Hydraulic synchrony of spawning sites amongst Earth's riverine fishes

Earths riverine fishes utilize a suite of reproductive guilds, broadly following four guilds: nest guarders, broadcast pelagic spawners, broadcast benthic spawners and nest non-guarders 1,2, and these guilds utilize different mechanisms to aerate eggs 3,4. Globally, river fishes populations are declining5, and spawning habitat rehabilitation has become a popular tool to counter these declines6. However, there is a lack of understanding as to what classifies suitable spawning habitats for riverine fishes, thereby limiting the efficacy of these efforts and thus the restoration of the target species. Using data from n = 220 peer-reviewed papers and examining n = 128 unique species, we show the existence of a hydraulic pattern (defined by Froude number (Fr), a non-dimensional hydraulic parameter) that characterizes the reproductive guilds of riverine fishes. We found nest guarders, broadcast pelagic spawners, benthic spawners, and nest non-guarders selected sites with mean Fr = 0.05, 0.11, 0.22, and 0.28, respectively. Some of the fishes in this study are living fossils, suggesting that that these hydraulic preference patterns may be consistent across time. Our results suggest this hydraulic pattern can guide spawning habitat rehabilitation for all riverine fish species globally in absence of specific spawning habitat information for a species, where resource managers can establish the reproductive guild of the species of interest, and then apply the specific hydraulic requirements (Fr range) of that reproductive guild, as presented herein, in the rehabilitation of the target species.