46 U. S. BUREAU OF FISHERIES 
were examined in September of the same year and found to be mar- 
ketable. The meats were fat and filled the shell cavity and the shells 
were thickened by a deposit of white chalky nacre. Another planter 
replanted poor oysters from beds farther up the York River to the 
lower river and noted marked improvement. These examples offer 
conclusive proof that the upper York River environment contains 
harmful elements that are not found in the lower river and waters 
removed from the York. 
Samples of oysters have been taken from various parts of the York 
River and from several places on the Piankatank River and examined 
by Dr. Galtsoff for the presence of the sporozoan parasite known to 
infest oyster tissues. The parasite was found in oysters from locali- 
ties where conditions were good as well as from localities where they 
were poor, showing that infection with this sporozoan is not respon- 
sible for the poor quality of the upper York River oysters. 
The relative spawning activities of the York and Piankatank River 
oysters can be judged by the abundance of free-swimming larvae and 
amount of setting when conditions are favorable. Examination of net 
plankton samples taken throughout the summer of 1936 disclosed zones 
of abundance or scarcity of the swimming stages. In general it can 
be said that the abundance decreases with the distance from the mouth 
of the river toward the head. In the lower York larvae were plenti- 
ful from the time the 1936 studies began in June until they ended in 
late October. The figures taken from counts of the net plankton 
samples arranged chronologically show two peaks of spawning, one in 
early July and the other in late September. The samples from the 
upper part of the river show a decidedly low count. The maximum of 
the upper-river spawning occurred during the last part of August and 
extended through the first week of September. An abrupt drop in 
average number of larvae per sample appeared in the vicinity of 
Purtan Bay and above. In the control river, the Piankatank, the 
same condition prevailed, spawning being heavier in the lower part 
of the river than in the upper. However, the differences between the 
upper and lower sections of the river were less marked than in the 
York River. 
The setting of larvae followed the same line as the spawning figures 
with the lower part of the river receiving the heaviest set. There 
was an exception, however, in the York when a rather heavy set was 
recorded for the areas near and inside Purtan Bay. The set in the 
York River, except at station 20, was less than that noticed in the 
Piankatank River. The set was of commercial magnitude at several 
stations in the lower York and the lower-river station in the 
Piankatank. 
A very complete program has been followed in obtaining a picture 
of the hydrographical conditions of the York and Piankatank Rivers 
through 1936. The examination of the water consisted of observations 
on temperature, salinity, pH, dissolved oxygen, phosphorous, turbid- 
ity, and net- and nanno-plankton content. Tidal cycles at Yorktown 
and at West Point were studied and the isohalines of the York and 
Piankatank Rivers were determined. 
The percent saturation of the water with oxygen was found to be 
consistently low in the vicinity of West Point. A period of low 
oxygen occurred during the latter part of July at. which time many 
