356 



Part III. — Twentieth Annual Report 



The increase during the third year appears to be a little greater, 

 and amounts to between 80 and 85 mm. ; and the average size when 

 three years old is probably about 23 cm. {^-y^ inches), allowing for a 

 slight increase from November to March. During the next year the 

 increment is about the same, so that a plaice when four years of age 

 (in March) probably measures about 32 or 32-5 cm. (12| inches). In 

 connection with this and the later groups it is, however, necessary to 

 distinguish the males from the females, because the rate of growth now 

 begins to be markedly different. 



It appears fairly certain that in the plaice the males and females are 

 approximately eqvial in numbers and in size, the disparity in the average 

 number and average size of the sexes in the species as a whole, such as 

 has been shown to exist,* being due to the slower growth and earlier 

 death of the males. The proportion of females to males, taking all sizes, is 

 about 142 to 100, and the ratio of length about 114 to 100, the mean 

 length of 1355 males examined being 13'74 inches, and of 1932 females 

 15 "62 inches. During the present investigation I endeavoured whenever 

 possible to separate the sexes and record the results apart. In the 

 smallest plaice, i.e. of the first summer, it nearly always requires 

 microscopic examination to distinguish the sexes, although sometimes 

 the minute pouch which the ovary forms may be detected through the 

 translucent flesh. t In going through a number of specimens, however, 

 it happened that some cases occurred in which I failed to determine the 

 sex, being sometimes unable to find the reproductive oi'gan ; and hence 

 in most of these collections the sexes were not completely separated. In 

 two cases they were, and they are included in the following table, which 

 brings together the numbers and measurements in some other cases. 



The disparity of the sexes in the first case is probably exceptional, 

 because in the next series they are almost equal in numbers, and in the 

 third series sliglitly in excess. The observations cannot be regarded as 

 showing absolutely the exact relation, but it is fairly certain that 

 the sexes are at first almost equal in numbers. Their equality in 



* Fulton, Eighth Ann. Rep. Flshen/ Board for Scotland, Part iii., p. 348 ; Jhid.. Tenth, 

 V- 239. 



f Among this series a few months old (in October) the largest eggs in several examined, 

 and measuring 62, 65, 58, 54, 5C mm., had a diameter of -015 and "017 mm. In some 

 ciught at the same time measuring 85 mm., the eggs were -035 mm. ; in some, 91, 94 

 mm., the largest were -045 mm. ; in some, 169, 166 mm., .08 mm. The same sizes were 

 observed on 30th April. In such specimens the testis is a thread-like .slightly frilled band. 



