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TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



fishery statistical investigations, that is, the material 

 studied consists of samples, and these are collected by 

 apparatus which does not, as a rule, give a sample really 

 representative of the natural population which we wish 

 to investigate. Fish samples, as well as plankton samples, 

 are taken by means of nets which catch some things, but 

 do not catch others, or which catch some things in much 



Fig. 4. Age-groups of Plaice in the Eastern waters of the Irish Sea. 

 Figures on the vertical axis are percentages of all the fish of each 

 group caught. Figures on the horizontal axis are the lengths of the 

 fish. The smooth lines in the figure pass near the points actually 

 plotted. The broken lines represent the probable shapes of the curves 

 for Groups I and IV if very large series of measurements were obtainable. 



greater proportions than they catch others. We assume, 

 for instance, that the ratio between the numbers of 

 copepods, peridinians and diatoms taken in a plankton net 

 are just the ratios in which these organisms exist in the 

 sea ; or that the ratio between the numbers of plaice of one 



