SEA-FISHERIES LABORATORY. 249 



fish were recaptured during" the months of April, May, 

 and June; yet, even in December of the same year, two of 

 these plaice were recaught in Barrow Channel. 



Now it is perhaps as well that not too much import- 

 ance should be attached to the measures of " intensity 

 of fishing " deduced from marked fish experiments. It 

 has been claimed that if (say) 25 % of the fish liberated in 

 some particular experiment are recaught within (say) 

 one year, then 25 % of all the fish inhabiting the area 

 fished over by the boats, among which were they that 

 recaught the marked fishes, must also have been caught. 

 That is, the percentage of the marked fish recaught is also 

 the percentage of all the fish caught. This conclusion 

 would be, generally speaking, a reliable one if all the fish 

 marked and liberated survived in a healthy condition, and 

 if they were evenly distributed over the area fished 

 commercially. Usually this is not the case, and in so far 

 as some of the fish marked are injured and do not survive, 

 and all are unevenly distributed, so far does the argument 

 lose its force. 



In this particular case the plaice were liberated in a 

 narrow channel with such a great rise and fall of tide 

 that the area covered by the sea varies enormously. The 

 tidal streams are very rapid. The trawlable area, and 

 that over which stake nets are set, are restricted, that is, 

 only a rather small part of the navigable channel is 

 trawled, and there are very few places on which stake nets 

 are usually set. The fishermen do not, then, go to look 

 for the fish; they do not change their fishing grounds 

 as the fish are more or less abundant. They wail lor I he 

 fish to come to their nets, so to speak. In these circum- 

 stances it seems as if they really did sample a fish popula- 

 tion, which moved about over the whole of the area, 

 according to the tides. The fish marked were selected, 



