THE INVISIBLE FOOD OF FISH.
RECENT experiments on the food of the oyster show that the oatmeal commonly given to "fatten" them causes them to lose weight and die, and that flour, often used for the same purpose, soon poisons them, though, on the other hand, the typhoid bacillus is destroyed by passing through the oyster's alimentary canal.
The latter discovery will be good news to the owners of oyster-beds. But the study of bacteriology is a new one. What strikes the average reader as more curious is the lateness of the discovery that the food commonly used in shops to fatten oysters disagrees with and kills them. Yet it is only one of the results of what, until recently, was a very general ignorance of the main food- supply, not only of shell- fish like the oyster, but of all the swarming vertebrate-fishes of the sea, except such as are entirely carnivorous and live by preying upon other fish. The food of river-fishes was better known; but what was, until recently, thought to be their princi- pal food now appears to play only a limited part in their main- tenance, and the common fisherman's view, that river-fishes, like robins, live mainly on worms and grabs, with a change to may-fiy in the season, and occasional feasts of ground- bait and paste, is almost as far removed foam fact as the showman's description of the elephant's diet as consisting mainly of cakes and hay.
But the case of the river-fish did not settle the obvious pro- blem suggested by the question of the food-supply in the sea. The sea, except in the shallow-water fringe along the shore, is devoid of vegetables. It contains in general no growth of weeds and plants to harbour swarms of possible food-creatures with their eggs and larva, and where such vegetable growths do occur, as in floating weeds of the Sargasso Sea, a race of ,fish and crustaceans at once appears, limited to that locality, and obviously fed from that source alone. Neither does the sea, except in certain areas, greatly abound in vertebrate fish. You may catch large fish at any point on the voyage in the narrow seas, from Gibraltar down the Mediterranean to Aden. But the open seas are not full of the fry of fish which might form a goad supply for others, and in the Atlantic, except on the Newfoundland banks, there are no fish found near the surface at all. A bucket of Atlantic water is to the eye simply a vessel of transparent brine, unfouled with weed, void -of fish, and, in most cases, not visibly infested with any form of floating marine organisms. Yet at any moment shoals of fish numbering millions of individuals may elect to enter this apparently foodless waste ; the herring- shoals disappear into the deep Atlantic, and return in good condition, oily and exuberant, and the whales find sufficient food to make them the " fattest " creatures in creation.
Many of the whales are carnivorous ; some, including the
ight" whale, have long been known to live on small sea- crustaceans, which were supposed to be found in exceptional numbers in the Arctic Seas. But the ease of the typical "shoal" fish, such as pilchards and herrings, offered special difficdties. When caught near land they were often found to have been living on sand-eels, roe, and small fish and crabs. But in the greater number of cases the contents of their stomachs were quite unrecognisable, and the immense size of the shoals increased the difficulty of believing that on the migration they could live on fish or on vegetable food. For fish moving in a serried shoal extending for a square mile, and perhaps thirty feet deep, it would be impossible to find room to chase and capture smaller fish, even supposing these could be found in sufficient numbers to form food for the numberless herrings. The rapidity and order of their movements make it equally unlikely that they should stop to chew and browse on vegetables, even when arrived in the shallow seas where seaweeds grow. It was just possible that previous to spawning the fish could for a time abstain from food. But it was equally certain that after spawning their numbers were the same, and that they must require food, and that in large quantities, in an area no greater than that occupied by the herring or pilchard shoal, so long as the fish remained in that formation. The explana- tion is that the microscopic creatures which are in parts of the Atlantic massed so thickly in the water as to discolour the surface, and give abundant food for the whale, are present, not so thickly, but in numbers comparable to the motes in the air, in all parts of the sea. For the purposes of the herring and the pilchard, and countless other vertebrate- fish, shell-fish, and zoophytes, the upper waters of the sea are in fact a nutritive soup, teeming with food exactly suited to their needs. These microscopic creatures are the basis of all the larger life of the ocean, and in a great degree of the growth and increase of fresh-water fishes. Some of these tiny creatures are water-fleas, others like carapaced shrimps, others occupy shells like miniature bivalves, others are forms of the one-eyed microscopic monster of the ponds, the Cyclops. All are of prodigious fecundity, and proof against astonishing changes of temperature, and the eggs and young, the microscopic offspring of the water-midgets, per. vade every drop of the surface-ocean, the rivers, and the ponds. In fresh water the common water-fleas often discolour a pool; they produce three broods a month, with forty or fifty eggs in a brood ; other species swarm on the leaves of every water- growth, and adhere to every filament of the fad and conferrx.
The Cyclops will, it is calculated, beget four hundred and forty-two thousand young in the course of a year; and the Cetochilus, or "whale-food," is said, even in the Firth of Forth, to form almost exclusively the food of the herrings and the sea-living salmon and salmon-trout. Their existence is one of the greatest economic triumphs of nature. They are the creatures which dispose of the refuse of the world in the sea, and keep it sweet. Dead vegetable and animal matter feed these entomostraca, and they are converted without further machinery into the food-fishes of the world, or at one remove, when these are eaten, as for d for other fish, such as the tunny, the cod, and the mackerel, which follow the herring-shoals. Nothing short of assimilation in the digestive organs of fish seems to kill these en tomostraca. They swarm in the distilled brine of the salterns on the Solent. Their eggs are proof against frost, and survive being baked by the sun. They even come to life without being fertilised. Yet they undergo infinite changes of form, and their cast shells are piled like billows of dust on parts of the Cornwall coast. Detached and self-supporting they wander over the whole ocean, swimming mainly upon the surface. At times they descend to the deeps, and this, it is surmised, causes the temporary disappearance of fish, which necessarily follow them. Their countless numbers are also recruited by the microscopic larvm of fixed shells. The barnacle, for instance, begins life in this form, taking its place in the ingredients of the "sea-soup" as a one-eyed swimming crustacean, then growing a pair of eyes, and finally settling down as a fixture in proper barnacle style.
In rivers they are almost the sole food of all young fish, and probably the main resource of the older fish when other supplies fail. In the first days of spring, the creatures in every stage, eggs, larva, and perfect, though microscopic entomostraca, swarm in the water, on the mud, and on the foliage of the water-plants. At such times even trout feed mainly on them. In the Hertfordshire streams the trout are then said to be" tailing." They push their heads down into the weed, and raise their tails, which wave about in the weeds, or even above the surface of the water. They are eating the weed bare of the clinging film of microscopic larvaa, of water- fleas, Cyclops, and other fresh-water entomostraca. The trout is the most easily fed of all fish, being greedy, omnivorous, and not afraid of artificial food, such as bread or paste. But the kind of food with which it is supplied makes a vast difference in its growth. Experiments made on trout showed that when fed upon worms only they grew slowly ; others fed on minnows did better, but a single fish fed upon insects weighed twice as much at the end of the experiment as a pair of those reared upon worms and minnows respectively. For feeding all young trout the mscroscopio food is now admitted to be best of all. Mr. Armistead, in a recent lecture on "Fish Culture" at the rooms of the Royal Institution, dwelt on the necessity for making separate pools, full of weeds and plants, in which the millions of entomostraca might increase, and serve as food for the young trout below. Carp were formerly believed to be veg( table-feeders, and the carp-ponds of Germany used to be drained and planted with rye as carp-food. So it was, but only as being itself food for the microscopic millions. The carp chews the water-weed, sucks off the insects, and then spits it out again. It may be doubted whether there are any of our common food-fishes, except the grey-mullet, which are vegetable-feeders ; and by a curious reversal of the rule obtaining among birds, the fish- eating, or " entomostraca-eating," fishes excel in flavour the vegetable-feeders. The red-mullet, which lives mainly on "sea-soup," is among the dainties of the table, while the grey. mullet is almost worthless.
This unseen but omnipresent source of food, nourishing either directly or at second-hand almost every creature of sea and river, from the tiny jelly-fish to the " right " whale, explains the truth of the old saying, that an acre of sea is worth four acres of land. In the words of a recent writer,— " No other source of food can compare in economic value with this. Even the smallest pools and ditches swarm with the entomostraca, and wherever life can find a lodging in the water they are found in countless numbers ready to become the food of the higher animals, and able, by their surprising rapidity of reproduction, to maintain their mini-
berg. Without them the life of the fresh-water fishes would become impossible, and lacking their innumerable swarms, the schools of herrings and other sea-fish would hardly be able to exist."
Previous page
