Evidence of Introgressive Hybridization between Bull Trout and Brook Trout

Abstract

Hybridization between native bull trout Salvelinus confluentus and introduced brook trout S. fontinalis occurs over a wide geographic area in the western United States. We described the extent to which introgressive hybridization has occurred between these species using biochemical and molecular genetic techniques in samples collected from five streams in western Montana. We found that about three-quarters of the hybrids detected were male, first-generation (F₁) hybrids. Most of the rest were backcrosses to the parental species, indicating that F₁ hybrids can reproduce. We found no evidence of hybrid swarms in which all individuals were of hybrid origin. Our results suggest that both the reduced fertility of F₁ hybrids and the reduced survival of their progeny prevents these species from forming such hybrid swarms. We also found that hybridization between bull and brook trout tends to occur predominantly between female bull trout and male brook trout, indicating that hybridization represents greater wasted reproductive effort for bull trout than for brook trout.