POPULATION DYNAMICS OF THE ATLANTIC HERRING 25 



and efficiency of the fishing fleets, but they asume that (a) exploitation of 

 the Southern Bight spawners is confined to the southern North Sea and 

 Dogger fisheries; (b) there are no major differences in 'selectivity' between 

 the fishing fleets. 



It is known from the results of tagging experiments (Hoglund, 1955; 

 Wood et ah, 1955) that some fish present in the northern North Sea in the 

 summer are subsequently fished later in the year in the southern North Sea 

 fisheries. However, it is not possible, from biological data available at 

 present, to identify them and determine their distribution and abundance in 

 this region or their proportion in the northern North Sea catch. Also, 

 information on the time and spatial distribution of age-groups in the southern 

 North Sea indicates that age selection differences exist both in time and 

 between the fishing fleets exploiting the southern North Sea stock. 



These observations illustrate the current difficulties and uncertainties 

 associated with the measurement of the effort exerted on the Southern Bight 

 spav^oiing stock. 



Similar problems arise in effort measurement for the northern North Sea 

 fisheries. Of particular importance in this area is the marked age stratification 

 of the exploited stock over the fishing grounds, especially over the north- 

 western North Sea drift net grounds (Parrish & Craig, 1957; Parrish et al, 

 1953-9)- This raises difficulties in the allocation of effort to the age-groups 

 for which the estimated mortahties refer. 



However, a first assessment of the effort exerted in this region from the 

 published statistics of the principal fisheries suggests that, as in the Southern 

 Bight, the total effort increased between 1946 and 195 1-4, due to the growth 

 of the trawl fisheries, and thereafter decreased to some extent. 



THE RELATION BETWEEN MORTALITY AND EFFORT 



When the estimated total mortality rates for both regions (Fig. 8) are viewed 



in the light of these effort changes, it is evident that 



(i) They did not increase markedly during the period of increasing effort in 



the period up to 1953. In the Buchan area they were mostly low up to 1952 



(negative values were obtained between 1949-50 and 195 1-2), but they were 



consistently high in later years. 



(2) In both areas, but especially in the north-western North Sea, they 



fluctuated widely from year to year. 



On a superficial inspection of the mortahty and effort data, it is not possible 

 to assign clearly the major changes in total mortahty rate to its fishing 

 mortality components, generated by the fishery. This difficulty is at the root 

 of the current dilemma in the interpretation of the mortahty changes, and 

 in estimating the magnitudes of the fishing and natural mortahty rates. The 



