THE NATURALIST.

Otago Witness, Issue 2664, 5 April 1905, Page 68

THE NATURALIST.

Destroyers of Fish. — Plants and Animals Which Prey Upon Them.—

When one comes to consider the countless millions of fishes which are born each year in the fresh and salt waters of the (globe, the brain reels at their numbers, and one wonders how they can all secure space enough in which to move around or food enough to keep alive the vital spark. A wise Providence has, however, so arranged 1 matters that overcrowding is prevented by epidemics, which kill them off by thousands, while the ravages of man, ths predatory species of their own kind, birds, four-footed beasts, insects, and even plants, help to keep their numbers within wholesome bounds. The fishing methods employed by these various animals and plants are in many instances very interesting, and in some cases so effective tLat they have be-en adopted by man. One- of the most remarkable destroyers of fish is undoubtedly the bladderwort (Utrioularia), a plant with a handsome yellow flower, and common in ponds and ditches, only occasionally being found in rivers. This is a pelagic, or free floating plant, and its roothke branches extending downward are covered with numerous capillary and much divided leaves, which are interspersed with tiny green vesicles. formerly, these vesicles were supposed 1 to bo filled with water, by which means the plant was kept afc the bottom until the time of flowering, when the water gave place to air, and the plant then rose lo the surface to allow its bloom io expand ; but it is now known that they have- a tcrriblo use but litll© dreamed of by the early botanists. Their form is that of a flattened ovoid sao, and when viewed under a low-power microscope they look exactly like a human stomach. They are all attached at their hinder extremities each by a very short and

fine pedicle, or footstalk, in the axil of the leaves. Each, too, has an opening at the opposite free extremity, somewhat quadrangular in outline, from either side of which project two branched antennae. On either side of the opening several lon^ bristles project outward, and these brittle*, together with the branches of the antenna?, form a sort of hollow cone surrounding tho entrance, and act as a guide for tho \mwary victim. The entrance is closed by a valve, which, being attached above, slopes into the cavity of the bladder, and is attached to it on all sides except its lower margin, which is free, and forms one side of the slit-like opening leading into the blsddor. Fish enter the bladder by bending inward the lower free- edge of the valve, which, owing to its being highly elastic, shuts again immediately. The edge is extremely thin, and fits closely against the edge of tho collar, both projecting into the bladder, thus makinig it extremely difficult, if not impossible, for any animal to escape after once entering. The plant has no digestive apparatus, and probably derive? nutriment from absorption of the fish when decaying. Generally, the fislws caught by the head, although some are seized by the tail. An idea of the destruction wrought by this plant can be gained when it is slated that in two days' time one of them destroyed 150 young perch confined in a small alass aquarium with it.

Next to man, birds are probably the most destructive to fifeh life. Very many species feed entirely, or almost entirely, on fish, and among ihey have as many ways of capturing their prey as liu.iian fishermen have.

Among the four-footed beasts fishing is generally a side issue, to be practised only when other food is scarce. While the salmon are running in the streams of the Pacifio Coast, bears ace to be found along them in numbers. Standing motionless on the bank, the bear waits jatiently until a fish comes within reach, when, quick as a flash, ho dashes his great, flat paw into the water and throws the fish out of thestrpam almost before it has time to even think of dansrer.

The South American jaguar is fond of fish. He is a patient, unerring fisherman, watching far long periods from some vantage point, which may be either a fallen tree trunk extending into the stream, or a,t the bank's edge, until a victim ai^pears, when, with a lightning blow, he hurls the fish out -on the bark.

Alligators, otters, sea lions, seals, wal-rusc-s, porpoises, mink, muskrats, water rate, etc., count their finny victims by the thousands.

Certain species of snakes ara also fond of fish. This is especially true of the moccasin, water adder, and blaHc watersnake. Certain insects also forage among the finny tribes, destroying both eggs and young fish. Water beetles, especially the rapacious dytiscus, will combine in parties of six or sev*en, and, attacking a small fish, literally eat it alive, while they often eat holes into the larger fish. The larvje of the beetles and dragon flies also devour eggs and young fish. The waterbug, called bclostoma by entomologists, is also a fisheater.

Several observations of mosquitoes t'estioying young fi=h have been recorded. In one a number of young trout, with the yolk sac still unabsorbed, we~o swimming around in a sunny pool. Every few minutes they would poke their heads out of the water in order to breathe. A cloud 1 of n osquitoes was flj'ing around just over tho pool, and when a head appeared above tho surface a mosquito would light down and immediately transfix tho trout by inserting its proboscis or bill into tho brain of the fish, which seemed incapable of escaping. When the mosquito would fly away the trout would turn over on its back and float down tha stream dead.