182 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Plotted on a logarithmic scale, these values describe the curve given in figure 11. It 

 is interesting to note that the change in slope approximately coincides with yolk sac 

 absorption. 



For ready comparison the growth curves of mackerel and of these other species 

 are assembled in figure 12. In all of them, length was used as an index of size. Mass 

 or volume would be a more nearly true index. However, if there is no change in 

 form, length would serve well to test for logarithmic growth since a certain power of 

 length would be proportional to the mass or volume, and in logarithmic plots the 

 only difference between the two would be a difference in vertical scale. Since the 

 mackerel and haddock undergo little change in form during early life history, a simple 

 logarithmic curve well fits their growth as indicated by length. The herring larva, on 

 the other hand, is slender and almost eel- like when young, growing stouter as de 

 velopment proceeds. This being true, length overestimates size early and under- 



- 20 



15 



z 



x 

 10 | 



6 



5 l( 



AGE, DAYS 



Fiouee 11.— Growth of northern pike during early life. Data from Embody, 1910. 



estimates it later. This may be the reason for the nearly linear arrangement of points 

 when lengths are plotted directly against age. Further, the change in slope when the 

 logarithms of lengths are plotted against age suggests that the change in form is 

 greatest at about 30 days of age when the herring is about 18 mm. long. The growth 

 of the northern pike, too, shows a change in slope. In this instance it approximately 

 coincides with yolk sac absoption, hence this might as easily be a real change in growth 

 rate due to difference in food availability or assimilation rather than an apparent 

 change due to altered form. Evidence from the information available on these several 

 species supports the view that growth in the early life of other fishes, as well as the 

 mackerel, is logarithmic in character and at a uniform percental rate throughout this 

 stage of life except when there is a change in mode of living (e. g., yolk sac absorp- 

 tion) and that the use of length as an index of size may complicate interpretation of 

 growth rates when there is considerable change in form. 



