LIFE HISTORY OF LAKE HERRING OF LAKE HURON 395 



Alterations in the conditions of (growth in the bay would affect the growth rate 

 of the 1-year herring more than that of the older groups. The growth rate of the 

 1-year fish would not be altered necessarily in the same way from year to year as 

 that of the older fish, as the environmental growth conditions (bay and open lake) may 

 not have been the same for the two groups; but if the environment of the second and 

 older age groups was virtually the same in any one calendar year why did the 2 and 

 3 year fish show alterations in their growth rate while the older herring did not? 

 Or, in other words, if the second and older age groups had been subjected to the same 

 environmental conditions, as I believe, would not these age groups show the same 

 kind of changes in their growth rates? There are at least two probable reasons why 

 they would not: (1) The younger (2 and 3 year) herring may commence the new 

 year's growth earlier in the spring than the older fish. This was found to be true in 

 the marine herring {Clupea Tiarengus) by Dahl (1907), in the lemon dab {Pleuronedes 

 microcephalus) by Storrow (1916), in the haddock (Gadus xglefinus L.) by Samundsson 

 (1925), and in the Atlantic salmon by Menzies and Macfarlane (1926). In that 

 case changes in the conditions of growth in the bay would affect the growth of the 

 yoimger herring more than that of the older. (2) In general, the total amount of 

 annual growth becomes progressively less with age. Slight alterations (inasmuch as 

 these older herring remained in the bay for a short period, most of it when growth is 

 not taking place, alterations in conditions could have aft'ected growth only slightly) 

 in growth rate can not be detected as readily in the average measurements of the 

 older fish as in those of the younger. Thus, even though the growth rates of all the 

 age groups (II and above) of the Saginaw Bay herring were affected by changed 

 environmental conditions of growth, the measurable effect would become progres- 

 sively less with each older age group and finally disappear. I believe that for these 

 reasons the alterations in the growth rates of the 2-year herring (Table 39) were more 

 noticeable than those of the 3-year fish, and that no consistent changes in growth 

 rates occurred in the older fish. As the 2 and 3 year herring had lived under similar 

 conditions of growth in any one calendar year, it is to be expected, as was actually 

 found to be true, that the kind of changes in growth for the same calendar years 

 would be identical in these two age groups. 



We may now ask. What affected the growth rate of herring of years I to III in 

 Saginaw Bay? As it has been shown that a low growth rate of fish 1 to 3 years of age 

 prevailed in the bay in the years 1915 to 1918, inclusive, as compared with that of 

 the years 1919 to 1922, inclusive, two alternatives are possible: (1) The low rate of 

 growth of 1915 to 1918 is the normal or usual one and prevailed before 1915. In 

 that case something happened in 1919 to better hitherto prevailing normal growth 

 conditions. (2) The growth rate of 1915 to 1918 was abnormally low; a higher rate 

 prevailed before 1915 and was resumed in 1919. In that case some factor unfavor- 

 able to growth was effective in the years 1915 to 1918 but not before or after 

 those years. 



COMPARISON OF GROWTH RATES EXISTING BEFORE AND AFTER THE PERIOD 1915 



TO 1918 



To decide between these alternatives it is necessary to compare the growth rates 

 of the Saginaw Bay herring existing before and after the period 1915 to 1918. Was 

 the growth rate of 1919 to 1922 merely a resumption of that prevailing before 1915? 



