Specific gravity of ova and larvae of pelagophilic broadcast-spawning cyprinids

Abstract

Many cyprinids native to streams and rivers of the US Great Plains belong to a reproductive guild that spawns by broadcasting non-adhesive semi-buoyant ova into the current. After fertilization these ova increase in size and buoyancy and remain suspended in the water column as long as sufficient flow is available. After hatching, early larval stages are incapable of sustaining buoyancy. They rely on a swim-up movement and flow to keep in suspension until the swim bladder is fully developed, which takes about four to six days post-hatching depending on temperature. It is believed to be important that ova and larvae remain in suspension to prevent settling to the river bottom where they are susceptible to being buried by shifting sediments. Specific gravity can be used as a means of estimating the minimal flow required for ova and larvae to remain in suspension. Previous studies have measured the specific gravity of ova from a small number of these cyprinids, yet no study has measured the specific gravity of larvae. The purpose of this study was to measure specific gravity of fertilized ova and larvae from five broadcast-spawning cyprinids using a calibrated, density-gradient column ranging from 0 to 30 ppt salinity concentration (ova) or 30 to 125 ppt salinity concentration (larvae). Mean specific gravity of fertilized ova ranged from 1.0050 to 1.0080, and were similar to measurements of specific gravity from previous studies. Mean specific gravity of larvae one day post-hatch ranged from 1.0440 to 1.0459 and were significantly denser than ova. Given that larvae are significantly denser than ova and likely require higher flow to stay in suspension, I conclude that the hypothesized effects of river discharge on reproductive success of pelagophilic broadcast-spawning cyprinids most likely is attributable to survival of early larval stages.