Popular Science Monthly/Volume 20/December 1881/Deterioration of American Oyster-Beds II
|←Notes||Popular Science Monthly Volume 20 December 1881 (1881)
Deterioration of American Oyster-Beds II
By Francis Winslow
|Physical Education XII→|
Last in series
AN oyster-bed, in its natural and undisturbed state, consists of a long, narrow ridge of shells and oysters, lying generally in brackish water, on and surrounded by sticky bottoms, a mixture similar to clay and mud being the most favorable.
The form and area of the bed are variable, but, naturally, the length is greater than the breadth, and the greatest dimension is usually in the direction of the current. The bed itself is made up of masses of shells and oysters, covering areas of different sizes, and separated from each other by mud or sand-sloughs, though frequently it is unbroken, and the animals spread evenly and continuously over the entire area. This separation and detachment of the groups of animals are much more noticeable upon a bed which has been much worked. Upon an unworked bed, or one in its natural state, the oysters grow in clusters, and are firmly cemented to the bottom. Upon a bed which has been frequently dredged, the animals are single, and have no hold upon the bottom.
The fauna of the bed varies with the locality; in the Chesapeake it is somewhat sparse.
The bottom is usually of clay or mud, and of sufficient consistency to support the oysters; certain varieties of bottom are more favorable than others, but, except within wide limits, the character of the bottom does not appear to be of great importance, so long as it is sufficiently firm to prevent the animal from sinking into it and thus smothering.
The oyster thrives best in slightly brackish water, and the finest varieties are usually found in water of a lower specific gravity than that of the sea, but the density is not important, except within wide limits, though the specific gravity should not fall below 1·01 at 60º Fahr., distilled water being represented by 1·00. The main necessity is that the water should contain a sufficient amount of lime to furnish the animal with the principal constituent of its shell.
The mature marketable oyster is so well known that it is unnecessary to describe it; but the following will assist the reader, who is not acquainted with the anatomy of the animal, to a correct appreciation of the facts stated and terms used in subsequent pages:
That part of the oyster usually known as the heart is a muscle, called the abductor muscle; its office is to keep the valves (or shell) closed, and prevent the ingress of deleterious matter.
The two valves are hinged at the round, blunt end of the shell, and between this hinge and the abductor muscle lies the body of the oyster or visceral mass, which is made tip of the light-colored reproductive organs and the dark-colored digestive ones, packed together in one continuous mass. The mouth of the oyster is that part nearest the hinge, and what is usually called the "beard" of the animal is known as the "gills."
The oyster lies on its side in the shell, and the minute animal and vegetable organisms contained in the water, and which form the food of the animal, are passed along between the gills to the mouth by the action of myriads of small vibrating hairs, called cilia. These cilia cover the surface of the gills, and, by a rapid and simultaneous motion in one direction and a slow one in the opposite, cause a strong current to set into the lips of the valves, bringing in not only what is suitable for food, but many other minute organisms and particles which thus come in contact with the gills and what they may hold.
The European oyster (Ostrea edulis) and the American oyster (Ostrea Virginiana) are varieties of the same family, and, though differing in several particulars, are not so dissimilar but that the conditions favorable to the growth and life of one may be considered as equally so for the other. With each variety the formation of the generative matter is gradual, and the spawning-season of both is during the early summer months, its advent depending probably upon the temperature, the higher temperature hastening and the lower retarding that event. No particular temperature can be assigned, as it depends greatly upon the locality, but departures from the normal temperature of the spring and summer months will have the effect described. In the same way an increased or diminished density of the water, whether due to change of temperature or to the addition of water of greater or less specific gravity than that usually surrounding the animals, has probably a similar accelerating or reverse effect upon the spawning, though as yet we can not speak with certainty upon this point.
Generally, both in Europe and America, the spawning-season may be said to be from June 1st until August 15th, though variations of the temperature and density of the surrounding waters may expand or contract that period considerably.
All authorities upon the early stages of the European variety concur in the statement that the young oyster or "spat" is formed by the fertilization of the eggs of the female while within the shell of that animal, and that the "spat" is held between the gills, and thus protected by the parent until the shell has formed. Many authorities are also of the opinion that the parents are hermaphrodites, and it is here that the first difference between the European and American varieties is noticed. Professor W. K. Brooks, in his recent paper upon the embryology of the American oyster, states that, though he has during the past two years examined a very large number of oysters, he has never found any evidence of a union of the two sexes in the same animal; and my own more limited observations have likewise failed to discover any such indication.
The second and most material difference is in the manner of impregnating the eggs of the female. According to the best authorities the fertilization of the eggs of the European variety is accomplished by the passage of the male fluid into the water, and thence between the valves and gills of the female, where the eggs, having been expelled from the ovaries, are lying ready for fertilization. The young resulting from the union of the ova and spermatozoa are held and protected within the gills of the female until the shells have formed and until they are quite well advanced in development, having at the time of their expulsion locomotive powers of their own, which enable them to swim about and seek an appropriate place for attachment.
The American variety, according to the observations of Dr. Brooks, differs in this, that the young oyster is not found within the gills of either parent, nor does the fertilization take place within the shell, but the contents of the generative organs of both sexes are expelled into the water, there to stand the chance of coming into contact. My own observations support this theory, inasmuch as relates to the presence of young within the gills, I having examined those parts of a large number of oysters very carefully, and with an utter failure to discover an embryo oyster, even during the height of the spawning.
It is evident that a large measure of protection is afforded the young of the European variety by the inclosing shells of the parent, and that this protection is given during the most precarious stages of their existence, while the ova and spermatozoa of the American oyster are not only left to a happy chance for their successful union, but the resulting young are exposed, unprotected, to all the vicissitudes of climate and to the ravages of all enemies.
After the formation of the shell, and the development of the locomotive powers, the young of both varieties begin their search for a permanent resting-place or point of attachment. The swimming period is of short duration, and the powers of locomotion are not very great; consequently, such points of attachment must soon be obtained, or the young oyster perishes. Any moderately rough, hard substance, provided that the surface is clean, is suitable for attachment, and such objects, when exposed for the purpose of attracting the young brood, are called "cultch." Pieces of wood, boughs of trees, planks, stones, old shells, tiles, and fagots, have all been successfully used. It is only necessary that the surface should be hard and clean, and, if dark colored, so much the better. Upon finding the "cultch," the "spat" attaches itself firmly, and is thenceforward, so far as its own power is concerned, located for ever; all future movement and change of position are due to causes, natural or otherwise, over which it has no control.
The development now is one of ordinary growth, the animal having passed through its embryonic life; its organs are formed and are in active operation; it is but immature. The increase in size and the development of the different parts, or, in other words, the growth, are much more rapid with the American variety than with the European. Though we have no record extending over any length of time, and none of different localities, yet the observations during the summer of 1879 in Chesapeake Bay show that in the first three months of existence the oyster in that locality increases in size from a hardly visible speck to an average length of one and a quarter inch, and a few were over two inches long. The lower and attached side grows the most rapidly, and during this first period the growth is mainly in length; there is, of course, a development in all directions, but the greatest is toward the lips of the shell. After the first year, the increase in size is not so rapid, and oysters of two or three years of age are about two inches broad and three inches long, though those dimensions will vary considerably in different localities on account of the different conditions to which the animals are subjected; in three years at the most the American oyster is considered mature, and will present all the characteristics of those found in the markets. With the European variety the growth is much slower, and at maturity they are very much smaller than the American oyster: but this difference is immaterial to the question under consideration, and all that is necessary to notice is the consequence of the different methods of propagation.
With all animals Nature strives to provide against the destruction of the young after birth by insuring a sufficient number to allow for all ravages; and the greater the dangers to which the immature of any species is exposed, the larger will be the number provided to meet those dangers. Hence, as the embryo European oyster receives some protection and the American none, it is inferred that the number of American embryos in any community will be subjected to greater danger, and consequently it is probable that a larger number of eggs and spermatozoa are provided, that the production may not be less. If this is not the case, then the reproduction upon the European beds is much greater than upon our own. Let us see if the investigations, conducted both in this country and abroad, support this conclusion.
Professor Möbius, in his work upon the oyster and oyster-culture, estimates the number of eggs spawned by the European variety as about 1,800,000, and his estimate is supported by Eyton, in his "History of the Oyster and Oyster-Fisheries."
Professor Brooks estimates the possible number of eggs spawned by the American variety to be as large as sixty millions, and the average number to be over nine millions, or the American oyster spawns about nine times as many eggs as the European. The number of male cells is so great that, even if it were possible, it would be unnecessary to estimate them; but, from the comparison of the numbers of eggs spawned, it will be seen that Nature supplies with the American variety a much larger number of eggs, in order that the deficient protection afforded them after fertilization may, as far as possible, be remedied.
It would seem that the growth of a bed and increase in the number of oysters would be immense did we not know that the greater the production of the germs in all forms of life the greater the mortality among those germs; and, therefore, any conclusions as to the impossibility of destroying the fecundity of the oyster-beds, when such conclusions are based upon the large number of eggs spawned, must be erroneous. The number of embryos surviving and maturing can not be accurately stated for either variety, as we have but four data from abroad bearing upon the question, and, until the last year, none at all from this country. Professor Möbius has collected the results of the official examinations of the Schleswig-Holstein beds, and from their inspection has come to certain conclusions, and, as these results are the only ones known to us, it may be interesting to briefly describe the method of arriving at them.
The observations were made on the beds by officials of the Government from 1730 to 1852, and were conducted in the following manner: Each bed was dredged over in three or six places, according to its size, and the oysters taken were divided into three classes and carefully counted. The classes were denominated "marketable," "medium," and "young growth." The "marketable" oysters were those full-grown and mature, from seven to nine centimetres in length and breadth, and eighteen millimetres thick. The "medium" were half-grown oysters, from sixteen to eighteen millimetres thick, and less than nine centimetres in breadth; the "young growth" were those one or two years old.
From these observations, which were made annually, Professor Möbius discovers that there was on an average 421 medium oysters to every 1,000 marketable ones—that is, out of every 1,421 oysters 1,000 were full grown. The average of all observations differed very little from the number given by each, and consequently shows that there was but a slight fluctuation in the proportion in one hundred and twenty-two years. The "medium" oysters are considered by Professor Möbius to be those descendants of the "marketable" ones that have survived their most precarious years of existence and escaped their principal enemies, and are consequently likely to reach their full growth. They thus represent the total number of oysters spawned which have survived in the struggle for existence. From his experiments Möbius decides that about one million eggs are spawned by each oyster, and that about forty-four per cent, of the oysters on a bed spawn each season.
It is evident from the above that in an assemblage of one thousand oysters four hundred and forty million eggs would be voided every season, and of the resulting embryos four hundred and twenty-one would survive; or, 1,045,000 eggs would perish where one survived. But the "medium" oysters also spawn, though they send forth a much smaller number of eggs, and Möbius estimates that four hundred and twenty-one in the community would produce about sixty million, or the fourteen hundred and twenty-one would spawn together about five hundred million eggs, and from these five hundred million only four hundred and twenty-one oysters would be produced; or, where one oyster arrived at maturity, about 1,185,000 eggs or oysters perished!
As heretofore no examinations of the oyster-beds of this country have been made, there is no data similar to the foregoing upon which to base an estimate. As already pointed out, the mortality among the American embryo oysters must be much greater than among the European, and, in order that there might be some idea of the decrease in number and increase in size of our own variety when in the natural beds, there were deposited at various points in Tangier and Pocomoke Sounds, Maryland and Virginia, during the summer of 1879, a number of earthenware tiles as "spat"-collectors. Many of these were removed or destroyed, but from those left in place, by carefully counting at intervals the number of oysters attached, we have been able to estimate the decrease in numbers during the early months of existence.
The inspections of the "spat" collectors showed that the oysters continued attaching until about the 20th of August, and that the largest number attached about the first of that month; between the 23d of August and 10th of October the mortality was shown to be fully fifty per cent.; future examinations of the "spat"-collectors will probably show a diminished mortality, as the animal will be better able to protect itself as it increases in size, but the destruction among the unprotected, delicate embryos must be immense, and, as it is as great as fifty per cent, after attachment, it must be much more serious prior to that event.
From observations made during the summer of 1879, I find that, on a natural, unworked bed, the ratio of young oysters, or those over one and under two years of age, to those mature and over that age, is as one to two, or in a community of fifteen hundred there would be one thousand mature and five hundred young oysters.
There is no reason to suppose that, circumstances being favorable, this entire number does not spawn, for, of all those I have examined, I have never in the spawning-season found an animal that did not contain the generative matter, or that had not recently expelled it. Numbers send forth the products of generation at unfavorable times and in either an unripe or overripe condition, and some fail to void the fluids at all, but only unfortunate combinations of natural causes have those effects; and it is probable, if not absolutely certain, that in most spawning-seasons all the animals spawn. Dr. Brooks estimates the number of eggs voided by the American oyster at from nine to sixty millions; for convenience, we may take ten millions as the average number, which is probably less than what is actually the case. The thousand mature oysters in the community would, then, spawn ten billion eggs, and, as the young European oyster has been found to spawn about one third as many eggs as the mature animal, we will consider the same to be true for the American variety. The five hundred young would, then, spawn 1,600,000,000 eggs, or the total number in the community would spawn 11,600,000,000 eggs, from which would result five hundred oysters, or about twenty million eggs or oysters would perish where one was preserved!
It is evident, then, that the vast number of eggs spawned by the oyster is no assurance that even a small proportion of them will reach maturity, or that any external or abnormal agency, natural or unnatural, may not be sufficient to destroy the beds by removing either young or old oysters, or in other ways preventing them from reproducing their kind.
My personal observations convince me that the beds of Pocomoke Sound at least are in a condition very similar to the French beds before they were subjected to the action of protective laws; and, as the French beds have, by a wise and efficient protection, been made to yield again a profitable return, it will be instructive to see how that protection is afforded. Artificial cultivation in any way is not considered, as not being pertinent, and because, in however bad a condition our own fisheries may be, the time has not yet arrived when we will be reduced to those laborious and expensive methods of obtaining a supply of oysters.
The French Government assumes control of all oyster-beds and fore-shores. As occasion may seem to require, an entire bed or part of it may be reserved from dredging for a certain time, that time being decided by the local commission. The general practice seems to be, to buoy off a third or fourth of a bed each year, which portion is only sufficiently dredged to remove weeds, mud, vermin, etc. The remainder of the bed is open to all licensed persons for a certain specified time. The following year another part of the bed is reserved, and occasionally portions are reserved for longer periods than usual. The local commission decides all matters pertaining to the beds in their vicinity, and is composed of the following officers: the inspector of the fisheries, the commander of the fishery guard, two gardes maritimes, and one fisherman, master of a boat.
The following are the most important regulations, made for the guidance of the commission by the Minister of Marine: The beds should not be opened for fishing until the "spat" has acquired strength to resist the action of the dredge—until the end of January, for example. When a bed has well-established breeding qualities, a fourth or fifth part of its entire area should be set apart as a reserve, and dredging over such part entirely prohibited. A fishery guard-boat should, whenever practicable, take part in the working of each bed. When a bed is foul, or encumbered with weeds or other matter noxious to the development or adherence of the "spat," it should be opened for dredging until cleaned. Beds on which there is never any production of "spat" should be opened all through the season. After the working of any bed is over, it should be carefully examined, and, if necessary, the "cultch" replenished. The close-time is between the 1st of May and 31st of August, and is strictly observed.
The above regulations and their strict observance have caused a great improvement of all the beds on the French coasts; but one instance will be sufficient to show the effects of a protective policy when understandingly conceived and rigidly enforced. In 1870, through over-fishing, the beds in the Bay of Arcachon had become entirely exhausted; but, by the strict protection afforded them, their fecundity has once more become so great (in 1876) that the waters of the bay from June until August are filled with the young swarm. On a bed, when dry at low spring ebbs, and comprising 26·7 acres, there were taken by forty or fifty persons, in some two and a half hours, about sixty thousand oysters. That part of the bed was immediately buoyed, and no more fishing allowed during the season.
Having seen what protection has been afforded the French beds, and with what success, let us see how that experience can be best used, together with such knowledge on the subject of our own fisheries as the investigations conducted during the past two years have given us. The deterioration of an oyster-bed and its impaired fecundity will be shown: 1. By the general appearance and condition of the beds and animals, the former being broken up with mud and sand among the shells, and but little healthy, growing sponge; the oysters will be large and single, the shells covered with worms, and much broken and bored in many places. Very few barnacles will be found, and the general appearance of the shells will be one of decay. 2. The ratio of "young" to "mature" oysters will be abnormally large or small; should it be greater than as two to one, or less than as one to one, it may, until we have more information on the subject, be considered abnormal. 3. The amount of débris in the bed will be very large, and, should it exceed fifty per cent, of the contents of the dredges, it would be unusually large. 4. The number of oysters on the beds will be found to decrease each year, though slight improvements may from time to time occur. 5. The discovery of unusual inhabitants of the beds, other than the oysters, or the disappearance of those usually found, or, in general terms, marked changes of the fauna of the beds, are an indication of deterioration.
If a bed or any number of beds present these peculiarities, in order to prevent their entire destruction it will be necessary to insure the animals some adequate protection. The best remedy for any evil is the removal of the cause; and, should the deterioration of the beds be due to excessive fishery, the prohibition of such fishery would be the readiest and most certain means of arresting the deterioration: but, as a large number of the poorer class of people is dependent, in one way or another, upon the beds, or the oyster industry, for support, it would be impossible to prohibit all fishing of any extensive area without causing great distress, and the working of the beds can consequently be only restricted and not prohibited.
As the number of dredging-vessels employed directly in the fishery is, for a time at least, constant, it follows that, the larger the area open to these workers, the less exhaustive will be the fishery of any particular locality, for the fishermen will naturally seek the most productive fields for labor, and leave old and worn-out beds for those newly discovered and well stocked. Therefore, one of the best means of affording protection to the overworked beds is the discovery of new ones, as thus without prohibitory laws the fishery will be transferred from the former to the latter. Fortunately, the area covered by oysters along the coast of the United States is so great that at present, when any bed or locality ceases to give an adequate return, the fishery is transferred to other points—as, for instance, those vessels accustomed to work in Tangier and Pocomoke Sounds have left that locality for the Potomac River and the beds in the Chesapeake bay.
In time, however, the areas covered by oysters will be known, and all future extension of the fishing-ground must be an artificial one of those known areas.
In searching for new beds they will probably be found off the mouths of creeks or rivers, and not distant from beds already known. They generally exist as long, narrow ridges, very thickly stocked and having their greatest dimension in the direction of the current. Those areas where the bottom is sticky and the change from deep water sudden are most likely to reward a search.
The extension of the old beds can be effected by depositing suitable "cultch" upon the bottoms contiguous to the bed, and thus afford a place for the attachment of the drifting "spat." Stones, ballast, old pieces of earthenware, water-pipes, and old shells form excellent cultch, and, if any of these are scattered about the beds, a good catch may be confidently expected. Care must be taken to deposit the cultch upon those bottoms sufficiently consistent to support it, and also to make the deposit upon such areas as lie in the direction of the tidal currents, so that the young brood, rising from the natural beds, may be carried over the newly exposed cultch. If a number of mature oysters are deposited with the shells, they will materially assist in the extension of the beds. The cultch should be exposed late in the spring, so as to insure its cleanliness, as that is very desirable.
The consumption of the oyster is constantly increasing, and as the demand increases so will the disposition to fish the beds, and, should there be any failure of the supply, the increased price consequent upon that failure will induce even more exhaustive fishery; and it will become so great, if it has not already, that only strict protective laws, rigidly enforced, will be sufficient to protect the beds, and prevent the destruction of the industry.
There is, however, another means of maintaining the fecundity of the beds, which merits consideration.
During the summer of 1879, Professor W. K. Brooks was successful in securing, by artificial means, the fertilization of the eggs of the female, and in protecting the offspring for some time. Though, owing to various unforeseen combinations of natural causes, and to the accidents incidental to all tentative work, he has not been successful in maintaining the embryos until such a time as they could be deposited upon the beds with a certainty of survival, yet he has accomplished sufficient to show that the impregnation of the female cells can be easily and certainly achieved by a very simple process; and, as probably the greatest loss of the young is due to the failure of the ova to meet the male fluid at the proper time, any method which will insure such contact and protect the embryos, for even a limited period, is of great value, and well worthy of the attention of those interested in the preservation of the oyster-fishery.
Any protection afforded the young oyster assures the maturity of a greater number, and, as the beds are failing from a want of reproduction, due to the absence of mature oysters, any method which will insure the maturity of an abnormal number should be brought, if possible, to a point of practical benefit. It would be useless to give a synopsis of the laws of the several States, so far as they relate to the oyster-fishery, as long as we are ignorant both of the condition of the beds and of the amount of observance of the law. In the locality which has been investigated an inefficient law is entirely disregarded. An oyster-guard exists, but pays no attention to the duties assigned it, and the fishery is governed solely by the demands of the market and the necessities of the oystermen. What should be done in this locality, now that the condition of the beds is known, will be apparent to any one; and the sooner other localities are subjected to a similar examination and the condition of the beds determined, the better.
So far as it is possible to give an opinion which is suitable to all localities, it would seem that any legislation looking to the protection of the oyster-beds should comprise the following:
An organized and systematic effort to discover new beds and to extend the old ones. Continued experiments, in order to ascertain the possibility of artificially raising the oysters in sufficient numbers to restock impoverished areas. The maintenance of a commission to have charge of all matters pertaining to the fishery—the commission to be composed of intelligent men, having special knowledge of the subject and allowed considerable power, and to be so appointed and constituted that their acts will be influenced by no considerations other than those for the good of the beds. Under the control of the commission should be a fishery-guard, of sufficient power and importance to enforce the regulations of the commission.
The regulations of the commission should have in view the following desired results: The prevention of exhaustive dredging. The reservation of those beds or parts of beds upon which there is a large number of young growth. The prevention of the removal of the young growth from the beds. The close observance of the close-time, which should include the outer limits of the spawning-season. The cleansing of the beds before the advent of the young brood. The exposure of suitable "cultch" when a bed has been long worked, and the destruction of starfish, drills, or other enemies that may exist on the beds.
While I am unable to say with certainty that the beds of other localities than the one I have examined have been overworked, yet I should infer that such was probably the case; and, considering the surprising results of the investigation of one locality, and one which was supposed to be the most productive in the country, if not in the world, the investigation of others can not be too soon undertaken.
The probability of all our beds arriving in time at the forlorn condition of the European oyster-banks has been foreseen, and Professor Möbius warns us of the consequence of our exhaustive fishery in the following words: "In North America the oysters are so fine and so cheap that they are eaten daily by all classes; hence they are now, and have been for a long time, a real means of subsistence for the people. This enviable fact is no argument against the injuriousness of a continuous and severe fishing of the beds. . . . But as the number of consumers increases in America the price will also surely advance, and then there will arise the desire to fish the beds more severely than hitherto; and, if they do not accept in time the unfortunate experience of the oyster-culturists of Europe, they will surely find their oyster-beds impoverished for having defied the bioconotic laws."
How nearly correct he has been in his prophecy I have attempted to show, and it is with the intention of seconding his advice, and with the hope that it will be heeded, that this article has been written.
- At least one variety of the European oyster is reproduced in a manner similar to our own. During the summer of 1880 I successfully impregnated the eggs of several female oysters, taken from the Bay of Cadiz, Spain. The embryo were maintained alive for eight days, and their development was entirely similar to that of the American variety.
- Derived from bioconosm, a word signifying complete within itself.