76 U. S. BUREAU OF FISHERIES 
quantities, are often more insidious and far reaching in their effects 
on fish life than other more obvious pollutants which kill fish at 
once. Collectively, therefore, the pollution investigations determine 
the suitability of particular waters, both in the stream and in the 
hatchery, for various types of fishes, the detrimental actions of spe- 
cific substances, and reliable means for measuring the condition of the 
fish themselves. 
This year’s work has shown, among other things, that very small 
quantities of arsenicals and several other materials introduced into 
or placed on waters as mosquito larvacides, impair the growth and 
nutrition of fishes in these waters in addition to building up serious 
hazards of lethal poisoning; that certain combinations of some of 
the less common minerals which are found in small quantities in 
various western streams are responsible for internal injuries to trout 
and other fishes in these waters, and that these internal conditions 
are correlated with decline in weight and finally with the death of the 
fish in such streams; and that small quantities of several inorganic 
salts quite widespread in southern and western waters are the cause 
of low productivity and on occasion of the sudden death of large 
numbers of fishes in fish hatcheries, streams, and reservoirs receiving 
such waters. 
Because of these findings, and other similar studies in progress, 
the listing of streams throughout the United States which can be 
designated as unsuitable for fish plantings, .or in which low produc- 
tivity may be expected because of these biochemical factors, is in 
progress. 
As impounded waters are particularly subject to concentration of 
the various minerals and other compounds which constitute natural 
hazards to fish and other aquatic life, the detailed chemical and bio- 
logical studies which have been in progress for some time at Elephant 
Butte Reservoir, N. M., and Lake Mead, Nev., have been continued 
throughout the past year. 
Acute pollution problems continue to claim considerable attention, 
and specific cases of paper-mill pollution have been studied in Flo- 
rida, South Carolina, North Carolina, and Minnesota; of phosphate- 
mine pollution in Florida; of copper, lead, and zinc mining opera- 
tions in Idaho, and Montana; of combined industrial pollution in 
New York and Mississippi; and of placer gold mining in Oregon. 
Reports on these investigations have been submitted or are now in 
preparation. This unit has continued to advise both public and pri- 
vate agencies on matters of pollution and water conditions, and many 
State officials, various industrial concerns, and numerous private in- 
dividuals have been given specific answers to particular problems 
throughout the year. 
ICHTHYOLOGICAL INVESTIGATIONS 
Fishes of the Canal Zone and Panama.—The study of specimens 
and data collected in 1935 and 1937 was continued by Dr. Samuel F. 
Hildebrand. The study of the fresh-water material was completed, 
and the results have been embodied in a publication entitled “A New 
Catalogue of the Fresh-Water Fishes of Panama.” In this catalogue 
127 species are listed, including 9 new ones. The origin and distri- 
