50 D. A. HANCOCK AND A. C. SIMPSON 



J. B. Cragg: Are there any sample areas outside the range of the oyster- 

 catcher? 



D. A. Hancock: No. We are attempting to assess and predict natural 

 mortalities in order to increase yield of this particular area (Llanrhidian 

 Sands), and the oystercatcher is just one factor we are studying with an 

 open mind. 



D. Chitty: Are good spat years uniform round the coast? 



D. A. Hancock: There are some *bumper years' in which everywhere 

 has good brood settlement. Others are much more local — from river to 

 river and site to site. 



M. E. Solomon: After a 'bumper year' does a population peak carry 

 through to the adult stage or does it get flattened out? 



D. A. Hancock: After a good settlement year of cockles the physical 

 saturation of the settlement area may lead to reduced settlement in subse- 

 quent years, even if these are potentially good. But causes of irregular 

 recruitment are more fundamental than this. These year peaks remain 

 evident for a number of years. 



N. Waloff: Are the 'good settlement years' due to high fecundity? 



D. A. Hancock: In oysters good conditions of temperature and food 

 supply seem to be the keynote. My colleague, Mr G. D. Waugh, has shown 

 that the temperature must reach a certain level for the liberation of larvae 

 and must remain above i8°C for a period for larvae to develop. Also food 

 supply (flagellates) is important if early larvae live to reach settlement stage. 



E. D. Le Cren: With the selection of cockles as they reach marketable 

 size, is there any evidence to show that the size limit has led to the evolution 

 of slower growing strain? 



D. A. Hancock: It is important to discover where recruitment is 

 coming from — it may come from unexploited stocks nearby. Certain parts 

 of cockle beds are not exploited because stocks are slow growing for environ- 

 mental reasons. These grow well if transplanted and much egg production 

 may come from these. 



