28 B. B. PARRISH 



of the 'natural factors' hypothesis is that the natural mortality rate has 

 increased following the growth change. 



G. V. Nikol'skii: Is there any evidence of the influence of gill-net 

 selectivity on the 'quality' of the stock? For example, one would expect 

 that the thickest of the smaller fish and thinnest of the large fish would be 

 caught. This might have important long-term effects on the genetical 

 composition of the stock. 



B. B. Parrish: There is unfortunately no detailed information either on 

 the selection of 'quality' characteristics of the fish, nor I feel do we know 

 enough of the determinants of 'quality' to assess this type of selectivity 

 effect. It is an important problem which warrants close examination, but I 

 do not envisage an easy or early solution. 



A. Milne : In recent years large fleets of Russian fishing vessels have 

 concentrated off the Shetlands in spring. Have their activities contributed to 

 the recent changes in the herring stocks in the North Sea? 



B. B. Parrish: The Russian fishery in the spring has in fact taken a 

 relatively small part of the total annual herring catch from the North Sea. 

 Furthermore, in the spring, fishery in this region is centred largely on 

 members of the Atlanto-Scandian herring tribe and not the main North Sea 

 stock. However, the Russian data are included in the assessment of total 

 North Sea catch and fishing effort. 



B. N. K. Davis: Are the continental trawl fisheries and the British 

 drift net fisheries exploiting the same herring stock? 



B. B. Parrish: This is an important point which I have not been able 

 to deal with in the time at my disposal. The problem of stock identity of 

 the North Sea herring is still the subject of intensive investigation in Europe. 

 Current evidence indicates that the trawl and drift net fisheries in the southern 

 North Sea together exploit immediate pre-spawning and spawning members 

 of a 'Downs' herring group, spawning off the Belgian and French coasts, 

 while the fisheries in the northern North Sea are centred on the earher 

 maturity stages of this group and menibers of a 'Bank' group, spawning in 

 the north-western North Sea and on the Dogger Bank. 



