • Development of female conformation
    (body shape, muscles)
  • More stable pond temperature
  • With a male-only pond, depth is not
    as important. We still need
    predator protection and
    can use pond design
    instead of merely depth
    to thwart predators.
    Temperature can also be
    managed. With a shallower
    pond, it is easier to
    catch and remove excess
    fish! Pond management can become
    easier, and easier chores are more likely
    to be done. After all, a koi pond is a form
    of aquaculture. It needs to be managed
    or maintained in order to remain healthy
    for the future.

    If fish are to be added to an optimally
    stocked pond, then fish need to be removed
    to maintain status quo. The optimal result
    for everyone — fish and human — is a
    safe, low-stress and workable stocking
    density that all may enjoy.

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