160 REPORT OF THE SECRETARY OF COMMERCE 
POLLUTION INVESTIGATIONS 
Field and laboratory studies have been continued over a wide area 
of the country for the purpose of analyzing stream and hatchery 
waters from the standpoint of their suitability for various types of 
fishes and of determining the harmful actions of specific pollutants. 
Methods of determining the physiological condition of the fish them- 
selves have been markedly improved. 
Many of the findings of the past year are of considerable interest 
and importance. For example, it has been found that arsenicals and 
other materials commonly used as mosquito larvacides impair the 
growth and nutrition of fishes even though used in very small quan- 
tities, and may also build up serious hazards of lethal poisoning. Jt 
has also been demonstrated that small quantities of many substances, 
normally present or introduced into streams and lakes, may have a 
cumulative effect over a period of time that is even more detrimental 
to fish life than many more obvious pollutants. Certain minerals 
found in small quantities in various western streams and several in- 
organic salts that are common in southern and western waters exert 
this type of cumulative action. 
Problems arising from the concentration of minerals and other com- 
pounds hazardous to fish and other aquatic life in impounded waters 
have been investigated at Elephant Butte Reservoir, N. Mex., and 
Lake Mead, Nev. ; 
Acute pollution problems were investigated during the year in Flor- 
ida, South Carolina, North Carolina, Minnesota, Idaho, Montana, 
Oregon, and Mississippi. Specific types of industrial pollution 
studied include that produced by paper mills, phosphate mines, and 
copper, lead, zinc, and placer gold mining operations. 
CONSTRUCTION OF FISH-PROTECTIVE WORKS 
A section on hydraulics has now been established under the super- 
vision of the Division of Scientific Inquiry, with an experienced engi- 
neer for the design of fish-protection facilities. During the year the 
engineer assisted in designing and supervising the installation of fish 
screens being constructed with the aid of P. W. A. and W. P. A. funds 
on certain Federal power and irrigation projects in the Northwest. 
Important consulting services on screens and ladders were also 
afforded to other agencies. 
SHELLFISH INVESTIGATIONS 
Scientific investigations have been directed toward increasing the 
cultivation of oysters, improving the quality of oyster meat, and 
standardizing the raw and canned product. 
In the Long Island Sound area, where the collection of a good crop 
of seed oysters is of paramount importance to the industry, regular 
bulletins were issued during both the 1938 and 1939 seasons advising 
oystermen when spawning and setting might be expected. To aid in 
protecting valuable beds from starfish, surveys of the distribution of 
this oyster enemy were made and the results communicated to oyster- 
men, permitting more efficient eradication. 
