Fish are susceptible to quite a few ailments and pests; but one pest constitutes a greater threat than all others combined. And sadly, I know of no satisfactory remedy. That pest is the heron. Should you happen to live anywhere near a river or heronry, then sooner or later youris likely to be visited by one of these birds. Even a small sheet of water is very conspicuous from the sky and a heron in the vicinity can hardly fail to notice it. They are shy birds, easily frightened by the appearance of human beings. But what they lack in courage they certainly make up for in perse-verance. At odd, quiet hours, especially around dawn, a heron will not hesitate to land in a built-up area if he knows there is food for the taking. Goldfish are easy prey and a heron can devastate, indeed wipe out, one’s entire stock of fish. If he does not do so on one visit, be assured he will return so long as there are more fish to be caught. In my experience, the most likely time for this unwelcome visitor to arrive is in winter when food may be scarce, especially after heavy rain when the local river is in a muddied spate.
In many countries, herons are protected by law. And rightly so, for they are graceful, lovely creatures in flight, and their tranquil, placid silhouette on many a river bank or estuary — if not in the garden — is surely a sight worth preserving. You could remove your fish to an indoor aquarium to protect them from this danger during the winter months. But this may not be feasible if you have a large stock of fish, and besides, it does not protect the fish from a possible visit in summer. A small pond can be netted over in winter, but again the heron may arrive when the net is off; and if you are growing plants around the margin of the pool, then you have the added irritation of the plants growing through the net in early spring before you may feel it is safe to remove it. And bear in mind too that a heron has no compunction about sticking his long beak through netting with even small holes. I have known the birds both to walk on plastic netting and raise it up at one corner. So the netting should be firmly secured at the water’s edge and raised above the pond by placing a pole or similar object in the centre of it. The only foolproof method of keeping herons away is not often practicable. If your pond is enclosed by walls or rustic poles (with very narrow gaps between vertical and horizontal poles), then you may be able to stretch plastic netting over the whole enclosure. The poles or walls must be situated so as not to interfere with the amount of sunlight the pond receives.
Cats take a malicious pleasure in standing by the pool’s edge and whipping fish out. Friendly fish who come up to the surface on the sight of a human being may well do the same for a cat. A well-aimed missile will put off most cats from repeating the experiment.
There are a number of insects which may attack small fish and fry. The Great Diving Beetle, Dystiscus marginalis, and others of the genus, areand will off fish, snails, tadpoles and insects. They fly from pond to pond so they cannot be entirely excluded. The Water Boatman, the most common of the species being Notonecta glauca, has a powerful pair of stylets with which it can make short work of tadpoles and small fish. The Water Scorpion,Nepa cinerea, takes hold of its victim with a strong pair of front legs and then sucks the blood of its captive like a vampire. The larva of the dragonfly possesses a so-called mask, a kind of jaw with powerful hooks.
These are used to snatch and grip passing prey, fish being among the victims.
To prey and be preyed upon is part of the unalterable cycle of the pond, indeed it is of the essence of all natural life. To break the cycle is well nigh impossible. Hand picking or removing by net a number of these creatures is possible — in particular the Diving Beetle — but their eradication is not. From the many eggs that mature goldfish may lay and which hatch out, only a tiny proportion reach adulthood. Only the most stringently controlled conditions can raise the survival rate significantly. The pond owner must, I think, be satisfied with the way of nature in what is a relatively natural environment. Adult fish can be expected to contain the population of these enemies. And although enemies to fish, the activity of the Water Boatman and the Water Scorpion as they scurry about the pond, as well as many other insects, is surely an intrinsic element of interest in any water garden.
Remedial action should, however, be taken in the case of an infestation of Anchor worms or fish lice. The Anchor worms attach themselves to the skin of the fish and there is usually a slight swelling at the point of entry. They can grow up to almost 2 cm. Fish lice are very much smaller, 5-6 mm., and when they attach themselves to a fish, it is common to see the fish dashing wildly around the pond for no apparent reason. In both cases remove the affected fish and, holding it firmly in a wet cloth — but without squeezing it unduly — take out the parasite with a pair of tweezers. Treat the point of entry with iodine or a little household disinfectant; or paint the parasite with paraffin oil which will kill it. Examine the rest of your fish stock and treat in the same way as necessary.
These parasites may be introduced with plants taken from the wild. Before placing in your pond, new plants can be disinfected with potassium permanganate. A few grains of the chemical in a bucket of water in which the plants can be submerged will serve the purpose. A complete pond can also be disinfected, and potassium permanganate will also cut down the algae growth and improve the clarity of the water. The danger is, of course, that the fish will be poisoned by the chemical, fry in particular, and so far as clear water is concerned the measure can only be regarded as temporary. So I would use it only as a last resort. Make up a saturated solution of the chemical and introduce it to the pond at the rate of two teaspoonfuls for every ten litres of water.
White Spot is the common name of another parasite made evident by the rash of white dots that may run over the skins and fins of the fish. It should not be confused with the little dots that may appear on the gill plates of cock fish during the mating season. It is more common in tropical tanks than outdoor pools. The White Spot parasite is dependent upon hosts, so that if you can remove your livestock from the pool, hopefully the parasite will be eradicated. The affected fish are best placed in running water for a week or more, if you can rig up an aquarium under a tap. Alternatively, add ten to fifteen drops of 2l/2 per cent solution of mercurochrome to every ten litres of still water.
A salt bath is the answer if you find your fish suffering from leeches, easily recognized by their worm-like appearance with a disc at either end, and by their length: up to 2.5 cm. Begin with four teaspoonfuls of salt (sea salt is best if obtainable) dissolved in every ten litres of water, and increase the amount of salt daily unless the fish shows signs of distress. This is also the stock treatment for the more serious problem of fungus (although your aquarist can supply a fancy range of chemicals for this, as for many fish ailments. Some chemicals, however, may tend to make your fish infertile). The fungus Saprolegnia ferox is normally present in water, but fish are unlikely to become infected with it unless they are wounded in some way or are in a poor state of health. Overfed fish are less resistant to infection than those properly fed. The infection is visible as greyish-white strands, or as if cotton wool was adhering to the fish. As well as the salt bath, which should be changed daily, the affected area on the fish can be painted with diluted iodine.
Constipation should not be a problem with outdoor fish which have a varied diet. When it occurs the symptoms are sluggishness and the excrement is lengthy and knotted. Hard and dry foodstuffs and overfeeding are often the cause. Fish may right themselves, but you can transfer them for twenty-four hours to a tank containing Epsom salts at the rate of six tablespoonfuls to every six litres of water. Dried foods should be avoided for a while and chopped earthworms substituted.
For dropsy, evident by the manner in which the scales of the fish stand out and the general puffy appearance of the fish, there is no known cure. Similarly, fish whose sense of balance has been affected are usually best destroyed. One can try placing the fish in a tank heated to about 20-22°C,it on earthworms and trying Epsom salts, but the results are rarely successful. Despite the long list of ailments and parasites to which fish can fall victim, one can expect few casualties among fish that are not overfed and which live in a well-oxygenated pond.
A problem of which I have been the unwitting cause is worth mentioning, for many pond owners must be in the same. No text book of which I know warns against it. That is the danger of removing fish, and not only small ones, when removing clumps of overgrown weed. If you grow Elodea, for example, fish can easily lurk unseen in this weed and when a large number of strands are drawn up in a bunch, a fish trapped in the middle is quite powerless to make his presence known. Having transferred sizeable clumps of weed to tanks I have, on occasion, been surprised to find a fish or two swimming round in the tank a few days later. Their fate would have been quite different had the weed been intended for the rubbish dump. So do examine your surplus weed with care.