LIFE HISTORY OF LAKE HERRING OF LAKE HURON 335 



groups of these two year classes. It is decidedly conspicuous, however, in these 

 age groups in the 1919 year class. In this year class the calculated lengths of the 

 6-year fish are noticeably lower in most years than the corresponding lengths of the 

 5-year fish. Our data, then, do not appear to be sufficient to enable us to decide 

 definitely whether Lee's "phenomenon" is present in the scale-diameter measure- 

 ments or in the calculated lengths of the fifth and older age groups of a year class. 



Can the assumption that a selection occurs in the matured age groups, whereby 

 comparatively slow-growing individuals are introduced into the fourth and fifth 

 age groups, explain the apparent contraction and expansion of scales? It is apparent 

 that such a selection could account for the progressive decrease in the scale incre- 

 ments with age in the first two years of life of the herring of age groups III to V, 

 inclusive; but it is not clear how this selection can explain why the presumably slow- 

 growing fish of a year class should, in their third or fourth year of life, become the 

 fast-growing fish of their year class, and why the presumably slowest-growing fish 

 should change into the fastest-growing. In other words, it can not explain the pro- 

 gressive increase in the scale increments of the fourth and later years of life with age. 



Another factor — sexual maturation — may be involved here, however. Spawn- 

 ing takes place in the lake herring in November or December. The annual ripening 

 of the sexual products and the attendant change in the habits of the fish may cause 

 an earlier cessation of body and scale growth in mature fish than occurs in immature 

 fish. The growth zone of the year may be narrower in the scales of mature fish 

 than it would have been had the fish remained immature. That sexual maturation 

 retards growth is shown in some fish (for example, the salmon) by a spawning mark. 

 It is also common knowledge that in fishes (at least in those of the northern latitudes) 

 the first prominent break in the curve of growth generally occurs in that year in which 

 a large percentage of individuals reach sexual maturity. This holds, also, for the 

 lake herring, whose curve of growth, as shown in Figure 39, bends sharply in the third 

 year. Sexual maturation and retardation in growth are probably positively corre- 

 lated in the lake herring. 



As the lake herring reach sexual maturity in either the third, fourth, or fifth 

 year of life, it follows that the fourth and older age groups (practically all individuals 

 in the Bay City samples were sexually mature) include individuals that were immature 

 in their third year. These immature fish, which, according to Lee's first suggestion, 

 were the slower growing individuals of their year class, were not retarded in their 

 growth in the third year by sexual maturation. The fast-growing, mature, 3-year 

 fish may have been retarded sufhciently in their third year growth rate to allow the 

 slow-growing, immature individuals of their age group and year class to approach 

 closely their growth rate, to equal it, or to exceed it. These immature individuals, 

 on becoming sexually mature in their fourth or fifth year and joining the mature 

 fourth and (or) older age groups, would then affect the averages of the third-year 

 scale increments of these age groups in such a way that they would be slightly lower 

 than or equal to or exceed the average of the third-year scale increment of the mature 

 3-year fish of the same year class. A retardation in the growth of the sexually 

 mature 3-year herring could account for the increase in the width of the third year 

 growth zone in the scales of the 4-year fish, but, as I shall show shortly (p. 336), not 



