SEA-FISHERIES LABORATORY. 181 



This method may appear somewhat artificial, but is 

 not so in reality to any great extent, as it merely means 

 that the values of the numbers for the months 

 immediately before and after each given month are 

 allowed an influence in determining the new values for 

 that month. 



Now if we proceed to construct curves showing the 

 variation in the average hourly catches of plaice and 

 soles made per month on the banks, we notice a distinct 

 similarity in the shapes of these curves, i.e., where there 

 is an increase in the number of plaice, there is also an 

 increase in the number of soles, and vice versa. We do 

 not, however, know whether the rate of increase is the 

 same in the two cases, that is, for instance, whether a 

 10 per cent, increase in plaice would correspond with 

 a 10 per cent, increase in soles. Now, in order to find 

 whether this rate of increase is the same or different, we 

 may take, instead of the actual numbers of plaice and 

 soles caught, the logarithms of those numbers, and the 

 curves constructed from the latter values show the relation 

 between the rates of increase and decrease, that is to say, 

 if the two curves have the same shape and height the 

 rate of increase is the same in the two cases. This has 

 been done (figs. 1, 2) for the monthly variation of 

 plaice and soles on the Mersey banks, and also for the 

 monthly variation of average hourly catch of soles with 

 shrimp-trawl and six inch mesh fish-trawl. 



We are now in a position to attempt to draw con- 

 clusions from the results obtained by the above methods. 

 The data are not sufficient to warrant any conclusion 

 regarding yearly variation except in the case of numbers 

 of plaice and soles caught by the shrimp-trawl net on the 

 banks in the summer months. The curves plotted from 

 these values seem to indicate a complementary relation- 



