Reproductive cycle of northern largemouth bass Micropterus salmoides salmoides

The seasonal dynamics of the northern largemouth bass (Micropterus salmoides salmoides) reproductive cycle was investigated in a Midwestern United States population. Mature largemouth bass (n = five males and five females) were collected each month over a one-year period. Environmental cues (water temperature and photoperiod) were recorded in situ using data loggers. Temporal changes in plasma sex steroid hormones (testosterone [T], 11-ketotestosterone [11-KT], and estradiol-17β [E₂ ]) and vitellogenin (Vtg), gamete development (histology), and organosomatic indices were highly associated with temporal water temperature and photoperiod dynamics. Female plasma concentrations of T and E₂ and organosomatic indices exhibited similar temporal patterns, with peaks occurring in early summer. Gamete development in females was associated with increases in gonadosomatic index (GSI) and Vtg. In males, plasma concentrations of T and organosomatic indices exhibited bimodal peaks; high peaks occurred in the spring for T and GSI and in the fall for viscerosomatic and hepatosomatic indices. Male gamete development and GSI closely tracked temporal patterns in plasma T, with GSI lagging behind the T cycle by approximately one sampling period. Plasma concentrations of 11-KT were elevated in late winter and spring and were considerably higher than T throughout the reproductive cycle. Unlike the Florida largemouth bass (M. salmoides floridanus) advanced gamete development was observed in both sexes before winter, which may reduce the immediate energy required for gamete development in the spring. Results from this study also provide baseline information necessary for off-season spawning procedures and endocrine-disruption studies in northern largemouth bass.