SEA-FISHERIES LABORATORY. 207 



and two years of age taken in the six-inch trawl net is also 

 the ratio between those fishes living on the fishing grounds. 

 Unless we had some means of investigating the " catching 

 power " of plankton nets and fish-trawls, we should have to 

 stick to this postulate; but we do know that a plankton 

 net of No. 20 silk does not give us a real sample of 

 copepods or fish eggs, while it may give us a real sample 

 of the larger diatoms; and we also know that a trawl-net 

 of 6-inch mesh gives us a real sample of plaice of Group II, 

 but not a real sample of Group I or Group IV. The same 

 source of error is inherent in each of the plankton and 

 fish-nets: the copepods and fish eggs enter the plankton 

 net, and are, to some extent, swirled out again, and the 

 smaller diatoms pass through the meshes of the silk; 

 while the larger plaice of Group IV enter the 6-inch trawl 

 net, and may escape out from its mouth, while the small 

 ones of Groups and I escape through the meshes. It 

 seems easy enough to remedy this defect in the collecting 

 methods : we might use simultaneously plankton nets of 

 different meshes, or fish-trawls of different mesh dimen- 

 sions ; and as a matter of fact, this has been attempted. 

 Plankton nets of coarse and fine silk are employed, and 

 fish-trawls of \, 4, 6, and 7-inch meshes are employed, for 

 the collection of pelagic organisms and of fishes. But 

 again, we must consider whether or not we have employed 

 just the right sorts of nets, and whether we have obtained 

 just the right numbers of samples from each. Do, for 

 instance, parallel hauls with plankton nets of (say) 10 and 

 20 silk, or parallel hauls with ^-inch and 6-inch trawl- 

 nets sample the plankton and fish population? We are, 

 I think, able to answer this question in regard to the fish 

 samples, but I do not know of any easily-available means 

 of answering it in the case of the plankton samples. And 

 yet, until we can answer the questions indicated, and then 



