406 BULLETIN OP THE BUREAU OF FISHERIES 



As the 1-year herring hatched in Saginaw Bay remain there longer than the older 

 fish that migrate into the bay, the former were already subjected to the pollution 

 in 1915 while the latter were first subjected in the fall of 1915 or the spring of 

 1916. This would explain why the growth rate of the 1-year fish was first reduced 

 in 1915 and that of the older groups in 1916. (See p. 397.) All age groups in 

 Saginaw Bay were subjected to the chemical pollution in 1916, 1917, and 1918 — 

 the 1-year group presumably throughout the growing season, the older age groups 

 in the fall and also in the spring, when the pollution was most concentrated and most 

 severe. By the spring of 1919 the pollution had abated to such an extent as to permit 

 the 1-year herring to retiu-n partly to their normal rate of growth. The absence of 

 the concentrated pollution in the spring of 1919 allowed the older age groups to return 

 to their normal growth rate. It was not until 1921 that the 1-year herring apparently 

 regained their normal rate of growth. This may be accounted for in two ways: 

 (1) According to Mr. Harrison, Bay City chemist, the dichlorobenzol, which is a 

 heavy Hquid, could be seen to He as a separate layer on the bottom of the river in 

 various places. It is plausible to believe that this deposit was dissipated gradually 

 by solution into the river water and by current and wave action, so that normal 

 conditions in the bay were restored slowly. (2) The chemical pollution may have 

 reduced the abundance of plankton, the food of the herring. The restoration of the 

 normal supply in all probabLhty would be gradual. It is not surprising, therefore, 

 that the normal growth rate of the 1-year herring returned slowly. Neither does 

 it seem remarkable that normal growth rate returned suddenly in the older fish. 

 They were subjected only for a short period each year at a time when the pollution 

 was most concentrated. When this severe pollution ceased the period of exposure 

 was too brief to allow the relatively mild pollution to retard the growth of these fish. 



It is realized that there were various other industrial wastes that entered the 

 Saginaw River and found their way into the bay. The wastes from some of these 

 industries doubtless were increased as production grew during the war period; but 

 with the close of the war, industries in general continued to operate by the same 

 methods. They produced other materials of similar kind, sometimes in reduced 

 quantities. There is no reason to suppose that the quality of their wastes changed 

 when the war closed, although the quantity may have been somewhat less; but the 

 waste of the Dow chemical works was qualitatively different during the war period 

 from what it v/as before or after, in that it contained dichlorobenzol. 



The growth alteration of the Saginaw Bay herring appears, then, to be correlated 

 with the temporary pollution by the Dow Chemical Co. by wastes containing dichloro- 

 benzol. The period of retarded growth coincides exactly with the period of the 

 pollution. The presence of this pollution explains all the facts in the growth history 

 of the herring, and no known fact is inconsistent with this explanation. If this 

 chemical pollution is not responsible, then the coincidences of the critical dates and 

 data are truly remarkable. 



