A review of the hybridisation potential of brown trout with particular reference to invaded environments, and a case study from Newfoundland

Hybridisation is a complex process that begins with mating. Females have more to lose with each hybrid fertilisation than males, so they should avoid it. Even if females choose con-specific males as preferred mates, they often cannot control which additional males release sperm during spawning. Polyandry is ubiquitous and may result in hetero-specific sperm competition between males of different species. In such cases, cryptic female choice (the ability to bias paternity towards certain males under sperm competition) is the last line of defence to prevent hybridisation of her eggs and is highly adaptive if it enables con-specific sperm preference. Such seems to be the case with the hybridisation of Atlantic salmon (Salmo salar) and brown trout (S. trutta) in their native Europe. Under hetero-specific sperm competition, hybrid fertilisations in these fish are reported to be reduced via ovarian fluid-mediated cryptic female choice. It is not known, however, whether the strength of this mechanism is dependent on reinforcement and thus, the historical sympatry/allopatry of hybridising populations. Brown trout are one of the world's worst invasive species. Ecological impacts arise through competition with other species (e.g. Galaxids in the southern hemisphere, Oncorhynchus in western North America). Eastern North America contains native salmonids that evolved in the absence of brown trout but have gametes that are compatible. The 140 -year-old brown trout invasion of Newfoundland is ground zero to study these potential interactions. Their relatively low spread rate across the island may be the result of inherent poor productivity, but data suggest it could also be a function of hybridisation with native Atlantic salmon and brook char (Salvelinus fontinalis).