350 BULLETIN OF THE BUREAU OF FISHERIES 



from various localities on Lake Huron and by me in 1922 at Oscoda, Mich. Doctor 

 Koelz's collections are usually small, taken for taxonomic purposes, and my con- 

 clusions based on them must be accepted with such reserve as the relatively small 

 numbers impose. 



ABUNDANCE OF AGE GROUPS AND YEAR CLASSES IN THE SAMPLES' 



AGE GROUPS 



As stated on page 268, the Saginaw Bay herring samples of 1921, 1922, and 1923 

 were taken in pound nets set at Tobico about 3 miles west of the mouth of the Sagi- 

 naw River, while those of 1924 were taken in pound nets set at Tobico, Nayanquing, 

 Au Gres, and Gravelly Point, the last three bars being, respectively, about 9, 25, and 

 32 miles north of the mouth of the Saginaw River. (See fig. 1.) Each collection was 

 taken on or near the spawTiing grounds and is composed principally of fish that would 

 have spawned shortly, perhaps in a week or so. 



In each of the years 1922 and 1923 herring were collected on a single date and 

 in a single locality (one large sample), while in 1921 one large sample was taken on 

 October 29 and several smaller samples were collected on the other days of the period 

 October 26 to November 4; in 1924 herring were collected on various dates through- 

 out the height of the spawning run until ice conditions made fishing impossible. The 

 Tobico, Nayanquing, and Gravelly Point collections of 1924 each comprise two or 

 more small samples. The small sample of Au Gres herring was taken on a single 

 date. I shall later (p. 385) give reasons for concluding that the character of the vari- 

 ous samples taken at the same locality does not change consistent!}' as the season 

 advances, so that the Tobico collection of 1924 is entirely comparable with the 

 sample's taken at the same point in earher years. I shall also (p. 387) give reasons for 

 my belief that the herring of Tobico and Nayanquing belong to the same population. 

 The herring taken at Gravelly Point and at Au Gres may belong to the Tobico and 

 Nayanquing races, but the data do not show this indisputably. The Tobico and 

 Nayanquing material, therefore, is treated as a homogeneous collection and as such 

 is strictly comparable with the material taken in 1921 to 1923, while the samples 

 from Gravelly Point and Au Gres are considered separately. The Au Gres collec- 

 tion, however, which comprises relatively few individuals, may not be representa- 

 tive for all age groups. Data of these fish, therefore, are considered in a more or less 

 incidental way. In a strictly comparative study of the samples of different years the 

 Tobico and Nayanquing material alone of the 1924 samples is considered. 



' A year class refers to fish hatched in the same year; an age group to fish of the same age. Thus, the year class I9I4 includes 

 all individuals hatched in 1914, irrespective of their age when captured: age group IV includes fish in their fourth year of life. Fish 

 of the same year class captured in different years belong to different age groups. 



