PARAMETERS OF MARINE INVERTEBRATE POPULATIONS 49 



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DISCUSSION 



T. B. Reynoldson: Have you any data on the relation between cockle 

 growth rates and the presence of parasites, such as trematodes? 



D. A. Hancock: No. The level of infection seems to be fairly general, 

 but the labour involved in studying its relation to growth would be pro- 

 hibitive in large samples. 



J. B. Cragg: With high densities this parasitism does seem relevant. 



D. A. Hancock: I agree that very high densities may be prone to rapid 

 spread of disease. Normally there is a very rapid and more or less exponential 

 falling off in numbers of cockles per square metre as growth proceeds. 

 When the site is full of animals there must be either reduced growth or 

 greater mortahty or both. 



E. D. Le Cren: Is plankton density a relevant factor? 



D. A. Hancock: I know of no critical work on the food of cockles 

 but flagellates are likely to be important. Competitive effects seem to occur, 

 and it seems that growth rates fall off at high densities, well before the 

 substratum is full. 



A. Macfadyen: Oystercatchers [Haematopus ostrakgiis) are said to take 

 only two-ring cockles — does this mean that they do no damage? 



D. A. Hancock: No. The average density of two-ring cockles in one 

 sample area fell from over 1,000 per square metre to 77-0 per square metre, 

 i.e. over 90 per cent in one square during one year. The mortality of brood 

 cockles was only 50 per cent and of cockles older than two-rings less than 

 this. Thus a high selective mortality occurs at two years and is thought to 

 be due to oystercatchers. The relationship of these high mortalities to the 

 total population is being calculated. 



