POPULATION DYNAMICS OF THE ATLANTIC HERRING 27 



DISCUSSION 



J. B. Cragg: Is any country carrying out standard trawling operations 

 which will give unbiased control data? 



B. B. Parrish: No. At present the sampling of the herring stocks in the 

 North Sea is conducted principally by each country from its commercial 

 fisheries. Also, there is the problem of varying accessibility of herring to 

 different gears, which makes sampling by a standard gear in all areas and 

 seasons very difficult. However, steps have been taken recently to undertake 

 conjoint international herring surveys for specific purposes, and such a 

 conjoint trawling survey is at present in progress in the North Sea to study 

 the distribution and abundance of the pre-recruits of the North Sea herring 

 stocks. 



D. H. Chitty: What data are the mortality estimates which you have 

 shown us based on ? 



B. B. Parrish: For both the East Anglian and the north-western North 

 Sea data, the mortality rates are estimated from the abundance indices of 

 successive year-classes between ages four and seven. In the slide I have 

 shown the average annual mortality rate between ages five and seven and 

 the corresponding values between ages four and five. 



K. R. Ashby: Can any of the changes in the herring stocks which you 

 have described be attributed to increased pollution of the North Sea since 

 the war? 



B. B. Parrish: There is certainly no evidence to suggest that this can 

 have played any substantial part in the recent changes in the herring stock. 



E. D. Le Cren: Is there any substantial competition for food between 

 the pre-recruit and older age-groups, which might have contributed to the 

 dechne in abundance of the older fish ? 



B. B. Parrish: The main food items of the pre-recruit and adult herring 

 is the same, namely Calanus, but the competition between them is likely to 

 be very small because of the spatial separation of the pre-recruit and adult 

 components of the stocks. 



E. D. Le Cren: It has been shown for some freshwater fish species that 

 growth rate and natural survival rate are inversely proportional. Is there 

 any evidence that this is so for the herring ? 



B. B. Parrish: There is unfortunately no direct evidence of it, but this 

 sort of mechanism is clearly of special significance to the observed changes 

 in the stock composition. The changes in mean size for age since 1950 have 

 been very striking, especially in the northern North Sea, and the substance 



