MANUAL OF FISH-CULTURE. 145 



should be discolored enough to prevent the fish from seeing the nets, 

 but not thick, say from 10 to 20.* An occasional freshet reduces the 

 temperature and prolongs the season ; however, with an equal number 

 of fish in the rivers, clear water is probably more advantageous for 

 natural increase, as a large i)roportion of naturally deposited eggs must 

 perish from suffocation under the mud in seasons of freshet. 



THE WEATHER AND SPAWN. 



The development of eggs within the ovaries is hastened by heat and 

 retarded by cold. In a warm season fish ready to spawn are more 

 numerous early in the season than in a cold one, and the period for 

 obtaining them is apt to close earlier. The eggs, not only after they 

 are deposited and impregnated, but before they leave the body of the fish, 

 are affected by the temperature of the water, often being "blighted" 

 or "rotten riiie." This phenomenon was observed as far back as 1873. 

 It occurs on the water reaching 80° to 81°, or with a rapid rise. On 

 the other hand, a sudden fall in temperature has been observed to 

 arrest natural spawning, produce blighted egg^, and to destroy those 

 in the hatching- vessels. Continued low temperature is also disastrous 

 to fishing. 



An abnormally inferior quality of the Potomac Eiver eggs was noticed 

 during the full period of operations in 1800. The bulk of the run of shad 

 made their appearance on a rapidly ascending temperature, and the 

 eggs were injured within the parent fish, more than half perishing 

 before conversion into fry. The rise in temperature was greater than 

 had been recorded in the eleven years preceding. The run of shad 

 increased proportionately, the catch at one seine increasing from 100 

 to 800 in 21 hours. A snowstorm on April 7 — morning air temperature 

 35° F. and mean air temperature 46° — was followed by heavy frost on 

 April 9, the morning air temperature on the last-named date being 34°. 

 The river water on April 10 was 40°, rising to 48° on A])ril 12 and to 

 71° in the afternoon of April 21, thus gaining 25° in 10 days. After 

 April 21 the catch of shad fell otf to such an extent that fishing was 

 no longer profitable. 



The water of the Potomac early in March is usually of a temperature 

 of 36° to 40°, rising to 52° to 58° about the middle of April, when the 

 spawning period begins, and at the end of May, the close of the i)eriod, 

 it averages from 65° to 70°. 



STRIPPING AND FERTILIZING THE EGGS. 



In stripping the eggs the shad is lifted with the right hand and 

 caught above the tail with the left. All slime and loose scales are, 

 removed l)y going over the fish two or three times in quick succession 

 with the right hand. The head is carried to the left side under the 



*The condition that permits the discernment of objects at a distance of 10 to 20 

 inches beneath the water surface, the method of registration employed by the Waehr 

 ington (D. C.) a»iueduct oUice. 



F. C. R. 1897 10 



