LAKE HERRING OF GREEN BAY, LAKE MICHIGAN 



101 



Table 12. — Percentage of lake herring with completed annuli 

 collected from pound nets during period of annutus forma- 

 tion 



[Number of flsh in parentheses) 



Table 13. — Relation between onset of new growth and total 

 length of lake herring taken in pound nets during period of 

 annuhts formation, by age group 1948-51 

 [Length in inches] 



(1937); rock bass, Ambloplites rupestris, by Hile 

 (1941); and the Atlantic herring, Clupea harengus, 

 by Hodgson (1924). 



It appears also that within an age group the 

 smaller fish tend to start the season's growth 

 earlier than do the larger ones (table 13). With 

 only three exceptions (age group IV of the May 8 

 sample and age groups III and V of the June 19- 

 21 collections — the last two age groups repre- 

 sented by few fish), the lake herring that exliibited 

 new growth had averaged smaller at the end of the 

 preceding season than had fish whose current 

 season's growth had not yet started. In some age 

 groups the current-season increment was suffi- 

 ciently great to eliminate the original difference of 

 average length. 



The relatively long period of annulus formation 

 (at least 6 weeks and probably longer) and the 

 correlation between age and the onset of new 

 growth necessitate care in the determination of age 

 for fish captured early in the growing season. For 

 some individuals it may be difficult or even im- 

 possible to decide whether marginal growth repre- 

 sents a small full-season increment of the preceding 

 year or unusually rapid growth made during the 

 current season. 



PROGRESS OF SEASON'S GROWTH 



The data on the amount of growth and on the 

 percentage of the season's growth completed on 

 various dates of capture (table 14) exhibit some 

 irregularities in trend, and on some dates rather 

 large discrepancies occur among the figures for 

 different age groups.* To some extent these irreg- 

 ularities and discrepancies can be attributed to the 



' Collections of June 21-22, 1960, were omitted because of abnormal scale 

 slie which might have affected the computation of growth Increments— see 

 footnote 4 (p. 100). 



Scale margin 



May 26, 1948: 



Age group III: 



Not growing-. 



Growing 



Age group IV: 



Not growing.. 



Growing 



Age group V: 



Not growing.. 



Growing 



May 13, 1949: 



Age group III: 



Not growing.. 



Growing 



Age group IV: 



Not growing.. 



Growing 



June 21-22, 1950: 

 Age group III: 



Not growing.. 



Growing 



Age group IV: 



Not growing.. 



Growing 



May 8, 1951 : 



Age group III: 



Not growing.. 



Growing 



Age group IV: 



Not growing.. 



Growing 



June 19-21, 1951: 

 Age group II: 



Not growing- 

 Growing 



Age group III: 



Not growing. 



Growing 



Age group IV: 



Not growing. 



Growing 



Age group V: 



Not growing. 



Growing 



Number 

 offish 



308 

 13 



Length 



before 



start of 



growth ' 



10.31 

 10.11 



10.64 

 10.37 



11.58 

 11.40 



10.63 

 10.12 



10.81 

 10.39 



10.02 

 10.17 



10.70 

 10.44 



10.68 

 10.66 



11.11 

 11.29 



7.30 

 8.24 



10.85 

 10.06 



10.75 

 10.39 



11.08 

 11.48 



Total 

 length at 

 capture 



Incre- 

 ment of 



new 

 growth 



' Length of growing flsh calculated by scale measurement of recently formed 

 annulus. 



small numbers of fish in certain age groups. 

 Another possible source of bias lies in the fact that 

 collections were made in different calendar years 

 in which the progress of growth may have been 

 dissimilar. A real correlation appears to exist, 

 however, between age and percentage of growth 

 completed in samples captured during the period 

 of annulus formation. In collections taken before 

 July 13, for example, the percentage of growth 

 completed decreased as age increased with only 

 one exception — the Ill-group of the June 19-21 

 samples. In this collection, the Il-group probably 

 was not representative, because the actual amount 

 of growth and the percentages were both smaller 

 than in May. 



Another factor bearing on the data of table 14 

 that should be mentioned, even though it cannot 

 be evaluated, concerns the validity of the base 

 employed for the computation of the percentages. 



