24 REPORT OF THE COMMISSIONER OF FISHERIES. 



passage of the fish from the sea to the fresh waters of the spawning- 

 grounds at the river heads, with the belief that light will be thrown on 

 the causes of death. The work so far done has been largely of a pre- 

 liminary nature, consisting mainly of measurements of a series of fish 

 and their principal organs; the collection of specimens for chemical and 

 biological analysis; the determination of the rate of respiration, the 

 rate and force of the heart, and the blood pressure, and the measure- 

 ment of the electric conductivity and freezing point of the blood. All 

 the data have been obtained from fish taken, respectively, in salt, 

 brackish, and fresh waters, and at progressive stages of reproductive 

 activity, for the purpose of comparative study when the material has 

 been completely worked up and anatyzed. A special report covering- 

 certain phases of the study has recently been published. 



The spoonbill catfish. — This large but rather coarse food fish of the 

 Mississippi Basin, known to science as Polyodon spathula, is the object 

 of rather important local fisheries, and as it is liable to commercial 

 extermination its feeding and breeding habits were made the subject 

 of investigation during the summer of 1904. The work has cleared 

 up certain misapprehensions as to the feeding habits of this fish, but 

 concerning the breeding season and habits nothing positive was deter- 

 mined, not one of 1,500 fish examined having significantly developed 

 sexual glands. 



Food of dogfishes. — For several years there have been conducted 

 investigations upon the food of certain fishes of little or no food value, 

 though of considerable indirect importance, and this work has been 

 continued in 1905. Two species, the smooth dogfish and the horned 

 dogfish, which were studied in southern New England, have been shown 

 to be so destructive to food species as to be a distinct menace to the 

 fisheries. 



The smooth dogfish feeds principally on large crustaceans, nearly 

 all of which are of direct economic value, and conspicuous among 

 which is the lobster. Estimating the number of smooth dogfish in 

 Buzzards Bay as 100,000, which is conservative, and allowing each 

 dogfish one lobster in three days, there would be represented a 

 destruction of 150,000 lobsters in one month, or 750,000 during the 

 five months of the presence of the dogfish in the region. 



In the vicinity of Woods Hole the principal food of the horned dog- 

 fish is a little jellyfish, but observations on other parts of the coast 

 indicate that not only food fishes but the nets and lines of the fisher- 

 men are destroyed. Ground fishing in Boston Bay in 1903 yielded an 

 average of $3 a day per man during- July and August, but in 1904 the 

 horned dogfish was present in such great numbers that it was impos- 

 sible to catch anything else. When fish of value were taken they 

 were torn in pieces by dogfish before they could be landed. Herring, 

 mackerel, and other food fish are torn from the gill nets by this 



