Fishes of the Western North Atlantic 461 



effect being corroborated by the fact that their concave surfaces are sheathed with a glan- 

 dular tissue resembling the poison glands of the venomous European weaver {Trachinus 

 draco) J'^ 



They may be anywhere between the surface and bottom to depths as great as 90 to 100 

 fathoms, and perhaps even deeper. But they have never been found to be pelagic in the 

 ocean basin. On the other hand, while their ability to survive in brackish water has been 

 proved by experiment, and although they have been reported in at least one river" in 

 Denmark, they do not normally enter fresh water, and specimens placed in fresh water 

 died within a few hours." 



Relationship to Temperature. Spiny Dogfish do not appear on the United States coast 

 in spring until the temperature of the water has risen to about 6° C, and most of them dis- 

 appear from the inshore belt west and south of Cape Cod by the time the surface has 

 warmed to about 15°, either moving into deeper cooler water nearby or northward to 

 colder seas. Similarly, during their summer stay farther north they are seldom taken in 

 water warmer than about 1 5° or colder than 6 or 7°, unless temperatures within this range 

 are to be found a few fathoms shallower or deeper. Their northward advance along 

 the coasts of Newfoundland and Labrador clearly appears to follow the vernal warning 

 of the coastal waters, and their autumnal reappearance west and south of Cape Cod coin- 

 cides roughly with the date when the surface has cooled to 12 to 15° i few if any are seen 

 anywhere along the coast, north or south, after the surface has chilled below about 6° or 7° ; 

 and the winter temperature ranges between 6° and 1 1° on the bottom along the offshore 

 belt, where the majority of the stock is now known to spend the cold months. 



The foregoing, added to similar data for European waters, shows that the seasonal 

 migrations north and south, and between shoal water and deep, are chiefly thermal in 

 character, i.e., to avoid extremes of temperature, either lower than about 7° or 8 ° or higher 

 than about 1 2° to 15°. The thermal relationship of the Spiny Dog is thus analogous with 

 that of the Mackerel, except that its optimum thermal range is slightly lower. 



Breeding Habits. Females, with young nearly ready for birth, are taken in New- 

 foundland waters in early autumn (October), in the Gulf of Maine in late summer and 

 autumn, near Woods Hole and New York in autumn, oflF Virginia and North Carolina 

 in January to February. This, with the fact that very young specimens are rarely seen 

 anywhere on the coast, suggests that most of the young are born from late autumn through 

 the winter on the oflFshore wintering grounds. However, the season of production may 

 extend through the spring in some years, as evidenced by recent captures of young Dogfish 

 with the umbilical scar still discernible, near Woods Hole in June, in the Gulf of Maine 

 in summer, and in schools at the entrance to Long Island Sound in July. It may even 

 extend sporadically into summer, which is proved by a catch of 74 adult females which 



32. For details, see Evans (Philos. Trans., [B] 212, 1923: 27). 



33. The fact that they were reported there with cod and Merlucciui (Feddersen, Naturh. Tidt^kr., [3] 72, 1879: 

 68, 69, footnote i; Bean, Amer. Nat., 14, 1880: 525-526) indicates that the bottom water in the stream io 

 question was salt, or at least brackish and not fresh. 



34. For accounts of experiments on the survival of Spiny Dogfish in brackish and fresh water, see Scott (Ann. N. Y. 

 Acad. Sci., 33, 1913: 30, 60). 



