FISHES

J.D. McPhail

ORIGINS OF THE FISH FAUNA

Eighteen thousand years ago the Canadian Montane Cordilleran Ecozone was covered in ice. Here-and-there an occasional mountain peak protruded above the Cordilleran Ice-sheet but these steep, barren nunataks did not support fish. Consequently, with the advance of the Wisconsinan ice sheets, the interglacial fish fauna of this region was either destroyed or pushed into unglaciated regions called glacial refugia. Thus, the fish species now found in the Montane Cordilleran Ecozone are relative newcomers --- recent (postglacial) immigrants from ice-free regions. The geographic distributions of about 75% of the species now found in the MCE include only a single glacial refugium. and it is assumed that they survived glaciation in this refugium (Table 1). In contrast, the geographic ranges of about 25% of the species include more than one ice-free region. Presumably, the preglacial distributions of these species were fragmented by the advancing ice and the isolated fragments survived in more than one refugium.

Three major ice-free regions contributed fish to the MCE. To the north lay the Bering Refugium. An arctic desert --- intensely cold but without enough precipitation to build a major ice sheet. The Yukon was the master river of this refugium and, although none of the fish now found in the MCE survived glaciation exclusively in the Bering Refugium, a molecular study (Redenbach 1997) of the Arctic grayling, Thymallus arcticus, indicates this species colonized the MCE from the north. The Arctic grayling apparently survived glaciation in more than one refugium, and relict populations in northern Michigan (now extinct) and Montana indicate survival in the Mississippi-Missouri system as well as in the Bering Refugium.

To the south and west of the Continental Divide, lay the Pacific Refugium. Relative to the Bering Refugium this was a benign environment. Tundra extended only a few kilometers south of the ice margin and the climate of much of what is now Washington, Oregon, and Idaho was similar to the present climate. The Columbia was the master river of this refugium, and the source of over half the species that now occur in the MCE. The distribution of the northern squawfish, Ptychocheilus oregonensis, is typical of species that survived in the Pacific Refugium.

To the south and east of the Continental Divide, lay the Great Plains Refugium. This vast area to the south of the Laurentide Ice-sheet extended from the foothills of the Rocky Mountains east to the Mississippi River. The distribution of the white sucker, Catostomus commersoni, is typical of a fish that colonized the MCE from the Great Plains Refugium. The master rivers of the Great Plains Refugium were the Mississippi and Missouri rivers. Although these rivers are connected, their confluence lies far to the south of the maximum extent of glaciation. Thus, for some species (e.g., northern pike, Esox lucius) the two river systems were sufficiently isolated that there was little gene-flow between populations in the two drainages and there are now slight, but consistent, genetic differences between pike in the two systems (Seeb et al 1987). The Great Plains Refugium harboured almost four times the number of fish species as the Pacific Refugium (Cross et al 1986; McPhail and Lindsey 1986), yet less than a quarter of the species that now occur in the MCE colonized the region from the Great Plains. There are two reasons for this discrepancy. The obvious reason is that most of the MCE lies west of the Continental Divide and for fish mountains form almost an insurmountable barrier. A less obvious reason is that most Great Plains species are adapted to relatively low gradient environments, and in the MCE even those waters in the MCE that were easily accessible to Great Plains species (e.g., east-slope drainages) have high gradients within the MCE.