POPULATION DYNAMICS OF THE ATLANTIC HERRING 19 



southern North Sea herring fisheries and stocks. For convenience the two 

 components of the change are discussed separately. 



THE RECRUITMENT CHANGE 



As shown in Fig. 6 the recruitment change in both regions resulted from an 

 increase in the abundance of three-year-old herring in the catches rather 

 than a decrease in abundance of fours. This increase was most marked in the 

 north-western North Sea, where it more than compensated for the decrease 

 in abundance of the older age-groups, and so gave rise to the relatively high 

 unit catches after 195 1. The abundance of four-year-olds, following this 

 increase, remained at about the same level as before, in both regions. 



It is appropriate to examine first whether this change was the result 

 principally of man-made influences as postulated for the southern North Sea 

 by Hodgson (1956, 1957), or whether it constitutes a natural change in the 

 pattern of recruitment. 



Hodgson attributes the reduction in the three- to four-year-old survival 

 rate to the fishing mortahty generated by the industrial fishery in the south- 

 eastern North Sea, which grew rapidly from about 5,000 tons in 1950 to 

 over 100,000 tons in 1955. He postulates that the faster growing component 

 of each year class, which recruits the fisheries at three years of age are outside 

 the area exploited by the industrial fishery, which is based, principally on 

 the slower growing members, recuiting at four. 



It is difficult however to reconcile this hypothesis with the available 

 statistical and biological data. The following items are of particular relevance, 

 (i) Danish investigations show that the industrial fishery is centred principally 

 on two-year-old herring (II Group) (Bertelsen & Popp Madsen, 1953-?; 

 Gushing, 1959) which mostly leave this region at the end of their second 

 year of life. In recent years, between 70-80 per cent of the exploited pre- 

 recruit stock has been of this age. 



(2) The biological characteristics of these pre-recruits are similar to those 

 recruiting the adult fisheries at three years of age. Danish scientists (Popp 

 Madsen, personal communication) have estabhshed a relation between the 

 relative abundance of the smallest members of successive year-classes as two- 

 year-old pre-recruits on the nursery grounds and as three-year-old recruits in 

 the southern North Sea fisheries. 



(3) The results of international tagging experiments on these pre-recruits, 

 organized by the International Council for the Exploration of the Sea in 

 1957 and 1958 gave estimates of fishing mortahty, generated by the industrial 

 fishery, of approximately 14 per cent and 15 per cent respectively. Estimates 

 of the loss rate using other methods (Gushing, 1959) are of the same order of 



3 



