Page:Popular Science Monthly Volume 20.djvu/162

150 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