462 SOME RESULTS OF THE INTERNATIONAL FISHERY INVESTIGATIONS. 



Such a " law," however it may apply to the conditions off the coast 

 of Holland, is much too definite to apply to other localities. It is 

 altogether incorrect when applied to some areas off the coasts of 

 Britain. It is too often forgotten that all these statements of distribu- 

 tion are made with respect to areas where a long-continued fishery for 

 plaice has been carried on, and where the influence of man as fisherman 

 is continually exerted in reducing the numbers of large plaice. Large 

 fish are more easy to catch, and must necessarily be fewer than smaller 

 fishes ; again, the fisliery is, roughly speaking, most intense near the 

 land, and decreases in intensity as we proceed further out to sea. Tliis in- 

 fluence of fishing must operate in bringing about, to some extent at least, 

 the distribution of plaice with size varying according to the depth of the 

 sea. It is far otherwise in the few regions which are natural " plaice 

 grounds," and where the law prohibits trawling. There, instead of 

 a distribution such as is indicated above, we may find that plaice of all 

 sizes and ages are living together on the same restricted portion of sea 

 bottom. 



The determination of the age of a plaice has been arrived at by 

 means of two methods. One is that of Petersen, and depends on the 

 analysis of an extensive catch of plaice according to the sizes of the fish. 

 If, say, some thousands of plaice captured on the same (restricted) 

 fishing ground be individually measured, it will be seen that there are 

 far more fishes of certain sizes than those of the intermediate sizes. 

 Such a method of estimating the ages of tlie fishes forming a single 

 catch cannot be easily understood without an example, and the diagram* 

 (Fig. 6), which is based on a catch made by the Marine Biological 

 Association off Mablethorpe, will make the reasoning clear. The figures 

 on the vertical line represent the numbers of plaice taken, and those on 

 the horizontal line represent the sizes of the fish. At the point of the 

 curve marked a line drawn horizontally shows that about 360 plaice 

 were captured, which had an average length of about 5| centimetres 

 (a little over 2 inches) ; at the point I about 85 plaice were taken with 

 an average length of 10 centimetres (4 inches); then at the point II 

 60 plaice were captured with an average length of 15 centimetres 

 (6 inches). That is to say, in this catch three predominant sizes of 

 plaice were present, 2 inches, 4 inches, and 6 inches. In fact we have 

 three groups or " schools " of fish, each of which resulted from a 

 different year's spawning. The Group consists of fish less than one 

 year of age, Group I of fishes over one but less than two, and Group II 

 of plaice over two but less than three years of age. 



The other method of determining age is that of otolith examination. 



* Wallace, Fishery and llydrographical Investigations, etc., Southern Area (cd. 2670), 

 1905, I). 208, tig. 4. 



