Reproductive Biology of Syntopic Blackside Dace and Southern Redbelly Dace in Two Kentucky Streams

When a rare endemic species and an ecologically similar, but more cosmopolitan species share habitat, understanding how they interact with each other is important for conservation efforts. This is especially true if both species rely on the same spawning habitat, which could potentially lead to competitive displacement or hybridization. It is known that the federally threatened Blackside Dace and common Southern Redbelly Dace are ecologically similar species, occur syntopically in Kentucky in the upper Cumberland River basin, and utilize some of the same host species for nest sites; however, it is unclear if the two fish species are spawning site competitors. This study examined reproductive interactions between these two species in the wild by 1) comparing spawning activities of both species in a stream where they are syntopic to those of Blackside Dace in a stream without Southern Redbelly Dace (i.e., spawning mode, habitat, co-occurrence in spawning aggregation), 2) discerning if spawning seasons of the two species differ temporally (based on stream temperature), and identifying differences in spawning and non-spawning microhabitats where the two species do and do not co-occur, 3) developing an ethogram of reproductive behaviors of the focal species and quantifying behaviors relevant to the question of reproductive competition between the two dace species from video recordings. All 8 spawning events observed in the Blackside Dace only stream and all 11 spawning events observed in the syntopic stream were in association with Creek Chub nests in stream habitats characterized as runs. Where the two species were syntopic, all spawning observations contained both species. The first and last observations of spawning activity in the Blackside Dace only stream occurred on earlier dates, and at lower temperatures (April 26th at 12.1 °C, and May 7th at 15.3°C respectively), than the corresponding dates and temperatures where the two species were syntopic (May 8th at 15.2°C, and June 18th at 18.9°C respectively). Spawning seasons for both species completely overlapped in the syntopic stream. However, the maximum number of individuals observed in spawning aggregations peaked at an earlier date for Blackside Dace (May 8th) compared to Southern Redbelly Dace (June 12th and June 14th) in the syntopic stream. The Blackside Dace only stream had significantly lower mean values for conductivity, water velocity, stream width, and canopy cover compared to spawning sites in the syntopic stream. Spawning sites in both streams had an absence of silt and lack of substrate embeddedness compared to non-spawning sites. Canopy cover was significantly greater at spawning sites in the syntopic stream compared to non-spawning sites. All behaviors from the ethogram occurred between and within species. Male-male (agonistic) behaviors and male-female (courtship) behaviors generally occurred at greater rates within species than between. Males of both species formed spawning clasps with females of both species, but successful spawning clasps usually occurred at higher rates with female Southern Redbelly Dace. Competition occurred between male Blackside Dace and Southern Redbelly Dace for spawning opportunities with females of both species, but there were no indications that these species compete for spawning sites in a way that displaced one species from nests. The results demonstrates how behavioral observations in conjunction with habitat analysis can inform management and conservation of imperiled species by revealing that when syntopic both of these species rely on the same spawning habitat, spawn in the same nest at the same time, and spawn with each other across species