BODY MEASUREMENTS IN THE WEAKFISH, CYNOSCION REGALIS. 1 45 



The lengths were measured by means of a centimeter scale placed on the fish; 

 width and depth were taken with the aid of spring calipers, using the same scale. For 

 the measurement of width and depth it was necessary to secure points that would not 

 be influenced by the amount of food material in the stomach. The abdomen of the sque- 

 teague is extremely elastic, and its volume varies considerably with the stomach con- 

 tents. Significant measurements in this region of maximum depth are likewise impossible 

 after the removal of the contents of the stomach. The places selected fulfilled the 

 requirements suggested and were found to be sufficiently near the maxima for our 

 purposes. 



The curves shown in figure 4 were derived from the data obtained. For every 

 specimen total length was plotted as abscissa and the other measurements detailed 

 above as ordinates. ° From the resulting straight lines it is at once apparent that there 

 is a simple relation between the dimensions of the external parts of the fish and its total 

 length. It is clear that with increasing length there is a constant, directly proportional 

 increase in all the body measurements taken. 



From the slopes of the lines the rates of growth of the corresponding parts relative 

 to the growth of the total length may be calculated. Using the units shown on the plot, 

 the "tangent" of any line is determined by dividing the vertical distance between two 

 points on this line by the horizontal distance. These tangents are as follows: 



Standard length o. 



Body 



Tail 



Head 



Depth 



Width 



840 

 530 

 273 

 215 



13s 



From this it is obvious that, of the body parts, the body has by far the greatest rate 

 of growth, while the width has the least. It is also clear that the head and tail have 

 approximately the same rates of growth, and that the depth and width also grow at 

 about the same rate. It is, of course, to be understood that when the "rate of growth" 

 is mentioned, we do not mean "rate" with regard to time, but relative growth per unit 

 increase in total length.. Thus, for every 10 cm. increase in total length the standard 

 length will increase 8.40 cm., the body 5.30 cm., the tail 2.73 cm., the head 2.15 cm., 

 the depth 1.3 cm., and the width 1.15 cm. 



RELATION OF BODY MEASUREMENTS TO WEIGHT. 



From the regularities shown in the previous section we may conclude that there 

 exists a relation between any body measurement and weight similar to that which 

 exists between total length and weight. Yet another relation, however, may be demon- 

 strated. Since depth and width are each equal to a constant multiplied by the total 

 length, we may substitute in the formula for the derivation of weight,*" depth, and 

 width divided by their respective "tangents," and thus secure a formula for the weight 

 in terms of length, width, and depth. This formula is W = k. I. w. d. By direct calcu- 

 lation from figures 4 and i, fe = o.55i3±o.oo88, and the equation becomes weight = 

 (0.5513 ±0.0088) (length) (width) (depth). 



a Here also many of the points represent duplicates and triplicates. & See p. 143. 



