Total vertebral counts ofLycodapus mandibularispopulations in Georgia Strait, British Columbia, are significantly lower than those of southern populations off Oregon and California. The highest temperatures and lowest salinities at 200 m or deeper, where the species is most abundant, are also in Georgia Strait and correlate with the lowest counts. The Strait's deep water originates from tidal mixing of intermediate and estuarine surface waters of lower salinities and higher temperatures, whereas the mechanism in southern latitudes, which instead provides unmixed cooler and saline deep waters, is oceanic up-welling. It is suggested that differences in environment associated with different degrees of upwelling and mixing may cause the lower counts in the Strait of Georgia.Key words:Lycodapus mandibularis, Zoarcidae, meristic variation, inlets, upwelling, temperature, salinity, northeast Pacific Ocean, zoogeography