72 Carrying Capacity, Productivity, and Growth 



with 1500 bluegill fry weighing a total of 1.0 pound and in November 

 when the pond was drained 200 pounds of bluegills were collected, the 

 fish production for that season would be 199 pounds plus as additional 

 poundage of fish that had been lost through natural causes during the 

 season. In addition, flesh added to fish that later died and decayed or 

 were eaten by predators, must be included in a total production estimate. 

 Thus, if this same pond were restocked in March of the next season with 

 203 pounds of bluegills and drained in November, with a population 

 weighing 204 pounds, the production the second year would be 1 pound 

 plus a poundage lost as above. 



The term "production," therefore, is more definitely associated with 

 yield than with standing crop, although it is never entirely synonymous 

 with yield unless one is willing to assume that, for a given period, there 

 has been a complete replacement of fish flesh equal to that removed 

 through predation and "natural" deaths as well as through human pre- 

 dation ( fishing, netting, spearing -^ ) . According to Carlander,^^ "Since the 

 annual rate of turnover probably varies less from one fish population to 

 another than does the standing crop, standing-crop data are probably 

 fairly good estimates of fish production." However, Clark ^^ states that, 

 "the magnitude of the standing crop at any moment does not give a 

 measure of the rate at which production is going on since it (standing 

 crop) is determined by the difference between the rates of production 

 and destruction over the whole previous history of the population up to 

 the time considered." (material in parentheses mine) 



Food Conversion 



Various kinds of fish are able to convert foods (of several kinds 

 palatable to them) into flesh at various rates. Maximum efficiency in food 

 conversion is attained when food is available for consumption at a rate 

 between maintenance requirements and the maximum a fish is able to 

 eat in a specified period. When food is scarce, a fish may expend too 

 much energy in finding the food and, therefore, be unable to approach 

 maximum efficiency. Where food is super-abundant, the fish may consume 

 more than it can digest and assimilate so that a loss of efficiency results. 



Thompson ^^ stated that at 70°F 2.5 pounds of minnows are required to 

 produce one pound of bass. When larger amounts were fed to the bass, 

 the food was used less efficiently and conversion values were 3.8 for 

 largemouth bass and 4.5 for smallmouth bass.^^ 



Markus ^^ has shown that the rate of digestion in largemouth bass is 

 very slow at temperatures below 65°F, but that it increases rapidly be- 

 tween 65° and 90°F. Tliompson ^^ used Markus' temperature-digestion 

 rate curve along with mean monthly temperatures at each of seven dif- 

 ferent localities within the range of largemouth bass in the United States 



