PROGRESS IX BIOLOGICAL INQUIRIES, 1937 15 



tion if any winter as far eastward as the outer edge of the Nova 

 Scotian banks. Both southern and northern contingents leave strag- 

 glers behind them along their migratory routes so that some mack- 

 erel, usually the smaller adolescent individuals, populate the coasts 

 of southern New England and Nova Scotia, even though the main 

 bodies are in the Gulf of Maine and the Gulf of St. Lawrence. 



llonzontal distribution. — During the summer sojourn mackerel 

 predominantly inhabit the open waters over the inner part of the 

 continental shelf. Important numbers are regularly found neither 

 in enclosed bays nor in far olf shore positions. The waters of the 

 deep central portions of the Gulf of Maine are particularly avoided 

 and catches are only occasionally made over the offshore banks. 

 Larger individuals tend to be farther offshore than smaller ones. 



Vertical distribution. — The marked thermocline established in 

 spring and persisting through summer forms the lower boundary 

 of mackerel distribution, the underlying colder water imposing an 

 effective barrier to downward movement. Variations in position 

 and intensity of the thermocline affect the success of the fishery. 



ISehooling habits. — Mackerel are usually aggregated in dense 

 schools — a form of association that probably is advantageous in feed- 

 ing on plankton. The lesser swimming ability of smaller individuals, 

 resulting from the relation between volume of muscle and surface 

 area, causes a marked but variable schooling according to size. 



Feeding habits, — The American representatives of the species feed 

 almost entirely on plankton, but additional observations are needed 

 relative to large offshore individuals, which, like their European 

 relatives, may subsist on small fishes during the late summer and 

 autumn months. 



/Spawning. — The inner portions of the continental shelf between 

 Cape Hatteras and Cape Cod form the most productive spawning 

 ground; next is the southern half of the Gulf of St. Lawrence. 

 Spawning in intermediate areas is of slight importance excepting 

 perhaps the southern part of Massachusetts Bay which has high con- 

 centrations of eggs in spite of its small area. Spawning takes place 

 in April and May south of Cape Cod, in May and J'u.e in Massa- 

 chusetts Bay, and in July and August in the Gulf of St. Lawrence. 



Early life history and growth. — Eggs hatch in 3 to 8 days, depend- 

 ing on temperature. The earliest post-planktonic individuals to 

 be found during the course of the investigations were 2 to G inches 

 long in the latter part of July. By the end of the first season 

 mackerel are 7.5 to 10.25 inches long; by the end of the second season, 

 10 to 14 inches long. Thereafter, growth is slower. By the tenth 

 year mackerel average about 18 inches in length, and weigh 2 to 2i^ 

 pounds. 



Although many of the more important features of mackerel biology 

 are fairly well understood, and by themselves will furnish a reliable 

 catch forecast in some years, the fact that environmental conditions 

 exert a profound effect upon the actual catch was well illustrated this 

 year. In the annual prediction for 1937, it was estimated that the 

 abundance of mackerel would be about 80 percent of the 193G level. 

 The actual catch, however, reached about 35 percent of the 1936 level. 

 The difference appears to have been due chiefly to inability of purse- 

 seine fishermen to find fish in the usual localities. 



7G6_'2— 38 3 



