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Part III. — Seventeenth Annual Report 



group at that time to range between 5| inches and 8J inches, and having 

 an average size of 6*963 inches. Also in this case the identification is 

 well justified, owing to a number of specimens of the younger and the 

 older groups being attached. Further confirmation of this group is 

 contained in Petersen's Table VI., where the records from the coast south 

 of Hals for May 25th show the group to range between 4| inches and 6J 

 inches, and having an average size of 5*366 inches, and also in the 

 lecords for June 29th : 4 \ inches to 6 \ inches, with an average of 5*762 

 inches. Local variation may possibly exist and influence these groups to 

 a certain extent ; but in all the cases here quoted it is necessary to 

 recognise the "2 " group in its development from May to September. The 

 difference in size between the "1" group and the "2" group appears to be 

 about 2*535 inches, which is an indication of the second year's growth. 



In Dr. Petersen's tabled records this " 2" group is not equally well 

 demonstrated. The probable cause of this has already been explained 

 (p. 242). It has in some cases been mixed up with the neighbouring 

 " 1 " group, when misleading data have been produced and caused Petersen 

 to combine these two groups into one large " 1 " group with a range of 

 5 or 6 inches. From these deductions apparently Dr. Petersen has 

 concluded that the smallest mature plaice in the Danish waters (about 7 

 inches long) are in their second year only (p. 6), while according to my 

 interpretation of his records those fishes should be at least three years 

 old.* According to my calculations this is also the probable age of the 

 smallest mature plaice on the East Coast of Scotland, and if 

 it were known that the plaice becomes mature at the same age in the 

 different seas (which is probable), another strong proof would exist in 

 support of my theory of the growth-rate of the plaice and the probable 

 age at maturity. 



In order to compare the apparent growth-rate of the plaice in the 

 Kattegat with my observations on the same subject on the East Coast of 

 Scotland, the various data, as deduced from D. Petersen's Tables, have 

 been represented on Plate I. (No. 2). The figures 1 to 13 have been 

 arranged according to the date of observation and the average size of the 

 group in question. (For furthur particulars see Table IX.) These figures 

 have been connected by the dotted line (No. II.), which should be 

 considered an approximate illustration of the growth of the plaice in the 

 Kattegat at the various seasons, and extending over nearly three years. 

 Owing to the want of sufficient data, it is impossible to say whether the 

 course of this line is in all details correctly drawn ; but for the purpose 

 of a general comparison sufficient information is available. It will be 

 seen that the growth-rate of the plaice in the Kattegat is considerably less 

 than on the Scottish East Coast, and that this difference appears to 

 increase as the fishes grow older. 



It is of the greatest importance that these investigations regarding the 

 growth-rate of the plaice should be continued, and also be extended into 

 deeper water than was the case at Dunbar. The difference of opinion 

 regardiug the age at which the plaice becomes mature ought to be cleared 

 up, as it is a question of considerable importance in several respects in 

 connection with the fisheries, and the only method by which this may be 

 satisfactorily attained is to carry on systematic fishing from shallow to 

 deep water, so that all the animal series may be demonstrated, and their 

 age accurately determined also, after the earliest stage of maturity. 



* In the Fourth Report (1893), p. 6., Dr. Petersen says— "It seems that the plaice, 

 everywhere, generally become mature in their third year, some already in their second, 

 . . ." and in the Sixth Report (1895), p. 14, it is stated— " A plaice is, ordinarily, not 

 mature till its fourth year. . . ." While I have shown why this first statement musi 

 be wrong (and also Dr. Petersen's classification of most of his groups), I find that my 

 own interpretations of the Tables, as well as the observations made at Dunbar, correspond 

 with the latter view. It appears that a considerable number of mature plaiee are found 

 in the Kattegat about eight and nine inches long, and if these fishes are considered to be, 

 at least, in their fourth year and not yet full-grown, the annual growth cannot be nearly 

 so large as suggested by Dr. Petersen. 



