CH. IX] THE PRODUCTIVITY OF THE SEA 199 



percentage of the marked plaice recaptured represents roughly 

 the percentage of all the plaice (of the same range of size as the 

 marked fish) caught on the same fishing grounds. That is, it has 

 been argued, that if (as has often happened) 25 per cent, of the 

 marked fish liberated are recaught on a certain fishing ground, 

 25 per cent, of all the plaice which were present on the same area 

 must have also been caught. For the marked plaice do not 

 usually suffer from the operation of affixing the label, and they 

 distribute themselves over a wide area. But again the validity of 

 this method has been denied by those whose genius confines 

 itself to the task of criticism. It is contended that the marked 

 plaice segregate themselves and that the percentage caught is 

 more or less accidental and depends on the distribution of the 

 fishing boats. But the advocates of the method claim that the 

 number of the marked fishes recaught is a measure of the intensity 

 of fishing ; and the results of such experiments made in England 

 and on the continental side of the Xorth Sea indicate that about 

 25 per cent., on the average, of all the marked plaice liberated 

 are again caught within the year after the date of liberation. 

 Perhaps it would be straining the results of these experiments to 

 maintain that man annually catches one quarter of all the 

 marketable plaice in the sea, but it is not really improbable that 

 such may be the case. 



By far the most satisfactory evidence of the extent of 

 depreciation of a fish population is to be afforded by quantitative 

 plankton investigations such as were carried out by Hensen in the 

 " Nordsee Expedition " of 1895. We cannot, by any means, 

 directly estimate the absolute number of (say) plaice in a sea area 

 such as the Xorth Sea, but we can estimate the number of the 

 eggs spawned by these fishes resident there, and from annual 

 variations in the number of eggs it is possible to estimate the 

 variations in the number of mature spawning plaice. We have 

 seen that this method requires considerable care and must be 

 carried out on a large scale, but it is quite a practicable one, and 

 is, in addition, theoretically quite a sound method. It also affords 

 in an indirect way evidence of the extent of depletion of the 

 stock due to fishing operations. Hensen carried out extensive 

 quantitative plankton observations in the West Baltic and 



