454 U. S. BUREAU OF FISHERIES 



classes. This is a question of fundamental importance in fisheries 

 science, and its answer would be of inestimable value to the solution 

 of fishery problems. 



To provide data for the interpretation of findings on the mackerel- 

 spawning grounds, with special reference to the influence of ocean 

 drifts in transporting fish eggs and larvse, experiments on the rate 

 of development of mackerel eggs were carried out by Leonard G. 

 Worley during the spring of 1931. He found that the eggs of this 

 species hatched in 61/2 days at 11° C. and 2 days at 21° C, with 

 corresponding inoubation periods for intermediate temperatures. 

 In addition, it was found that the limits of successful hatching were 

 between 10° C. and 22° C, with the maximum percentage hatching 

 at 16° C. 



COD 



Studies of the migratory habits and other phases in the life his- 

 tory of the cod were continued by William C. Schroeder during 1931, 

 though field work was limited by the requirement of personnel and 

 vessels on other projects. Marking experiments were made in the 

 following localities: (a) Woods Hole, Mass., January 8-9; (h) 

 Nantucket Shoals, August 10-11; and (c) Mount Desert, Me., June 

 21-29. 



The results in general resembled those of the preceding years, cor- 

 roborating earlier conclusions that many of the cod which summer off 

 southern Massachusetts migrate westward in the fall to winter off 

 Rhode Island and the Middle Atlantic States region, but that the 

 shore cod of eastern Maine remain for the most part localized, a few 

 of them moving north and east to Nova Scotia. 



A total of 45,452 cod was tagged at sea from 1923 to 1931, of 

 which 3,204, or about 7 per cent, were reported recaptured, the re- 

 turns from some lots of fish being as low as 1 per cent, while from 

 others they have been as high as 35 per cent. In addition, 2,223 cod 

 were tagged and liberated at the Woods Hole fisheries station from 

 1926 to 1931, of which about 3%: per cent were reported recaptured. 



A new type of tag made of two small, thin celluloid disks, re- 

 sembling the Scottish plaice label, was devised by R. A. Nesbit in 

 connection with his study of tagging methods useful for Middle 

 Atlantic fishes. This celluloid tag was used for tagging cod during 

 1930 in place of the metal-strap tag employed in previous experi- 

 ments ; and although but little more than a year has elapsed, it gives 

 promise of being much more efficient than the old one. For ex- 

 ample : Whereas only about 3 per cent of the Monel-metal tags w^ere 

 recovered the first year from cod tagged on Nantucket Shoals during 

 1923-1929, the returns from the celluloid tags used in that locality 

 in October, 1930, amount to 15 per cent for the succeeding 12-month 

 period. While these favorable returns demonstrate the superiority 

 of the new style tags during the first year of release, it still remains 

 to be seen whether they persist during later years. 



The results so far obtained from this new disk tag make it desirable 

 to continue cod-marking experiments on the prolific offshore grounds, 

 from which only meager data have been obtained; chiefly, perhaps, 

 because of the inefficiency of the metal-strap tag. An important 

 objective is determination of the sources of replenishment for the 



