No abstract

Density-related changes in age at maturation, sex ratio of recruits, growth, and fecundity at very low density result in spawning populations which comprise many older females and few young males and in which egg production per lake herring (Coregonus artedii) is increased by a factor of five. Age of maturation is delayed at low density for females, but not for males. Thus, males dominate the youngest age classes but females become increasingly more abundant from age 2 + through age 6 +. A model of lake herring population structure showed that although delayed maturation accounts for many characteristics of depressed populations, it does not explain overall population femaleness. The model was used to evaluate effects of differential adult mortality, recruitment failure, and changes in the sex ratio of recruits, and only the latter provided an adequate explanation for overall femaleness. Despite the apparently compensatory nature of these responses, the slow and unsteady recovery of depressed Lake Superior populations during the last two decades indicates that one or more additional factors play important roles in determination of year class strength.