Extreme female predominance in the bloater (Coregonus hoyi) of Lake Michigan in the 1960's

The sex composition and other biological characteristics of the bloater changed substantially during recent decades of continuous ecological change in Lake Michigan. The percentages of females increased from 72% of the bloaters samples in 1928-32 to 95% in 1963, and ranged from 94 to 97% in 1964-69. The unusual predominance of females was established at an early age, unquestionably before age III. Bloaters grew faster in southeastern Lake Michigan in 1960-69 than in 1954 and earlier, and were less abundant there in 1966-69 than in 1962-65. The average age of female bloaters from trawls increased from 3.5 years in 1964 to 6.0 years in 1969. The increase in average age followed a heavy drop in fishing intensity and was accompanied by a decline in recruitment. The percentage representation of age-III bloaters, for example, decreased from 62.4% in 1964 to 2.5% in 1969. The changes in biological characteristics of the bloater may partly be symptoms of a fish population that is poorly adjusted to the changing ecology of Lake Michigan.