PARAMETERS OF MARINE INVERTEBRATE POPULATIONS 



39 



^15 



5 



z 



3 



o 



2 5 



-'"^fs 



70 80 90 



CARAPACE LENGTH mm 



Fig. 7. — The percentage increase in length on moulting of male and female European and 

 American lobsters of 55-110 mm carapace length (6-5-12 in. total length). Data from (a) 

 south-east Scotland (Thomas, 1958); (b) North Wales (Simpson, 1957); (c) Pointe du Chene, 

 Canada (Templeman, 1948); (d) Grand Manan, Bay of Fundy, Canada (Wilder, 1953); 

 (e) Lismore, Nova Scotia (Wilder, 1953). 



RECRUITMENT 



Egg production in commercial shellfish may be extremely high (Table II) 

 and in all except the whelk the larvae have a pelagic phase. 



Variation in the strengths of year classes is very pronounced among shell- 

 fish, and in cockles, oysters, and clams appears to take the form of a series of 

 poor year classes, with good settlement and survival occurring only at 

 intervals of several years. Waugh (1957) found that the spatfall of oysters in 

 the Rivers Roach and Crouch was only moderate in 1950, and light in 

 subsequent years until 1957, when there was a good settlement (Fig. 8). On 

 Llanrhidian Sands there was an abundant settlement of cockles in 195 1, after 

 which annual recruitment was only slight until 1958, when settlement was 



Table II. Fecundity of shellfish 



* Brackets contain numbers of juvenile whelks emerging. 



