Climate change and abundance cycles of two sympatric populations of smelt (Osmerus mordax) in the middle estuary of the St. Lawrence River, Canada

Commercial catches of two ecologically distinct sympatric smelt (Osmerus mo rdax) populations segregated along the two shores of the St. Lawrence middl e estuary exhibited inverse patterns with periodicities on the order of 30 years. The influence of water level in the St. Lawrence River and air tempe rature, chosen to reflect variations in hydrology and climate, differed mar kedly between the two populations. Analyses revealed that both water level and temperature were generally positively related with north-shore smelt la ndings and negatively related with south-shore smelt landings. For both pop ulations, a number of significant climatic factors contributing to variance in smelt landings were lagged by one to three years relative to the year o f landings, indicating that climatic variables influenced smelt recruitment . The contrasting role of hydroclimatic variables in driving these abundanc e cycles is likely related to differential exploitation of estuarine habita ts; the south-shore population is associated with shallow shoal habitat, wh ereas the north-shore population is associated with deep channel habitat. T he responses of the two smelt populations also reflect the fundamental ecol ogical differences existing between shoal and channel habitats, indicating that future climate change may differentially affect other populations or s pecies that are segregated between these two habitats.