214 Recent reports of fishery authorities. 



are found to exhibit.* These differences are seen on comparing the 

 average sizes of plaice which have just arrived at maturity, and also the 

 average sizes of mature (grown-up) plaice {i.e., three years old and over) 

 from different localities. Thus while in the Baltic the average size 

 of mature plaice is about 10 inches, it is 11 inches in the Lesser Belt, 

 and 12-13 inches in the Cattegat. Whether this gradual decrease 

 in the average size of the mature fish, as we pass from the Cattegat 

 to the Baltic, is due to a corresponding gradual change in the conditions 

 favourable to growth, or whether it implies a migration of the larger 

 plaice from the Baltic towards the Cattegat, is not certain, but there are 

 reasons for thinking that the plaice of the Baltic do not enter that sea 

 in any numbers till they are one year old, so that to speak of a race of 

 plaice peculiar to the Baltic would be erroneous. 



Further, as we pass from the German Ocean to the Baltic, there is 

 a gradual decrease of the size at which plaice become ripe for the first 

 time. If to these differences others {e.g., in the number of fin rays) be 

 added, the existence of separate races is still unproved. For seeing that 

 the eggs and fry of all the plaice are pelagic, and must in consequence 

 all be mixed together, the appearance of one form of plaice in the Baltic 

 and another in the Cattegat, must be due either to the ftict that the eggs 

 of one form cannot live when carried into the territory of the other, 

 or that the differences between the two forms are wholly ontogenetic. 

 Of these two alternatives our author is inclined to accept the second. 



Perhaps the most interesting part of Petersen's paper is that in 

 which he describes his method of determining the rate of growth of 

 plaice. By fishing at any given time of year in a number of different 

 places, at different depths, and with nets of various kinds, and 

 measuring all the fish caught, Petersen found that the fish were grouped 

 about certain maxima corresponding to the most common lengths of the 

 fishes born in successive years. These groups he calls the " group," 

 consisting of fish less than one year old, the " 1 group " between one 

 and two years old, the " 2 group " between two and three years old, and 

 the " 3 group " consists of fish three years old and over. 



Leaving for a moment the question of how far this method of 

 determining the rate of growth of the fish, and the probable age of any 

 particular individual, is a sound one, we may pass on to a brief resume 

 of the life history of the plaice in Danish seas as traced by Petersen. 

 The spawning season lasts from November to April, with a maximum 

 in January and February. The larvae, so long as they retain their yolk 

 sac, are 6-7 mm. long. " When the yolk sac is absorbed, and the fish 

 have become unsymmetrical and compressed, with their left eye sitting 



* Cf. Cunningham. "North Sea Investigations" — this Journal, vol. iv. nos. 1 and 2 ; 

 especially no. 1, pp. 23-2.'), and no. 2, pp. 97-lOS. 



