Can cryptic female choice prevent invasive hybridization in external fertilizing fish?

Polyandrous mating systems result in females mating with multiple males,
generating opportunities for strong pre-mating and post-mating sexual
selection. Polyandry also creates the potential for unintended matings and
subsequent sperm competition with hybridizing species. Cryptic female
choice allows females to bias paternity towards preferred males under
sperm competition and may include conspecific sperm preference when under
hybridization risk. The potential for hybridization becomes particularly
important in context of invasive species that can novelly hybridize with
natives, and by definition, have evolved allopatrically. We provide the
first examination of conspecific sperm preference in a system of three
species with the potential to hybridize: North American native Atlantic
salmon (Salmo salar) and brook char (Salvelinus fontinalis), and invasive
brown trout (Salmo trutta) from Europe. Using naturalized populations on
the island of Newfoundland, we measured changes in sperm swimming
performance, a known predictor of paternity, to determine the degree of
modification in sperm swimming to female cues related to conspecific sperm
preference. Compared to water alone, female ovarian fluid, in general, had
a pronounced effect and changed sperm motility (by a mean of 53%) and
swimming velocity (mean 30%), but not linearity (mean 6%). However,
patterns in the degree of modification suggest there is no conspecific
sperm preference in the North American populations. Furthermore, female
cues from both native species tended to boost the sperm of invasive males
more than their own. We conclude that cryptic female choice via ovarian
fluid mediated sperm swimming modification is too weak in this system to
prevent invasive hybridization and is likely insufficient to promote or
maintain reproductive isolation among the native North American species.