Chapter 9

TROUT. THE FRIENDS AND ENEMIES OF THE FISH CULTURIST

The creatures which are sometimes found in and around rearing ponds containing ova or young fish are very numerous, and it is advisable that the fish culturist should have some knowledge of them. It is for this reason, that while I cautioned my readers against the creatures which are dangerous, and enumerated some of those most serviceable as food, I left detailed descriptions of these enemies and friends of the little fish, in order that I might deal with them in a separate chapter.

Among the worst enemies of both ova and fry is the _Dytiscus marginalis_, whether this insect be in the larval or adult stage. I think that I should hardly be wrong in going even further and saying that _D. marginalis_ is very dangerous to trout early in their yearling stage. The accompanying illustration shows a larva of _Dytiscus_ which has caught a young trout. This illustration is taken from a photograph of a specimen lent to me by Mr. F. M. Halford, and both the fish and the larva were alive when they were caught. Unfortunately the trout is a little shrivelled, and the legs of the _Dytiscus_ have been broken. _D. marginalis_ lays its eggs in the stems of rushes. The larva, when hatched, makes its way out, and proceeds to lead a predatory life. The larva when full-grown is about two inches long, and is quite the most rapacious creature which lives in our waters. The adult beetle is also purely carnivorous, but is perhaps not quite so rapacious. It would, however, probably attack a larger fish.

The largest of English water beetles is _Hydrophilus piceus_. This beetle is not, in the adult stage at least, carnivorous, but the larva, which is about half an inch longer and considerably fatter than that of _D. marginalis_, is carnivorous. It may be told from the larva of _Dytiscus_ not only by its size, which is hardly a reliable point for discrimination, but by the smaller size of the head in comparison to the rest of the body. The claws, with which _Hydrophilus_ seizes its prey, are, too, considerably smaller than those of _Dytiscus_. This larva should be kept out of the rearing ponds with just as much care as that of the more voracious _D. marginalis_.

With the kingfisher I have already dealt at some length, so that I need say but little more with regard to it. One of the worst features in this bird's character is that it will go on killing many more little fish than it can possibly eat. As I have before said, it is surprising how these birds will appear in considerable numbers where a fish hatchery is started, even in localities where they have before been considered rare. I have already described how the ponds should be protected from their ravages.

Herons do a great deal of harm to fish ponds, even when the fish have got well into the yearling stage. I have on one or two occasions known of herons wounding trout of at least a pound in weight. Besides the actual damage they do by killing fish, they put all the other fish in the pond off their feed through frightening them. After a heron or kingfisher has been about a rearing pond the little fish will not feed for a considerable time, sometimes even for days. Notwithstanding their very evil proclivities, both herons and kingfishers are very interesting. A kingfisher, if he catches a fish which is a little too big for him to swallow whole, will knock the head of the fish, which he always catches by the middle of the body, against a stone, in order to kill it, or at least to stop it struggling; it might otherwise in its struggles escape, as the kingfisher can only swallow a fish head first. There are stories which tell how herons sometimes pluck small feathers from their breasts and, floating these feathers upon the water, catch the trout as they rise to it; it is supposed that the trout takes the feather for a fly. Personally, I do not think that much credence should be attached to the latter story.

Other birds, usually found on or near the water, are also likely to do much harm to the ova and young fish. Almost every creature which is found near the water seems to have a great liking for the ova of fishes. All the wading and swimming birds are to be dreaded by the fish culturist. They will, all of them, eat ova in enormous quantities, and many of them will also eat the little fish.

Besides birds, small larvæ of several insects will eat, or at any rate kill, the ova in considerable numbers. Caddis-worms are among these larvæ which eat ova. This seems to be one of the few cases in which nature is just, for caddis-worms are taken very readily by even small trout. Large trout will take them very greedily, cases and all. Therefore, I should advise the fish culturist to cultivate them as food for the fish he is rearing, but to be very careful that they do not get into the rearing boxes or hatching trays when he has ova in them. The caddis-worms kill the ova by making a small hole in them and sucking some of the contents out; from this hole some more of the contents escapes, and as it comes into contact with the water becomes opaque.

Caddis-worms are the larvæ of an order of four-winged flies commonly known as sedges, caddis-flies, or water-moths. The latter appellation is of course a misnomer, as these flies (_Trichoptera_) have nothing whatever to do with moths. They resemble moths, however, in that they have four wings which when at rest lie in much the same position as do those of moths, and as many of them have their wings thickly covered with hairs, this resemblance is sometimes very marked. The larvæ (caddis-worms), being eagerly sought as food by many fish, and having very soft bodies, make for themselves cases. Some of these cases are made from small sticks, some from little pieces of stone or sand, and some from a mixture of all of these substances. As these cases resemble such small pieces of rubbish as are frequently found in streams, care should be taken that they do not get into the hatching trays containing ova.

Many of the water beetles, and practically all of their larvæ, will attack the ova; they should therefore be carefully excluded from the hatching trays. As there are about 114 different species of beetles in the family of _Dytiscidæ_ alone, my readers will appreciate my reason for not attempting to enumerate them. It will be a sufficient warning to state the fact that they are all carnivorous, and their relative sizes is the only thing which will decide whether the beetle will eat the fish, or the fish the beetle.

Very similar to beetles are some of the water-bugs. They may, however, easily be distinguished from beetles, as the outer or anterior wings of the bugs cross each other at their lower ends, while the elytra of beetles, which much resemble the horny, anterior wings of some of the water-bugs, meet exactly in the middle line. These water-bugs, though some of them are excellent food for even the small fish, will attack the ova, and therefore they should be kept out of the hatching trays. The fish culturist should, however, whenever it is possible, cultivate such of these water-bugs as are good food for the fry in separate ponds, as I have before recommended. The best of these water-bugs are _Corixæ_. Others, such as the water-boatman, water-scorpions and pond-skaters, are not of any value as food for the fish.

The larvæ of _Ephemeridæ_ are very good food for the fish, and should be cultivated in separate ponds if possible, and some turned into the ponds containing the little fish occasionally. A fair proportion should, however, be kept in the ponds and protected, so that a good stock of the flies may be available next year.

The larvæ of _Ephemeridæ_ may be obtained in many streams, and are best caught with a fine gauze net. Some of them swim, but most are generally captured with such a net at the bottom of the water among the _débris_. Eggs of _Ephemeridæ_ may be obtained sometimes from another locality if they cannot be got on the spot. These should be carefully preserved for the first year at any rate, and a good fly may thus be introduced into a water where it was before unknown.

I have already spoken of the fresh-water shrimp and the water-flea (_Daphnia pulex_). These valuable articles of diet should be introduced whenever it is possible. _Daphnia_ must be reared in a stagnant pond, the fresh-water shrimp (_Gammarus pulex_) in running water, with plenty of weeds.

Other useful creatures besides those snails and mussels which I described in a previous chapter, are the water-louse (_Asellus aquaticus_), _Cypridæ_, and _Cyclops quadricornis_. Asellus is very similar in size and shape to the common garden-louse, which is found in decaying wood. It will live either in stagnant or running water. _Cypridæ_ are very much smaller, being generally only as large as a large pin's head. They have a bivalve shell which makes them look something like a small mussel. They are, however, very active, swimming by means of two pairs of legs. They also possess two pairs of antennæ and one eye. (The species belonging to the genus _Candona_ of the family _Cypridæ_, do not swim.) _Cyclops_ is another very small crustacean, shaped like a large-headed club. It swims very actively, and, like the _Cypridæ_, is an excellent article of diet for very young fish. Both these crustacea live in stagnant water, and must, therefore, be kept in a separate pond, whence they may be taken as required to be given to the fry.