CONSIDERATION OF INDIVIDUAL FISH 6i 



quently. In males the advent of maturity is one or even two 

 years earlier than in females. 



The fact that sexual maturity is first attained by plaice of different 

 sizes in different. parts of the North Sea is one reason why certain 

 investigators have been led to put forward theories of distinct 

 races of plaice. Several such theories have been suggested, some 

 have been modified by their authors as a result of further investi- 

 gation. It is doubtful whether we possess . sufficient information 

 of the biology of the plaice to justify confidence in the existence 

 of " races " of plaice in the North Sea.^ 



Probably the best method of determining the growth and age- 

 groups in the case of the plaice is by measuring large numbers of 

 individuals caught together on the same ground. This method 

 has been followed with success by Johnstone* in the Irish Sea, 

 about i6,0G0 plaice having been examined by him in the years 

 igoghis- 



As a result of these investigations the modal lengths of plaice in 

 the inshore waters of the Irish Sea are given as follows : 



Plaice of age-group o (that is, over o and under i year old) are on 



the average 2f inches long. 

 Plaice of age-group I (that is, over i and under 2 years old) are on 



the average about sj inches long. 

 Plaice of age-group II (that is, over 2 and under 3 years old) are on 



the average about 7I inches long. 

 Plaice of age-group III (that is, over 3 and under 4 years old) are on 



the average loj inches long. 

 Plaice of age-group IV (that is over 4 and under 5 years old) are on 



the average 13 inches long. 



Large numbers of measurements of individual plaice have been 

 made by the English, Scottish and German authorities, not only 

 on the fishing boats, but in the markets as well. Hundreds of 

 thousands of measurements have been recorded and tabulated, 

 and the higher mathematics utilised to clear up the vexed question 

 of the destruction of immature fish. 



To give one instance, the Germans have conducted since 1904 a 

 series of " scientific market measurements," that is to say, they 

 have measured and weighed large numbers of representative samples 

 of sea fish landed at the markets by the commercial trawlers. 

 Measurements were also made on board the trawlers of plaice and 

 other fish, so that a comparison might be made between the catches 

 made on the trawlers and the fish landed at the markets. Roughly 



» See Wallace, " Report on the Age, Growth and Sexual Maturity of the Plaice 

 in certain parts of the North Sea." Board of Agriculture and Fisheries. Fishery 

 Investigations. Series II. Sea Fisheries, Vol. II, No. 2. London, 1915. 



• See especially Report for 1913 on the Lancashire Sea-Fisheries Laboratory, 

 Liverpool, 1914, p. 78, et seq. 



