6 Investigations on the Life-History 



From these weighings the total weight of muscle and the weight of 

 "thick" and "thin" could be calculated, as shown by the following 

 example from the Receiving- Book : 



No. 23. Aberdeen, 27th May 1896. 

 From Maidh of the Dee. 



Clean fish. Sea lice. Stomach empty. Intestine contained yellow 

 mucus and tape-worms. Pyloric appendage coated with fat. Gall 

 bladder distended. 



Length, 67 cm. Girth, 36'5 cm. Depth, 15 cm. 



Weight, 3,425 grms. 



of Skin on one side, . . . 110 grms. 



of Viscera, 103 grms. 



of Ovaries, 16 grms. 



of Trunk Muscle of one side, . 1,087 grms. 



of Rest of Fish, .... 2,005 grms. 



The weight of the trunk muscles was thus 2174 grms., and of this 

 a quarter, or 543, was "thin," and the remainder, or 1631, was 

 "thick." 



(c) Method of comparing Fish of different sizes. 



In dealing with this mass of material the first question which had to 

 be considered was how to make the results obtained from one fish 

 comparable with those obtained from another. The fish received varied 

 in length from 66 to 108 cm., and in weight from 2675 to 12,670 grms. 

 To compare the various parts of a fish of 3000 grms. with one of 10,000 

 grms. it is obviously necessary to reduce the results to some common 

 measure. 



In previous observations the weight of the various organs has usually 

 been expressed simply as a percentage of the total weight of the fish ; 

 but, if the changes in substance of two structures such as the muscle and 

 ovaries one of which steadily loses weight while the other steadily, gains 

 it are to be compared, obviously to calculate the changes in muscle, in 

 terms of the weight of the fish, will give a fallacious idea of the extent 

 of these changes. 



A more constant standard is the length of the fish, and in his 

 Annual Report, No. 14, 2-12, Mr. Archer has used this standard. 



But the weight of the fish varies not as its length but as some factor 

 of its length approaching the cube, on the rule that two bodies of similar- 

 shape and of the same material vary as the cube of their length or some 

 like dimension. Thus a fish of 100 cm. in length is not twice as heavy 

 as a fish of 50 cm., but is eight tunes as heavy. On trial, in a large 

 number of cases of fish of different sizes, it was found that the cube 

 gave satisfactory results, and it was therefore adopted as the factor. 



The standard length, or unit of length, selected was 100 cm. the 

 ordinary length of a salmon of about 301bs. This was selected simply 

 because it yielded convenient figures. 



To obtain the weight per unit of length the proportion thus is : 



Actual length, cubed : standard length, cubed : f actual weight : x. 



Table I. shows the correspondence of the results obtained in this 

 way with the actual weights observed in the very large number of fish 

 included in Table XIV., p. 27 of Vol. XIV. (1896) of the Fishery 

 Board Reports, Part II. 



