By Stephen M. Meyer
Q. I have a problem with herons killing the koi in my outdoor pond. How can I discourage these unwelcomed dinner guests? Also, my pond is fed by a natural spring and the overflow drains out. As a result, fresh water circulates through the pond continuously. Can my pond hold more fish than a pond that recirculates the same water all the time?
A. The problem of predatory birds feeding on koi and goldfish is one that many pondkeepers find difficult to solve. In most states, it is illegal to harm the birds — many of which are on endangered species lists. Unfortunately, there is no good answer to this question.
The simplest solution is to put a net over the pond. This works well, but of course is rather unsightly. Special mesh nets are sold for just this purpose because fish farms are constantly at the mercy of predatory birds.
Cranes and herons tend to fish from a standing position. Therefore, making the pond deeper can help considerably.
A very different alternative is to set up a bird scare. This can be a simple device that blasts a loud noise every 5 or 10 minutes. The problem is that neither you nor your neighbors will appreciate this solution after about 15 minutes.
Some fish farms have been very successful using waters jets on a timer. Every 10 minutes or so, a powerful water jet skims over the pond, about a foot off the surface. The herons apparently find being blasted with this stream of water quite annoying and move on.
Still another solution is to add a pair of swans to the pond. Swans grow quite large, but they are vegetarians and will not harm the fish. They will, however, keep other birds out of a small fish pond. The drawback here is that swans eat a lot of plant material and the water will tend to foul over time. Of course, if you have a continuous freshwater feed, this will not be a problem.
Lastly, my own solution to the heron and kingfisher problem is my dog. She just sits and waits for one of these birds to land, and off she goes into the water. Although she has never caught a bird, I can say that none of the birds have caught my fish! A friend of mine tried placing plastic pink flamingos in the pond, but that did not work.
On the question of fish load, whether or not your pond will hold more fish because it has a freshwater feed depends on many other factors. The most important is the pond volume turnover rate. In the end, my answer must be "no." If you want your koi to thrive and grow to full adult size, your goal should be to keep a minimum number of fish in the water. The rule of thumb I use is an absolute maximum of approximately two 13-inch koi per 260 gallons, but an ideal load would be about one 8-inch koi per 260 gallons.
Don't fall into the trap of measuring the quality of your fish collection or fishkeeping prowess by the number of fish you can keep in a small space. The correct measure is the health and longevity of the animals.