SEA-FISHERIES LABORATORY. 169 



10 and 25 cms. in length, can escape out through the meshes. 

 Thus the fish-trawl tends to under-sample the smaller fish 

 inhabiting the ground over which it was dragged. 



We cannot allow for this by using both a shrimp-trawl 

 and a fish-trawl at the same time, for we don't know exactly 

 how much time to give to each method. We cannot employ 

 a compound net, that is, a small-meshed one laced round about 

 a wide-meshed one, for the small-meshed net restricts the flow 

 (or draught) of water through the large-meshed one and so 

 impedes the action of the latter. Table 15 shows that we get 

 a different result for the Age-group I, according as we use the 

 shrimp-trawl or the fish trawl. Therefore we have added 

 together the results of the fishing of these two instruments 

 and then made some rather arbitrary corrections, which, 

 however, are probably " quite all right." Thus we get the 

 Column " I " of Table 14. 



Observe here that we are dealing with a characteristic 

 " small-plaice " area. We might sample Luce Bay, the Solway 

 spawning grounds, Beaumaris and Ked Wharf Bays, each at 

 its appropriate season, and get much bigger plaice. Still, in 

 regard to the operation of any restrictions or prohibitions, 

 Table 14 shows us what are the kinds of plaice which will be 

 affected. 



It will be seen that a plaice of length of 25-5 cms. may be 

 1, 2, 3, or 4 complete years of age — and this represents an 

 apparently wide range of variation. But the table also shows 

 that, in all, 181 plaice between 25 and 26 cms. in length were 

 measured (that is, 181 out of 7,724 fish that were examined) ; 

 that 3 of these belonged to Group I, 148 to Group II, 29 to 

 Group III, and 1 to Group IV. Therefore, the chance that any 

 plaice caught at random belongs to Group II are 148 in 181 ; 

 that it belongs to Group I, 3 in 181 ; that it belongs to Group 

 III, 29 in 181, and that it belongs to Group IV, 1 in 181. 

 These are the " odds " in favour of the ascription of the fish 



