Natural Hybridization between Steelhead Trout (Salmo gairdneri) and Coastal Cutthroat Trout (Salmo clarki clarki) in Two Puget Sound Streams

Abstract

A genetic investigation of anadromous trout populations in the Puget Sound area revealed numerous juvenile individuals from two streams with electrophoretic phenotypes consistent with those expected for hybrid descendents of steelhead trout (Salmo gairdneri) and coastal cutthroat trout (S. clarki clarki). The likelihood of hybridization was evaluated with a hybrid index measuring the relative probability that the combined genotype for a particular fish at several diagnostic loci could have arisen by random mating within each of the two Salmo species. The distribution of hybrid index scores among fish from the two creeks clearly demonstrated the genetic distinctness of the two species and the intermediate genotypic composition of the unknown fish. We concluded that these electrophoretically intermediate fish were natural steelhead–cutthroat hybrids based on their restricted occurrence at specific sample sites in only 2 of 23 streams surveyed, the linear distributions of juveniles from the two parental species within each stream, and the distribution of hybrid index values for a hatchery population of known mixed ancestry. Further, from estimates of gametic disequilibria and the absence of a consistent excess of heterozygotes we suggest that backcrossing may have occurred. The existence of these natural hybrids raises many questions concerning the biological bases for maintaining species integrities in regions of sympatry and indicates the need to fully understand the biological consequences of present and future management practices.

Individual contributions to pooled-milt fertilizations of silver catfish Rhamdia quelen

“…Considering that all tested males exhibited high fertilization rates in the control crosses, the obtained results suggest the dominance of some males over others during the pooled-milt fertilization. Spermatozoa competition in the fertilization of an egg pool with pooled milt of Atlantic salmon was observed by Campton & Utter (1985). Witlher (1988) found dominance of some males of Oncorhynchus tshawytscha in crosses with pooled milt, with variations of 1.4 to 75.6%.…”

“…Paternal effects on early life history are often obscured and unexplained when sperm from multiple males are mixed during fertilization (Rideout et al, 2004). Previous research has demonstrated that spermatozoa competition in pooled-milt results in different paternity contributions from potential male parents, which reduces the effective size of the broodstock (Campton & Utter, 1985;Allendorf & Ryman, 1987). Reported variations in the fertilizing capacity of pooled-milt were confirmed by the verification of family relationships for Chinook salmon Oncorhynchus tshawytscha (Walbaum) (Gharret & Shirley, 1985;Withler, 1988;Withler & Beacham, 1994), rainbow trout Oncorhynchus mykiss (Walbaum) (Gile & Ferguson, 1995;Herbinger, 1995), Atlantic halibut Hippoglossus hippoglossus (L.) (Ottesen et al, 2009) and common carp Cyprinus carpio (L.) (Kaspar et al, 2008).…”

Evaluate the Life History of Native Salmonids in the Malheur Subbasin, Fiscal Year 2008 Annual Report.

“…We used a simple hybrid index (Campton and Utter 1985;Rubidge and Taylor 2004) to classify individuals as pure bull trout, pure brook trout or bull trout 9 brook trout hybrids. For each individual the number of brook trout alleles present was summed and divided by the total number of alleles.…”

Yakima/Klickitat Production Project Preliminary Design Report, Appendix A: Refined Project Goals and Harvest Managment Plan : Experimental Design Plan : Genetic Risk Assessment.

“…Natural interbreeding between steelhead and cutthroat has been reported in the state (Campton and Utter 1985), so there is also the posslbiiity of Type 3 risk. Habitat preferences probably iimlt the opportunity for Interactions between the two species, so the risks are probably negligible.…”