vealed fish remains in only three stomachs. Five 

 stomachs contained about 4 ounces of a light-gray, 

 custardlike material, evidently fowl well advanced 

 in digestion. Most of the stomachs, however, were 

 filled with clear fluid; only a few stomachs were 

 empty and flaccid. 



Fifty stomachs examined at Pt. Judith, R.I., in 

 July 1959, contained mostly amphipods {Lepto- 

 cheirus) and occasional fish remains. 



In July 1961, John M. Hoberman found silver 

 hake (Merbwociua bilinearis) in the stomachs of 

 dogfish collected in Ipswich Bay during a cruise of 

 the Delaware. Silver hake were abundant in the 

 area at the time. 



Fishes and rock crabs {Cancer) were the prin- 

 cipal food items of 33 spiny dogfish collected in 

 June 1963 off Block Island, R.I. The dogfish were 

 examined aboard the. research vessel Albatross IV. 

 Sixty percent contained fish, 33 percent contained 

 rock crabs, and 7 percent contained squid. Rec- 

 ognizable food items included squirrel hake, 

 Vrophycis chuss; silver hake; winter flounder, 

 Pseudopleuronectes americamis ; and sculpin, My- 

 oxocephalus sp. 



It is evident spiny dogfish have no food pref- 

 erences, but eat nearly anything that moves. It is 

 evident too, they are opportunistic feeders, prey- 

 ing on whatever species are abundant and avail- 

 able. Their catholic food habits probably con- 

 tribute greatly to the species' biological success. 



AGE AND GROWTH 



The traditional techniques used in fishery 

 biology for age determination are not, unfortu- 

 nately, applicable to the spiny dogfish. The dog- 

 fish does not have scales suitable for examination, 

 and being a cartilaginous fish, it has no true bones 

 in which visible growth zones are formed. Dog- 

 lisli otoliths, unlike the calcareous otoliths of the 

 teleosts, are simply aggregations of sand particles 

 loosely joined in a gelatinous substance, and thus 

 offer no opportunity for detection of growth zones. 



A possible solution to the problem of determin- 

 ing the age of the dogfish is presented in an ob- 

 scure Russian paper (Kaganovskaia, 1933) read 

 in English translation. Briefly, the paper notes 

 thai the dorsal spines of t he dogfish are marked 

 with annulations apparently related to growth 

 periodicity (tig. 4). The Russian biologisl had 

 collected the dogfish from the waters around 



Sakhalin. At first she examined the vertebrae, 

 but the barely noticeable rings in them became 

 even less visible after treatment. Cross sections 

 of the teeth and of the dorsal spines were ex- 

 amined, but without success. The teeth showed 

 no zones, and the spines were found to have an 

 internal cavity along their entire length. The 

 enamel coating of the spines, however, had mark- 

 ings, ". . . which doubtless represent annual 

 deposits." 



Figure 4. — Photograph of a dogfish spine showing the 

 annulations. 



The makeup of the spine is quite similar to the 

 makeup of a mammalian tooth. Daniel (1934) de- 

 scribes the structure of the spine as follows : 



For almost half its length the spine is buried in the 

 Integument. The buried part is designated as the root or 

 base and the exposed portion the crown or spine 

 proper . . . 



. . . The spine contains a large central cavity which 

 when in place fits over a cartilage of the tin skeleton. 

 The walls of the spine are made of dentine which in the 

 crown consists of a double layer. The more superficial 

 layer is bounded anteriorly and laterally by a layer of 

 enamel, but enamel does not extend over the posterior 

 groove which fits up against the basal cartilage of the 

 fin skeleton. A more or less compact layer of pig- 

 ment . . . separates the enamel ... in front from the 

 layer iff dentine. 



Kaganovskaia (1933) did not try to validate 

 the spine markings as year marks although she 

 noted that the spines of fish less than 1 year old 

 were light gray in color and had no markings. She 

 examined a sample of rear dorsal spines (the 

 posterior spines are more clearly marked than the 

 anterior spines) from 210 dogfish, 380-1,180 mm. 

 in length, and reported their ages as "2-25 years. 



536 



U.S. FISH AND WILDLIFE SERVICE 



