1 72 Memoir Sears Foundation for Marine Research 



or four only. But they are correspondingly large when born, for those in a mother of 

 about 15V2 feet (4,700 mm.) measured respectively 1,500 and 1,550 mm. (about 5 

 feet)." Others, perhaps from smaller mothers, are considerably smaller at birth, judging 

 from the small sizes of free-living specimens repeatedly recorded. 



Habits. The Thresher is a typically pelagic species, most often seen at least a few 

 miles offshore, but often coming close in to pursue small fish. It is com.monly described as 

 usually keeping near the surface. We have seen Threshers jumping on one occasion, these 

 being easily identified by their long tails. But while it is from specimens seen at the surface, 

 or taken in nets set shoal, that the majority of records of its occurrence emanate, it is 

 equally certain that it may descend to moderate depths on occasion, since there is record 

 of at least one specimen captured on hook and line at 35 fathoms in British waters.^^ 



It feeds chiefly, if not solely, on whatever smaller schooling fishes may be avail- 

 able; in North American waters most commonly on mackerel, bluefish {Pomatomus)^ 

 shad (Alosa) and menhaden (Brevoortia), of which it destroys great numbers; no doubt 

 it feeds on herring also, as well as on bonito and squid. In North European waters its diet 

 includes pilchards, garfish, etc. Twenty-seven mackerel have been recorded from a speci- 

 men 13V2 feet long, and one-half bushel of garfish (Belone) from another in Scottish 

 waters. The method by which it captures its prey is highly specialized; in general accounts 

 it has been described repeatedly as using "its whiplike tail to splash the water, while it 

 swims in narrowing circles round a school of fishes, which are thus kept crowded together 

 until the moment of slaughter . . . Sometimes a pair of threshers work together . . . "^° 

 That it also uses its tail on occasions to stun a prospective victim is proved by eyewitness 

 accounts of one in Irish waters rising and killing a wounded sea bird with a stroke of its 

 tail, then swallowing it,'* and of another at La Jolla, California,'"' injuring a single small 

 fish by lashing at it repeatedly with its tail. Perhaps it is hardly necessary at this late date 

 to remark that the time-honored tradition that the Thresher leagues with the swordfish 

 to attack whales, which was doubtless based on its being confused with the killer whale 

 (Orca), has long since been relegated to the category of myth. 



Presumably, young are produced throughout its geographic range, very small speci- 

 mens having been taken off southern Florida on the one hand and in New England waters 

 on the other. No information is available as to season when the young are born, or when 

 mating takes place. 



Abundance and. Relation to Man. Of late years the Thresher has not appeared in 

 sufficient abundance anywhere along the Atlantic coast of America to be of any commercial 

 importance one way or the other. However, when it gathers in any numbers in pursuit 

 of small fish, as is said to have happened at times in the past, it has been an annoyance 

 to mackerel fishermen by becoming entangled in their nets. This is a frequent occurrence 

 in British waters where the Thresher is a more familiar species. It is entirely harmless. 



21. Vaillant, Bull. Soc. philom. Paris, (7) 10, 1885: 41. 22. Day, Fish. Gt. Brit., t, 1880-1884.: 302. 



23. Nichols and Murphy, Brooklyn Mus. Sci. Bull., j, 1918: 21. 



24. Blake-Knox, Zoologist, (2) i, 1866: 509. 25. Allen, Scierce, N. S. $&, 1923: 31. 



