POPULATION DYNAMICS OF THE ATLANTIC HERRING i3 



(2) At the same time there was a marked change in the size and age composi- 

 tion of the catch. This consisted of: 



(a) A change in the relative abundance of three- and four-year-old fish in 

 the catch. Whereas in pre-war years and post-war up to 195 1, four-year- 

 olds were, on average, about twice as abundant as three-year-olds of the 

 same year-class, after 195 1 the three-year-olds become the most abundant 

 age-group and the ratio of numbers was reversed. 



(b) A reduction in the abundance of the age-groups older than four. This 

 was very marked after 1952, and in 1955-7 the average abundance of these 

 age-groups was only about a quarter of the pre-1952 level. These changes 

 are illustrated in Fig. 6. 



(3) Corresponding with the decrease in abundance of the older age-groups 

 after 195 1 there was an increase in the apparent total mortality rate amongst 

 the fully recruited age-groups. These data are shown in Fig. 7. 



(4) Associated changes were also detected in the mean lengths of the exploited 

 age-groups. These are shown in Fig. 8, which shows a greater mean length 

 for age post-war than pre-war and a further marked increase after 1949. 



It is evident, therefore, that the overall change was a complex one, 

 embracing the pattern of recruitment to the fishery and the abundance and 

 loss rate of the fully recruited age-groups. 



These phenomena are not confmed to the British East Anglian fishery ; 

 they have also been detected in the drift net and trawl fisheries of other 

 countries in the southern North Sea (Ancellin, 1955; Krefft, Schubert & 

 Sahrhage, 1955; Zijlstra, I957)- 



During this period of major decline and change in the East Anglian fishery, 

 the total landings of herring from the Southern Bight increased. This was 

 due to the rapid growth of trawling on the spawning grounds after 195 1. 

 It increased from between 150,000-180,000 tons in the period 1946-50, to 

 about 250,000 tons in 1953 (Gushing & Burd, 1957, p. 20). 



THE NORTH-WESTERN NORTH SEA HSHERY 



The British drift net fishery in the north-western North Sea between May 

 and early September is centred in two .main regions: (i) in the outer reaches 

 of, and eastwards from, the Moray Firth, and off the Scottish east coast (the 

 Buchan fishery); (2) to the east of the Shetlands (the Shetland fishery). Of 

 these, the Buchan fishery is now the largest, and has produced over 80 per 

 cent of the total Scottish post-war herring catch in the north-western 

 North Sea. Drift net and trawl fisheries of other European countries are also 

 conducted in this region, the most important being the German, Swedish, 

 Dutch, Belgian and Polish trawl fisheries on the 'Fladen' and *Gut' grounds, 

 and the Dutch and German lugger fisheries (Fig. 3). 



