606 Weight and Length of Brook - Trout. 



shape as a pounder. It is unnecessary to add that these conclusions 

 can only be true under similar conditions. We cannot compare well- 

 fed trout with half-starved ones, nor trout full of spawn with those 

 not in that condition. Nor have we any right to suppose that figures 

 deduced from observation on Lake Superior trout will apply to those 

 caught elsewhere. 



The growth of a trout takes place in three dimensions — length, 

 breadth, and thickness ; and if the growth is symmetrical, each of 

 these dimensions will increase in the same proportion. Thus, if one 

 fish is twice as long as another, he will also be twice as thick and 

 twice as deep. He will, therefore, be eight times as heavy. In 

 other words, the weight varies as the cube of the length. 



If, then, we divide the cube of the length of a trout by the cube 

 of the length of a pound trout, we shall, if the trout grows symmet- 

 rically, obtain the weight of that trout in pounds. 



We see by the table that the length of a pound trout is thirteen 

 inches, but as this number is only founded on one observation, it will 

 not do to base our calculations upon it. 



We can, however, from the length of a four-pound, three-and-a 

 half-pound, three-pound, and two-and-a-half pound trout, calculate 

 what the length of a pound trout ought to be. We find that the 

 numbers obtained from all these four starting points agree exactly ; 

 and hence we obtain the number 13.17 inches as the length of a 

 pound trout. The cube of 13.17 is 2286; and hence, if w==the 

 weight in pounds, and / — the length in inches, of any trout : 



P 



2286 



The correspondence between the numbers calculated by this 

 method and those found by observation is rendered still more strik- 

 ing, if we express them graphically, by representing the length on 

 a horizontal scale, and drawing at each inch a perpendicular propor- 

 tion to the weight. * * * 



The result will be a regular curve, almost coincident with that 

 obtained from the formula given above. 



Our trout were almost all caught in the lake, off rocky points, 

 and at the mouths of small streams. They were in excellent con- 

 dition. The average weight of our whole catch was two and a half 

 pounds. 



