20 B. B. PARRISH 



magnitude. This loss rate is much too small to account for the 'loss* of four- 

 year-olds from adult stocks (Gushing, 1959). 



Thus the evidence suggests that the recruitment change in both regions 

 was not a direct consequence of the industrial fishery, but was the result of 

 a natural change (Gushing Sc Burd, 1957; Gushing, 1959; Parrish & Craig, 

 1957). The main features of the mechanism of the change are considered to 

 be as follows: 



(i) The movement of herring from the nursery areas to deeper water is a 

 function of size, which also governs the time of onset of first maturity. (The 

 existence of a general biological relation between growth and first maturity 

 in fish is discussed by Beverton & Holt (1959).) 



(2) Since the war, and especially since 1949, the growth rate of the adolescent 

 herring has increased, apparently as a result of an increase in the abundance 

 of food (Gushing & Burd, 1957; Burd, 1958). In consequence, a greater 

 proportion of each year-class has reached the 'critical' size for migration and 

 maturation in their third year of hfe, than before, and moved from the 

 nursery area to the 'adult' fishing grounds at this age. Gushing & Burd 

 (1957) show that after 1951, over 90 per cent of each year-class recruiting 

 the East Anglian fishery did so at three years of age. Data from the Buchan 

 fishery provide a similar figure. 



This hypothesis does not deny any effect of the industrial fishery on recruit- 

 ment to the adult fisheries, but allocates it principally to the fish recruiting 

 at three years of age. The results of the international tagging experiment 

 provides estimates of this effect as a reduction in total recruitment of about 

 15 per cent. The data available at present do not provide any clear indication 

 of its allocation between recruits to the northern and southern North Sea 

 fisheries respectively. 



THE MAGNITUDE OF RECRUITMENT 



This mechanism accounts for the main facts of the change in recruitment 

 pattern in both the northern and southern North Sea, which tends to support 

 the view that the adults fished in both of these regions originate mainly from 

 the same, or closely neighbouring nursery grounds, subject to similar 

 environmental conditions and changes. However, while the change in the 

 general pattern of recruitment has been the same in the two regions, there 

 have been important differences in total recruitment between them, especially 

 in recent years. In particular, the total abundance of three-year-olds, recruit- 

 ing the north-western North Sea fisheries (Buchan) increased after 195 1 

 relatively more than in the Southern Bight (East Anglia). Estimates of the 

 relative abundance of each year-class at an age immediately prior to recruiting 



