Science in Conservation During War Times 61 
The most important requirement for continuation of a spe- 
cies is a surrounding in which it can reproduce. For the many 
species of fish this requirement varies so widely that no general 
description of suitable spawning grounds can be given; however, 
most of the species will spawn on riffles, in vegetation, in cav- 
ities, or on cleaned gravel areas. If a continuation of the species 
is desired, suitable nest locations must be available and the breed- 
ing adults must be protected at the time of nest building, spawn- 
ing, and nest guarding. Eggs will smother if silt is allowed to 
settle on them, and this often happens when silt-laden waters 
cover spawning areas following a sudden downpour in an eroded 
watershed. * 
After the eggs have hatched and fry have emerged from the 
nest they are attractive morsels for slightly larger fishes and other 
animals. If, then, these fry are to survive, shelter is a "must," 
and the cover which serves the requirements of adult fishes can- 
not serve also the fry. Finer-texture cover is needed for small 
fish — weeds in shallow water providing optimum conditions, for 
here in shallow water the small fish are in their feeding area, the 
large fish avoid it, and the plants attract food. Such cover is 
necessary for young fish until they mature enough to compete 
with the larger fish in the main stream. 
The cardinal necessities for fish production may or may not 
be present in proper proportions to constitute a suitable habitat 
for fish. 
M 
miles of water, the only way in which a full set of scientific facts 
can be accumulated is to study each stream in a standard way 
and the gathering of scientific data on a stream or body of water 
by trained personnel using standard methods and unbiased judg- 
ment constitutes an aquatic survey. This is necessary if facts 
relative to all waters are desired, for though each stream is well 
known to some individual, there is no one familiar with all 
waters. Furthermore, observations made by several individuals 
may vary widely and may be biased, with the result that not 
facts, but opinions suiting the fancy of the individual are obtained. 
Actual measurement of pool depth in relation to' its length 
and volume, along with the pool-riffle ratio, gives a fair index of 
the living room aflforded fishes in a stream. Chemical analyses 
of the waters show the compounds present as well as their con- 
centration. Qualitative and quantitative studies made on the bot- 
tom fauna available for fish food, added to the free swimming 
forage forms such as minnows, Crustacea and insect larvae, give 
