Popular Science Monthly/Volume 35/July 1889/The Artificial Propagation of Sea-Fishes
|THE ARTIFICIAL PROPAGATION OF SEA-FISHES.|
By Prof. W. K. BROOKS,
OF JOHNS HOPKINS UNIVERSITY.
SOME years since the writer was much impressed by an article by Prof. Huxley, in "The Popular Science Monthly," on the artificial propagation of food-fishes, in which he recognizes the value of the economic results which have followed the culture of the fishes of inland waters, but gives very emphatic expression to Ms belief that man's influence, either for good or for bad, upon the infinite wealth of the ocean, is so very slight as to be absolutely without significance. He argues that an oceanic species which is rich enough in individuals to resist all the enemies which prey upon it can be in no danger from man. If, he says, it is able to hold its own in the fierce struggle with the natural conditions of its existence, the loss of the few individuals, which are all that the human fishermen are able to capture, can not possibly lead to its extermination, nor even exert any noteworthy influence upon its abundance; nor can man, he argues, by artificially fertilizing a few million eggs, and by rearing a few million young fishes, cause any appreciable increase in the abundance of a species which includes countless millions of adult fishes, each of which has the power to leave behind it millions of descendants.
As compared with the natural reproductive power of the cod-fishes upon the Grand Banks, the efforts of man to artificially increase the supply sink into absolute insignificance, and Huxley's statement of the case seemed to me at the time to be convincing; but I have recently been able to investigate the subject for myself, and I am now satisfied that his opinions are not beyond question. As I am well aware that their influence has been far-reaching, and has much to do with current views, I take this opportunity to state my reasons for the change in my own opinion, as I wish to call attention to what I now consider a serious fallacy in his argument. If man's destructive influence were similar in kind to that of the other enemies of marine food-fishes, it would undoubtedly be quite true that the numbers destroyed by him are as nothing when compared with those which are destroyed in other ways; but the danger which comes from man's influence is fundamentally different from all the natural dangers to which sea-fishes are exposed, since it is modern or recent, and has, therefore, failed to be recognized and provided against during the evolution of these animals. In this sense man's influence is unnatural, while all other dangers are natural. The danger from man is not only modern, but also totally anomalous in the rapidity of its approach. It has not grown up gradually and imperceptibly, but has swept over the entire ocean with a speed which leaves no chance for the production of compensating adjustments by the slow process of selection. If it were to remain without change, or were to change very slowly, there can be no doubt that all the species which were not quickly destroyed would ultimately be brought into adjustment, and would from that time on be able to resist; but what animal can become adjusted to an enemy who is able, in less than a generation, to increase his power by such inventions as the steamboat, the electric light, and the dynamite bomb? To marine food-fishes man is a catastrophe, not a natural enemy, and the natural methods of maintaining the harmony between oceanic animals and the slow geologic changes of the ocean bottom are of no avail against him.
A study of the destructive forces of nature shows that man is peculiar in other all-essential particulars. It is a well-known fact that of all the marine animals which fall a prey to enemies, or become the victims of accidents and diseases, all but an infinitesimal percentage are destroyed during infancy or youth. As soon as the eggs of a fish are laid, the process of decimation begins, and it is initiated on a scale which would quickly sweep the species out of existence if it lasted long; but, fortunately, it does not, and each day in the life of a young fish brings with it an enormous increase in the chance for a long life.
During the early stages of development the young fish is totally defenseless, and at the mercy of enemies and accidents; and, although each pelagic fish lays enormous numbers of eggs, not a single one could escape if the embryonic period were long. Natural selection has been constantly acting for untold ages to shorten it, however, for in each generation those eggs which developed most rapidly have most frequently escaped destruction; and as the fishes which hatched from these precocious eggs have inherited a tendency to produce similar eggs, the embryonic life has gradually grown short, and most pelagic eggs now develop so rapidly that it is not unusual for them to hatch within twenty-four hours after they are fertilized. After they are hatched the transparency and activity of the little fishes add greatly to their safety, although each school of young fishes is constantly encompassed on all sides by a host of enemies. I have found, by watching for an hour from the end of a wharf a school of some eight hundred or a thousand young fishes, that one of them fell a victim each minute to the enemies of the air or of the water. While the death-rate is vastly less than it was during embryonic life, it is great enough to put an end to the entire school in a single day, were it not for the fact that each time a bird swoops down upon the little fishes out of the air above, and each time that a predacious fish darts in among them out of the depths and carries off a victim, the survivors profit by the new experience, and become more alert and vigilant and better able to escape future danger. While it is not possible to give figures, there can be no doubt that the chance for long life increases by a high geometrical ratio with age. Among salt-water fishes the death-rate is enormous at first, but it grows less and less as the individuals grow older; and the natural death-rate of adult fishes is infinitesimal as compared with the death-rate of the young. A high birth-rate has its advantages, since it gives an opportunity for selection, and thus contributes to the maintenance and gradual evolution and improvement of the standard of the race. Each adult fish is a survivor, picked out or naturally selected from among thousands or even millions of less favored brothers and sisters; and while many of the accidents which overwhelm the eggs and young are of such a character that individual peculiarities count for nothing against them, we can not doubt that, on the whole, the alert and energetic and intelligent fishes are most likely to escape, and to grow up to maturity and to bear descendants. A high rate of increase does unquestionably aid evolution by selection, but the well-known fact that it is reduced in all species with low death rates shows that its primary and most important purpose is to compensate for the loss from accidents and diseases and enemies, and to insure the perpetuation of the species.
A young fish with a million brothers and sisters must, before it reaches sexual maturity, be in imminent peril of life a million times before it is able to reproduce its kind; and the million perils are so grouped that most of them face it at the beginning of its life, and grow less and less frequent as it becomes older. The perils of a fish may be compared to a pyramid which tapers from a broad base in infancy to a pointed apex in mature life, and each species must be made up of individuals of all ages in a similar numerical ratio to each other. The perils of each individual fish seem to be accidental, but their average for the entire species conforms to exact numerical laws, and the number which die during the first day, the second day, and so on, of their lives, must be about the same, season after season. During the slow process of evolution the birth-rate of each species has been so regulated by selection that, after the natural mortality has been provided for, there shall be enough survivors in each generation to maintain the species and to keep the area which it inhabits stocked with as many adults as it can support.
All the natural sources of mortality are thus provided for. As each species is slowly and gradually brought into harmonious adjustment to the conditions of its environment, its birth-rate, like all its other attributes, is regulated and adapted to meet all the natural demands upon it. Now what happens if, after each one of the natural enemies has claimed its victims, a new enemy not provided for by Nature suddenly attacks the few adult survivors which Nature has provided to perpetuate the species? What happens when the last drop falls into the brimming bucket? What happens when the proverbial last straw is put on the load? It may be quite true that, for each codfish which man catches, the natural enemies destroy a million. That has no bearing on the subject. Nature has provided for the destruction of the million. Before their birth they were destined to premature death. The one was reserved by Nature for another purpose.
If the destructive influence of man had been gradually brought to bear, and had kept pace with the evolution of the species, natural selection would have provided a remedy, and the birth-rate would have been correspondingly increased; but this has not been the case; and, while man might not be able to make any impression on the broad base of the pyramid, we must remember that he does not attack the base, but the pointed apex. The fact that sea-fishes are so enormously prolific is entirely irrelevant. Their high birth-rate is an adjustment to their natural environment, while the influence of man is a new factor which has not been provided against.
It is difficult to get statistical information regarding marine animals, but there is ample evidence that they may be exterminated by man. The Bahama sponge-fishermen complain that they are now compelled to make long voyages and to visit remote banks for sponges which in former years could be gathered in abundance near the seaports. It is well known that, just before the oil from the wells of Pennsylvania came into common use, the sperm whales had become so scarce that they were in imminent danger of extermination. The scarcity and the high price of sea-fishes in the vicinity of large seaport towns are unquestionable; and the shore-fisheries of the New England coast, to which Cape Cod owes its name, have been so completely destroyed that, when the Cape Cod fishermen caught, a few months ago, in their nets some of the young codfishes which had been hatched in the Fish Commission laboratory at Wood's Holl, they brought them to the naturalists as specimens of a new and unknown species. The destruction of sea-fishes may require many years, but there is no animal on earth large enough to be valuable as human food which can long survive the attacks of a new unnatural enemy armed with the energy, the resources, and the intelligence of civilized man. Fortunately, the qualities which render him the most resistless of enemies also enable him to become a producer as well as a destroyer; and, while the fear of him and the dread of him is upon every beast of the earth and upon every fowl of the air and upon all the fishes of the sea—while they are all delivered into his hands, and are powerless to resist him—he alone of all animals is able to make good the destruction caused by his ravages, and to increase, by agriculture, by domestication, by selection and improvement, and by artificial propagation, the animals and plants which he destroys.
Can these influences be brought to bear upon marine animals? Can human intelligence and skill and power over Nature be so employed as to make quickly, by artificial means, that slight adjustment in the birth-rate of food-fishes which would have been brought about more slowly by natural agencies if man had long occupied his present rank among their enemies?
Looked at in this way, the proposition certainly does not seem to be impracticable; and, while human efforts in this field are of too recent a date to furnish positive evidence, I believe that I have shown that there is no a priori impossibility and no logical basis for a negative answer to the question. The results which have already been reached by the artificial propagation of certain sea fishes, like the shad, which make periodical visits to fresh water, are extremely interesting, as they furnish indirect evidence which is very conclusive. They prove that human influence produces very prompt and decidedly advantageous results in the case of these fishes, and thus give us every reason to hope that equally valuable results will follow—a little more slowly, perhaps—from our efforts to increase the supply of more strictly marine species.
In the year 1880 the fisheries census and special investigations which were carried on under the direction of the United States Fish Commission proved that there had been a most rapid and alarming decline in the value of the shad-fisheries in the rivers and bays and sounds of our Atlantic coast, and that there was every reason to fear that in a few years the shad would be utterly exterminated. The adult shad is an oceanic fish, but each spring it enters one of the inlets or bays and makes its way up to the fresh-water streams to reproduce its kind. The supply of shad for the market is caught during this spring migration, when the fishes enter our inland waters plump and fat after their winter's feast upon the abundant supply of food which they find in the ocean. As they spend the greater part of each year gathering up and converting into the substance of their own bodies the innumerable minute marine organisms which would be of no value whatever to man without their aid, and as their natural instincts impel them to bring to our very doors this great addition to our food-supply, their economic value is very great, as they put at our service a vast area of the surface of the globe which would otherwise be entirely beyond our control. The extinction of the shad would, therefore, be a national calamity.
In 1880 the fishermen believed, apparently with good reason, that the rapid decline was due to improper methods of fishing—to the erection of pounds and weirs along the shores of the salt bays and sounds, where the fishes were captured in great numbers long before they had reached their spawning-grounds. It was urged that, if these obstructions were removed, and all the shad were permitted to reach fresh water before they were captured, enough eggs would be deposited each year to keep up the supply, but that the destruction of such great numbers in salt water must necessarily result in extermination. This seemed to be good logic, but in the spring of the year 1888 more shad were caught in salt water than were caught altogether in the year 1880 in both fresh and salt water; and yet the shad-fisheries are now increasing in value from year to year, while in 1880 they were in danger of destruction.
To what is this change due? In 1880 the United States Fish Commission began systematically and upon a large scale the work of collecting the eggs from the bodies of the shad which were captured for the market in the nets of the fishermen. These eggs were artificially fertilized and hatched; the young fishes were kept for a few days in captivity in glass jars; they were then set at liberty in the fresh-water streams, and the waste of eggs was thus prevented. This work has been prosecuted steadily for eight years, and the results are briefly summarized in the following table:
|YEARS.||Shad captured in
salt or brackish
|Shad captured in
The money value of the excess in 1888 over the total catch in 1880 is more than $700,000. The conditions are now more unfavorable than ever to natural reproduction, and there can be no doubt that, if no shad had been produced by man since 1880, and if all the other conditions had been as they are, the fisheries would now be valueless. The mature shad which run the gantlet of all the pounds and traps in the lower waters, and finally reach the mouths of the rivers, are excluded by dams and other obstructions from all the streams which are of most value as feeding-grounds for the young; and the area which is now available for spawning is restricted to the lower waters of the rivers, which are so assiduously swept by drift-nets and seines that each fish is surely captured soon after its arrival, and before it has had an opportunity to deposit its eggs. The number of eggs which are naturally deposited is now very small indeed, for, while the take upon the spawning-grounds has increased from 1,600,000 in 1880 to 3,600,000 in 1888, the take in salt water has increased from 2,500,000 to 5,000,000, and the shores of our bays and sounds are now so thoroughly lined with both nets and pounds that the number of shad which reach the spawning-grounds at all is proportionately much less than it was eight years ago, and more shad are now taken each year in salt water, where spawning is impossible, than were taken altogether in 1880. The fact that, in spite of all this, the value of the fisheries has increased eighty-five per cent, seems to prove that the shad is now entirely an artificial product, like the crops of grain which are harvested on our farms.
If any one doubts whether this result is due to man's efforts, we have more conclusive evidence. Previously to 1870 no shad were found in the Pacific Ocean or in any of its tributaries. Between 1870 and 1875 the United States Fish Commission introduced a few young shad into the Sacramento River. The number was very small, but the little fishes made their way down to the Pacific to feed and grow large and fat, and to return at last to the fresh water to reproduce their kind. Some of them came back to the same river, but others, following the warm Pacific current, wandered farther north into other rivers, until now the shad is in some places sufficiently abundant to furnish profitable fisheries, and it is distributed along more than three thousand miles of the Pacific coast of North America, and is still spreading northward in such a way as to indicate that it will in a few years be found in the rivers of Asia, so that the descendants of the shad of the Chesapeake Bay will increase the food-supply of China. If such noteworthy and valuable results follow the artificial culture of a fish which spends the greater part of its life in the ocean, and there obtains its food, is there any reason why man should not also make good his destruction of species which are more strictly marine?
The great increase in the shad-fisheries during the last eight years has been effected by the use of means which, while effective, are very crude and primitive as compared with those of modern agriculture, for example, and we must look for great improvements and a vastly greater return in the future. A farmer who did nothing more than to save and sow wild seeds which would otherwise be lost on sterile ground or killed by frost or damp or eaten by birds and insects, would no doubt effect a slight increase in the food-supply, but his efforts would be very far behind the requirements of modern agriculture. His harvest would be as nothing compared with that of the farmer who sows improved seed; cultivates, protects, and nourishes his seedlings, and thus increases many hundred-fold the bounty of nature. Can similar improved methods be applied to the harvest of the sea? The Superintendant of the United States Fish Commission, Prof. Marshall McDonald, is now trying on a large scale experiments which will furnish an answer to this question, and the result will be eagerly looked forward to by those who are interested in pure science, as well as by those who value nothing except economic results. The young shad which are reared from the artificially fertilized eggs are usually turned out into the streams soon after they are born to shift for themselves. Many of them perish from accidents and the attacks of enemies, while others are forced to struggle for an insufficient supply of food. All horticulturists and breeders of domesticated animals know that the size and vigor and vitality of a plant or animal depend to a great degree upon its treatment during its infancy and youth, and that a stunted or injured infant seldom becomes a valuable adult plant or animal. Last spring about half a million young shad were placed soon after hatching in a large pond in Washington, and were carefully tended and fed and protected from enemies during the whole of the period which the young shad spends in fresh water. The young fishes prospered and grew rapidly, and nearly all of them were still alive when the time for migrating to the ocean came in the fall. The gates of the pond were then opened one morning, and all day long the silver stream of young shad poured out through them and started on the long journey down to the sea. All naturalists will look forward with the greatest interest to the time when these fishes return, bringing back with them to the fishermen of the Potomac the wealth of food which they have gathered in the ocean. In the mean time we may indulge the hope that the strong constitutions which they have acquired during their carefully nurtured youth will enable them to excel their less favored brothers, and that when they reach our market they will have some of the excellence of our improved garden products.
But this is not all. These shad were reared from selected eggs. The adults which enter our waters first in the spring are most valuable to the fishermen, since they are put upon the market at a time when fresh fish are scarce and high priced. Our experience with garden vegetables justifies the expectation that the eggs of early shad shall themselves give birth to early shad. Now, all the young fishes which were put into the Fish Commission pond were hatched from eggs taken from the earliest shad of the season, and, if this process of selection be pursued for a few years, we may feel confident that the Potomac River will soon abound in shad of extra quality at the time when fine shad are hardest to get and most valuable.