POPULATION DYNAMICS OF THE ATLANTIC HERRING 21 



at age three, before and after the recruitment change, can be made for each 

 region from the data of the abundance of the three-, four- and five-year-olds 

 in successive years in the fisheries, and from values of the natural and fishing 

 mortahty rates operating in these years (Gushing & Burd, 1957). Such 

 estimates for the year-classes 1927 to 1934, and 1947 to 1953, recruiting the 

 East Anglian (Gushing, 1959) and Buchan fisheries respectively, are given in 

 Fig. 9. These show that the recruitment change was followed by a marked 

 increase in estimated year-class strength in the north-western North Sea while 

 in the south, it did not rise above the earlier level. At present the explanation 

 of this difference in total recruitment is not clear. Of particular importance 

 is the problem of the biological identity of the recruits to these two regions. 

 Are they members of distinct stocks of herring or do the spawning products 

 from all North Sea spawning concentrations form a common 'pool' of pre- 

 recruits, the time and direction of movement of which is governed by 

 external factors (e.g. growth rate, rate of maturation, hydrographical 

 features, food supply) ? 



As stated earlier, the available information points to the vicinity of the 

 Dogger Bank, as the nursery ground from which the bulk of the recruits to 

 the northern and southern North Sea fisheries are derived, but it is not yet 

 clear to what extent, and at what stage in their growth histories these com- 

 ponents become segregated. 



In view of the importance and complexity of this problem, steps have been 

 taken recently, under the auspices of the International Gouncil for the 

 Exploration of the Sea, to organize a conjoint international study of. the 

 distribution, abundance and biological characters of the adolescent and adult 

 herring concentrations in all parts of the North Sea. Amongst the questions, 

 for which it is hoped an answer will be provided by this investigation, are 

 the following: 



(i) Does the south-eastern North Sea nursery ground constitute the principal 

 source of recruits to the northern and southern North Sea fisheries, or are 

 the two regions recruited from separate nursery areas? 



(2) Gan the pre-recruits of the northern spawning groups be distinguished 

 from those to the south during their nursery ground phase ? 



(3) At what time and by what route do^the recruits leave the nursery grounds 

 for the northern and southern North Sea respectively ? 



(4) What are the growth characteristics of the adolescent fish, and are these 

 related to the magnitude of recruitment to the north and south ? 



The answers to these questions have an important practical as well as 

 biological significance. Studies on the Southern Bight spawning grounds 

 have provided evidence of a marked decrease in larval production in recent 

 years (Bridger, 1959), which is associated with the reduction in adult stock 



