164 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



places of least intense spawning are areas with broken bottom where tidal and general 

 circulation produce extensive vertical turbulence, drawing cold water from the depths 

 to the surface, thereby delaying the vernal warming of the upper strata, as a rule, 

 until the mackerel spawning season is over. As nearly as may be determined from the 

 information on hydrographic conditions (Bjerkan, 1919, pp. 379-403, Bigelow, 1928, 

 pp. 550-585) and on spawning times and places (see above), the dividing line between 

 good and poor spawning areas may be drawn at a vernal temperature of about 8° C, 

 (46° F.). The areas that receive little or no spawn are, during the spawning season, 

 usually colder, and those that receive much spawn are usually warmer than this 

 temperature. 



NUMBER OF EGGS SPAWNED AND SIZE OF SPAWNING STOCK 



A rough estimate of the total number of eggs spawned in the region between Cape 

 Cod and Cape Hatteras can be made from the data of the 1932 survey of spawning. 

 The average catch during the fust seven cruises was about 1,000 eggs per 17 square 

 meters of sea surface (table 19), or an equivalent of about 200 million eggs per square 

 nautical mile. Taking 25,000 square miles as the areas surveyed, this would amount 

 to a total of 5,000 billion eggs. Since tbis figure is based on the average concentration 

 during a 50-day period, and since the period of incubation would average about 7 days 

 at the prevailing temperature, there must have been about 7 renewals or approximately 

 35,000 billion eggs spawned to maintain this average concentration. From a curve of 

 numbers of eggs taken in successive cruises, it appears that perhaps one-seventh should 

 be added to allow for the fact that the cruises did not begin early enough or extend 

 late enough to include all the spawning. This raises the figure to 40,000 billions eggs. 

 These are in all stages, and it may be computed from mortality rates of eggs (table 7) 

 that this would be equivalent to 1.6 times as many newly-spawned eggs. Applying 

 this factor, the final estimate of eggs spawned in this area in 1932 becomes about 

 64,000 billion. 



It is difficult to appraise the reliability of this estimate because of the uncertainty of 

 its components. Judging these as well as may be, it appears that at best it may be 

 within 25 percent of the true value and at worst only witbin the true order of magni- 

 tude. But this is only personal judgment, and since it is impossible to study statistical 

 probabilities, there is utility in testing the result by deriving a related statistic from 

 an entirely different source. 



During 1932 the catch of mackerel on or near spawning grounds during the 

 spawning season; that is, in area XXIII (Fiedler, Manning, and Johnson 1934, p. 96), 

 and in area XXII, west of Nantucket Shoals during April, May, and June, was about 

 13,000,000 pounds. From unpublished records on size composition of this catch, it 

 appears that about 10,000,000 pounds of it consisted of fish of spawning size, and that 

 their average weight was nearly 1.9 pounds. Thus, a take of about 5,000,000 spawners 

 is indicated. 



To estimate from this the size of the spawning stock it is necessary to know what 

 percentage this was of the spawning stock in 1932. This may be done only in an 

 indirect manner. The 1923 class of mackerel, after reaching spawning age, declined 

 at a rate of 20 percent per year as measured by the catch per purse seine boat during 

 the four seasons, 1928 to 1931 (Sette, 1933, p. 17). Tlus decline was so steady that it 

 probably should be ascribed to mortality rather than to other causes, such as changes 

 in availability. Of course one cannot be sure that the spawning population in 1932 

 was subject to the same mortality as the 1923 class during the previous years, but 



