On one three-spined stickleback sample from Surnadal, near Trondheim, G. alexanderi was detected. It has been suggested to be present in Western Norway, but without precise locality data.
This species is an important addition to the Norwegian fauna because it is a Pacific species, found mainly on the West Coast of North America and in Japan and the Pacific coast of Siberia. In Europe, there is an unconfirmed record from Germany near Berlin (Glaser, 1987) and a number of authenticated records from the UK (Harris, 2008), where the species is thought to be a glacial relict.
The working hypothesis is that this species was brought to the Atlantic basin with the initial expansion of the sticklebacks from the Pacific over 100 000 years ago (Orti et al. 1994). Recolonisation of rivers from the sea by sticklebacks after glaciation has principally led to colonisation by G. arcuatus and G. branchicus. How then did G. alexanderi persist in Norway and the UK during the last ice age?
