44 D. A. HANCOCK AND A. C. SIMPSON 



that a study of the sex ratios in certain length groups of Crustacea may afford 

 a means of approximate estimation of the mortahty rate. 



NATURAL MORTALITY 



Natural mortality is computed as the difference between total mortality and 

 fishing mortality, and cannot be measured directly in exploited populations. 

 In unexploited populations, some of the causes of mortality will not be 

 represented in the estimate, namely the damage to the stock by fishing gear, 

 the effects on mortality of overcrowding, and possible changes in predation 

 rates on undisturbed or concentrated stocks. Exposed sedentary populations 

 have the particular value that natural mortality can be estimated directly,and 

 some of the causes of error in assessing natural mortality in unexploited 

 populations can be estimated. Experiments to this end are in progress on the 

 Llanrhidian Sands cockle beds. Areas of lo metres square have been fenced 

 off and fishing prohibited within them. Regular sampling within these areas 

 has allowed the calculation of natural mortality rates in varying conditions 

 of exposure and density (Fig. lo). The effects of fishing methods may be 

 examined by marking off an area, estimating its population by sampling and 

 then subjecting it to fishing. The effect of raking and sieving cockles, and 

 the fate of cockles passing through the sieve can then be determined, after a 

 period of tidal coverage, by relating the yield from the area to the original 

 density and the number remaining. 



Unlike mobile species, which are either unlikely to be subjected to extreme 

 conditions or are able to move away from them, sedentary inshore popula- 

 tions have only a very limited power of evading adverse environmental 

 conditions. A combination of unfavourable circumstances frequently results 

 in heavy or catastrophic mortalities. This is specially true of intertidal 

 populations which are subjected to the greater extremes of physical condi- 

 tions found in the terrestrial environment. Flourishing beds of exposed 

 shellfish are frequently found decimated by frost or unusual heat, storms, 

 winds and strong currents (Wright, 1927). Sublittoral populations are not 

 immune from extreme conditions. The severe winters of 1939-40 and 1947 

 were responsible for great mortalities amongst oyster populations on the 

 east coast (Cole, 1940; Knight-Jones, 1952). Extremes of salinity are danger- 

 ous, particularly when combined with extremes of temperature. As men- 

 tioned previously, a feature of oyster fisheries is that nursery grounds are 

 frequently at the heads of estuaries, where a combination of low salinities 

 and low temperatures causes heavy winter losses (Waugh, 1957). This 

 obviously influences management of the fishery, and young oysters are 

 collected before winter and relaid in areas more favourable for survival. 



