PAPJVMETERS OF MARINE INVERTEBRATE POPULATIONS 43 



first (Fig. 3), while the remainder may not reach market size until after the 

 winter period of about six months, during which there is no growth. Al- 

 though there is, with sedentary populations, the possibility of very efficient 

 removal down to a very low density this, in fact, rarely happens, except in 

 certain areas where cultivated oysters and mussels may be relaid in high 

 densities, all of one age or size, which are subsequently harvested efficiently. 

 With cockles, patches of high density are fished down to an uneconomic 

 level and the fisherman then moves on to another good patch. Areas between 

 are subsequently fished, but not systematically. Oysters are dredged until 

 catches become uneconomic, but because of the inefficiency of the method 

 many oysters will remain, as will the lobsters, crabs and whelks left behind 

 after catches in pots have dropped below a certain level. Individuals remaining 

 do not normally collect up together or shoal as do fish, so that they will not 

 be fished again until their numbers have been increased by recruitment and 

 growth. This means that shellfish stocks can seldom be completely fished 

 out, and this becomes important if it is considered that a certain level of 

 spawning individuals is required to ensure proper recruitment to the fishery. 

 A knowledge of the contributions made to the larval population by other 

 stocks is necessary before this can be decided. 



It has already been stated that on reaching maturity the growth of female 

 lobsters becomes less than half that of the males. Thomas (i955^) l^^s shown 

 that this results in the sex ratio altering strikingly with the size of the lobsters 

 and that the curve relating sex ratio to size will also change with the degree 

 of exploitation of the stock. This is demonstrated in Fig. 9. Thomas suggests 



30 7o ANNUAL MORTALITY 



70 7o ANNUAL MORTALITY 



INCHES 



Fig. 9. — Theoretical sex-ratios among lobsters of different sizes when subjected to 30 per cent 

 and 70 per cent total annual mortalities (from Thomas, 19556, Table I). 



