Associations between Pre-Spawn Concentrations of Plasma Sex Steroids and Reproductive Potential of Wild Common Carp at the Level of Individuals and Populations

Fall photothermal change drives gonadal growth for spring reproduction in many temperate teleosts. Sex steroids promote gametogenesis and seasonal patterns of gonadal growth generally parallel those of plasma sex steroids. Thus, sex steroids are an important link between proximate environmental cues and reproduction. It is often unclear, however, if variation in life history (e.g., fecundity) caused by natural or anthropogenic challenges is associated with individual variation in sex steroid production. This study examined associations between fecundity and plasma sex steroids (11-ketotestosterone, estradiol-17β) in Common Carp of Lake Mead (NV/AZ) under various contaminant-exposure scenarios. Pre-spawn (spring) ovarian follicle numbers (absolute fecundity) and gonadosomatic index (GSI) were estimated in females, and GSI in males served as index of fertilization potential. Sex steroids positively correlated with fecundity/fertility indices in individual males and females. Chemical analyses of individual males showed differences in whole-body contaminant concentrations among sites, which covaried with individual steroid levels and GSI. Population (site means) differences in steroids and fecundity indices in males and females paralleled population differences in contaminant exposure. In conclusion, individual and population variation in fecundity/fertility indices of Common Carp caused by anthropogenic contaminants is associated with and may be partly driven by individual variation in sex steroids.