146 
BULLETIN OF THE BUREAU OF FISHERIES 
There are two aspects in the study of mortality problems in relation to pollution 
of water. One is to determine the cause of the mortality, that occurred some time 
ago, the second is to find out how the pollutant agent may affect marine life and what 
are the expectations of the fishing industry if the pollution is permitted to continue. 
From the point of view of conservation, the second problem is of far greater impor- 
tance, while the parties involved in the controversy regarding the causes of mortality 
are primarily interested in the first one. Although the present report fails to give a 
defin i te answer to the first question, it supplies sufficient data regarding the possible 
dangers of oil pollution to oysters. 
One must bear in mind that actual conditions in the sea may be much more 
complex than they appear to a casual observer. There is a possibility, for instance, 
that because of unfavorable meteorological conditions, attacks of parasites, or other 
unknown factors, oysters already have become weakened or diseased. In that case 
an additional adverse factor, as for instance a small concentration of a toxic sub- 
stance in the water, may have been responsible for the mortality, although under more 
favorable circumstances the oysters might have been strong enough to withstand it. 
Being ignorant of the past history of the case and not being able to duplicate condi- 
tions that existed at the time of greatest mortality, the biologist is unable to determine 
with certainty the cause of it. He can provide, however, sufficient evidence regarding 
the toxicity of the suspected pollutant, the manner in which it affects the organism, 
its fate in the ocean, and from all this information provide a substantial basis for 
outlining the methods and policies of future conservation. 
These considerations and the lack of funds to carry out field observations on the 
large scale contemplated in the original project, made it necessary for the authors to 
concentrate their attention on the experimental studies of the effect of oil on the 
behavior of the oyster and on the growth of diatoms, which constitute the principal 
part of the oyster diet. 
It was thought desirable, however, to present first the preliminary field observa- 
tions made by H. F. Prytherch in 1933 and the result of the survey made by R. 0. 
Smith in March and April 1934. 
PRELIMINARY FIELD INVESTIGATIONS, 1933 
By H. F. Prytherch 
Terrebonne Parish includes practically the whole oyster-producing region be- 
tween Barataria Bay and the Atchafaiaya River and is the westernmost section in 
which good oysters are obtained in considerable quantities. According to the 
Bureau’s fisheries statistics for 1930 (Fiedler, 1932), this parish exceeded all others 
in oyster production and furnished over 360 thousand bushels, valued at $156,213, 
or approximately 30 percent of the total State crop. Virtually all of the oysters 
produced here come from private beds, leased from the State, most of which formerly 
were natural beds that had become depleted through overfishing and the destruction 
of small seed (spat) by natural enemies. Through the leasing and cultivation of 
these areas production has been maintained and a better grade of oysters produced. 
The usual procedure is to stock the leased areas with seed obtained from natural 
beds in other coastal sections such as Sister Lake, Bay Junop, Barataria Bay, etc. 
The seed are planted quite densely at reported concentrations of 700 to 900 barrels 
per acre which does not allow a great deal of room for subsequent growth and increase 
in volume. This practice, however, has been generally successful for many years and 
is apparently caused by the fact that the oyster beds are elevated to some extent 
