LAKE HERRING OF GREEN BAY, LAKE MICHIGAN 



107 



ence on either annual variations in temperature or annual 

 variations in population density suggests that possibly 

 these variations in growth depend closely on both factors, 

 and that the failure of these factors fo operate in the 

 same direction in the same year tends to obscure the effect 

 of each of them. 



Van Oosten (1929) found no correlation between 

 annual fluctuations in first-year growth and annual 

 fluctuations in the air. temperatures during the 

 growing season for Saginaw Bay lake herring. 

 More recently, Svardson (1951) has shown that the 

 growth of whitefish in Sweden was greater in 

 hot summers than in cool. 



The data for the Green Bay lake herring (table 

 22) give no evidence of a definite relation between 

 fluctuations of growth and deviations of mean 

 air temperatures or population density over the 

 8-year period 1944 to 1951. 



Table 21. — Annual calculated growth increments of lake 

 herring from pound nets in southern Green Bay in January 

 or February, 1949-52 



Table 22. — Deviation of growth, air temperature during the 

 growing season (May-October), and abundance of lake 

 herring in southern Green Bay, from the average for the 

 8-year period, 1944-51 



' Mean monthly deviations of air temperatures for the period May-October 

 recorded by t^ S. Weather Bureau at Green Bay, Wisconsin. 



2 Percentage deviation fioni average catch per unil-iif-efTort in pound and 

 gill net.s computed from Wisconsin commercial catch records for southern 

 Green Hay (Wisconsin commercial fishing district M-1). 



Discrepancies in calculated growth 



The systematic discrepancies among calculated 

 growth histories of different age groups already 

 noted for the Green Bay lake herring are a frequent, 

 almost regular, characteristic of data on growth 

 of fish. These differences occur among different 

 age groups of the same year class as well as among 

 age groups of different year classes. The pattern 

 of the discrepancies varies from species to species 

 and stock to stock. Most common is that which 

 goes under the name of Lee's phenomenon of 

 "apparent decrease of growth," in which the esti- 

 mates of lengtii at the end of various years of life 

 decrease with increase in the age of the fish on 

 which the estimate is based. In this "typical" 

 situation, tlie calculated lengths in the earlier years 

 of life show the greatest disagreements. More 

 recent authors have tended to depart from this 

 definition and to apply the term "I^ee's phenom- 

 enon" to all systematic discrepancies among 

 calculated lengths. 



The literature on causes of Lee's phenomenon, 

 in both the restricted and the broader sense, is 

 extensive and to a considerable degree contro- 

 versial. A review of the subject at this time could 

 serve little purpose.' It may be useful, never- 

 theless, to list the principal factors that liave been 

 offered in explanation of systematic discrepancies 

 in calculated lengths. These several factors, the 

 significance of which will become clearer from later 

 discussions, are as follows: 



1. Use of wrong formula for growth calcula- 

 tions. 



2. Selective action of fishing gear. 



3. Biological segregation on basis of size or 

 maturity. 



4. Higher mortality rate (natural or in the 

 fishery) of the fish with the more rapid growth. 



In the consideration of discrepancies among 

 calculated lengths of Green Bay lake herring, the 

 first of these items is not significant since the 

 validity of the method of calculation was estab- 

 lished by a study of the body-scale relationship. 

 The effects of gear selectivity (item 2) can be 

 rendered insignificant by confining studies of 

 growth discrepancies to samples taken by pound 

 nets, which, as has been pointed out, were capable 

 of capturing fish smaller than the smallest herring 



« See Van Oosten (1929) and lllle (1936) for detailed discussions of the prob- 

 lem. 



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