144 THE WEALTH OF THE SEA 



dant species of fishes. Some of our waters have be- 

 come so strongly polluted that adult fish, even, are 

 unable to survive at or near the source of contami- 

 nation. It is highly improbable that young fish, or 

 the minute organisms upon which they feed, could 

 live in such waters. It is evident, then, that a study 

 of the effects of wastes thrown into the waters must 

 go far beyond a test of the toxic effects they have 

 upon adult fishes. It is also clear that the absence of 

 dead fish does not necessarily indicate an absence 

 of pollutions harmful to the fisheries. It is evident 

 furthermore that pollutions may affect our fisheries 

 to a far greater extent than is generally known. 



Pollution and a consequent shortage of food is 

 not, however, the sole cause for the decline of some 

 of our most important fisheries. Over-fishing, fishing 

 during the spawning season, reckless destruction of 

 young and undersized fish by certain types of fish- 

 ing gear, all contribute toward the same end. The 

 clearing of forests and drainage • for agricultural 

 purposes, too, are contributing causes. Dams and 

 other obstructions which prevent fish from reaching 

 their spawning grounds are not to be overlooked as 

 important causes for the decline, for* such obstruc- 

 tions are especially detrimental to the perpetuation 

 of the species which come from the sea and asce'nd 

 fresh-water streams to spawn. The salmons, shad, 

 alewives, striped bass, and sturgeon, all included in 

 this class, are of first-rate importance among food 

 fishes. Interference with the migrations of these 

 species to their spawning grounds, and pollutions 

 preventing the hatching or the development of the 



