38 D. A. HANCOCK AND A. C. SIMPSON 



and in mussels (Baird & Drinnan, 1957) and cockles there is a positive 

 correlation between rate of growth and the period of immersion. The 

 problem of density dependent effects on the growth of sedentary molluscs 

 requires careful investigation in view of the variable growth rates which 

 may occur as a result of other factors. Under favourable conditions, very 

 high densities may occur, leading to competition for food and space. 

 Kristensen (1957) concluded that the growth rate of second-year cockles of 

 2,000 per square metre density was reduced by competition. On Llanrhidian 

 Sands, the substratum was found to be physically saturated by 3,000 second- 

 year cockles per square metre, with reduction in growth rate occurring at 

 densities less than this. 



Owing to the difficulties in determining the moult increment and moult 

 frequency of Crustacea in the field, annual growth rates are not at all fully 

 understood. Most work has been done on the lobster, and in general terms 

 the moult frequency decreases until at maturity the moult occurs approxi- 

 mately annually in males and approximately every second year in females. 

 For the mature female, a year without carrying eggs alternates with one 

 during which eggs are carried, with moulting occurring shortly after the 

 eggs are shed, each second year. Just prior to maturity, the moult increments 

 of both sexes of lobsters are similar, but after maturity the moult increment 

 of the female decreases, so that with the reduced moult frequency the annual 

 growth becomes considerably less than half that of the males. Fig. 7 sum- 

 marizes the information at present available on the moult increment of 

 lobsters and shows a marked similarity betweeen the areas studied. The lack 

 of information on moult frequency from area to area means that it is still 

 not known whether moult frequency or moult increment is the more 

 important in causing differences in annual growth rate between different 

 areas. 



A feature of bivalve molluscs is that the whole volume is not always an 

 accurate indicator of the volume of meat within. This assumes importance 

 when predicting yields from a shellfish fishery. Cockles, for example, have 

 the best 'condition', i.e. ratio of meat volume to internal volume of the 

 shell, in midsummer (Fig. 6). The condition falls markedly from about 

 August onwards, and does not begin to improve again until April. Spawning 

 begins in late April to May, but by this time the rapid assimilation of food 

 materials soon hides any change in the meat volume, which increases rapidly. 

 A hundredweight sack of cockles has been found to contain 50 per cent more 

 meat in July than in February. The condition of shellfish from different areas 

 may be quite different. This fact has been made use of during oyster cultiva- 

 tion, in which oysters are transplanted first to areas known to encourage 

 rapid shell growth, and then to beds which give good growth of meat. 



