We developed a population database and classification system based on a combination of molecular genetic data, habitat variables and management issues to assist in defining Conservation Units for steelhead trout in BC. Steelhead distribution was mapped on a 1:50,000 digital watershed atlas. The smallest independent units considered in our system are persistent populations defined as populations that have independent demography and are large enough to have a low risk of extinction. These units are intended to correspond closely with the Viable Salmonid Population definition used by the National Marine Fisheries Science agency in the US. Observed patterns of genetic differentiation in BC supported the use of tributaries of the ocean and large rivers as the theoretical population unit for steelhead. The Watershed Atlas was used to identify these populations. Watershed area and anadromous stream length were used to estimate capacity, and populations in watersheds with maximum run sizes of <20 adults were not considered persistent. Instead, we call these ephemeral populations. This list was reviewed by Regional biologists for obvious errors based on local knowledge.

A variety of watershed characteristics accessible through the Watershed Atlas (e.g. gradient, size, snowfield size, lake area, distance from ocean) can be used to define groups of populations that share similar habitats and, presumably similar selective regimes for adaptive traits associated with these habitats. Two habitat characteristics (migration difficulty, velocity of rearing area) were chosen to explore possible distribution patterns associated with similar selective regimes. Populations with similar migration difficulty are strongly clustered geographically whereas those with similar velocities are not. Populations that share common management issues and policies can be defined using information such as hatchery stocking, commercial harvest data, land use data and angler use statistics. We believe that this approach will facilitate the identification of rare and unusual ecological types, as well as aid the process of managing hundreds of populations with almost no population status data.

Finally, we define Conservation Units (CU’s) for populations identified in the database based on sequential splitting of populations into major groups based on phylogenic lineages (i.e. major phylogenetic groups or MPG’s), run-timing (summer versus winter) and major water drainages. This resulted in the identification of 27 CU’s for steelhead populations in BC. This approach combines the use both of phylogenetic and phenotypic variation to determine units.