— Q — LATER STAGES: PLEURONECTIDS 



is shown in a sliglitiy different form in tiie Leman Region, whilst the curve J'or the Fle- 

 mish Bight is simple. 



The phenomenon is certainly suggestive of a southern race with a low average-size- 

 at-maturity, extending northwards and the more active males mingling in gradually de- 

 creasing proportion with the larger northern race. 



DOGGER REGIOn. MALES 



on,n 

 Fig. 



100 

 90 



SO 



70 

 60 



so- 

 lo 



30 



3 25 27 29 31 



35 37 39 4-1 13 



CW 21 23 25 27 Z3 31 33 35 37 39 41 «3 «S 'n 49 



Diagrams showing percentages of immature and mature Plaice at different sizes. (After Wallace). 



In last year's Report occasion was taken to point out the state of knowledge with 

 regard to the question of two or more races of plaice in the North Sea and the econo- 

 mic importance of further organized researches upon the subject, Tlie details of the relation 

 of age to maturity are of the same nature as those of size, but here a smaller number 

 of factors (perhaps six generations) are involved and whilst the. tendencies towards 

 maturity graduate slowly in the case of size they must necessarily tend to become accen- 

 tuated at one or more ages. In other words, the fish of a particular generation will all 

 tend to come to maturity in the same spawning season, a deferment till the next season 

 or a precocious development in the antecedent season involving abnormal characters. 

 Hence it is found that there is a sudden increase in the percentage proportion of mature 

 in the critical year, indicating that in that year the greatestnumber ol plaice became mature 

 for the first time. 



The same sequence is found in the case of size, and the males at first maturity are 

 relatively younger (usually about one year) than the females from the same ground. There 

 is a difference of about one year in the age at first maturity between the northern and 

 southern parts of the North Sea, as previously conjectured by Kyle. 



Other results are as follows : — 



■-(l). The youngest mature males were in their, second year and females' in their third, 

 whilst the oldest immature males were, in the. case of the Dogger, over six years old 

 and in the females over five years old. 



(2) In the Dogger Region the majority of males became mature when 5 years bid, 

 the females when 6 years old. 



In the Flemish Bight the majority of females became mature at 5 years old, and in 

 the English Channel at 4 years old, the males at 3 years old. 



It then becomes of importance to find out whether size or age" is the primary deter- 

 mining factor in the attainment of maturity. Does a plaice attain maturity because it has 



