RAPPORTS. XIII C2: THOMPSON 



18 



"/o 



100 

 90 



10 20 



Fig. i8. The Total Catch of Cod 

 by small-mesh net: Firth of Forth, 1903 — 1909 



SDctntitnElits 



of hauls, of any one species of fish. The example above given is a simple one, because 

 it deals with a single and homogeneous group of fish. When the group is not homo- 

 geneous, but contains several maxima or 'modes', and especially when (as we shall se in 



some of our Moray Firth curves) the 

 entire catch is composed of two very 

 diverse groups with the modes far 

 apart, then these points of the curve 

 become much less reliable, and the 

 whole method becomes less satisfactory. 

 Mr. J. Johnstone, in certain of his in- 

 vestigations upon Plaice for the Lanca- 

 shire Sea-Fisheries Committee, has made 

 use of the same method, but has drawn 

 his curves, point by point, for each 

 successive centimetre. To do this is 

 extremely laborious, and I am inclined 

 to think that the additional details so 

 furnished are perhaps better represented 

 by curves of the ordinary kind, such 

 as those shewn in the preceding figures of this Report. Suffice it then to say, that the 

 curves now under consideration, which we call summation curves, because at each point 

 they show us the to- 

 tal percentage of fish 100 

 that are above or be- 

 low the size corre- 

 sponding to that point 

 on the curve, are very 

 easily prepared , and 

 give us in a broad and 

 general way a very 40 

 simple view of the com- 

 position of the catch. 

 In Table II, I show 

 the medians, quartiles 

 and other points on 

 which our curves are 

 or may be based, for 

 the various stations in 



the Firth of Forth, and the same Table also includes the average catch of Cod per hour, 

 and also the percentage of Cod (by number) to the total catch of all fishes by the otter 

 trawl on the same stations. To illustrate the summation curves themselves we have in 

 figure 19 the three curves drawn for stations II, IV, and VIII. The two former stations 

 lie, as has been said, in the bays at opposite sides of the wider part of the Firth of 

 Forth, while Station VIII lies at the mouth of the Firth of Forth, south of the Isle of 

 May. It will be seen from the curves that the average composition of the catch is very 



80 



20 







10 



20 



30 



40 



50 



60 



70 



80 



90 



100 



Fig. 19. Diagram of the percentage numbers of Cod, 

 at and over each centimetre-size, from three stations in the Firth of Forth. 



