Biotic and Abiotic Influences on Nest-hatching Outcome in the Japanese Fluvial Sculpin, Cottus pollux

Synopsis I examined the interrelationships among the timing of reproduction, physical characteristics of nests and hatching outcome of male Japanese fluvial sculpin, Cottus pollux, in a Japanese mountain stream with flood disturbance. Breeding took place from mid-February to early April, with bimodal peaks of breeding activity. Male body size had a significant effect on the timing of initial reproduction and mating success. Hatching outcome, by contrast, was not affected by male body size and male mating success. The timing of flood occurrence varied between years, and most nest-hatching failure was observed within 1 week after floods. Failed nests were exposed to faster current velocity than successful nests, and degree-days of failed nests negatively correlated with the magnitude of flood flow experienced. These data suggest that the timing and magnitude of floods substantially affected hatching outcome of the species. I also discuss the adaptive significance of bimodal breeding activity of males from life-history perspective in the face of variable stream flow conditions.