172 Theories and Techniques of Management 



There are several genera of filamentous algae that grow luxuriantly on 

 the pond surface to form a thick blanket that may almost completely 

 cover the pond. Lawrence ^^ lists Pithophora as a nuisance form for this 

 reason in the southeastern states, and Hansen el al.^- describe a pond in 

 southern Illinois that was nearly always partly covered with a floating 

 layer of Rhizoclonium. Hatchery personnel in northern states are some- 

 times bothered with Htjdrodictyon, an alga in which the elongated cells 

 are arranged in the form of a net with 6-sided mesh. Small fish become 

 entangled in these algal nets and die because they are unable to escape. 



Some algae that grow on rocks and submerged concrete are dangerous 

 to bathers and wading fishermen because they create slippery footing and 

 often cause waders to fall. One type of Spirogijra with very coarse fila- 

 ments is notoriously slippery, and I once saw a bather slip and sit down 

 at the top of a steep, spirogyra-covered lake spillway and slide entirely 

 to the bottom before he could stop. Needless to say, he repeated the act 

 until the algae as well as the seat of his bathing suit was practically gone. 



Control of Algae. Algae are very sensitive to copper and for many 

 years crude copper sulfate crystals dissolved in water and sprayed on 

 algae or dragged in a sack behind a boat has been a standard method of 

 algae control. In soft water, 1 ppm or less was toxic to algae, but when 

 used in hard water the copper ions united with carbonate in the water 

 to form an insoluble precipitate that was useless in killing algae. Thus, 

 it was necessary to use a much stronger dosage ( 5 to 12 ppm ) in order to 

 control algae. At dosages higher than about 12 ppm the copper became 

 toxic to fish. Because the hardness of water varies a great deal it is difficult 

 or impossible to define a dosage, and only trials will allow one to discover 

 the amount needed for an effective treatment for a specific water. 



Copper citrate is sometimes used in algae control work. This copper 

 compound is much more expensive than copper sulfate, but a dosage of 

 0.5 to 1 ppm is usually sufficient to kill algae. Copper citrate is also more 

 toxic to fish than is copper sulfate. 



CMU [3-(p-chlorophenyl)-l, 1-dimethylurea] has been recommended 

 as a deterrent to algal growth after an established bloom has been killed 

 by other chemicals.^^' ''^ 



Loss OF Fish Production Through Rooted Vegetation 



There is some evidence that dense stands of submersed rooted aquatic 

 plants may bind up nutrient materials throughout the growing season,^ so 

 that they are not available for the production of phytoplankton and the 

 organisms that feed upon phytoplankton. This, in turn, may be reflected 

 upon the fish through an eventual reduction of their food supply. 



An apparent relationship between fish yields and increasing stands of 

 Potamogeton foliosus and P. nodosus is shown in Table 6.4.^ The area of 



