Limitation by Nutrients in Nature 291 



In fertilized Loch Craiglin plaice accomplished 2 years' normal 

 growth in 1 year (Fig. 8.3), and flounders accomplished 5 to 6 years' 

 growth in less than 2 years. 



Growth of phytoplankton resulting from the fertilization of ponds 

 may produce further efi^ects of ecological importance. Dense 

 "blooms" automatically cause the disappearance of benthic plants by 



Apr. June Aug. Oct. Dec. Feb. Apr. June Aug. Oct. Dec. Feb. Apr. 



Age 1 year 2 years 



Fig. 8.3. Diagram showing the greater growth of the flounder, Pleuronectes flesus, 



in Loch Craiglin (upper curve) after fertilizer was added compared with normal 



growth in an unfertilized loch {lower curve). (Data from Gross, 1947.) 



cutting off the light from the bottom. Bottom vegetation is generally 

 undesirable in farm ponds that contain both prey and predator species 

 of fish because its presence allows too many small fish to escape 

 capture by the large fish. On the other hand, the growth of the 

 planktonic algae must not be allowed to proceed to such a point dur- 

 ing the summer that the decomposition of the algal material after the 

 growing season will exhaust the oxygen supply and kill the fish during 

 the winter. For this reason the addition of fertilizers must be cur- 

 tailed in northern regions where the winter is long. Carefully con- 

 trolled experiments carried out in relation to practical fish farming 

 have provided valuable quantitative data on nutritional and other 

 ecological relationships involved in the aquatic environment. At 

 the same time the failure of hasty or ill-conceived fish farm practices 

 form an admirable illustration of the necessity for a thorough under- 

 standing of ecological principles as a background for successful con- 

 servation or cultivation of biological resources. 



