136 NORTH SEA INVESTIGATIONS. 



fact, as the production of new plaice is not constant, but confined to 

 a particular part of the year, the spawning season, it naturally follows 

 that if sufficiently numerous measurements are made, the waves of 

 production ought to be perceptible in the greater abundance of plaice 

 at certain sizes, separated by regular intervals, each group thus dis- 

 tinguished representing the progeny of a single year. 



It is better, when possible, to consider only the female sex in 

 applying this method, because the sexes of the same age are of different 

 sizes. The length at which the maximum number of specimens is 

 found, in the sample from oft' Amrum on December 20th, is 13 in., 

 while in the sample from off Nordeney in November, the corresponding 

 length is between 11 and 12 in. Now we have not samples of all 

 the other plaice in the same region at the same time, but we know 

 that there were smaller, younger plaice nearer shore, and larger further 

 seaward, and may reasonably consider the above to be the mid-size 

 of the plaice which were completing their second year, which would be 

 two years old in the following spawning season, about February. This 

 conclusion is supported by the fact that they were all immature. 



I am obliged to confess that I cannot altogether follow Petersen's 

 arguments. He gives in his tables two samples taken in the Limfjord in 

 the beginning of December, the mid-size of one being 14 in., of the 

 other 12 in. to 12| in. He takes 14 in. as the mid-size of what he calls 

 group 2 from these samples, by which he apparently means that 

 they are at the end of their third year. In my judgment, these fish 

 closely resemble those I have examined from the German Bight, 

 their mid-size is clearly about 13 in., and there is no reason to suppose 

 that they are completing their third year, I consider them to be 

 just at the end of their second year. 



Petersen places the mid-size of his 2 group in the Limfjord, in July^ 

 at 10 in,, of his 1 group at 5iin., meaning by the former, plaice two 

 years and some months old, by the latter those of one year and 

 some months. I cannot see what reason he has for placing the 

 middle of the 2 group at 10 in., as the largest number of specimens 

 in his sample is at 9 in. With regard, then, to the larger northern race 

 of North Sea plaice, there is a very distinct difference of opinion 

 between myself and Dr. Petersen, which is most clearly exhibited in 

 our conclusions concerning the and 1 groups, that is of fish in their 

 first summer and their second. Dr. Petersen criticises my observa- 

 tions in a note on p. 23 of his memoir, and there entirely ignores 

 the fact that though I have not exhibited my data by the graphic 

 method in tables quite similar to his, yet I have used the principle 

 of the mid-size in separating the groups. It is true that I did not 

 attempt to apply this principle to any extent to the 2 and 3 groups, but 



