Ecology and evolution of sympatric sticklebacks (Gasterosteus): spatial segregation and seasonal habitat shifts in the Enos Lake species pair

Abstract

A pair of stickleback species (Gasterosteus) coexist in Enos Lake on Vancouver Island. Spatial segregation between species and seasonal habitat shifts were investigated by means of monthly surface and bottom samples. During the summer there was clear spatial segregation; both sexes of the benthic species were inshore, reproductive males of the limnetic species were also in the littoral zone, and adult limnetic females and nonbreeding males were at the surface. There were also seasonal habitat shifts. Benthics of both sexes moved onshore in the spring and remained throughout the summer. In the autumn, after the lake turned over, large numbers moved offshore to deeper water and appeared to disperse. It is suggested that this shift in the benthic species is an adaptation to the seasonal oxygen cycle in the hypolimnion. The limnetic species appeared at the surface in spring and remained there throughout the summer and early autumn. In late autumn they disappeared from the surface and did not reappear until next spring. During the winter small numbers were taken in bottom traps. Apparently limnetics move away from the surface during the winter. The adaptive significance of this shift remains unclear.