Linking hydrological and thermal regimes to the reproductive success of riverine fishes in the lower Tallapoosa River

Abstract

The periodic-stochastic nature of rivers coupled with environmental phenomena produces inherent variable conditions important to maintaining ecosystem function and supporting biodiversity. However, when hydrological alteration creates abnormal flow and temperature patterns, this can severely affect pivotal life stages in a fish’s life that are integral to spawning, recruitment and growth. The Tallapoosa River is a major tributary to the Alabama River and has been heavily modified for hydropower, consumptive use, flood control, and recreation. Our research focuses on the southernmost dam, Thurlow Dam, that is associated with cold-water releases and operated with a minimum flow release requirement of 34 m3/sec. Our research aimed to evaluate and assess the effects of hydrological and thermal conditions associated with hydropower on the maturation, spawning success, hatch success, and daily growth of target fishes representing nest-guarding and crevice-spawning reproductive strategies: Alabama Bass Micropterus henshalli, Largemouth Bass Micropterus nigricans, Bluegill Lepomis macrochirus, Longear Sunfish Lepomis megalotis, and Blacktail Shiner Cyprinella venusta. We quantified maturation and spawning success using both histology and an oocyte maturation index. We used Bayesian regressions to assess the relationships between maturation, successful spawning, and hydrological, thermal, and temporal factors. We quantified hatch success and daily growth rates using otolith microstructure analyses. We used logistic regressions and linear regressions to evaluate hatching success and daily growth rates, respectively, related to hydrological, temperature, and temporal variables. Our results showed hydropower driven shifts in the spawning phenology of nest-guarding and crevice-spawning fishes at both adult and juvenile life stages. Specifically, our data showed the occurrence of a protracted spawning season with autumn and winter spawning among our nest-guarding species in a warmwater system. We identified temporal variation in spawning efforts and spatial differences in spawn timing between mainstem and tributary sites. Our model results indicated that successful reproduction and successful juvenile survival of riverine fishes was related to a combination of flow and thermal metrics. Collectively, consideration of both the timing and frequency of hydropeaking events by implementing ramping rates and selective withdrawals could contribute to improving reproduction and juvenile recruitment.