492 EEPOKT OF THE COUNCIL. 



B. LABOEATOKY WOKK. 



Age and G-rowth of Plaice. — During the last year a report lias 

 been issued dealing with the size and age of plaice at maturity in the 

 North Sea and English Channel. A report has been in preparation 

 dealing with observations on the age and size of over 19,500 plaice 

 collected at different seasons and in different years over a wide area 

 of the North Sea and in the western part of the English Channel. 

 The ages of these fishes have been determined by examination of their 

 otoliths or ear-stones. The investigation of this material has enabled 

 the average size of plaice of given age on many different fishing 

 grounds to be determined, and has lirought to light some interesting- 

 differences. 



Considerable pains have been taken to determine the true average 

 growth of plaice in the region between the English and Dutch coasts, 

 a task which is complicated by the circumstance that the size of plaice 

 of the same age varies according to the distance from land. This 

 difficulty has been overcoine by determining the ages and sizes of all 

 plaice caught in continuous lines of trawlings extending from the 

 Dutch coasts into the open sea. The results of several series of 

 observations along these lines agree very closely. They show, among 

 other things, that the average growth of plaice in this region during 

 the first three years is at the rate of 6-7 cm. a year ; plaice of three 

 years old averaging 20-21 cm. (about 8 inches) in length. In the 

 western part of the Channel growth of young plaice is more rapid, 

 the average length at three years old l)eing 28 cm. (11 inches). 



Plaice do not arrive on the Dogger Bank in any consideraltle num- 

 bers until they are about four years old. In the following year, 

 judging from the average size of five-year-old plaice in this region, 

 they grow faster than plaice of the same age in the southern and 

 eastern parts of the North Sea. This observation is in harmony with 

 the results of the transplantation experiments. 



A somewhat sudden diminution in the average rate of growth takes 

 place at the age at which the majority spawn for the first time. In 

 the western part of the Channel these phenomena occur about two 

 years earlier than in the central part of the North Sea. 



xA.n investigation of the proportions of the sexes at different ages 

 in collections from the Nortli Sea and English Channel has also 

 brought to light interesting differences, which also appear to be 

 associated with the age at which maturity first occurs in the two 

 sexes in the two regions. 



A comparison of the number of plaice of different ages caught per 



