40 



D. A. HANCOCK AND A. C. SIMPSON 



good. Kristensen (1957) reported that 1947 was an extremely favourable 

 breeding year for cockles and many other species, but spatfall in the Dutch 

 Waddensea in following years was so poor that this remained the dominant 

 year class until 1953. This pattern of frequent poor year classes may be 

 related to the many factors which must be favourable for the survival of 

 estuarine pelagic larvae if a good year class is to result. The native oyster in 

 Britain is a viviparous hermaphrodite living towards the northern limit of 

 its distribution, and it has been found that in cool summers in which the 

 water temperature scarcely reached 20° C few larvae are liberated. As with 

 most small pelagic larvae, food can be of critical importance and Waugh 

 (1957) has found that survival of oyster larvae is related to the abundance of 

 suitable flagellates. Oysters require a suitable substrate for settlement, and 



Pje 8_ — Numbers of oyster spat which settled on ten complete oyster shells on Purleigh Shore 

 oyster ground, River Crouch, 1948-59 (from data supplied by G. D. Waugh). 



there can be intense competition for this by barnacles and other animals. 

 There is evidence (Cole & Kjiight-Jones, 1949) that oyster larvae are gre- 

 garious and settle most readily where older oysters are already present. 

 Together with these two latter requirements for settlement, the movement 

 of the water in estuaries at the times the larvae are planktonic may be 

 extremely important for survival. Spring tides and strong winds from 

 particular quarters could remove all the larvae from an estuary, so that few 

 remained where adult oysters and suitable substrates are present. Young 

 oysters may also suffer severe mortalities soon after settlement. Waugh 

 (1957) found a 50 per cent mortality among oyster spat during the fourteen 

 days following settlement, and even greater mortalities have been recorded 

 (Knight-Jones, 1952). In 1953 on one oyster laying, 73 per cent of spat which 

 had reached fmgernail size were dead by the December after settlement. 

 Fifty-eight per cent of the spat had been eaten by the American whelk tingle 

 (Hancock, 1954). Cockles, too, have a high mortality during their first year 



