82 NOTES ON A FISHING VOYAGE TO THE 



The Average Size at First Maturity. 



The average size at first maturity, that is to say, the size at which 

 equal numbers of mature and immature fish occur, I find to be about 

 40 cm. (see diagram, page 79), in the case of the females. 



In regard to the males, owing to the virtual absence of immature 

 individuals, this size cannot be determined. The indications, however, 

 from the material available, point to this size being not far remote 

 from 31 cm. 



These sizes correspond closely with those Dr. Wallace kindly in- 

 forms me he has determined from 895 females and 561 males for the 

 central grounds of the North Sea (Dogger, Flamborough Off' Grounds, 

 Clay Deep, etc.), viz., 40 cm. for females and 31 cm. for males. The 

 correspondence is remarkable, and would hardly seem a mere 

 coincidence. It is evident, however, that the plaice of the Barents 

 Sea mature at a much later age (p. 85). 



Age. 



The age investigation presents many difficulties. It is evident that 

 the rate of growth is extremely slow, and this not only renders the 

 distinction of year groups impossible by a study of the length 

 frequencies (Petersen method), but it is also reflected in the otoliths, 

 on which the annual rings are so narrow and crowded togetlier that 

 only in the case of the smallest fishes found has it been possible to 

 estimate the age with a degree of certainty. 



I made a small collection of otoliths on board the Roman, and 

 although this material is quite insufficient for obtaining an estimate 

 of the rate of growth, it can be seen that this is exceedingly slow, even 

 during the years before maturity is attained. 



A noticeable feature of these otoliths is the contrast of the com- 

 parative width of the white and dark rings, the latter being exceedingly 

 narrow. 



The physical conditions which apparently regulate the deposition 

 of these respective rings have been studied in the case of plaice from 

 the Baltic and North Seas. 



Various investigators* have found that the white ring first shows 

 itself in spring, when the temperature of the water commences to rise 

 and the fish to feed. In late summer and into the autumn (the period 



* Reibisch. Ueber die Eizahl bei Plcuronectes platessa und die AUersbestimmung 

 dieser Form aus den Otolilhen. Wiss, Meeres. Abt., Kiel, N. F. Bd. 4, 1899. 



Maieu. Beitrdge zxir AUersbestimmung der Fische ; Arb. d. wissen. Komm. f.d. Intern. 

 Meeres. No. 5. Bd. viii., 1907. 



Wallace. Loc. cit. 



