452 U. S. BUREAU OF FISHEEIES 



they are due to natural causes beyond his control of such nature 

 that they may be foreseen and turned to advantage by appropriate 

 adjustments in the fishery. 



The work to date has shown that the changes in abundance have 

 been due primarily to the variable number of mackerel added to the 

 stock by each year's spawning. This probably is because conditions 

 for survival of the very young are much better in some years than 

 in others, so that sometimes many mackerel grow to commercial 

 size, while in other years very few survive the first weeks or months 

 of existence. In years of good survival so many mackerel are added 

 to the stock that a period of plenty ensues; and, conversely, during 

 a series of years when survival is poor, so few are added that a 

 period of scarcity follows. 



This working knowledge of the mechanism of fluctuations has 

 been attained gradually through seven years of close observations 

 on the volume and age composition of the market landings of 

 mackerel and from periodic surveys of the spawning and nursery 

 grounds of the species by the bureau's fishery research vessel 

 Alhafross II. 



The investigations were continued during 1931 under the direc- 

 tion of Oscar E. Sette. The data on market landings were collected 

 mainly at New York during May and June, at Boston from June 

 to October, and at Gloucester during November and December. 

 These were augmented by observations at Cai^e IVIay during April, 

 at Newport during May and June, at Woods Hole from May to 

 October, and during the summer at various minor ports in the 

 Middle Atlantic region. 



During the year the fleet landed 3,056 fares of mackerel, aggre- 

 gating 36,490,847 pounds. By interviewing the masters of fishing 

 vessels the date, locality, and quantity caught were ascertained for 

 1.329 fares, and samples of mackerel were measured from 856 fares. 

 These, together with 10,883 mackerel measured from the pound net 

 fishery, made a total of 35,068 measurements for the season. Scale 

 samples were taken from 1,280 mackerel. Many mackerel-vessel 

 captains continued to cooperate by keeping detailed logs of their 

 activities. Thanks to the kindness of the Atlantic Biological Sta- 

 tion, St. Andrews, New Brunswick, Canada, 26 samples, affording 

 2,623 measurements of gill-net and trap mackerel, were collected at 

 Pennant, Nova Scotia. 



The 1931 work has supplemented our previous understanding of 

 mackerel fluctuations in numerous respects, but perhaps the most 

 striking evidence of the approach to thorough knowledge is the grati- 

 fying outcome of the forecast on probable abundance during 1931.'-^ 

 Before discussing this subject in detail, it should be explained that 

 the fishery draws mainly on the mackerel that are 2 years of age or 

 older. Mackerel younger than this weigh less than a pound, are deli- 

 cate, spoil readily, and can not compete in the open market with the 

 larger and more desirable mackerel. They usually are sold in a sepa- 

 rate category known to the trade as blinks and tinkers. Because of 

 the peculiar combination of economic and biological conditions that 



2 Outlook for the mackerel fishery in 1931. By Oscar E. Sette, Bureau of Fisheries, 

 Fishery Circular No. 4, August, 1931 (approved for publication May 26, 1931). 



