POPULATION DYNAMICS OF THE ATLANTIC HERRING 23 



THE DECLINE IN ABUNDANCE OF THE FULLY RECRUITED AGE-GROUPS 



This component of the recent changes in the northern and southern North 

 Sea was due to a decrease in abundance of the older age-groups and also 

 an increase in the total mortality rate between them (Figs. 6 and 7). The 

 latter observation indicates that the decline in abundance was not simply a 

 direct consequence of the recruitment change (due to additional fishing 

 mortality experienced by each year-class between ages three and four) or to 

 changes in relative recruitment. It shows that other influences gave rise to 

 an increase in the loss rate amongst the fully recruited age-groups. The 

 identification of the causes of this loss rate are therefore crucial to the interpre- 

 tation of the fishery changes. 



The total mortality rate is composed of two components, fishing mortality, 

 which is generated by the fishery, and hence constitutes the chief man-made 

 influence, and natural mortality, which is the loss due to natural agencies 

 (defmed here in its broadest sense to include loss due to emigration and 

 changes in availability and accessibility). Natural deaths (or other losses due 

 to natural causes) in the sea cannot be measured directly, nor, usually, can 

 the agencies causing them be identified and measured. So, in investigating 

 the causes of the total mortality rate change, it is necessary to examine the 

 extent to which it is due to its fishing mortahty component, through changes 

 in the fishing intensity. This method of assessment has been used in numerous 

 fishery investigations in recent years; its theoretical basis is described by 

 Beverton & Holt (1957). 



For the application of this method, unbiased estimates of total apparent 

 mortality rate and accurate measures of effective fishing intensity are required. 

 The provision of both of these for the North Sea herring present major 

 difficulties, especially the measurement of fishing effort (fishing intensity is 

 the fishing effort per unit area). 



THE MEASUREMENT OF FISHING EFFORT 



The main, current difficulties in obtaining accurate measures of fishing effort 



for the North Sea herring fisheries stem from two principal sources*: 



(i) The complexity of the fisheries in composition, distribution, fishing 



power and 'efficiency'. 



(2) Variations in, or uncertainty concerning the distribution, availabihty and 



accessibility of the stocks fished in different parts of the North Sea. 



The fisheries in the northern and southern North Sea are exploited by two 

 main gears, trawl and drift net. In the main these are separated in space and 



* The lack of published effort statistics for some countries is also a current difficulty, but it can 

 be overcome by the estimation of 'equivalent effort units' for these countries, using catch data. 



