416 BULLETIN OF THE BUREAU OF FISHERIES 



51. Tlie low growth rates that prevailed among the herring of Saginaw Bay 

 during the period 1915 (1916) to 1918 were abnormal; in general, higher rates pre- 

 vailed before 1915 (1916) and were resumed in 1919. 



52. The low growth rates of the Saginaw Bay herring during 1915 (1916) to 1918 

 were due to some unfavorable local conditions of growth in Saginaw Bay. 



53. The growth alteration of the Saginaw Bay herring during the period 1915 to 

 1922 is explained best as due to the temporary chemical pollution in 1915 to 1917 by 

 the Dow Chemical Co., of Midland, Mich., by wastes containing dichlorobenzol. 

 Temperature, hght, and fishing intensity as factors of growth do not explain all the 

 data satisfactorily and therefore do not appear to have been the controlling factors 

 in the growth of these fish. 



54. Rough computations based on growth data and herring statistics show that 

 the indirect economic losses, due to retardation in growth, in the herring fisheries for 

 the years 1917 to 1923, inclusive, totaled 4,450,224 poimds, with a value to the 

 fishermen of $135,753. These indirect losses are believed to have been occasioned 

 principally by the chemical pollution of 1915 to 1917. 



CONCLUSIONS 



This study shows that the structural characters of the scales of the coregonoid 

 fishes of Lake Huron are so clearly recognizable as to permit their use by tJie scale 

 method. It sliows, furtlier, that the fundamental assumptions underlying the scale 

 method are warranted in so far as they apply to the lake herring {Leucichthys artedi 

 Le Sueur). The scale method is therefore valid when applied in a study of the life 

 history of the lake herring. The Ufe history of the lake lierring that occur in Lake 

 Huron is described in detail in this paper for the first time. 



