24 B. B. PARRISH 



time, and in some regions exploit different age and length components of 

 the fished stocks. This is particularly marked in the northern North Sea, 

 where the drift-net fisheries (principally British, Dutch and German) are 

 centred in the north-western area in the months May to July and the trawl 

 fisheries (principally German, Swedish, Polish and Dutch) take place later 

 in the year, on the Fladen and Gut grounds. Similarly, in the southern North 

 Sea, the drift net fisheries operate principally off the East Anghan coast, 

 while the trawl fisheries are centred on the spawning grounds off the Belgian 

 and French coasts (see Fig. 3). Also, throughout the inter-war and post-war 

 periods, the relative sizes of the trawl and drift net fisheries have changed 

 substantially (Fig. 2), and the areas fished have been extended; further, the 

 size and engine power of the trawling fleets have increased, and the introduc- 

 tion of fish detection methods (e.g. echo-sounders) has probably led to an 

 increase in the 'efficiency' of the fishing units (Gushing, 1959). It is clear 

 therefore that the total effective effort exerted each year on the fully recruited 

 age-groups must take into account the differences in fishing power and 

 * selectivity'* between the trawler and drifter fleets and the changes in fishing 

 power and efficiency of each. 



For all except the selectivity problem, these difficulties can be handled in 

 two ways: 



(i) By choosing one fishery and method of capture for which detailed catch 

 and effort statistics are available, and estimating total effort in 'equivalent 

 units' by raising its effort by the ratio of its catch to the total catch (GuUand, 

 1955). The chosen fishery should be a major component of the total fishery, 

 and one in which changes in fishing power and/or efficiency during the period 

 in question are known. 



(2) By determining the relative fishing powers, efficiencies and selectivities 

 of the fishing units in all components of the total fishery, and combining the 

 corrected effort units. 



The first of these methods has been used by Gushing (1959) hi compiling 

 the total effort for the fisheries in the southern North Sea. He used the 

 statistics of the East Anglian drift net and German trawl fisheries as standard 

 and estimated total effort in terms of British drifter shots and German 

 trawlers fishing days respectively. An 'efficiency' index was used to adjust 

 for estimated changes in the efficiency of the drifter fleet in recent years. 



These two sets of effort data follow approximately the same course during 

 the post-war period. Effort increased to a peak in the years 195 1-4 and there- 

 after decreased, at least up to 1958. 



These estimates take account of the major changes in the fishing power 



* 'selectivity' is used here in its widest sense, and includes selection of stock strata (size-groups, 

 age-groups, etc.), as well as the 'inherent' selectivity of the gear (e.g. mesh selection). 



