216 KECEXT EErORTS OF FISHERY AUTHORITIES. 



intervening seas, has suggested tlie view that an emigration of three- 

 year-old plaice takes place from the Baltic. 



Turning now to the question of the accuracy of the method employed 

 for determining the rate of growth of fish, it seems clear that while the 

 existence of the natural groups, each varying about a most common 

 length, correspond to the average size of fish of tliat age is clearly 

 shown ; a reference to Petersen's tables leaves the impression that 

 considerable uncertainty exists as to where one group ends and another 

 begins, and as to the exact position of the most common length for each 

 group. And this is practically admitted, when the remark is made 

 that " besides distinguishing the sexes, we ought also properly .... to 

 fish the same number of specimens of each annual series, in order not to 

 efface the boundary lines between them." How can we be sure that 

 this is done ? * Tn cases where fish of all the different ages can be 

 fished " in one draught" the difliculty is no longer present. 



It was mentioned above that plaice less than 1 year old were not found 

 in the Baltic. Hensen has, however, shown that the eggs of the plaice 

 are found there ; and the absence of the young fish is accounted for by 

 the peculiar hydrographical conditions which obtain in that sea. For 

 experiments made by I'etersen, in Copenhagen, on living plaice eggs, 

 proved that the highest specific gravity at which all the eggs sank 

 was 1'0120 10° C, corresponding to a salinity of 1-44 per cent. ; and 

 Hensen's investigations prove that '^almost every month there occurs 

 such a low salinity that the eggs must sink to the bottom." If, as is 

 probable, the eggs, on sinking to the bottom, are killed, the absence of 

 the young fish is clearly accounted for. 



The same does not, however, hold for the turbot, the brill, or the 

 flounder. The fry of these fish are sometimes met with in multitudes 

 on the shores of the Baltic, while in the Cattegat young flounders 

 are found in company with young plaice. AVith regard to the turbot 

 and the brill the explanation given is that the pelagic fry of these fish 

 are more hardy than those of the plaice, and so can live in water 

 of a lower salinity. 



With regard to the food of plaice of different ages a short summary of 

 the main facts is given by our author. Thus, during the pelagic stages the 

 food consists chiefly of copepods; and even when the fish have grown to 

 1\ to 1 ^ inches, " Co23epoda, Cladocera, Ostracoda, and the larvcv of 

 bivalves" may form their food. 



At a length of 2 in. to 3 in., however, the diet has changed, and 

 now consists of Idothea, Gammaridce, smaller Annelida, and the fry of 

 bivalves." When the fish have grown to a length of 3 to 5 in., " they 



[• Cf. for a more detailed criticism of Petersen's results Cunningham's paper in this 

 number of the Journal pp. 136-138.] 



