696 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [14] 
it by count, we can estimate the total number of embryos from the weight 
of the entire mass, which is also known. In this manner I estimated 
the number of embryos in each of five full-grown Schleswig-Holstein 
oysters, caught in August, 1869, and found that the average number was 
1,012,955. 
4.—WHY ARE OYSTERS NOT FOUND OVER ALL PORTIONS 
OF THE SEA-FLATS? 
It is now clear that the fruitfulness of the oyster is extraordinarily 
great, and that the extension of oyster-beds over the entire surface of 
the sea-flats does not fail of being accomplished from a lack of young 
oysters, but from other causes. It then becomes our duty to investigate 
into the characteristics of our sea-flats; in order to determine whether 
some portions are more suitable for the growth of oysters than others; 
and whether the saltness, temperature and movement of the water, the 
amount of food which it contains, and the nature of the ground compos- 
ing the oyster-banks, differ in any respect from these same features as 
observed in other places over the bottom. 
The saltness of the upper layers of the water of the open North Sea is 
from 3.47 to 3.50 per cent.* The water of the sea-flats is slightly less 
salt, being only from 3 to 3.3 per cent.t Here upon our sea-flats, and in 
other European coast-seas, where the water is less salt, the oysters ac- 
quire a much finer flavor than upon the ground of the open North Sea,t 
where they live in water 35 meters or more in depth, with a percentage 
of salt of about 3.5. 
That coast-water is, then, the most desirable for oyster-culture which 
*Dr. H. A. Meyer has published a paper concerning the saltness, temperature, and 
currents of the North Sea in the ‘‘ Bericht der Commission zur Untersuchung der deut- 
schen Meere tiber die Expedition zur chemisch.-phys. und biologischen Untersuchung 
der Nordsee, 1872. Berlin, 1875.” (Report of the commission for the investigation of 
the German Ocean upon the expedition for the chemico-physiological and biological 
investigation of the North Sea.) (Specific weight and saltness, page 18.) 
tI have myself repeatedly determined the temperature and saltness of the water dur- 
ing investigations of the oyster-beds of the sea-flats; and since 1872 the commission 
for the investigation of the German Ocean have caused regular stated observations 
to be made, which, since 1874, have appeared under the title ‘‘Ergebnisse der Beo- 
bachtungs-Stationen an den deutschen Kiisten iiber die physik. Eigenschaften der 
Ostsee und Nordsee. Berlin, 1874, 1875, 1876.” (Results of investigations into the 
physical characteristics of the North and East Seas made at observation stations 
along the German coasts.) 
{ Many oysters are taken north of Germany and Holland, east of England, and in 
the channel between England and France. The German fishermen of Blankenese and 
Finkenwirder, near Hamburg, who fish with great dredge-nets for flounders, turbots, 
and soles out from the mouth of the Elbe, often dredge oysters along with their fish- 
The oyster-grounds of the open North Sea lie mostly from 33 to 34 meters beneath the 
surface of the water. They begin with a small stretch to the southeast of the island 
of Heligoland, extend from this island in a west-northwest direction, and form a ter- 
iitory 15 to 22 kilometers broad, which spreads out far to the west. Fishermen from 
Holland and Germany dredge for oysters here, especially during the months of August, 
