A MTDNA ANALYSIS OF SPATIOTEMPORAL DISTRIBUTION OF 2 SYMPATRIC LARVALPOPULATIONS OF RAINBOW SMELT (OSMERUS-MORDAX) IN THE ST.-LAWRENCE RIVER ESTUARY, QUEBEC, CANADA

Ecological isolation through resource partitioning is invoked as a maj or factor for explaining the persistence of genetically distinct yet c losely related sympatric populations. Two genetically distinct sympatr ic populations of anadromous rainbow smelt (Osmerus mordax) exist in t he middle estuary of the St. Lawrence River. The persistence of these coexisting populations in sympatry is in conflict with current theoret ical concepts predicting population richness. In the present study, we performed mtDNA PCR-RFLP analysis of 922 larvae from 33 sampling stat ions to test the hypothesis that the larvae belonging to the two sympa tric smelt populations of the St. Lawrence middle estuary are spatiall y segregated and that such segregation may promote the persistence of the populations. Results clearly revealed spatial homogeneity in the r elative distribution of larvae from the two populations. Consequently, they did not support our working hypothesis that larvae belonging to the two sympatric smelt populations are spatially segregated. Two alte rnative explanations may account for the lack of spatial partitioning observed here. Competition may not be important enough to promote reso urce partitioning at the larval stage. Alternatively, resource partiti oning occurs, but not spatially. This study also demonstrated that the effect of historical events may have been as important as contemporar y ecological settings in determining generic population structure in s melt.