Aquatic Vegetation and Control Measures 173 



open water in this pond was reduced to 51.2 per cent of the total surface 

 area by a dense stand of P. foliosus and P. nodosus. Tlie fish yield was 

 reduced to 58.1 per cent of the yield taken during the year when aquatic 

 vegetation was hirgely absent, although during the year when the low 

 yield of fish \\ as taken the net fishing intensity was increased 359 per cent 

 and the angling intensity was increased 157 per cent. Swingle ^"^'^ in- 

 vestigated a pond that became filled with a heavy growth of naiad, 

 Najas guadalupensis. He concluded that the rank plant growths did not 

 reduce the hook-and-line yield. Evidence from the study cited above 

 indicated that the fish were actually supported in this pond at a lower 

 poundage than they had been before the dense stand of vegetation 

 developed. 



Table 6.4 Reduced yield of fish (in spite of increased fishing pressure) 

 associated with the spread of dense stands of rooted poxd- 

 WEEDS, Potamogeton foliosus and P. nodosus, in a pond in cen- 

 tral ILLINOIS.'-^ 



Area of Open 

 Water Not Filled Net-Fishing Angling 



Year Yield with Vegetation Intensity Intensity 



Per Cent 



Per Cent Per Cent of 1939 Per Cent 



of 1939 of 1939 Net- Net Man- of 1939 



Pounds Yield Acres Area days Fishing hours Angling 



Algae and rooted vegetation are in competition for available plant 

 nutrient materials and space in an aquatic habitat; when algae are 

 abundant they shade rooted aquatic plants and bind up the plant nutrient 

 materials within their cells. Similarly, rooted plants trap nutrients when 

 they become abundant and hold them from use by algae until the higher 

 plants die and decay and the nutrients are again released. Rooted vegeta- 

 tion often is able to suppress the growth of algae and, because it is 

 longer-lived and more stable than the algae, it tends to persist throughout 

 the growing season. This vegetation may die down in the fall when the 

 water becomes cold, but the release of nutrients in cold weather is of 

 little use to the trophic cycles of the lake or pond. 



Sudden Plant Die-offs. Occasionally progressive plant "die-offs" occur 

 in ponds and lakes. In two instances of plant die-offs that I have observed, 

 the deaths began at specific locations and spread to include all of the 

 rooted vegetation in a pond. One of these occurred in early August of 

 1941 at Fork Lake (Ilfinois).^ Here the vegetation involved was P. foliosus 



