PROGRESS IN BIOLOGICAL INQUIRIES, 1926 541 
Another line of investigation concerns the use of fertilizers as a 
means of increasing the basic food supply. 
In rearing bass and other predaceous fishes, the use of forage 
fishes affords a promising line of investigation. The introduction 
of forage fishes is designed to furnish a supply of small fish that 
will serve as food for the bass. This not only will tend to prevent 
cannibalism, but as these fishes feed on plankton and plant materials 
will greatly increase the amount of food available to the bass. 
During the past season three species of forage fishes—viz, goldfish, 
golden shiners (Notemigonus crysoleucas), and black-head minnows 
(Pimephales promelas)—were introduced into the bass ponds with 
excellent results. The black-head minnow promises to be especially 
desirable for the purpose, as it multiplies rapidly, continuing to 
spawn throughout the spring and early summer, so that a supply of 
small fish is always available. Furthermore, they are primarily 
bottom feeders, and consequently there is little direct competition 
with the young bass for food. Obviously, the use of forage fishes 
introduces a number of problems that are in urgent need of investi- 
gation. These include such questions as what is the most desirable 
species of forage fish to use with different game fishes, the advisa- 
bility of introducing two or more species of these fish in the same 
pond, and the proper methods of handling them to obtain maximum 
results. 
Other problems in pond culture that are being investigated relate 
to the number of brood fish required per unit area; the total yield 
of fish, both as to numbers and weight, that can reasonably be 
expected; and the advisability of rearing two or more species of 
food fishes in the same pond. 
As in the case of trout an attempt is being made to develop 
superior strains of pondfish by selective breeding, special emphasis 
being placed on rapid growth. 
PATHOLOGY OF FISHES 
As in previous years, special attention has been paid by the patholo- 
gist, Dr. H. S. Davis, to a study of the diseases that are causing 
serious losses at the trout hatcheries. ‘The Holden experimental 
hatchery affords exceptionally favorable conditions for such investi- 
gations, for here the fish are under close observation at all times and 
any outbreak of disease can be discovered in its early stages. During 
the summer of 1926, in addition to octomitiasis, which is now well 
under control, two infectious diseases appeared among the fingerling 
trout and caused considerable losses before control measures could be 
developed. 
The most serious losses were caused by an infection of the gills, 
which had not previously been recognized. The disease appeared 
during June and July among fingerlings of brook, rainbow, steel- 
head, and black-spotted trout, and also among some small landlocked- 
salmon fingerlings. It is due to an infection with bacteria, which 
form a luxuriant growth over the surface of the gills. They occur 
as long, threadlike filaments, which usually lie side by side so as to 
form a more or less continuous layer over the surface of the epi- 
thelium. The bacteria are colorless, transparent, and very difficult 
to distinguish, even under high magnification. They are most 
