Natural Reproductive Cycle of Northern Largemouth Bass in the Upper Midwest, with Applications to Off-season Spawning

Largemouth bass Micropterus salmoides are popular game fish in the United States. In addition to their popularity, largemouth bass function as top-level predators across their range, particularly in temperate small impoundment communities, where they regulate panfish size structure and abundance. In South Dakota, largemouth bass reproductive success and recruitment varies among populations, oftentimes yielding high density, slow-growing panfish populations in small impoundments. To address this concern, in addition to providing angling opportunities, northern largemouth bass M. s. salmoides fingerlings are produced for stocking purposes at Blue Dog State Hatchery using traditional pond culture methods. Fingerlings (~75 mm) are harvested and transported to stocking sites during late summer. Fingerling survival to age-1, however, is generally low in established fish communities, likely due to a combination of size and predation related mortality, losses due to environmental extremes, and insufficient overwintering resources. To improve age-0 fingerling survival in South Dakota waters, hatchery procedures directed at producing advanced sizes of largemouth bass need to be explored. Alternative culture methods (i.e., off-season spawning (OSS)) may facilitate the production of larger, age-0 stocking products for management applications in the upper Midwest. If successful, OSS of largemouth bass could provide advanced sized bass for earlier stocking dates. Stockings occurring earlier in the growing season would provide a window for age-0 fish to attain greater lengths and accumulate more energy reserves prior to winter, potentially enhancing survival. However, prior to implementing OSS, quantification of the physical cues (temperature and photoperiod) and physiological parameters which regulate reproductive processes in fishes is necessary. Therefore, the objectives for this study were 1) to assess the natural reproductive cycle of male and female northern largemouth bass, and 2) to develop an OSS method using results from objective 1 to produce advanced-sized largemouth bass fingerlings for management applications in South Dakota. To address objective one, the reproductive cycle of northern largemouth bass was documented in a small southeastern South Dakota impoundment to develop OSS methods. Mature largemouth bass (n = five males and five females) were collected each month via angling for one year. Water temperature and photoperiod was recorded in situ using data loggers. Monthly trends in plasma sex steroid hormones [testosterone (T), 11-keto-testosterone (11-KT), and estradiol—17β (E2)] and vitellogenin (Vtg), in addition to gamete development and organosomatic indices, were related to temporal water temperature and photoperiod patterns. Statistical analysis ( = 0.05) of reproductive parameters across months was conducted with Proc Mixed (SAS, 2004), followed by a Tukey’s post-hoc test. Models were significant (P \u3c0.05) for all parameters investigated in both sexes, indicating significant temporal differences existed in key physiological processes. In females, plasma concentrations of T and E2 and organosomatic indices exhibited similar temporal patterns, with peaks (P \u3c0.05) occurring in the spring. 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. Fall peaks in male reproductive parameters were not different from spring peaks (P \u3e0.05). Male gamete development and GSI also followed temporal patterns in plasma T, however increases in GSI lagged behind the T cycle by approximately one sampling period. Unlike T, elevated plasma concentrations of 11-KT in males during late winter and spring were different from fall concentrations (P \u3c0.05). Plasma concentrations of 11-KT were also considerably higher than T throughout the reproductive cycle, suggesting 11-KT is the primary androgen influencing spermatogenesis in this species. These physiological results were highly related to temporal changes in temperature and photoperiod. Although reproductive cycle similarities exist between the Florida M. s. floridanus and northern subspecies, the northern subspecies exhibited some characteristics typical of cool water teleost fishes (e.g., walleye Sander vitreus), which exhibit advanced gamete development prior to winter. Production and retention of gametes through winter periods may reduce the energy needed for gamete development in the spring. Additionally, females exhibited an E2-Vtg relationship which differed from the pattern commonly reported for temperate freshwater fishes. This E2-Vtg mismatch may be related to peak periods of endocytosis during the spawning season, where Vtg was rapidly removed from the blood stream and integrated into growing oocytes. Results from this study should provide a baseline for developing a compressed photothermal schedule to accommodate OSS by northern largemouth bass