Exploring Mechanisms Underlying Sex-Specific Differences in Mortality of Lake Michigan Bloaters

Sex-specific differences in mortality rates have been observed among freshwater and marine fish taxa, and underlying mechanisms can include sex-specific differences in (1) age at maturity, (2) growth rate, or (3) activity or behavior during the spawning period. We used a long-term (1973-2009) Lake Michigan data set to evaluate whether there were sex-specific differences in catch per unit effort, mortality, age at maturity, and length at age in bloaters Coregonus hoyi. Because bloater population biomass varied 200-fold during the years analyzed, we divided the data into three periods: (1) 1973-1982 (low biomass), (2) 1983-1997 (high biomass), and (3) 1998-2009 (low biomass). Mortality was higher for males than for females in periods 2 and 3; the average instantaneous total mortality rate (Z) over these two periods was 0.71 for males and 0.57 for females. Length at age was slightly greater (2-6%) for females than for males in different age-classes (3-6 years) during each period. Age at maturity was earlier for males than for females in periods 1 and 2, but the mean difference was only 0.2-0.4 years. To test the hypothesis that somatic lipids declined more in males than in females during spawning (perhaps due to increased activity or reduced feeding), we estimated sex-specific percent somatic lipids for fish sampled in 2005-2006 and 2007-2008. During 2005-2006, somatic lipids declined from prespawning to postspawning for males but were unchanged for females. During 2007-2008, however, somatic lipids were unchanged formales, whereas they increased for females. We found that sex-specific differences in Z occurred in the Lake Michigan bloater population, but our hypotheses that sex-specific differences in maturity and growth could explain this pattern were generally unsupported. Our hypothesis that somatic lipids in males declined during spawning at a faster rate than in females will require additional research to clarify its importance.