The spiny dogfish usually is found in large 

 schools composed of: (1) large, mature females; 

 (2) medium-size-fish, either mature males or imma- 

 ture females; or (3) small, immature fish of both 

 sexes in about equal numbers. The schools may 

 spend considerable time in an area and then move 

 rapidly from one area to another. 



In the Northwest Atlantic they move from the 

 southern part of their range, off North Carolina 

 to New York, northward with the advance of the 

 spring season. It is suspected they spend the win- 

 ter offshore in deep water. In addition to the 

 seasonal migrations, spiny dogfish take part in a 

 daily migration, rising to or near the surface dur- 

 ing the night and returning to the bottom during 

 the day. 



Tagging studies have had low rates of return 

 compared with tagging studies of commercially 

 valuable fishes. But the returns have shown that 

 the spiny dogfish is capable of long-distance migra- 

 tions — one individual travelled 4,700 miles — and 

 is long-lived since several tagged fish were at lib- 

 erty 7 to 10 years. In the New England area, the 

 recaptures suggest that spiny dogfish school to- 

 gether for long periods of time and return to the 

 same general area at about the same time of year. 



Food habits studies show that spiny dogfish are 

 primarily fish eaters but also feed on invertebrates, 

 both swimming and bottom-dwelling forms. Clu- 

 peoids, smelts, and chimeroids were the fishes 

 found most frequently in dogfish stomachs. They 

 also feed on shrimp, crabs, and squids and are one 

 of the few fishes that eat ctenophores. In gen- 

 eral, they are considered opportunistic feeders, 

 preying on whatever is abundant and readily 

 available to them. 



Age and growth studies based on interpreta- 

 tion of annulations on the dorsal spines suggest 

 the spiny dogfish is long-lived with some individu- 

 als attaining ages of 20 to 30 years. Based on the 

 spine readings, the growth rate is about 3% cm. 

 per year. Growth of tagged dogfish, however, is 

 less, usually about 1% cm. per year. At a given 

 length, female dogfish are slightly heavier than 

 male dogfish; mature and pregnant females are 

 the heaviest and longest individuals. 



It is difficult to make any. analyses of the dynam- 

 ics of dogfish populations because so much of the 

 basic life history information is lacking. More is 



known about the natality of the species than about 

 the mortality. 



The spiny dogfish is ovoviviparous. Sex ratio 

 of the developing fetuses is very nearly 1 : 1, and 

 presumably the young are born in the same ratio. 

 Sex ratio of the older fish varies with the sexual 

 maturity of the individuals; in general they tend 

 to group themselves by sex. Sexual maturity is 

 attained by males at 80-100 cm. in length and by 

 females at 100-124 cm. 



Mating takes place during the cold months, 

 probably on the wintering grounds, and the young 

 are born after a 2-year development. The num- 

 ber of young born per female varies with location. 

 In the Pacific it averages about 11 in the western 

 part and about 7 in the eastern part. In the 

 Atlantic it averages about 4 in the western part 

 and about 3 in the eastern part. At the time of 

 birth the young dogfish are about 20-30 cm. long. 



The natural mortality of the adults apparently 

 is low. In the Western Atlantic their principal 

 predators are the large sharks and large bony 

 fishes. Records of cannibalism are rare, and there 

 are few records of marine mammals feeding on 

 spiny dogfish. Fishing mortality, however, takes 

 a relatively high toll. 



In the United States the spiny dogfish has been 

 exploited mostly as an industrial fish for reduc- 

 tion to meal and oil. Immediately preceding and 

 during World War II, the groups on the west coast 

 were heavily exploited for their livers, an impor- 

 tant source of natural vitamin A. At the peak of 

 the fishery, in 1944, more than 40 million pounds 

 of dogfish were taken from Puget Sound. The 

 fishery collapsed when vitamin A was synthesized 

 in 1947, and today about 2 million pounds are 

 landed yearly. Most dogfish are caught with ot- 

 ter trawls or gill nets. 



There is a small and relatively steady market in 

 the United States for dogfish as human food. 

 Technological problems, however, have prevented 

 it from becoming more than just locally popular. 

 Fresh dogfish fillets have a flavor and texture 

 somewhat resembling those of haddock. It is a 

 popular food fish in parts of Europe. 



Management of the species is indicated to re- 

 duce the damage it causes to more valuable com- 

 mercial fisheries. Finding an economically at- 

 tractive use for the dogfish would be the most 

 worthwhile management method. Lacking this, 



LIFE HISTORY OF SPINY DOGFISH 



551 



