Resource partitioning in the St. Lawrence estuary

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The long term objective of this NSERC-supported research program has been to elucidate how the highly heterogeneous environments that characterise the estuarine transitional zone (where freshwater first mixes with seawater) influence the ecology and the evolution of estuarine fishes. We propose to pursue four major objectives. The first is to assess how the exploitation of the estuarine landscape during development generates the selective forces leading to independent phenotypic trajectories of two sympatric populations of smelt in the St. Lawrence estuary. We will sample two cohorts of smelt of both populations from hatching to reproduction (age 3+), thus providing a longitudinal developmental series for growth, morphological development, survival and sexual maturation as a function of depth, salinity and temperature gradients along transversal and longitudinal axes. The second specific objective is to test the hypothesis that tomcod, the most abundant fish of the ETZ, are morphologically specialized to exploit deep channel and shallow shoal habitat and exhibit a morphological divergence similar to that of the sympatric populations of smelt. Our third specific objective is to exploit this same sampling program to describe the spatial structure of the entire fish community as a function of the ETZ's physical gradients. The fourth specific objective is to focus on another tool with which we hope to reveal the partitioning of resources on a spatial and temporal scale. Stable isotope analyses of carbon, nitrogen and sulphur will be used to define the spatial extent of feeding of young-of-the-year and 1+ fishes. The same stratified sampling plan involving a grid of 30 stations spanning the environmental gradients of the ETZ will serve to map the distribution of all life history stages of all key transitional-zone fishes. These objectives are attainable thanks to the availability of a new sampling platform (the Lampsilis), which permits the sampling of shallow and deep-water habitat, and the use of a combination of innovative technologies including stable isotope analysis and molecular genetics.