1 8 B. B. PARRISH 



data are available) and 1952, this fluctuated widely, being higher in 1952 and 

 1953 than in the preceding years, but between 1952 and 1957 it fell sharply 

 to reach in 1957 a level less than a quarter of the 1947-52, and pre-war 

 average; (iii) corresponding with this decrease, the apparent total mortality 

 rate increased sharply after 1952, and remained at a relatively high level in 

 subsequent years (Fig. 7) ; (iv) increases in mean length of the age-groups took 

 place after 1949, similar to those observed at East Anglia. These were 

 particularly large among the four- and five-year-olds (Fig. 8). 



Therefore, although in this component of the north-western North Sea 

 fishery, the period after 1950 was not characterized by declining unit catches 

 as at East Anglia, similar changes took place in their composition. 



In the Buchan 'spawning' and Shetland fisheries, these changes were less 

 marked, but in both of them the unit catches of the older age-groups fell 

 off after 1954, and the apparent total mortality rate increased. 



Data from other countries fishing in the northern North Sea are less 

 complete than those for the British fishery, but statistics and age composi- 

 tions are presented for the German trawl fishery on the Fladen ground by 

 Krefft, Schubert & Sarhage (1955) and Schubert (1957), and for the Belgian 

 trawl fishery in the northern and central regions by Gilis (1957). Statistical 

 data for the Dutch lugger and trawl fisheries in the north-western North 

 Sea are also presented by Zijlstra (1959). As with the Buchan fishery, the 

 data for these fisheries show no striking decrease in catch per unit effort 

 in the period since 1950, but they exhibit the same general change in the 

 composition of the catch, especially after 1954. The general phenomena are 

 not, therefore, confined to the area exploited by the Buchan fishery, and are 

 common to the major north-western North Sea fisheries. 



INTERPRETATION OF CHANGES 



The results presented above show that since the war, similar changes have 

 taken place in the composition of the catches taken in the southern and 

 northern North Sea fisheries and they can be identified as {a) a change in 

 recruitment; (/;) a decline in abundance and an increase in apparent total 

 mortality rate amongst the fully recruited age-groups. These phenomena 

 have together resulted in a marked decrease in the age and size composition 

 of the exploited stock in both areas and a greater dependence of the fisheries 

 on the recruit age-groups. These changes are therefore of great practical 

 importance, and the principal question arising is whether they are the result 

 mainly of man-made or natural influences. 



In the remaining sections of this paper, the problems arising in answering 

 this question are discussed in the light of available data for the northern and 



