108 



marine animals. Everywhere the female is the dominant sex : 

 usually bigger in body, longer-lived, and predominantly the 

 more actively metabolic animal of the two. This is so in such 

 fish as the plaice, sole, turbot, etc., where the mass of material 

 converted into sexual products by the female is always much 

 greater than in the male. The ovaries in most fishes are bigger 

 than the testes. The herring, however, may be an exception, 

 for here the two " roes," hard (or ovaries) and soft (or testes), 

 are of the same size. 



The Sizes at which the Plaice hecowe Sexually Mature. 



Not so much has been done on this point as we would like, 

 but some observations will be found on p. 92. The smallest 

 mature female found was 26 cms. long and the largest immature 

 female was 42 cms. long. A general limit may be taken at 

 33 cms. : above that the majority of Irish Sea plaice are 

 sexually mature, while below it the majority are immature. 

 As to the males we are still rather uncertain, but the general 

 size at first sexual maturity (puberty) is less than in the case 

 of the females. 



The Migrations of the Plaice. 



Migratory fishes fall into several general types : — (1 ) Stur- 

 geon or shark, which roam over wide regions of ocean in an 

 arbitrary way. (2) Herring, mackerel, hake, sprat, etc., which 

 migrate at fairly regular intervals, the latter being determined 

 by sea- temperature, density of the water, and possibly other 

 physical events. The migrations may occur yearly, as in the 

 case of most herring fisheries ; or after longer periods of time, 

 as in the case of the Irish Sea winter herring fishery (30 or 40 

 years) ; or after periods of over a century, as in the case of 

 some of the winter herring fisheries off the Swedish coast. 

 (3) Plaice, sole, etc., where the migrations are small and occur 

 seasonally, the fish following fairly regular paths according 

 to the depth, nature of the sea-bottom, coast line, etc. 



