70 Carrying Capacity, Productivity, and Growth 



crease in standing crops with decreasing average depths (may not be 

 entirely significant), and a significant increase with increasing hardness 

 of the waters. There was also an increase in standing crop with an increase 

 in number of species present (Figure 4.1). 



70 



60 H 



50 



80H 



70 



60-1 



50 



80i 



70 



60 



50 



I I I I I I I I I I 



Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. 



MAXIMUM 



International 

 Falls, Minn. 



GROWING 



SEASONS FOR 

 FISHES 



Madison, Wis. 



11 to 12 months 



J I L 



J I L 



I I 



Figure 4.3. Variation in length of the fish growing season, based upon the 

 observation that growth in warm-water fishes is very slow at temperatures 

 below 55°F. (Records from U.S. Weather Bureau, 5-year average.) 



As stated previously, Moyle ^- was able to show that the standing crops 

 of fishes in Minnesota increased with increases of phosphorus, nitrogen, 

 and total alkalinity. 



Fish of Useful Sizes. In passing judgment on the value of a fish popula- 

 tion for sport angling, it becomes necessary to set up arbitrary standards 

 for fish of useful sizes as opposed to those too small to interest anglers. 

 Immediately, we enter an area of controversy among fishery biologists as 



