imately 2 or 3 million fish without any apparent 

 diminution of the supply." 



The results of increased exploitation of the 

 North European oceanic stock of dogfish are dis- 

 cussed by Aasen (1961, 1963, 1964a, 1964b). In 

 the period 1957-62, the Norwegian catch of dog- 

 fish increased by 50 percent. Returns from dog- 

 fish tagged in 1958-60 indicated an annual fishing 

 mortality rate of 7 percent and an annual natural 

 mortality rate of 20 percent. 



During the 1961-62 fishing season, there was 

 an annual mortality rate of 38 percent, (Aasen 

 did not separate fishing and natural mortality.) 

 And he reported (1964a) an average total instan- 

 taneous mortality rate of 72 percent in the years 

 1960-63. He interpreted the high figure as a 

 "danger signal." A measure of catch per unit of 

 effort for the period 1957-63 ". . . shows a heavily 

 declining stock [of spiny dogfish]." 



The evidence presented by Aasen in his several 

 papers, and by Barraclough (1953), shows that 

 long-term exploitation of the dogfish can produce 

 a real reduction in their numbers. Thus, increased 

 fishing effort, on the dogfish off the United States 

 could bring about some marked changes in the size 

 of the population. It is not unreasonable to sus- 

 pect that future fishery biologists might be called 

 on for studies to save declining dogfish stocks in 

 North American coastal waters. 



If increased use of the dogfish is not feasible, 

 either for food or industrial purposes, some other 

 method of control should be investigated. A rather 

 unique method — the sterile-male technique — has 

 been used successfully to control the screw- 

 worm, an insect pest of cattle, and may prove of 

 some Value in the case of the spiny dogfish. The 

 method is described by Knipling (1959) and, 

 briefly, involves sterilizing laboratory-reared male 

 screwworms through the use of gamma irradia- 

 tion. The treated males are released in the area 

 of infestation in the ratio of 5 to 10 times the num- 

 ber of normal males. Females that mate with the 

 sterile males lay infertile eggs, thus reproduction 

 is greatly reduced. The treatment has been suc- 

 cessful in eliminating the pest on the island of 

 Curacao and controlling it in Florida. Knipling 

 states that this control method is based on the 

 following biological principle: "The introduction 

 of sexually sterile but otherwise sexually vigorous 

 males, and to a lesser extent females, into the 



natural population of an animal species will have 

 greater influence in reducing the biotic potential 

 of the population than elimination of the same 

 number of individuals from the same population 

 by destruction or removal." The sterilization 

 could be done either by irradiation or with 

 chemicals. 



It would be difficult at the present time to try to 

 evaluate on a theoretical basis the sterile-male tech- 

 nique applied to the spiny dogfish problem. We 

 lack some very necessary knowledge of the life his- 

 tory of the species. For example : ( 1 ) How many 

 dogfish need to be sterilized to effect control (that 

 is, how many dogfish are present in the total popu- 

 lation), (2) how many times in its lifespan will 

 a dogfish mate (screwworms mate once), and (3) 

 does one male dogfish mate with more than one 

 female in a given season? 



The low fecundity of the spiny dogfish and the 

 long period of gestation are factors very much in 

 the favor of any control method that might be 

 applied. But mere control is not a very satisfy- 

 ing solution to the problem of dogfish abundance. 

 From a conservation standpoint it would be more 

 desirable, of course, to find a use for what is very 

 obviously a rich potential resource. It is possible 

 that in future years, when additional sources of 

 animal protein are needed for human food, we 

 may see development of an active fishery for the 

 species. Such a development would then remove 

 the dogfish from the pest classification. 



SUMMARY 



The spiny dogfish, SquaJus acanthias, is a small 

 shark of the family Squalidae. It is extremely 

 abundant locally and seasonally and has proved to 

 be a costly nuisance to commercial fishermen in the 

 United States. 



It grows to a maximum length of 100-124 cm. 

 and a maximum weight of 7.3-9.8 kg. The fe- 

 males usually are slightly larger than the males. 

 Two sharp spines, one anterior to each dorsal fin, 

 are the features that give the species its common 

 name and serve to separate it readily from the 

 smooth dogfish, Mustel/us en nix. 



Although the spiny dogfish is distributed in 

 many parts of the world, this report deals mostly 

 with the groups found in the Northwest Atlantic. 

 Pertinent information, however, is reported from 

 studies of the species made in other areas. 



550 



r/.S. FISH AND WILDLIFE SERVICE 



