828 ANIMALS AND THEIR ENVIRONMENT 



energy fixed by the algae in addition to tlie edible fish and Crustacea 

 harvested from the sea. 



Man could undoubtedly recover for his use much more of the bio- 

 logic productivity of the sea. Although he might be reluctant to eat 

 marine algae himself, they might be filtered from sea water and proc- 

 essed so as to be suitable as feed for cattle or some other gastronomically 

 acceptable animal. Careful studies by the U. S. Fish and Wildlife Service 

 of the fish population of George's Bank and other commercially fished 

 areas have led to recommendations about the rate of fishing and the size 

 of nets used which ensure that the fish are harvested at an optimal size 

 for greatest yield at present and in the fvuure. These areas, which had 

 been fished so extensively that some of the most desirable species were 

 reduced greatly in numbers, are now beginning to revive under careful 

 management. 



The shellfish— oysters, clams, shrimps and lobsters— present some- 

 what different and more difficult problems, for their habitat is more 

 limited than that of commercial fish and they are more affected by ad- 

 verse environmental changes. Oysters, whose food consists of algae or 

 detritus of a certain size filtered from the sea water by their gills, are 

 unable to use algae of a different size. Oysters were unable to survive 

 in certain bays of Long Island Sound when commercial duck-raising 

 was carried out on the adjacent shore. The wastes from the duck farms 

 were washed into the bays and the addition of this organic matter 

 changed the community ecology in such a way that the normal food of 

 the oyster, diatoms, were replaced by other algae which could not be 

 used by the oysters. Once an oyster bed has been seriously depleted it 

 may fail to recover even if seeded with oyster larvae, because the larvae 

 require a favorable surface for attachment and the most favorable is 

 the shell of an old oyster. In commercial oyster farms the larvae are 

 provided with artificial sites for attachment. Once they have become 

 attached they may be moved to other waters, even from one ocean to 

 another, to complete their growth in waters that are favorable for feed- 

 ing although not favorable for the reproduction of the species. 



373. Public Health 



Many aspects of the field of public health require the application 

 of ecologic principles; the prevention of the spread of diseases carried 

 by animals is an ecologic as well as a medical problem. The most 

 effective way of eliminating malaria, for example, is to eliminate the 

 particular species of mosquito which is the vector of the malaria para- 

 site, yet this must be done without destroying the useful insects of the 

 region. The mosquitoes which transmit malaria in different parts of 

 the world have quite different ecologic niches, and therefore measures 

 that may be effective in mosquito control in one region may be quite 

 ineffective in another. The malaria of the southeastern United States is 

 transmitted by mosquitoes living in marshes, Italian malarial mosquitoes 

 live in cool running water in the uplands, and Puerto Rican malarial 

 mosquitoes live in brackish (slightly salty) water. Careful ecologic sur- 



