Spatial Segregation in Spawning Sites of Coastal Cutthroat Trout and Steelhead and Implications for Hybridization

Introgressive hybridization is a conservation concern for many fishes and has caused declines in subspecies of native cutthroat trout in the interior western United States. Hybridization in these cases has been caused by anthropogenic introduction of non-native rainbow trout into habitat occupied by subspecies of cutthroat that evolved in allopatry.  However, it may also occur between sympatric native species where it is a natural process, the occurrence of which is determined by the effectiveness of natural reproductive isolating mechanisms.  Understanding the mechanisms that limit or promote hybridization between native species may offer important lessons for conservation in areas of anthropogenically-induced hybridization. In coastal watersheds of the Pacific Northwest native coastal cutthroat trout (Oncorhynchus clarkii clarkii) and steelhead (anadromous coastal rainbow trout—O. mykiss irideus) hybridize, yet maintain species integrity.  One explanation for the lack of complete introgression is spatial segregation of spawning areas between the species but this process is poorly understood.  To determine the degree of spatial reproductive segregation between coastal cutthroat trout and steelhead we collected and identified recently emerged juvenile trout using genetic markers, with the assumption that their distribution was representative of parental spawning areas.  In conjunction with the genetic samples, we collected physical stream habitat measurements in a spatially continuous framework of 35 reaches from tidewaters to headwaters in a small (20 km²) coastal watershed in Washington State.  Cutthroat, rainbow and hybrid trout comprised 35%, 42%, and 23% of fish collected, respectively. Spatial structure in species occurrence was apparent, resulting in varying proportions of each species depending upon location within the watershed.  Our results suggested strong habitat-based segregation of spawning areas between species and watershed drainage area contributing to each stream reach was the best predictor of species occurrence. Drainage area was correlated positively with steelhead occurrence and negatively with cutthroat trout occurrence.  Hybrid fry were found in areas occupied by both parental species but were more prevalent in transitional areas. A similar pattern was observed in older juveniles of both species but there was more overlap, suggesting substantial movement of trout post-emergence.  Our results offer strong support for spatial reproductive segregation as a factor limiting hybridization between steelhead and coastal cutthroat trout.