PROGRESS IN BIOLOGICAL INQUIRIES, 193 2 89 



the catch. This involved sampling the catch throughout the season 

 and estimating the numbers of fish of each year of age in the catch. 

 Since changes in size of the fleet might also be reflected in the annual 

 catch, statistics of the catch of individual vessels have also been com- 

 piled since 1925. 



In 1932 most of the sampling was done by F. E. Firth at New 

 York during May and June; at Boston from July to October; and at 

 Gloucester in November and December. Additional data were 

 secured at various minor ports from Maine to New Jersey. During 

 the year the mackerel fleet landed 3,802 fares aggregating 46,770,740 

 pounds. Of these, 2,020 fares representing 29,490,409 pounds were 

 recorded by locality of capture through interviews with captains; 

 31,352 mackerel were measured from 1,168 fares; and scales were 

 taken from 1,954 mackerel. 



The age composition of the mackerel population, as derived from 

 these data in 1932 and the previous 6 years, confirms the original 

 supposition that variability from year to year in production or sur- 

 vival of young mackerel is the cause of the ma j or fluctuations in the catch. 

 In every instance during the 7-year period an increase in catch has 

 followed a production of a large year class one or two seasons previous, 

 and each decline was preceded by a season or two in which subnor- 

 mal numbers of mackerel were produced. 



Knowing the mechanics of these fluctuations, it remains to devise 

 means of combatting their ill effects. At least three possibilities are 

 apparent: (1) Adjustment to the fluctuations through prediction of 

 future abundance, (2) lessening of the fluctuations by adjusting the 

 fishing intensity so that successful year classes will be spread over 

 enough years to bridge over the periods of subnormal production of 

 young fish, and (3) control of the production of young fish. 



Realizing the potential value of predictions the Bureau has been 

 accumulating experience along this line during the past 5 years. 

 From an appraisal of the rate of decline of each year class and the 

 likelihood of augmentation by the advent of successful classes, the 

 prospects for each season were estimated in terms of probable catch, 

 and the results made public in fishing trade journals during the first 

 3 years, and by means of Bureau fishery circulars in the last 2 years. 

 Of the 5 trial forecasts, 1 was noncommittal, 2 were approximately 

 correct, 1 (1930) was too high by 17,000,000 pounds or 28 percent, 

 and 1 (1932) was too low by 11,000,000 pounds or 42 percent. 

 These two instances of disparity between prediction and catch were 

 caused solely by unexpected changes in abundance of mackerel that 

 were 2 years old. In all the years of record the predictions of abun- 

 dance of mackerel older than this were reasonably fulfilled. 



The difficulties of predicting the abundance of 2-year-old mackerel 

 apparently have been brought about by the existence of at least two 

 distinct types of year classes, distinguishable by changes in availabil- 

 ity during the first 3 years of existence and by peculiarities of distri- 

 bution. If this is the case, an accurate prediction would depend on 

 the determination of the type in the yearling stage. From consider- 

 ations too involved to enter upon in this report, it is believed that 

 morphological studies of each year class, coupled with extensive 

 marking experiments of each year class in the yearling stage, might 

 enable such distinction to be made. Thus, it appears that an improve- 

 ment in the prediction of 2-year-olds is contingent on provision being 

 made for these two additional phases of st udy. 



