Emigration of the peamouth chub, Mylocheilus caurinus, across a dilute seawater bridge: an experimental zoogeographic study

In British Columbia, the postglacial distribution of the peamouth chub, Mylocheilus caurinus, is limited to certain mainland drainages except for several problematical island occurrences, including Vancouver Island, The saline waters of the 30-mi wide Strait of Georgia evidently act as a barrier to island dispersal. However, during periods of very high runoff from the Fraser River, a corridor of low-salinity water is established between the mouth of the Fraser River and Vancouver Island. For primary-division freshwater fish, this probably constitutes a sweepstakes emigration route consisting of a shallow surface gradient with terminal salinities occasionally falling to levels which peamouth chub can tolerate. In a simulated crossing of Georgia Strait, peamouth chub demonstrated superior survival capabilities in comparison to three other freshwater species. The study lends support to the idea that the presence of chub in the Nanaimo River system on Vancouver Island represents a natural postglacial range extension.