334 Part III. — Tireniieth Annual Report 



to be opened. Among flat-fishes, howevei', when they have reached 

 the adult size, it is necessary to deal with the sexes separately, 

 because the males then begin to grow much less rapidly than the 

 females. Fortunately the sexes can, as a rule, be distinguished by 

 simple methods. At about the spawning time the ovary can be, in 

 most cases, readily detected without the fish requiring to be opened. 

 At other times its presence may be easily revealed by the use of trans- 

 mitted light. The method adopted by me was to have the fish passed 

 seriatim in front of the acetylene lamp in the daikened deck-house ; 

 those obviously females or males were placed in separate baskets, and 

 doubtful specimens were set aside and opened. With some species, e.g., 

 lemon soles, the sexes cannot be determined in this way. 



Comparative Growth of Flat- Fishes and Round Fishes. 



One of the points brought out in the course of the investigation is 

 the great difference in the rate of growth of the flat-fishes and the 

 round fishes, the latter increasing in size with much greater rapidity 

 than the former from the earliest stages onwards. The haddock and the 

 plaice, and the whiting and the common dab, may be taken as examples, 

 since the spawning periods of these pairs coincide fairly well. At the 

 end of the first summer and autumn, when the first year's growth 

 is nearly completed, say early in November, the average length of the 

 haddock at Aberdeen is about 172 or 173 mm., or 6|| inches; some of the 

 year's fish may be as small as 4| inches, and others as large as about 

 85 inches. On the other hand, at the same date the young plaice 

 average about 65 mm., or a little over 2^ inches. Some may be as 

 small as about \\^ inches, and others as large as about 3| inches. 

 These measurements show a marked difference, but they do not exhibit 

 the contrast in growth so well as when the mass or weight is compared. 

 The normal weight of the average-sized haddock stated is a little under 

 1^ ounces (about 40*5 grammes), •while the weight of the average-sized 

 plaice after the first summer's growth is only about -y» of an ounce, or 

 2'5 grammes. The young haddock thus increases its weight on the 

 average sixteen times faster than the young plaice. Moreover, some of 

 the young plaice weigh only 0*8 gramme, and the heaviest is about 8 

 grammes, or a little over a quarter of an ounce ; while the smallest 

 haddock weighs about half-an-ounce, and the largest nearly 2| ounces 

 (76 grammes). 



These particulars are derived from the weighing of a large nimiber of 

 specimens of all sizes, and the construction of curves.* 



After the second summer's growth, when the fish are nineteen or 

 twenty months old, the average size of the haddock is about 276 mm., or 

 10|- inches, while some may be as small as about 8| inches or as large 

 as 12f inches. At the same period plaice of corresponding age average 

 about 145 mm., or 5| inches, some being about 4 inches and others as 

 large as about 6|. Here again the contrast in Aveight is striking. After 



* The weights of different specimens of the same length vary considerably, especially 

 the large ones. The abscissa in the diagrams represent length, and the ordinates weight, 

 and the smoothed curve enables the average weight of a fish whose length is known to be 

 readily ascertained. The curves vary for difl'erent species ; among the fiat-fishes dealt 

 with, for example, the heaviest in proportion to length is the plaice, then the common 

 dab, and, at a considerable distance, the long rough dab ; the witch is still lighter. 

 Among the round fishes the heaviest in proportion to length is the cod, then the haddock, 

 and then the whiting ; the plaice is heavier than the cod, whose curve corresponds 

 rather to the conmion dab. The curves do not everywhere agree with the rule that in 

 similarly-shaped bodies the masses vary as the cubes of the dimensions ; the proportions 

 appear to change somewhat with growth. The number of haddocks included in the 

 part of the curve above dealt with was 258, and the number of plaice '272. 



