84 Carrying Capacity, Productivity, and Growth 



constant weight throughout a growing season of 8 or 9 months if it were 

 hving under conditions that furnished no more than a maintenance diet. 

 There are aspects of a short growing season that may have an important 

 bearing on management practices. In some Michigan waters where the 

 length of the growing season does not exceed 5 months and ice and snow 

 cover the lakes for 4 or 5 months, bluegills seldom mature sexually until 

 the third summer of life.'^ This means that all phases of management that 

 bear any relationship to time of sexual maturity of the fishes must be 

 adjusted accordingly. 



Viosca ^- described the growth of warm-water fishes— largemouth bass, 

 spotted bass, and crappies— in the International Paper Company reservoir 

 at Springhill, Louisiana, where the growth period for these fishes is 

 limited by draining and refilling operations rather than the length of the 

 natural growing season. The lake basin (270 acres) is pumped full in 

 April and May, and newly-hatched fry are brought in with the water. 

 The lake is drained in September or October to make room for waste 

 water from the paper mill so the growing season for fry of various fishes 

 ranges from 5.5 to 6.5 months. Fish that enter the reservoir through the 

 pumps as fry in April and May produce a "field day" for anglers by late 

 summer. In 1949, largemouth bass (age group 0) ranged from 11.1 to 

 13.8 inches and 11.4 to 24.5 ounces; spotted bass ( 10 times as numerous 

 as largemouth) ranged from 6.1 to 10.6 inches and 1.4 to 11.6 ounces. 

 In 1950, crappies reached sizes as large as 9.8 inches and weights of 12 

 ounces in 6.5 months. These crappies averaged a weight increment of 

 1.8 ounces per month, which is very near their maximum rate of growth. 

 The accumulation of nutrients from the decayed paper waste added to the 

 productivity of this reservoir. 



There is evidence, both from laboratory feeding experiments and field 

 studies,^- that fish supplied with quantities of one or several live foods 

 alternate between periods (weeks) of heavy feeding with rapid growth 

 and periods of little or no food consumption with subsequent growth 

 stoppage. Fish, like other animals, find a heavy diet without variety 

 unattractive. 



In the laboratory experiments cited above, bluegills that stopped feed- 

 ing were being fed earthworms at the rate of 7 to 8 per cent of their 

 body weight per day. However, other bluegills receiving earthworms in 

 smaller quantity (3 to 5 per cent of body weight per day) did not stop 

 feeding. 



A continuous study of the ecology and growth of one or several species 

 making up a small fish population will show that there are times when 

 certain types of food are abnormally abundant and that this abundance 

 is often reflected in unusual growth. For example, in August, 1941, a 

 sudden die-oflF of heavy growths of submerged aquatic plants in a pond 



