PROGRESS IN BIOLOGICAL INQUIRIES, 1935 393 



present century. Tliough a record catch, it is not much larger than 

 others of recent years, for the mackerel population has been in a 

 phase of high abundance during the hist 10 years. The average an- 

 nual catch for the decade ending with 1935 was 41,000,000 pounds, 

 which is a marked increase over the previous decade with its aver- 

 age of 17,000,000 pounds, and in even more striking contrast with 

 tlie decade ending with 1915 when the average was only 11,000,000 

 pounds. But even the present high level is not to be regarded as the 

 apex of yield for in the decade ending with 1885, the average was 

 80.000,000 pounds, an amazing yield compared with present experience. 



This brings into high relief the dominating feature of the mack- 

 eral resource — its extreme fluctuations in yields. To find the causes 

 of these fluctuations and to discover means of turning this knowl- 

 edge to economic advantage has been the purpose of the research 

 on the mackerel fisherv. 



The studies which have been under way since 1925 have proven 

 that the changes in yield are mainly caused by changes in abundance 

 of mackerel, and only secondarily by variations in the rate of fish- 

 ing or by extensive migrations of the fish. Further, it has been 

 learned that the changes in abundance are due to great dift'erences 

 from one year to another in the numbers of young mackerel that 

 survive to commercial size. 



The present state of knowledge has been arrived at by the study 

 of changes in the mackerel popuhition by the sampling method, 

 in 1935, as in previous years, this work has been under the direction 

 of Oscar E. Sette, who^was assisted in the field work by Frank E. 

 Firth. Basic data collected were the statistics on daily landings of 

 each vessel of the fleet, information from interviews with fishermen 

 covering 1,371 of the 2.560 fares landed, and samples of 51,347 mack- 

 erel from 752 different catches. For purposes of determining age 

 composition of the populadon, length measurements were taken of 

 all samples. In addition, meristic characters such a fin spines and 

 rays were measured on 1,169 individuals to serve as an indication 

 of subdivisions or "races" in the population. 



In the laboratory, time available after meeting current demands 

 was spent in organizing for publication the results of 10 years' ac- 

 cumulation of data. This has involved refinement of the method of 

 statistical treatment to measure abundance, development of means 

 of treating length measurements of mackerel to provide suitable 

 accurate age-composition estimates, and their apj^lication to the 

 10-year series. In addition, some progress was made in analysis of 

 tow-net collections of eggs and larvae of the season of 1932. That 

 season is now known to have contributed but few recruits to the 

 stock and therefore must have been one of high "infant mortality." 

 I'he most commonly accepted theory of the cause of such year-class 

 failures has been that the newly-hatched young do not survive the 

 yolk-sac stage. But the data of 1932 do not indicate an appreciably 

 higher rate of mortality at this supposedly critical point than during 

 subsequent stages. If this is confirmed by examination of other 

 years' data, it will have important bearing on further search for the 

 causes of failures in year classes — a phenomenon occuring not only 

 in tlie mackerel but in many important food fislies. This problem 

 Jias been a perplexing one to the students of fisheries biology, and 



