394 BULLETIN OF THE BUREAU OF FISHERIES 



The accelerated growth rate of young fish in 1919 and later years presumably 

 was due to some improvement in growth conditions. Conditions relatively unfavor- 

 able to the growth of fish of the first three years apparently were present during the 

 years 1915 to 1918, inclusive, and conditions became favorable during the years 

 1919 to 1922 (1923?), inclusive. 



Why were only the herring of years I to III affected by alteration in growth 

 conditions? From the fact that Hanldnson (1914) found herring of the first year 

 in shallow water in Lake Superior it is reasonable to believe that herring hatched 

 in Saginaw Bay remain thei-e during the first year of life, or at least during the major 

 part of their first growing season, and this in spite of the fact that none have 'been 

 taken in the bay. It is not probable that upon hatching the young immediately 

 move out of the bay. If any of the older fish (age groups II and older) remained 

 in Saginaw Bay throughout the summer they would certainly be taken by the com- 

 mercial nets, which have been set in all parts of the bay without taking them in 

 commercial quantities; but no herring are taken in the bay after the big run in the 

 spring, which may continue until June in the vicinity of Bay Port, although 

 it lasts about one week only in April along the west shore of the bay. Yet 

 each fall large numbers of sexually mature herring (age groups II and above) 

 may be taken almost anywhere in Saginaw Bay.' The available evidence, then, 

 indicates that the older Saginaw Bay herring, both immature and mature (age 

 groups II and above), spend the greater part of their growing season in Lake 

 Huron proper. The immature fish older than 1 year may or may not migrate far 

 into the bay with the spawners. Very few of years II to V (Table 50) were taken 

 in my Tobico samples. It appears, then, that herring hatched in Saginaw Bay very 

 probably spend their first growing season in the bay, and that herring older than 

 1 year spend only the early part of the growing season there. The following discus- 

 sion assumes the correctness of this conclusion, and the conclusion itself is rein- 

 forced by the fact that it permits a consistent interpretation of the growth-rate data. 



If the same changes in growth conditions occurred throughout Lake Huron, all 

 the afl;8cted age groups collected in one locality should show the same kind of altera- 

 tion in growth rate in the same calendar years. Further, the herring taken at local- 

 ities on Lake Huron remote from Saginaw Bay should show, for the same calendar 

 years, a history of growth similar to that of herring taken in Saginaw Bay. The first 

 expectation does not appear to be substantiated by the fact (p. 393), as the 1-year 

 herring of Saginaw Bay grew progressively larger each year during the period 1919 to 

 1922, while the 2 and 3 year herring (which do not remain in the bay) each main- 

 tained a constant growth rate throughout these years. No suitable data are avail- 

 able by which to test the validity of the second expectation. As the effect on Saginaw 

 Bay herring in one locality is greatest in that year of life that, presumably, is spent 

 wholly in the bay, and as the alteration in the growth rate of age groups I to III did 

 not occur in the same manner in the same calendar years, we may conclude that the 

 alteration in the rates of growth of the Saginaw Bay herring under consideration was 

 due primarily to some local changes in the environment of Saginaw Bay. 



' Whether the main body of herring remains in the bay during the winter after having spawned is not Isnown. In general, rela- 

 tively few herring are talcen from the bay through the ice. As virtually no growth occurs in the winter, the whereabouts of the 

 herring during this season has no bearing on the present inquiry into the alteration of growth rate. 



