134 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



4. Scales for studies of age, growth, and year- 

 class strength were collected from 4,390 lake 

 herring taken from pound and gill nets. Investi- 

 gation of most phases of the life history was based 

 on catches of pound nets, which are less selective 

 with respect to size of fish than are gill nets. 

 Length records were obtained for all and weight 

 and sex data for most of the 2,039 lake herring 

 taken from experimental gill nets. 



5. Age determinations were made by examining 

 the magnified image of scales projected on a 

 screen. Fish with no annulus (year-mark) were 

 assigned to age group 0, those with 1 annulus to 

 age group I * * *. All fish were considered to 

 pass into the next higher age group on January 1. 



6. The maximum age of lake herring reported in 

 any population is XII ; the oldest fish from Green 

 Bay belonged to age group VII. The best-repre- 

 sented age groups in the various populations for 

 which there are published records are age groups 

 II to V; age groups III and IV were the most plen- 

 tiful in Green Bay. The commercial catch in Green 

 Bay was dominated by age group IV in the period 

 January to June and by age group III in July to 

 December. 



7. The age composition of lake herring from 

 pound nets was not representative of the popula- 

 tion, as young fish were seldom taken even though 

 the mesh sizes (1)2 to 2 inches, extension measure) 

 were small enough to hold them. Yearling lake 

 herring, as a rule, do not inhabit the relatively 

 shallow, inshore areas where pound nets are fished. 



8. The length of lake herring from the commer- 

 cial pound nets and gill nets varied little from 

 season to season. Even during the summer period 

 of rapid growth the effects of individual increases 

 in length were largely compensated by the selec- 

 tive destruction of the larger lake herring in the 

 fishery and by the shift to a lower average age. 



9. The relation between the total body length 

 in inches (L) and the magnified (X41) scale 

 diameter in millimeters (S) of Green Bay lake 

 herring is described by the formula 



L = 0.01615 + 0.05486 S 

 Since the intercept is so small, its value was 

 assumed to be 0, and lengths at the end of various 

 years of life were calculated from scale measure- 

 ments by direct proportion. 



10. Annuli are formed on scales of the Green 

 Bay lake herring in May and June. The progress 

 of annulus formation is irregular, possibly because 



of different local environmental influences. The 

 younger age groups and the smaller fish within an 

 age group tend to form annuli earliest. 



11. Growth within the season was described by 

 a sigmoid curve. Growth started about the first of 

 May and terminated near the end of October, 

 with the fastest growth in July. 



12. Males and females grew at the same rate. 



13. Selective destruction of fast-growing in- 

 dividuals was so great that seasonal differences in 

 style of growth were detectable, that is, lake 

 herring taken early in the year had grown faster 

 in earlier years than had fish of the same age 

 group captured later in the same year. 



14. Calculated length at the end of the first 

 year of life increased from north to south. These 

 first-year differences, almost surely of environ- 

 mental origin, were rapidly reduced by com- 

 pensatory growth in later years of life. 



15. Annual fluctuations in growth in length 

 indicated that conditions affecting growth of lake 

 herring in Green Bay changed little from year to 

 year. The growth rate was below average and 

 decreasing from 1944 to 1946, improved from 

 1946 through 1950, and then declined somewhat 

 in 1951. Growth was well above average during 

 the period 1949-51. 



16. The different age groups exhibited sys- 

 tematic discrepancies in calculated growth re- 

 sembling those commonly termed Lee's phenom- 

 enon of "apparent decrease of growth rate." 

 Selective destruction of the larger, faster-growing 

 fish by the commercial fishery was held to be the 

 most important of the various factors that may 

 have contributed to the discrepancies. 



17. Growth compensation takes place in Green 

 Bay lake herring. It was shown that growth 

 compensation will appear in the calculated growth 

 of fish that follow identical growth curves but 

 that are hatched at different times in the season. 

 It was also demonstrated that length rather than 

 age is the primary determinant of subsequent 

 growth of the individual, and hence that growth 

 compensation can occur among fish whose growth 

 curves are different. 



18. The general length-weight relation of the 

 Green Bay lake herring is described by the 

 equation 



log W^= -2.4386 + 3.0729 log L, 



