370 BULLETIN OF THE BUREAU OF FISHERIES 



SUGGESTED EXPLANATION OF RAPID GROWTH OF YOUNG HERRING IN 1919 IN 



SAGINAW BAY 



If we assume that improved conditions are responsible for the increased growth 

 rate ushered in by the year 1919, how may we account for the fact that only those 

 Bay City herring v/ere affected that were 3 years of age and younger? I beUeve this 

 is explained if we assume that the young herring hatched in Saginaw Bay remain there 

 during at least the first year and the early part of the second and of the later years 

 of life. The 2-year and even the 3-year herring may remain in the bay throughout 

 the year, though, as I shall show later (p. 394), this is not probable. It is known that 

 many 3-year herring join the spawning schools in the fall (p. 384) and depart with 

 them in the following winter or early spring to the summer feeding grounds in Lake 

 Huron proper. (See p. 394.) I shall later give reasons for my belief that growth con- 

 ditions were improved in Saginaw Bay during 1919 but not elsewhere in the lake. If 

 the above statements are true, all age groups were subjected during the growth sea- 

 son to the improved conditions of Saginaw Bay in 1919 and later years, the fourth 

 and older age groups for a short period in the fall and spring, the second and third 

 age groups during either the entire growing season or a part thereof, and the first 

 age group during the entire season. The measurable effect of any environmental 

 change that alters growth rate should be more noticeable in the years of rapid growth 

 than in those of relatively slow growth. The 1-year herring, with their large growth 

 increment, would, in general, show more clearly changes in the conditions of growth 

 than the 2-year fish; the latter would show alterations of growth rate more clearly 

 than the 3-year-old. In each older age group, as the growth increment decreased, 

 changes in it would be detected less easily and more likely to be obscured by other 

 factors affecting growth increments. Again, if the 1-year herring were subjected 

 to the improved conditions of Saginaw Bay throughout the entire growing season, 

 while the older fish were subjected to them during only part of the season, the growth 

 of the former naturally would be influenced more by these improved conditions than 

 that of the latter. As, then, the growth increment of the herring is greatest in its 

 first year of life and diminishes progressively in later years, any factor that tended to 

 alter growth rate should show larger measurable effects and therefore be detected 

 more readily in the first year than in later years. As first-year fish are believed to 

 spend a larger part of the growing season in Saginaw Bay than do older fish, growth- 

 controlling alterations in the conditions in the bay should affect them more than they 

 would older fish. They showed growth acceleration in 1919 and later years, over 

 that obtaining in 1918 and years immediately preceding. In the section of this 

 paper dealing with factors of growth in Saginaw Bay the probable causes of this 

 acceleration are discussed. 



LAW OF GROWTH COMPENSATION 



Gilbert (1914) concludes from a study of the computed growth increments of 

 some 4-year sockeye salmon (OncorhyncJius nerJca) that a compensation in growth 

 occurred in the third and fourth years of fife. That is, salmon that were large at the 

 end of the second year grew, on the average, more slowly in succeeding years than 

 the salmon that were small at the end of the second year, so that eventually all indi- 



