BIOLOGY OF THE ATLANTIC MACKEREL 207 



In stating existing theory, one can do no better than to quote Johan Hjort, who, 

 perhaps more than anyone else, was responsible for bringing attention to the impor- 

 tance of year-class success or failure as the explanation of fluctuations in the sea 

 fisheries. In 1914 he advanced, and in 1926 (p. 32) reiterated, the theory that: 



The rich year-classes appear to make their influence felt when still quite young; in other words, 

 the numerical value of a year-class is apparently determined at a very early stage, and continues in 

 approximately the same relation to that of other year-classes throughout the life of the individuals. 



It has already been shown that the observations on mackerel in 1932 are in harmony 

 with this theory (p. 204). 



Hjort (1926, p. 33) in discussing the great Norwegian cod and herring fisheries, 

 suggested further: 



As factors, or rather events which might be expected to determine the numerical value of a new 

 year-class, I drew attention to the following two possibilities: 



(1) That those individuals which at the very moment of their being hatched did not succeed in 

 finding the very special food they wanted would die from hunger. That in other words the origin of 

 a rich year-class would require the contemporary hatching of the eggs and the development of the 

 special sort of plants or nauplii which the newly hatched larva needed for its nourishment. 



(2) That the young larvae might be carried far away out over the great depths of the Norwegian 

 Sea, where they would not be able to return and reach the bottom on the continental shelf before 

 the plankton in the waters died out during the autumn months of their first year of life. 



Observations on mackerel do not support the first possibility. Mortality imme- 

 diately after hatching was little, if any, greater than at other times, and hence failure 

 of the 1932 class could not have been due to acute dearth of food at the hatching time. 

 If shortage of food was responsible, it had its effect either throughout the period of 

 planktonic existence or at the transition phase (9- to 10-mm.), well after the hatching 

 time. 



On the other hand, the second possibility has strong indications of support in the 

 mackerel data. Not only did the heightened mortality at the 9- to 10-mm. lengths 

 appear to be connected with drift of the larvae, but there also was a marked corre- 

 spondence between success of the year-classes 1930 to 1933, and the drift that they 

 must have experienced as the result of dominant winds in May of these four years. 



That drift may in general be an important influence on success of year classes is 

 further suggested by a similar finding for the American haddock (Walford, 1938, 

 p. 55), wherein the relative failure of the 1932 class corresponded with drift of larvae 

 away from Georges Bank, and relative success of the 1931 class corresponded with a 

 pattern of circulation that kept the larval population on Georges Bank. 



Thus, in the two instances where the events at sea have been traced, it was the 

 oceanic circulation that influenced the success of year-classes; and in the one case 

 where the course of mortality (in a failing year class) at sea was traced, it was not any, 

 if at all, higher at the hatching time, and hence failure could not be attributed to acute 

 shortage of food at this period. 



In addition to the actual facts observed and their contribution to the understand- 

 ing of year-class success or failure, the development of technique for determining mor- 

 tality rates can have significant influence on future development of fishery science. 

 If applied over a series of years, it woidd provide the data needed for separately evalu- 

 ating the correlation of the size of the spawning stock with numbers of resulting off- 

 spring, and the correlation of the survival of offspring with the contribution of the 

 year-class to the commercial stock. The predictive uses of such knowledge would be 

 of obvious value to the conduct of fishing operations and to the trade in fishery 

 products. But the value of such knowledge in formulating conservation policies would 



