Reproductive ecology of the plains minnow Hybognathus placitus in the Brazos River, Texas

The reproductive ecology of plains minnow, Hybognathus placitus, at two localities in the upper Brazos River was studied over a two-year period, January 2008 to December 2009. Using a combination of gonadosomatic indices, measurements of vitellogenic oocyte diameter, and histological analyses, I found that plains minnow spawning season lasts six months from, April through September. Spawning occurs multiple times throughout the spawning season and is often correlated with increases in stream discharge. The plains minnow population in the Brazos River spawns in synchrony, in relation to increases in discharge. However, these synchronized spawning events in relation to increases in discharge, within the population, are not only triggered by flood pulses, but also can be triggered by small increases in base flow. The histological analyses indicate that individual fish spawn asynchronously on a daily basis regardless of discharge. This indicates that the plains minnow belongs to the third genus of fishes that exhibit this type of spawning behavior (e.g. Arkansas River shiner Notropis girardi, smalleye shiner Notropis buccula, sharpnose shiner Notropis oxyrhynchus, and peppered chub Macrhybopsis tetranema). These species represent significant proportions of the ichthyofauna of the Brazos and Canadian rivers. Since, the plains minnow is widely distributed throughout the Great Plains, this suggests that synchronous and asynchronous reproductive strategies may represent the predominate spawning regime for pelagic broadcast-spawning cyprinids of the Great Plains.