PARAMETERS OF MARINE INVERTEBRATE POPULATIONS 



45 



The system of management of certain cockle beds is influenced by a know- 

 ledge of consistent mortalities, such as those due to excessive turbulence 

 caused by high winds from a certain direction. Mortalities among shellfish 

 due to physical conditions are generally greatest in the winter. 



Seasonal mortalities may also be related to the behaviour or migrations 

 of predators. The American tingle feeds on oysters only when the tempera- 

 ture exceeds 9°C (Hancock, 1959). The oystercatcher concentrates its 



2 



u 

 o 

 o 



li. 



O 3000 



(A 



cr 



Ui 



m 



2 2000 • 



SAMPLE STATIONS 



m'j 'j'aIsIoInIo 

 1958 



F I M I A I M I J I J 



1959 



N I D 



J IfImI 



Fig. 10. — Mortalities of cockles on Llanrhidian Sands 1958-60. Sample area 21-26 was not fished 

 till August 1959 when light fishing commenced followed by heavy fishing beginning in 

 November. Sample area i-io was protected from fishing in August 1959. Sample area 

 11-20 was heavily fished from November 1959. 



attention on cockle and mussel beds during the winter (Drinnan, 1957, 

 1958). Starfish and fish have a seasonal feeding behaviour on young molluscs. 

 Crabs and lobsters are probably particularly vulnerable whilst in the soft- 

 shelled stage. 



The natural mortality rate varies with age. In oysters and cockles there is a 

 heavy natural mortahty during the first year after settlement (Fig. lo). Apart 



