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Part III. — Eighteenth Annual Report 



that these average sizes are lOin. in the Baltic, llin. in the Lesser Belt, 

 and 12-1 Sin. in the Kattegat. These refer to both males and females 

 although the males are said to be somewhat smaller. These sizes (p. 3) 

 are said to represent the average size of the mature fish, but there seems 

 to be some discrepancy between the stated sizes, and what one may gather 

 from the Tables. En the Kattegat; for example, Table I.* shows that 

 immature specimens were obtained at 15 to 16in., and even as high as 

 19in. (These are in Danish inches — in English inches the sizes would 

 be a little less than half an inch larger,) It seems strange, therefore, that 

 the average-size of the mature specimens of the Kattegat should be so low 

 as 12- 13in. From the Tables, indeed, one would infer that the average 

 size at maturity was even greater than 12-13in. The data, un- 

 fortunately, do not lend themselves to the present mode of treating the 

 subject, and I am therefore compelled to leave the matter in an indefinite 

 state. From the resemblance that the plaice of the Kattegat have to the 

 North Sea plaice, and from their actual geographical continuity with 

 them — i.e., with those of the Danish coasts of the North Sea — it seems more 

 probable that 12-13in. is about the average size when they become ripe 

 for the first time. The average for the females is perhaps higher than 

 this, that for the males lower. 



In conclusion, a brief reference may be made to the usefulness of the 

 average-size when plaice become mature. Quite apart from the legislative 

 and practical aspects of obtaining by its means a reasonable size-limit for 

 the protection of the young immature fish, there is another aspect of far 

 greater importance. 



The average sizes at maturity and first-maturity might be put to effective 

 use in order to tell whether the fish on any area are really decreasing. 

 So far as known there are two effects of over-fishing — the larger fish grow 

 scarce, and the average size of the mature fish consequently decreases. 

 Whether the average size at first-maturity also decreases is not yet 

 definitely known, but it may not do so. The averages found from 

 Cunningham's data differed very little from those of my own, although 

 several years intervened. In the Kattegat, again, the fishing is so severe 

 that large specimens have entirely disappeared, and it is even doubtful 

 whether any plaice survive to spawn twice. Yet the average size at first- 

 maturity, on this supposition, is not much less than that for the North 

 Sea specimens. The average size at first-maturity probably depends upon 

 the surrounding conditions and not upon any change in the total number 

 of fish. If it does vary, however, decreasing with the decreasing number 

 of fish, its variations may be calculated in different years. 



The greatest and most certain sign of a decrease in the number of fish 

 on an area, apart from direct statistics, is to be found in the decrease of 

 the average size at maturity. This represents the average size to which 

 the plaice attain after they are mature, and if this size be compared with 

 the average size at first-maturity we should know what proportion of plaice 

 elude capture and spawn several times. By investigating these two 

 averages in different years wo should be able to tell whether the mature 

 fish are decreasing or not. For example, it has been shown here that the 

 average size at first-maturity for the female plaice of the northerly North 

 Sea is about 15*5 in., and Fulton has shown that the average size of the 

 mature specimens of the same region was about 20 in. some j-ears ago. 

 If this latter average still holds good, it is evident that more than half of 

 the mature plaice in this region must escape capture and spawn several 

 times, in spite of the refined methods of fishing now practised. 



The method of finding out whether the number of fish on an area is 



* Rep. Dan. Biol, Station, 1893. 



