SEA-FISHERIES LABORATORY. 179 



and with constantly changing nets. In a reliable series 

 of observations the length and duration of the drags 

 would be always the same. To compare drags of varying 

 lengths after correcting them for the length of the haul, 

 as is done by the Inspectors of Fisheries in the paper 

 already referred to, appears to us to be fallacious. For 

 the amount of fish caught by a trawl net is apparently not 

 proportional to the distance trawled over. That is, during a 

 3-hours drag over a uniform ground one third of the catch 

 will not be made during the last hour, for as the net 

 becomes filled with fish and other material its catching 

 power diminishes. This is because the weight of material 

 in the cod end may, with a certain construction of net, 

 tend to cause the irons and foot rope to bear less heavily 

 on the ground, perhaps to become lifted altogether. And 

 the more full the net becomes the less water will pass 

 through its meshes, and some kinds of fish entering the 

 mouth will have much better chances of escaping. 



In an ideal series of trawling observations the figures 

 obtained would represent the varying density of fish on 

 the bottom on the areas considered, and would represent 

 this only. But we have seen that the catch made may 

 be influenced by many causes such as the daily and fort- 

 nightly state of the tide, and the constants of the ap- 

 paratus employed, and these may vary quite independently 

 of any variation in the density of the fish. To correct the 

 observations for these varying conditions, such as would 

 be done in a quantitative physical experiment, is, in the 

 present state of our knowledge of the matter, quite im- 

 possible. And the distribution of the fishes on the bottom 

 is probably much more variable than is generally sup- 

 posed, so that to obtain results which would apply to even 

 a moderately large area, 100 square miles, for instance, 

 would necessitate a number of hauls being* made. Tt has 



