LIFE HISTORY OF LAKE HERRING OF LAKE HURON 393 



Huron, and (2) that Saginaw Bay produces, so far as known, the most rapidly growing 

 herring in Lake Huron. 



The above data suggest that the migrations of the herring are more or less local. 

 If the lake herring of two localities intermingled, their growth rates should be the 

 same. The different growth rates indicate that at least the majority of the herring 

 of the Saginaw Bay district keep apart from those of the Alpena district, and vice 

 versa, and that the Wiarton herring do not intermingle with those of Killarney in 

 Georgian Bay. 



FACTORS INVOLVED IN THE ALTERATION OF THE GROWTH RATE OF 

 SAGINAW BAY HERRING DURING THE PERIOD 1915 TO 1923 



A study of the factors that affect the growth rate of fishes in nature usually 

 requires a biological survey of the body of water involved, and this should cover a 

 period of consecutive years. Such a survey has not been made of Saginaw Bay, 

 and if one were to be carried on now it could hardly explain the past growth history 

 of its herring; but we know during which years of the period 1915 to 1923 acceler- 

 ation in the growth of the Saginaw Bay herring occurred, and there are certain factors 

 that are known to affect growth rate and concerning which data are available for 

 the period. It has seemed worth while, therefore, to correlate the data on these fac- 

 tors with the observed changes in growth rate in the hope that significant relations 

 would appear. The history of the growth of these herring is so unique that any 

 attempt to explain it is justified. The factors referred to are temperature and light, 

 fishing intensity, and the chemical pollution of Saginaw Bay by the Dow Chemical 

 Co. of Midland, Mich. 



RESUME OF THE GROWTH HISTORY OF SAGINAW BAY HERRING 



The significant facts of growth already discussed are the following: ^ 



1. In each of the years 1915 to 1918, inclusive, the rate of growth of herring in 

 the first year of life was the same. 



2. In the year 1919 the growth rate was increased in herring 1,2, and 3 years 

 of age. 



3. Neither in 1919 nor subsequently did_ the growth rate of fish older than 3 

 years increase. 



4. The acceleration of growth rate initiated in 1919 in fish 3 years of age and 

 younger continued in the years 1920, 1921, and 1922. 



5. In the period 1919 to 1922, inclusive, fish in their first year of Ufe, in general, 

 grew more rapidly each successive year. During this period there was progressive 

 increase of growth rate in 1-year fish, although in 1922 the increase was very slight 

 (1 millimeter). 



6. During the period 1919 to 1923, inclusive, the growth rate of the second and 

 third year fish did not increase in successive years but remained virtually constant 

 at the increased rate attained in 1919. 



7. The growth rate for the fourth and later years likewise tended to remain 

 constant in the years 1919 to 1924, inclusive. 



' For each statement refer to Table 39. 



