REPORT OF COMMISSIONER OF FISH AND FISHERIES. XXI 



as to the causes which have led to this result. These, as commonly 

 given, are principally the following : 



1. The decrease or disappearance of the food upon which the fish sub- 

 sist, necessitating their departure to other localities. 



2. A change of location, either entirely capricious or induced by the 

 necessity of looking for food elsewhere, as just referred to. 



3. Epidemic diseases, or peculiar atmospheric agencies, such as he£t, 

 cold, &c. 



4. Destruction by other fishes. 



5. The agency of man ; this being manifested either in the pollution 

 of the water by the discharge into it of the refuse of manufactories, 

 &c, or by excessive overfishing, or the use of improper apparatus. 



These we will now proceed to discuss briefly in their order, beginning 

 with, first, disappearance of the food. 



To this subject special attention was given in the course of the inves- 

 tigations of 1871 and 1872, as the suggestion was quite plausible, and 

 by many was believed to be of great weight. The dredging operations 

 under Professors Yerrill and Smith, were admirably calculated to test 

 this question, as the sea-bottom was raked in every direction by the 

 dredges ; and the towing and drifting nets revealed the extent and 

 comparative abundance of animal life in the surface-water or throughout 

 its depths. 



Fortunately for the proper solution of this question, an extensive 

 series of dredging operations had been conducted by myself in the 

 waters in the vicinity of Wood's Hole, as long ago as 18G3, when the 

 diminution in the abundance of the fishes had not made itself so palpa- 

 ble. As a general result, it may be said that, so far from there being 

 any scarcity of invertebrate life in the waters during the summer of 

 1871, as compared with earlier years, its actual amount was such as to 

 strike with astonishment every one in our party engaged in the in- 

 quiry. The dredge was never brought up from scraping the bottom 

 without being filled with worms, star-fishes, sea-urchins, shells, &c. 

 The location of numerous mussel-beds, of acres in extent, was estab- 

 lished; the towingnet would become almost filled, in a short time, with 

 embryos of crabs, worms, ascidians, &c, and, on several occasions, in 

 dredging off the coast, to a distance of twenty or thirty miles, the water 

 was found to be so thick with animal life that a bucket of water drawn 

 up would contain hundreds of specimens, the sea indeed appearing 

 like a thick mush of organisms. If any difference were appreciable 

 between the seasons of 18G3 and that of 1871, it was in favor of the 

 latter, possibly, indeed, because of the much less number of fishes calcu- 

 lated to reduce the mass. The validity, therefore, -of the assumption of 

 a diminution of food may be denied in the most positive terms. 



The second alleged cause, that of change of abode on the part of the 



fishes, has also received proper consideration ; but the most careful 



' inquiry failed to reveal any locality or localities along the coast where 



