SOME KESULTS OF THE INTERNATIONAL FISHERY INVESTIGATIONS. 407 



recaptured, and the difference in length is the growth during the 

 interval. As might be expected, there are considerable diflerences in 

 the average results so obtained. It now appears, from a discussion of 

 these experiments and others, that no average growth rate can be laid 

 down which applies to all plaice in the British and continental fishery 

 area. Some of these differences are summarized by Garstang.* On the 

 Horn lieef grounds, off the coast of Denmark, the greatest rate of 

 growth indicated by plaice in the course of a single year was about 

 5 centimetres (about 2 inches). On the other hand, certain plaice 

 caught on these grounds, and on the east coast of England, and then 

 transplanted to the Dogger Bank, showed a much greater increase in 

 length. In the case of some of these plaice, the growth in one year 

 was as much as 14 centimetres (nearly 5 inches). Analogous difl'erences 

 in the rate at which plaice grow are met with in very many of the 

 experiments made. 



The intensity of fishing on any fishing ground can be deduced from 

 these marking experiments. If we capture (say) 1000 fishes and 

 mark and liberate them on the ground from which they were taken, 

 then the proportion of these 1000 fishes which are recaptured within 

 one year from the date at which they were liberated is an indica- 

 tion of the degree to which this ground has been exploited by the 

 fishermen in the course of the year. For the 1000 marked fishes 

 may reasonably be assumed to have spread uniformly over the 

 fishing ground in question ; and if the fishermen capture (say) 250 of 

 them during the year, there seems no escape from the conclusion that 

 they have also captured 25 per cent of all the plaice, of the same range 

 of sizes as the marked fishes, which were originally present on the 

 ground. Of course, such deductions must be made very cautiously and 

 must depend on the consideration of fairly large numbers of fishes. 

 But, remembering this, it is certain that in this method we have a fairly 

 satisfactory means of ascertaining how far fishermen reduce tlie fish 

 population of a fishing ground, in the course of their ordinary operations. 

 There is only one other way of obtaining this information : by a con- 

 sideration of the number of eggs of the species of fish considered which 

 are produced on the ground during the spawning season, and this 

 method is very unreliable. Nevertheless, such an estimate, made by 

 A^ictor Hensen in the case of the West Baltic cod and plaice fisheries in 

 1895,f agrees very well with the average results of the fish-marking 

 experiments. Naturally the intensity of fishing on the various grounds 

 varies very greatly. + On the fishing grounds of the North Sea, Skagerak, 



* Eappis. et Proc.-verb., vol. iii., 1905, app. H, p. 14. 



t See Jenkins, Trans. Liverpool Biol. Sue, vol. xv. p. 312, 1901. 



X See Garstang, Rajypts. et Proc.-verb., vol. iii. app. H, p. 10, 1905. 



