POPULATION DYNAMICS OF THE ATLANTIC HERRING 17 



feeding, maturing fish until the onset of spawning in August and September. 

 Then a movement takes place southwards to the spawning grounds; a 

 proportion of the stock spawns off the Scottish east coast in August and 

 September, while others move southwards to spawning grounds on the 

 Dogger Bank and Southern Bight. After spawning, these herring move to 

 the deeper, northern regions, where they remain prior to re-invading the 

 north-western North Sea in the following spring. 



The 'recruit' components appear on the Buchan fishing grounds in April 

 and May. They subsequently spread over these grounds in June and July, 

 but do not usually appear in abundance on the Shetland fishing grounds in 

 that year. Although the nursery areas from which the 'recruit' component 

 is derived are not fully known, it is probable that part of it originates from 

 the major nursery area to the south and east of the Dogger Bank and part 

 from the juvenile concentrations off the Scottish east coast. 



It is evident from the above that the Scottish fishery can be subdivided into 

 three components, which differ with respect to the composition and bio- 

 logical state of the herring concentrations which they exploit. These are: 

 (i) The Buchan 'pre-spawning' fishery, which takes place between May and 

 July, and is centred on the feeding, pre-spawning concentrations of 'recruit' 

 and 'adult' herring. 



(2) The Buchan 'spawning' fishery, which is pursued in August and early 

 September on the banks off the Scottish east coast, and exploits the concentra- 

 tions of early spawning herring, derived from the pre-spawning concentra- 

 tions. 



(3) The Shetland fishery, which, like the Buchan pre-spawning fishery, 

 exploits the feeding, pre-spawning concentrations. Unlike the Buchan 

 fishery, however, it is usually centred on concentrations of the 'adult' 

 component. 



The post-war changes in these fisheries have been described by Parrish & 

 Craig (1957). They may be summarized as follows: 



(i) The catch per unit of effort of the Buchan pre-spawning and spawning 

 fisheries and the Shetland fishery did not decrease after 1950, as at East 

 Anglia. In the Buchan fishery in fact it increased after 195 1, while in the 

 Shetland fishery it decreased sharply from a high immediate post-war level, 

 but increased once more after 1954. These data are shown in Fig. 5. 

 (2) However, striking changes took place in the composition of the Buchan 

 'pre-spawning' fishery catches, resembling those observed at East Anglia 

 (Fig. 6). There was: (i) a reversal of the three-year-old to four-year-old ratio 

 after 195 1, which corresponded in time with the change in ratio at East 

 Anglia; (ii) a reduction in the catch per unit effort of the age-groups older 

 than four after 1952. Between 1947 (the first post-war year for which age 



