200 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



increased fishing would deplete a natural stock of fish. So it 

 must, of course, provided the increase is great enough ; but 

 the question we have here to consider is whether the increase 

 of fishing in the North Sea trawling fleet did actually lead to 

 the diminution in the catches which was observed in the year 

 following 1907 ? We anticipated, as everyone then did, in 

 1915-8 that the great decrease in trawling must have an effect 

 upon the fisheries, and such statistics as were then available 

 for the North-west of England were examined with that question 

 in mind. 



Fluctuations in the abundance of Plaice on the Lancashire 

 Fisheries, 1892-1920. 



From the year 1892 onward and till the present time the 

 condition of the inshore ground, or nursery, just off the estuary 

 of the Mersey, has been regularly examined. Experimental 

 hauls, made with a beam trawl-net of 25 to 30 feet in breadth 

 and having meshes of 6 inches, have been made several times 

 every month. Similar hauls with a shrimp trawl-net, of 

 2-inch mesh, were also made : all these experiments were 

 carried out by Capt. George Eccles, a highly experienced 

 fisherman. Taking them critically one sees that the methods 

 were not always strictly comparable, but this does not affect 

 the broad, general conclusions that are certainly to be drawn 

 from the data. The latter are recorded and are fully discussed 

 by K. J. Daniel,* and the numerical results need not be given 

 here in full. But a graph is necessary in order to see where 

 they lead. 



Fig. 16, then, is such a graph. Two series of " averages " 

 have been plotted : first, the average catch per annum 

 (smoothed by taking three-yearly averages yearly, and second, 

 the " half -ranges " calculated as in the following example : — 



No. of plaice caught in a haul to 50, 51 to 100, 101 to 150, etc. 



No. of times such a catch was made ... 13, 6, 1, etc. 



Sums of the above frequencies ... 37, 24, 18, etc. 



* Bept. Lancashire Sea-Fish. Laby. for 1919. 



