Population structure of sympatric anadromous and nonanadromous Oncorhynchus mykiss: evidence from spawning surveys and otolith microchemistry

Reproductive isolation between steelhead and resident rainbow trout (Oncorh ynchus mykiss) was examined in the Deschutes River, Oregon, through surveys of spawning timing and location. Otolith microchemistry was used to determ ine the occurrence of steelhead and resident rainbow trout progeny in the a dult populations of steelhead and resident rainbow trout in the Deschutes R iver and in the Babine River, British Columbia. In the 3 years studied, ste elhead spawning occurred from mid March through May and resident rainbow tr out spawning occurred from mid March through August. The timing of 50% spaw ning was 9-10 weeks earlier for steelhead than for resident rainbow trout. Spawning sites selected by steelhead were in deeper water and had larger su bstrate than those selected by resident rainbow trout. Maternal origin was identified by comparing Sr/Ca ratios in the primordia and freshwater growth regions of the otolith with a wavelength-dispersive electron microprobe. I n the Deschutes River, only steelhead of steelhead maternal origin and resi dent rainbow trout of resident rainbow trout origin were observed. In the B abine River, steelhead of resident rainbow trout origin and resident rainbo w trout of steelhead maternal origin were also observed. Based on these fin dings, we suggest that steelhead and resident rainbow trout in the Deschute s River may constitute reproductively isolated populations.