92 



the same variations will not necessarily be found in 

 different places. There is no doubt that this cause has 

 some eft'ect in producing the marked diiferences in the 

 catches from the same area on the same day, but it alone 

 is not enough to account for all. 



(3) Form, etc., of the Net and the Method of Use. 



It is obvious that with small nets, sweeping a small area 

 and catching few fish, the exact form and dimensions of 

 tJie apparatus must remain constant so that similar 

 samples will always be taken. But nets are frequently 

 lost or get damaged, and on every such occasion the exact 

 trim of the apparatus mav alter and its catching power 

 may change. The trawl net used for scientific observa- 

 tions is really a physical instrument, but it has apparently 

 never been regarded in this way, and the " constants " for 

 any particular apparatus determined. We do not know 

 of any experiments, except those of Mr. Dawson on the 

 catching power of the net in relation to the size of the 

 mesh, on the differences in catching power that alterations 

 in the length of the net, the beam, the foot rope, etc., will 

 make, although it is quite evident that for the purposes of 

 scientific trawling these should be known. Fishermen 

 know that apparently slight differences in the trim of the 

 net, the length and weight of the foot rope, or the amount 

 of " grip " given tO' the latter, for instance, may make 

 differences in the catching power of the net. So also the 

 exact method of using it. Two fishermen using the same 

 vessel and net and on a ground on which fish may be 

 reasonably supposed to be uniformly distributed will not 

 necessarilv get the same catches. It follows that in the 

 collection of any considerable series of observations from 

 which detailed conclusions are to be drawn, these con- 

 siderations must be borne in mind, as it must often 

 happen that the observations are made by different people 



