Distribution of fishes in the Thunder Bay area of northwestern Ontario since deglaciation, with special reference to the darters (Genus Etheostoma) and the Sibley Peninsula

Abstract

The prevailing theory explaining fish distribution in the western Lake Superior basin states that recolonization was from south to north following the retreating ice margins. Postulated access to post glacial Lake Superior is via the St. Croix River. However, as the ice masses retreated north of Lake Superior, several ice dams held back glacial Lake Agassiz. These were successively removed. Lake Agassiz, which covered much of Manitoba and northwestern Ontario, then catastrophically overflowed into the Superior basin. This allowed fish to gain access to western areas. The Sibley Peninsula and Thunder Bay area provide key evidence for studying the process of fish recolonization. A survey documenting the fishes of Sleeping Giant Provincial Park on the Sibley Peninsula allowed comparison of this area with previously surveyed areas (Isle Royale National Park and Quetico Provincial Park). The distribution of two darters, the Iowa (Etheostoma exile) and the johnny (E. nigrum), suggested that the Thunder Bay area was recolonlzed from both the south and northeast by utilizing Agassiz outlets. Creek chub (Semotilus atromaculatus), rock bass {Ambloplites rupestris) and common shiner (Notropis cornutus) are found at the edge of their range near or in the Kaministiquia basin suggesting either their late movement from the south into Lake Superior or low colonization powers. Colonization from the south is also suggested by johnny darter populations in the Kaministiquia basin. Geographic variation among johnny darter populations reveal that the Kaministiquia basin populations are meristically unique compared to those from other nearby areas. Karoinistiquia basin johnny darters are typically the scaly form and possess a high number of preoperculomandibular pores. Iowa darter populations do not exhibit much variation in merlstic characters but, similar to johnny darters, do exhibit disjunct distribution throughout the Thunder Bay area. Temperatures selected by Iowa darters during temperature selection trials were always cooler than those selected by johnny darters. Preference for cooler temperatures, along with survival in the Missouri refugium, allowed earlier movement northward following deglaciation. Iowa darters, inhabiting areas proximate to early Agassiz overflows, were deposited on both the Sibley Peninsula and Isle Royale. Analysis of rivers based on ichthyofaunal resemblance suggests Kaministiquia basin rivers and those to the south are very similar, likely due to similar points of origin for their ichthyofauna. Similarly, ichthyofaunal resemblance among Sibley Peninsula rivers suggests a common origin. Effects of Isostatic rebound reveals that the Sibley Peninsula contains more species than Isle Royale and the Huron-Porcupine Mountains if basin morphology is taken into account. Lakes on the Sibley Peninsula were elevated and Isolated even later than those of Isle Royale. Deep basins on Isle Royale allow for the existence of many large openwater species and cold adapted smaller species which can not inhabit the shallow lakes of the peninsula. Fishes now inhabiting the Sibley Peninsula and Isle Royale likely arrived in those areas during Agassiz discharges. It is suggested that the prevailing theory of the recolonization of the Lake Superior basin must be revised since Lake Agassiz overflows likely provided a means for fish to move into, rather than through, the Superior basin.