PARAMETERS OF MARINE INVERTEBPu\TE POPULATIONS 41 



of life (Fig. 10). Heavy settlement of young oysters frequently occurs well up 

 in estuaries, where low salinity and low temperatures often cause excessive 

 mortalities. 



In his methods of cultivating oysters, man tries to improve survival by 

 providing suitable surfaces for larval settlement in the form of cockle and 

 mussel shell, and he removes the young oysters from the vulnerable upstream 

 settlement grounds to safer growing grounds in deeper water and away 

 from winter freshwater run-off. More recently (Walne, 1959) experimental 

 work on artificial rearing of oysters is aimed at eliminating the problem of 

 *seed' supply. 



There are, however, cases among molluscs where recruitment is remark- 

 ably regular. On the rocky outcrops off Morecambe, Lancashire, the 

 recruitment of mussels is so regular and abundant that few mussels reach 

 more than about three years old before they are smothered with the regular 

 settlement and growth of the young ones. Occasionally, phenomenally 

 heavy spatfalls of mussels have occurred (Savage, 1956), when young mussels 

 covered the beds and smothered and killed practically the whole of the adult 

 stock. When this occurred in the Conway estuary in 1922 and 1940 a highly 

 productive fishery ceased until the young mussels had grown large enough 

 to be fished. 



With all populations which have pelagic larvae, the larvae from an 

 exploited or unexploited area may provide recruits to a distant fishery, so 

 that the exploitation of one area may affect recruitment to another. 



Recruitment to crustacean fisheries is also subject to considerable fluctua- 

 tions. Poor densities in the pink shrimp fishery in. 1949 were correlated with 

 the severe winter of 1947 (Mistakidis, 1957). The early stages of crabs and 

 lobsters are rarely encountered, but fluctuations in brood strength are 

 evident from the uneven recruitment to marketable stocks (Mistakidis, 

 personal communication), though with lobsters the variability in recruitment 

 is probably masked by the overlapping of year classes, since the stocks do 

 not reach commercial size until after about the fifth year of life. 



Of commercial shellfish, the whelk is the only species which has no 

 planktonic stage in the larval life history, the whole of which is passed 

 through in fixed chitinous capsules, from which the young emerge. In this 

 way many of the hazards of the developmental stages are avoided. Adverse 

 physical factors may still be an important cause of mortality — for example, 

 ground swell caused by high winds is said to cast the spawn ashore — but 

 there is less likelihood of catastrophic mortalities than amongst species with 

 free-swimming larvae. The abundance of the parent stock might here be 

 used with more confidence as an indicator of brood production, but the 

 relationship between recruitment and size of stock has not as yet been studied. 



