568 U. S. BUKEAU OF FISHElilES 



fashion, and indicate that mortality rate is very low once a brood 

 has passed its second year. 



Broods which should have been produced in the years 1924 to 1927 

 have been practically nonexistent. To be sure, the 1927 brood fur- 

 nished over a million pounds in 1928 and somewhat less in 1929 ; but 

 in the main, these year classes failed to produce enough survivors of 

 commercial size to be significant in the catch. 



The 1928 class furnished 22,000,000 pounds of mackerel in 1929 

 and 12,000,000 pounds in 1930. A year ago it appeared that the 1928 

 class was numerically superior to the 1923 class for it furnished 

 nearly five times as much fish in its second year as did the 1923 class 

 when it was of the same age. But the smaller catch of this brood in 

 1930 calls for a more modest estimate of its size ; unless fish of this 

 brood reappear in unexpectedly large numbers in the future. At 

 any rate, it is the most important contingent that has joined the 

 stock since 1923. 



It is too early to judge the importance of the 1929 class. The 

 6,000,000 pounds caught in 1930 indicate the advent of a brood of 

 some importance but the quantity of mackerel caught in the second 

 year of life seems not to be a very good index of their numerical 

 abundance, and judgment must await next year's fishery. 



It is obvious from the above observations that the production of a 

 numerically rich year class is an occurrence that takes place only at 

 intervals of several years ; but when a large brood is once established, 

 it is a dominant feature of the fishery through at least six years, 

 perhaps more. 



An understanding of the conditions which in one year may pro- 

 duce a rich class and in another may cause virtually complete mor- 

 tality of the current brood is important, both to the understanding 

 of the fishery as a resource to be conserved and for the practical 

 purpose of making more accurate predictions of the abundance of 

 mackerel. For this purpose the Albatross II made six cruises to 

 the mackerel spawaiing grounds during the period from April 3 to 

 July 18. Data Avere collected to afford a roughly quantitative esti- 

 mate of the natural production of mackerel eggs and of the survival 

 of the larvae, together with such observations on the temperature, 

 salinity, and plankton constituents as might be expected to furnish a 

 clue to the factors controlling the success or nonsuccess of the year 

 class. 



Later in the season, several short cruises to the mackerel grounds 

 in the offing of Cape Cod were devoted primarily to the collection of 

 data on the vertical distribution of plankton and its possible effect oi? 

 the availability of mackerel to the (commercial fishery. 



In accordance with the bureau's policy of making results of its 

 work available to the public as soon as possible, the information on 

 the year class composition Oif the catch during 1929 was summarized 

 and its implications as to the effect on the fishery during the ensuing 

 season was outlined in articles submitted to fisliery trade journals for 

 publication. The weight of evidence indicated a level of abundance 

 that was likely to furnish a catch of about 60,000,000 pounds, pro- 

 vided the intensity of fishing remained practically the same. The 

 total actually realized was 43,500,000 pounds or 27 per cent less than 

 the predicted amount. The discrepancy consisted almost entirely of 



