and dcpciuls in part on the deinh and IcniptTatiirc of 

 their habitats (Miller 1941). 



The tiibificid 2i-oniis do not leave the lake k)ttoni. 

 Many of tiieni occnr in tubes or cases, similar to the 

 habit of midge fly larvae. It is evident by the pres- 

 ence of sexually mature adults and the reproductive 

 cocoons that reproduction occurs principally during 

 the periods of autumn and, especially, spring over- 

 turns. There follows a large increase in numbers of 

 small immature worms (I"!ggleton 1931 ). 



It is obviously impossible to include a descrip- 

 tion of the life history of all sjiecies. The cisco or 

 lake herring has been selected to illustrate the life- 

 history of a lake fish (Cahn 1927, Fry 1937). 



The cisco spends much of the year in deep water, 

 feeding very largely on plankton, strained out by 

 specialized gill rakers. Because its food habits require 

 a large volume of water to be passed through its gill- 

 rakers, the fish swim almost constantly, usually in a 

 constant and definite direction, in schools of from 

 twenty to several hundred individuals. During hot 

 summers the cisco may leave the cool, deep, but oxy- 

 gen-poor carbon dioxide-rich waters and ascend into 

 the epilimnion. There they are sometimes killed in 

 large numbers by temperatures higher than they are 

 able to tolerate. The fish spawn in November or De- 

 cember, when the water temperature drops to 4°C. 

 For this purpose the fish move into water only one to 

 three meters deep, or even up into rivers. The males 

 precede the females by two to five days. When the 

 females arrive, several males consort with each. 

 When she is ready to spawn, the female descends to 

 within 20 cm of the bottom and sheds about 15,000 

 eggs. At the same time, the accompanying males dis- 

 charge sperm, and fertilization is completed. The 

 eggs are viscous and become attached to rocks or 

 bottom debris. Xo nest is made and no further atten- 

 tion is paid to the eggs. Incubation may last 10 to 12 

 weeks: hatching normally occurs in late March. The 

 young fish later return to deep water and reach breed- 

 ing condition in three years. Doubtless the slow rate 

 of development in this species is related to the low 

 temperature of the habitat. After spawning is com- 

 pleted, the adults may remain in shallow water until 

 water temperatures reacli 20°C. This temperature is 

 above their preferendum, although they can tolerate 

 temperatures up to at least 25 °C. 



APPLIED ECOLOGY 



Applied ecology involves the management 

 of lakes and the control of their resources for man's 

 benefit. Aside from their use in transportation, in 

 industry, and as sources of drinking water, lakes are 

 of importance to man for fishing, swimming, sight- 

 seeing, and boating. For swimming, clean, clear 



water with a sand bottom is desirable. Sewage and 

 industrial wastes must be diverted or eliminated for 

 reasons of health and the ajjpearance of the water. 

 .\lgal growth, when excessive, can be controlled with 

 cop|)er suljihate ; and rooted vegetation can be re- 

 duced by sodium arsenite treatment. When chemical 

 treatment of water is limited to low concentrations 

 .administered with discretion, there is generally no 

 great harm to fish ; some invertebrates, such as midge 

 tly larvae, mayfly naiads, and fresh-water shrinij), are 

 adversely affected (Machenthun 1958). 



\\'here there is excessive erosion of the surround- 

 ing upland, silting may render the waters of small 

 lakes turbid, decreasing the growth of algae, a basic 

 food substance for lake organisms. The rapid ac- 

 cumulation of silt on lake bottoms covers up bottom 

 organisms, clogs the gills of mollusks, and generally 

 reduces the lake's productivity. The obvious remedy 

 is the control of erosion at the source. 



The management of large lakes to the end of in- 

 creasing fish productivity is difficult because of the 

 area and depth of water involved. Where commercial 

 fishing is commonly practiced in large lakes, a care- 

 ful yearly catch record for each species should be 

 maintained. This will suggest regulations such that 

 annual cropping will not exceed annual production. 

 To improve fishing and increase productivity there 

 must be an increase in a lake's carrying capacity. 

 Carrying capacity depends on maintenance of good 

 chemical and physical characteristics of the water, an 

 abundance of food, plenty of breeding areas, and ex- 

 clusion of exotic predators. The drastic decline in the 

 annual yield of lake trout in the Great Lakes is at- 

 tributed to invasion by the predaceous sea lamprey. 

 Artificial fertilization of lakes presents greater prob- 

 lems than it does for ponds, but may eventually prove 

 practicable (Hasler and Einsele 1948). 



Pollution is usually a local problem in large lakes. 

 A moderate pollution of organic wastes may in fact 

 fertilize a lake and produce an increase in the plank- 

 ton and bottom organisms which serve as fish food. 

 Excessive pollution, however, must be controlled, as 

 it interferes with the use of the lake for recreation 

 and as a water supply. 



The smaller the lake the more practicable becomes 

 management of the habitat. The water level may be 

 manipulated, by damming, to increase the area of 

 shallow water available for spawning at certain sea- 

 sons, or lowered at other times to permit growth of 

 marginal vegetation or prevent spawning of unde- 

 sirable species. Artificial shelters or spawning areas 

 may sometimes be created, yielding a significant im- 

 provement (Hubbs and Eschmeyer 1938). In gen- 

 eral, rearing small fish in hatcheries for later release 

 has not proven economically practicable. Any pro- 

 posed introduction of exotic species should be investi- 

 gated with considerable skepticism. 



Lakes 77 



