| dc.description.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. | |
