62 Carrying Capacity, Productivity, and Growth 



were drained two years later, these fish still averaged about an ounce 

 each and the ponds' populations still weighed about 300 pounds per acre. 



In another series of experiments,"^"' three ponds were stocked with 1300, 

 3200, and 6500 bluegill fry per acre. When these ponds were drained in 

 November of the same year, the fish in the first pond averaged 4 ounces, 

 those in the second slightly less than 2 ounces, and those in the third 

 approximately one ounce. The total weight produced was approximately 

 300 pounds per acre in each of the three ponds. 



In 1939 these same authors ( Swingle and Smith •'^' ) published the re- 

 sults of other experiments having to do with the carrying capacity of 

 waters : 



"In May 1936, one pond was stocked with bluegill bream fry at the 

 rate of 26,000 per acre, weighing 2 pounds 5 ounces. Another pond was 

 stocked with year-old fingerlings at the rate of 13,000 per acre, weighing 

 180 pounds. . . . When drained the following November, the former pond 

 produced at the rate of 105 pounds of fish per acre and the latter at the 

 rate of 92 pounds per acre. One pond gained 103 pounds per acre, while 

 the other lost 88 pounds per acre due to overstocking." 



In a second experiment. Swingle and Smith used a pond of 1.8 acres 

 over a period of 2 years. In the spring of the first year (1935), they 

 stocked 4485 fish of eight common pond species, weighing 40 pounds, 

 9 ounces. At the end of that year they collected 22,069 fish weighing 293 

 pounds, and 4 ounces. Early in the spring of 1936 they stocked 236 fish 

 of the same species in this pond, weighing 24 pounds, and 7 ounces. At 

 the end of 1936 they collected 30,405 fish weighing 296 pounds, and 2 

 ounces. 



In these experiments the variation in total numbers and weights of 

 fish stocked seemed to have little effect upon the total weights of fish 

 found in the ponds at the end of one growing season. Rather the total 

 weights of these populations were adjusted upward or downward until 

 they approached a rather uniform level for individual ponds, probably 

 associated with the food production capacities of these ponds. 



Factors Affecting Poundage 



We have seen that the poundage of fish supported by a pond or lake of 

 constant size may remain fairlv constant, in spite of the numbers of 

 fish stocked. This is true only within limits; and the carrying capacity of a 

 lake or pond for fish may vary (1) with variations in the fertility of 

 water, (2) with age of water if age represents change in chemical com- 

 position, (3) with fertility of watershed soil if a change brought about 

 through erosion or artificial fertilization is carried to the pond in runoff 

 water, (4) with changes in the kinds of fishes, or in the relative abundance 

 of certain kinds and sizes of fishes. 



