indicator species provide the answer. For example, Euphausia pacifica is endemic in
the subarctic Pacific and northern California Current, while Euphausia eximia is an
eastern tropical Pacific endemic with a distribution usually just extending into the
southern end of the California Current. The distributions of these two species change
in complementary fashion between years of warm and cold conditions in the sea (Fig.
10.24). The euphausiid distributions (Fig. 10.25) show that advection is the main
factor. The data shown are for March–April, usually the period of maximum extent of
northward intrusion of E. eximia and, conversely, the minimum extent of southward
extension of E. pacifica.
Fig. 10.25 Comparisons of distribution patterns of a subarctic species, Euphausia
pacifica (left), and an eastern tropical Pacific species, Euphausia eximia (right),
between (a) 1958, a warm year, and (b) 1962, a more typical year. The northern
species was not carried so far south and the tropical species was carried much farther
north in the warm year.
(^) (After Brinton 1967b.)