VOLUMETRIC STUDIES OF THE FOOD AND FEEDING OF OYSTERS #« « »# From: BULLETIN, OF THE BUREAU OF FISHERIES, Volume XXVIII, 1908 Proceedings. of the Fourth International Fishery Congress: Washington, 1908 \ |) Seeegneeeseeesteenanssenssieeense}ssspesienseeenesassins=ssn=anpewsssrapsss=anspanasusossasenannsanssensersateanssssaneeneesennn-snseeneneenpeeemeentoeeeeennenene ee tn “yo WASHINGTON: 2oi2202 2: GOVERNMENT. PRINTING’ OFFICE, :.: 9: 3 :, 2 1910 Gass_ > H=A311- eae a7 VMOEUMETRIC STUDIES OF THE FOOD AND FEEDING OF OYSTERS #« « «# From BULLETIN OF THE BUREAU OF FISHERIES, Volume XXVIII, es Ee etinds of the Fourth International Fishery Congress : : Washington, pes Ss ensserenneneneneneeeeeseneee ee ee WASHINGTON : : : : : : GOVERNMENT PRINTING OFFICE ::::: : 1910 BUREAU OF FISHERIES DOCUMENT NO, 719 Issued May, 1910 D. OF D. MAY 18 1810 VOLUMETRIC STUDIES OF THE FOOD AND FEEDING OF OYSTERS * By H. F. Moore Assistant, U. S. Bureau of Fisherves m* Paper presented before the Fourth International Fishery Congress held at Washington, U. S. A., September 22 to 26, 1908 VOLUMETRIC STUDIES OF THE FOOD AND FEEDING OF OYSTERS: vt By H. F. MOORE, Assistant, United States Bureau of Fisheries. & Economically considered, probably the most important direct interrela- tion between a marine animal and plants is that existing between the oyster and its food. We have in the United States alone an industry valued at $18,000,000 per annum, which is immediately dependent upon the supply of microscopic vegetation in our bays and estuaries, a vast food resource useless to man in its original state, but of great present and still greater potential value when trans- substantiated into the flesh of oysters, clams, and other mollusks. Various investigations have shown that about 95 per cent of the food of the oyster consists of diatoms and that most of the remainder is composed of other equally minute plants or organisms on the more or less debatable borderland between plants and animals. The oyster obtains these microscopic organisms by drawing feeble currents of water between the open shells, straining them through the exceedingly minute orifices in its gills, and passing the filtrate by ciliary action into its mouth, which lies ensconced between two pairs of fleshy palps close to the hinge of the valves. Though the currents induced are feeble they are constant, and during the course of twenty-four hours the water thus minutely strained is many times the volume of the oyster. It is common knowledge among oystermen and oyster growers that differ- ent localities differ markedly in their powers or capabilities for growing and fat- tening oysters, and the results of various researches have shown that these diversities are correlated with the amount of food available to the sessile oys- ters. A deficiency may be due to a natural poverty of the waters, to an over- population of oysters, or to an absence of currents sufficient to carry the food within reach of the feeble external currents set up by the oysters themselves. Frequently all three of these factors are found to be involved where oysters grow slowly and fail to fatten. 1297 1298 BULLETIN OF THE BUREAU OF FISHERIES. Certain enthusiasts, some of whom should know better, have held forth the prospect of a time when the entire available bottom of our bays and sounds would be planted in oysters as densely as are the comparatively small areas now utilized. They fail to consider the fact that the natural fertility of the waters imposes some limit upon the production of oyster food, and that a vast increase in the oyster population, such as their imaginations contemplate, would undoubt- edly exceed the limits which nature has set. The microscopic vegetable life of our brackish bays and sounds is probably as abundant as it is capable of becoming under existing conditions. It is depend- ent primarily upon the quantity of certain mineral salts in solution, and is as strictly limited by the conditions as is the crop yield of a given area of land by the available salts in the soil. The soils can have their fertility artificially increased, but though experiments conducted by the author for the Bureau of Fisheries have shown that the same expedient is partially successful for limited areas of inclosed water, it can never be applied to open waters, as the fertilizer would be speedily carried away. In this connection, however, it is an interesting speculation whether our coastal waters are not to-day richer in fertilizing salts than they have been in the past. The denudation of our forest lands, the erosion due to faulty agriculture, the artificial fertilizers carried away from cultivated fields during periods of heavy rainfall, and the discharge of sewage rich in organic matter have undoubtedly added much to the available fertilizing content of our coastal waters, to the advantage of their microscopic vegetation. The question of food supply, its availability, and the quantity required for a given area planted in oysters is one of vital importance to the oyster culturist. Of the total oyster supply of the United States, about five-eighths, valued at over $10,000,000, is produced on planted beds, and the future growth of the industry is dependent upon the increase of the area of private bottoms under culture. With the extension of the planting industry to new localities and the inevitable congestion in places naturally favorable for growing and fattening oysters, the value of definite data upon this subject will be greater in the future than in the past. ; Empirical methods involving actual planting to determine the suitability of a locality are expensive and often wasteful, and operators with small capital are frequently deterred from taking the risk. Even though the work on a small scale may prove successful, an increase to a large commercial basis may overtax the food supply to such an extent as to make the growth of the oysters slow and their fattening impossible. A number of cases of this kind have come to the author’s attention, the most noteworthy being in Lynnhaven Bay, where the increase in the area planted, though the quantity per acre is exceedingly small, has made it almost impossible to fatten oysters properly on certain bottoms formerly satisfactory. FOOD AND FEEDING OF OYSTERS. 1299 As the economic importance of the subject merits, it has frequently been the matter of investigation and has probably attracted more attention from biolo- gists than has any other direct correlation between marine plants and animals. The nature of the oyster’s food was long ago determined and the work of the last twenty years has been hardly more than confirmatory of that which preceded it. Dean appears to have been the first to attempt the quantitative determination of the oyster food available in the water. He employed a chemical analysis of the water to determine the albuminoid ammonia content, assuming that the results would indicate the comparative food values of different regions. Subsequent investigators have recognized the grave defects in this method, and, including myself, have all followed the general method of Rafter. Water specimens of definite volume, usually 1 liter, have been collected either by means of a stoppered bottle or jug, from which the cork is pulled after it has been sunk to the bottom, or by a specially designed metal cylinder constructed on essen- tially the same principle. The suspended matter in the specimen, a large part of which often consists of sand, mud, and débris, is then concentrated in, say, 10 cubic centimeters of water by filtration through sand or precipitation in an Erlenmeyer flask after the addition of a few drops of formalin. A definite quan- tity of the filtrate is then removed after agitation and the food organisms counted in a Rafter cell, the calculated number of such organisms per liter being regarded as an expression of the food value of the water. This method has two defects, the first of which is that the water specimen is not drawn from the stratum tenanted by the oyster, but solely from a height of about 12 inches above the bottom. It would be possible to correct this defect by using a shorter, broader bottle or specimen cup, but as the water flows rather slowly into the necessarily narrow inlet, there would enter with it a considerable quantity of material stirred up when the instrument strikes the bottom. As the amount of this material would vary with the bottom, the impact, and the currents, a more serious source of error would arise and the results would become worthless. To obviate these difficulties I have designed the type of bottle illustrated in text figures1to5. It consists essentially of a brass barrel of a capacity somewhat over 1 liter, two conical valves, and a tripping device. The lower valve is fixed at a height of 2 inches above a broad base, which prevents the instrument from sinking in soft mud, but the barrel and upper valve slide freely on a central column or rod. ‘The instrument is set by engaging the lug (F) over the inclined surface (G) of the stirrup or tripping device (CDEG), which suspends the upper valve (B) and the barrel (A) so as to leave a gap of 2 inches between the two valves and their respective seats, the stirrup being maintained in position by tension on the cord by which the instrument is lowered. By rotating the cam (H) so as to pinch the cord between it and the collar on top of the upper 1300 E BULLETIN OF THE BUREAU OF FISHERIES. valve, the instrument may be locked in the set position, but it is automatically unlocked when it is raised by the cord. As the instrument is lowered there is a free flow of water through the barrel, so that at any given time its contents are taken from the stratum in which it SSF - L oy V5 r@ i LleLratiore Sechort Fic. 1. Fic. 2. Water specimen cup. rests. When bottom is touched the tension on the cord is relaxed, the tripping device instantly releases the upper valve and the barrel suspended from it, and they fall into their respective seats, inclosing a sample of water before it can be contaminated by the stirred-up bottom deposits. As the barrel is 10 inches FOOD AND FEEDING OF OYSTERS. I301 long, the water inclosed is a vertical column of the stratum lying between 2 inches and 12 inches above the bottom, and as the currents do not flow over the beds in horizontal strata, but roll over and over, this specimen is regarded as a fair sample of that in which the oysters are bathed. The instrument is now used in Massachusetts, Maryland, Virginia, and Loui- siana, and actual tests have shown that it takes a water speci- men much cleaner and freer from mud and extraneous materials than do the instruments previously employed. The other defect of the old method of determining the food value of oyster-producing waters arises from the practice of using the number of diatoms or organisms per liter as the measure of their productiveness. It is well known that diatoms, which usually constitute upward of 95 per cent of the food of oysters, mee eee eee differ greatly in size and the species vary in comparative abund- ance in different regions and from season to season in the same locality. Whena numerical expression is employed, it follows therefore that a multitude of small organisms may give an apparent superiority to a water specimen as compared with another containing a smaller number of a spe- cies of vastly larger size and much greater aggregate vol- ume, and my own ex- perience has shown cases where this error amounted to nearly 400 per cent. The method is attended with grave error as applied to even limited regions and is wholly untrustworthy as a basis of comparison between widely separated localities. It gives seemingly quanti- tative results, but these, not being volumetric, are decep- tive. Direct volumetric deter- mination can not be made on account of the presence of considerable volumes of sand, mud, and extraneous débris in the filtrate, these materials greatly exceeding the food organisms in volume. Grave attempted to overcome the difficulty by listing the food organisms by species, Jap Lien of 6) Lad Lhew of @ W Bolo Lien of © FIG. 4. Fic. 5. Details of tripping device of water specimen cup in figures r and 2. 1302 BULLETIN OF THE BUREAU OF FISHERIES. but this arrangement, though an advance on previous work, is not capable of comparative use, and any error in identification, not unlikely to occur with persons not diatomists, would be misleading to future investigators. To overcome these difficulties I have for several years used the following indirect method, which has given satisfactory results. The diatoms and other food organisms are collected and counted, as before indicated, and are listed by species, although their identification by their correct names is not essential. Careful outline camera lucida drawings are made of the zonal and valvular aspects of a number of specimens of each species, and their cubic contents are calculated by geometric methods from planimeter measurements of the draw- ings. ‘The average of a number of such calculations will give the average rela- tion of the volume to the product of length, breadth, and thickness of the species. Using this relation and the average of a number of micrometer meas- urements of the specimens themselves, a simple calculation will furnish an approximately correct expression of the average volume of the species in the region under investigation. If these volumes be employed as multipliers into the numbers of the respective species, determined from the counts in the Rafter cell, we have an approximately correct volumetric expression for the amount of the food content of each specimen of water. As the most convenient unit of measurement I have adopted Van Heurck’s ‘‘c. d. m.”’ (0.01 millimeter), the unit of volume being the cube of this, ‘cu. c. d. m.’’ (0.000,001 cubic milli- meter). The following is an illustration of the data required for each species: Synedra commutata (Matagorda Bay); average length, 4.7 c. d. m.; breadth, 0.5 c.d.m.; thickness, 0.5 c. d.m.; volume =o.6 (1 X b X t) =0.7 cu. c. d. m. This method sounds elaborate in its narration, but has not shown itself to be cumbersome in practice, and, moreover, it appears to be the only method so far proposed which gives data of real value. The results are directly compar- able with those obtained in other waters or with those reached in the same waters at different seasons. Five hundred or 600 determinations have been made in the past two years, and, for reasons shown below, the procedure gen- erally was found to require but little more labor than the older misleading and less accurate method. In oyster investigations it is customary to take a large number of water specimens at adjacent stations, and as the nature of the food content of each varies in quantity rather than in the character of the organisms, the measure- ments of eight or ten species will apply to all water samples from the locality. Only those organisms need be measured which examinations of the stomach contents of the oysters show to be important as food. The counts have to be FOOD AND FEEDING OF OYSTERS. 1303 made, whatever method be employed. In Matagorda Bay, where the present method was first used, about 150 specimens of water were examined and the additional time required was not over 10 per cent. The following table shows the results and the manner of tabulation, as well as the differences in results attained by the numerical and the volumetric methods: Foop VALUE OF WATERS OF MATAGORDA Bay.@ [Roman figures indicate volume of organisms, or food value. Bold-face figures indicate number of organisms.] A. B. (cy D. E. F. Between | Between Between Between Between Between N é Sand and High Mad Island, Shell Dog Island |Dog Island eo: Species. High Mound. | West, and | Island and} and Shell | and Pa- Mound and Lake Lake Mad Island| Island vilion signals. ‘signals. signals, | reefs. reefs. signal. ai). Coscinodiscuscrassius= => == one bo Pee seaee 917 ‘ 250 100 500 C 121,505 I41I,479 142,90 163,750 I4I, 750 122,500 2 WTS a { 3,483 4.042 4,083 5,250 4,050 3,500 - 3 TO, 500 17,502 17, 760 I2,000 51550 12,000 3 GSE SM aR IES SoS SaaS 1,750 2.917 2,960 2,000 925 2,000 é 2 nx 6,41 , 62 DE OOO | he eee | eee 4 | Navicula didyma__-_--__------------------ 2 378 258 2 875 £5000); | Sacuee ote oe ae 5 elliptical eee ee eee ase apne sorets oan I, 000 5,000 6 Brenatig =e ee ae ae eee { 333 7a Asn plora ovalis=.— = 2 2=2=--