30 



D. A. HANCOCK AND A. C. SIMPSON 



to average out these variations or, more usefully, to obtain reliable informa- 

 tion on the variations over the area under investigation. 



Intertidal sedentary populations may be sampled by methods similar to 

 those used in terrestrial ecology of plants. These populations have the 

 advantage that every individual is directly accessible and stationary, so that 

 the size of area sampled need depend only on the time available. Samples are 

 taken by removing areas of known size from the shore, and counting and 

 measuring their contents. Sampling stations may be arranged conveniently 

 in the form of transects, and subsequent observations can be made at the 

 same sampling positions. These methods have been used successfully for 

 assessing changes in cockle densities (Fig. i) and the distribution of mussel 



BURRY RIVER 



Fig. I. — Diagrammatic representation of the results of a survey of cockle resources. 

 Llanrhidian Sands, South Wales, May 1959. 



and clam resources on the shore. Sublittoral sedentary molluscs (oysters, 

 scallops) may be sampled by dredging, which is the commercial method of 

 exploitation. The efficiency of the gear may be calculated (Shelbourne, 1957) 

 and the accuracy of samples checked by diving (Baird Sc Gibson, 1956) and 

 by grab sampling (Mistakidis, 195 1). Diving may also be used for sampling. 

 Sampling by a dredge, as with a trawl, involves selection depending on ring 

 size or mesh. Mason (1957) found that commercial dredges select scallops of 

 three to four years and older. Baird & Gibson (1956) concluded that tooth 



