away and could not observe the delivery. An hour 

 later, about 2 cm. of the caudal fin of a pup could 

 be seen protruding from the female's vent. This 

 pup was not delivered, and the female died later 

 in the day. The average length of the pups was 

 21 cm., and each had a yolk sac attached. Al- 

 though their size suggests the pups were nearly 

 full term, presence of the yolk sac indicates they 

 were still developing. Presumably the female 

 aborted, perhaps from the shock of capture and 

 handling or perhaps from the water temperature. 

 The surface water temperature in the trap was 

 19.5° C. and the tank temperature was the same. 

 This is well above the temperature where the 

 species is commonly found. 



When the female was examined, post mortem, 

 the partially delivered pup was found to be held 

 by one dorsal spine in the anterior part of the 

 female's left uterus. The dogfish pup is morpho- 

 logically suited to head presentation in birth. No 

 doubt the caudal presentation noted here was ac- 

 cidental; the rearward sloping dorsal spines — 

 small as they are — deter smooth, tail-first move- 

 ment out of the uterus. It is not known if partial 

 delivery occurs among dogfish in the sea, and none 

 has been observed aboard our researcli vessels. 



Season of Birth 



The gestation period of dogfish is nearly 2 years, 

 although the authorities differ in the exact number 

 of months involved. Birth generally occurs in the 

 cold months of the year. Ford's (1921) extensive 

 studies at Plymouth led him to conclude that gesta- 

 tion occupies 21-25 months, with the fetuses ready 

 for birth from August to December. Hickling 

 (1930) confirmed Ford's findings. Templeman 

 (1944) suggested a gestation period of almost 24 

 months with birth between January and May. 

 Assen's (1964b) data suggest that in the offing of 

 the Orkney Islands, dogfish pups are ready for 

 birth in late November or soon after. 



A somewhat shorter gestation period for dog- 

 fish captured in the vicinity of Woods Hole is re- 

 ported by Hisaw and Albert (1947) who state, 

 "The gestation period apparently covers about 20 

 to 22 months and a female gives birth every other 

 year." The pups, they note, are born in the late 

 fall, somewhere south of Woods Hole. Bigelow 

 and Schroeder (1953) report a gestation period of 

 18 to 22 months with birth probably taking place 

 on the offshore wintering grounds, although some 



may be born in the spring and summer. Latham 

 (1921) confirmed that some may be born in the 

 summer when he reported many young dogfish 

 only a few hours old that were caught in a fish trap 

 in Long Island Sound in August. 



In the Pacific the gestation period is 2 years 

 with birth taking place during the winter (Hart, 

 1942), more specifically in November and Decem- 

 ber (Bonham et al., 1949). 



An individual female produces young only in 

 alternate years (Hart, 1942; Hisaw and Albert, 

 1947; Bonham et al., 1949; Clemens and Wilby, 

 1961). The data presented by Ford (1921), Hick- 

 ling (1930), and Templeman (1944) confirm the 

 broods-in-alternate-years conclusions of the work- 

 ers cited above. 



MORTALITY 



The spiny dogfish has few enemies and is canni- 

 balistic only to a very small degree. Thus, except 

 for disease, there is little to act as a deterrent to the 

 buildup of dogfish populations, and this no doubt 

 is one of the prime reasons for the vast numbers 

 of dogfish reported in one area or another. The 

 predators of the spiny dogfish are mostly the large 

 sharks and large bony fishes. In table 9, I have 

 listed the predators as reported in Bigelow and 

 Schroeder (1953) and noted certain appropriate 

 remarks. It should be stressed that in all but 

 tw T o of the instances the prey is specifically identi- 

 fied as the spiny dogfish. The remaining two in- 

 stances were simply listed as "dogfish" and possi- 

 bly may be the smooth dogfish. 



Table 9. — Predalors of the spiny dogfish 



Predator 



Mackerel shark (Lamna nasus) 



Maneater (CarchaTodon carcharias) 



Tiger shark (Galeocerdo cuvier) 



Blue shark (Prionace glauca) 



Barndoor skate (Raja laevis) 



Lancetfish f Alepisaurus ferox) 



Tuna (Thunnus thynnus) 



Tilefish (Lopholaiilus 



chamaeleonticeps) . 

 Goosefish (Lophius americanus) . . . 



Remarks 



Known to prey on spiny dogfish in the 



eastern Atlantic; probably Gull of 



Maine also. 

 One spiny dogfish, evidently torn off a 



line trawl. 

 Dogfish (species T) from one captured 



in Woods Hole. 

 Preys on spiny dogfish in northern 



waters. 

 Spiny dogfish from Woods nole records. 

 Small spiny dogfish eaten by Block 



Island specimen. 

 Swallowed whole dogfish (species ?) 



weighing 8 pounds. 

 One contained two spiny dogfish. 



One contained a spiny dogfish 1 foot 

 long and the vertebral columns of 6 

 others. 



Marine mammals apparently are not a threat to 

 the dogfish. In a study of the food habits of seals 

 (Fisher and Mackenzie, L955), dogfish remains 



544 



U.S. FISH AND WILDLIFE SERVICE 



