Behavioral Responses of Bluehead Chub and Mountain Redbelly Dace to Experimental Manipulation of Host, Associate, and Egg-Predator Abundances

In a behavior called “nest association”, mountain redbelly dace Chrosomus oreas spawn on gravel mound nests built by bluehead chubs Nocomis leptocephalus. The behavioral dynamics of this interaction may depend on biotic (e.g., host or predator presence/abundance) context. We sought to identify differences in timing of spawning and relative reclusive behavior (a surrogate for perceived risk) with an in situ experiment where factors included presence of chubs, dace, and abundance of egg predators (high or low) at two sites. Dace never spawned in the absence of chubs: adults never entered full breeding color or formed spawning aggregations, and contained mature ova at the end of the experiment. Dace may be obligate nest associates. Chubs spawned in the absence of dace, but were less reclusive in units with dace. Spawning was solely nocturnal in most chub-only experimental units, whereas chubs actively spawned and tended nests in the presence of dace. Among units with chubs and dace, chubs appeared to be more reclusive in units with higher egg predator abundance. Interspecific spawning appears to be beneficial to both species, corroborating previous classifications of nest association as a mutualistic relationship. The dynamics of this interaction appear to be context-dependent; further study is underway.